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KS01: Reclaimed water KS02: Managing your building services KS03: Sustainable low energy cooling: an overview KS04: Understanding controls KS05: Making buildings work KS06: Comfort KS07: Variable flow pipework systems KS08: How to design a heating system KS09: Commissioning variable flow pipework systems KS10: Biomass heating KS11: Green roofs KS12: Refurbishment for improved energy efficiency: an overview KS13: Refrigeration KS14: Energy efficient heating KS15: Capturing solar energy KS16: How to manage overheating in buildings KS17: Indoor air quality and ventilation KS20 KS18: Data centres: an introduction to concepts and design KS19: Humidification

ISBN 978-1-906846-26-8 Direct and accessible guidance from key subject overviews to implementing practical solutions

9 781906 846268 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

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ISBN: 978-1-906846-26-8

This document is based on the best knowledge available at the time of publication. However no responsibility of any kind for any injury, death, loss, damage or delay however caused resulting from the use of these recommendations can be accepted by the Chartered Institution of Building Services Engineers, the authors or others involved in its publication. In adopting these recommendations for use each adopter by doing so agrees to accept full responsibility for any personal injury, death, loss, damage or delay arising out of or in connection with their use by or on behalf of such adopter irrespective of the cause or reason therefore and agrees to defend, indemnify and hold harmless the Chartered Institution of Building Services Engineers, the authors and others involved in their publication from any and all liability arising out of or in connection with such use as aforesaid and irrespective of any negligence on the part of those indemnified.

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Note from the publisher This publication is primarily intended to provide guidance to those responsible for the design, installation, commissioning, operation and maintenance of building services. It is not intended to be exhaustive or definitive and it will be necessary for users of the guidance given to exercise their own professional judgement when deciding whether to abide by or depart from it.

Any commercial products depicted or described within this publication are included for the purposes of illustration only and their inclusion does not constitute endorsement or recommendation by the Institution. Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

CIBSE Knowledge Series: KS20

Authors Mike Farrell Gay Lawrence Race

Editor Ken Butcher

CIBSE Head of Knowledge Nicholas Peake

Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Contents

Symbols and units

Part 1: Psychrometry and the psychrometric chart

1 Introduction ...... 1 1.1 Why you need to know about the psychrometric chart ...... 2

2 Psychrometric fundamentals ...... 2 2.1 Humid air ...... 2 2.2 Dry air and its composition ...... 2 2.3 Atmospheric water vapour ...... 3

3 The psychrometric chart ...... 4 3.1 Dry bulb temperature ...... 6 3.2 Moisture content ...... 7 3.3 Percentage saturation and relative humidity...... 10 3.4 Wet bulb temperature ...... 11 3.4.1 Sling wet-bulb ...... 11 3.4.2 Screen wet-bulb ...... 12 3.4.3 The psychrometric equation ...... 12 3.5 Specific enthalpy ...... 13 3.6 Specific volume and density ...... 14 3.7 Dew point temperature ...... 14 3.8 Plotting state points on the psychrometric chart ...... 15 3.9 The CIBSE psychrometric chart ...... 16 3.10 Psychrometric calculations ...... 16 3.11 Using the chart for psychrometric processes ...... 19 3.11.1 Sensible heating and cooling...... 20 3.11.2 Humidification and dehumidification...... 21

Part 2: Using the psychrometric chart in practice

4 Design conditions ...... 23 4.1 Thermal comfort ...... 23 4.2 Internal design conditions ...... 24 4.3 External design conditions ...... 24

5 Psychrometric processes ...... 26 5.1 The room process ...... 26 5.2 Mixing air streams ...... 28 5.3 Air heating ...... 29 5.4 Air cooling ...... 31

Practical psychrometry 5.5 Air humidification and humidifiers ...... 32 5.5.1 Water spray types...... 33 5.5.2 Steam humidifiers...... 33 5.5.3 Psychrometric processes...... 33 5.5.4 Humidifier load...... 34 5.6 Heat recovery processes and heat exchangers ...... 34 5.6.1 Effectiveness/efficiency...... 35 5.6.2 Unit types...... 35

6 Applied psychrometry ...... 39 6.1 Centralised systems ...... 40 6.1.1 All-air system using all outside air...... 40

Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 6.1.2 All-air systems with recirculation...... 43 6.2 Unitary systems ...... 45 6.2.1 Dry unit operation ...... 46 6.2.2 Wet unit operation ...... 47 6.3 Local systems ...... 48

7 Further reading ...... 48

Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Symbols and units

A variety of suffices are used in addition to those shown to indicate various conditions (e.g. tai is used for indoor air temperature and tao for outdoor air temperature etc.). These are too numerous to list but are defined within the main text.

Air changes per hour N (h–1) Apparatus dew point ADP (°C) Contact factor/effectiveness E (—) Efficiency h (%) Enthalpy (specific) h (kJ/kg dry air)

Fluid enthalpy hf (kJ/kg) Latent heat of evaporation hfg (kJ/kg) Specific enthalpy of dry air ha (kJ/kg) Specific enthalpy of water vapour (gas)

at dry bulb temperature hg (kJ/kg) Flow rate (air): • — by mass ma (kg/s) • 3 — by volume Va (m /s) Flow rate (heat) F (kW) Latent heat flow FL (kW) Sensible heat flow Fs (kW) Mass of dry air ma (kg) Mass of water vapour mv (kg) Moisture content (vapour content) g (kg/kg dry air)

Room moisture content gr (kg/kg dry air) Saturated moisture content (i.e. moisture

content of saturated moist air) gs (kg/kg dry air) Outdoor moisture content go (kg/kg dry air) Percentage saturation m (%) Pressure p (kPa)

Air pressure pa (kPa) Atmospheric pressure pat (kPa) Saturated vapour pressure ps (kPa) Vapour pressure pv (kPa) Psychrometric plots: — apparatus dew point X — off cooling coil condition C — off heater condition H — outside condition O — mixed condition M

— return (exhaust) air R or Rret — room condition R Relative humidity (RH) f (%)

Practical psychrometry 2 Specific heat capacity cp (W/m ·K) Specific volume n (m3/kg) Temperature t (°C)

Air temperature ta (°C) Dew point temperature tdp (°C) Dry bulb temperature tdb (°C) Mean radiant temperature tr (°C) Operative temperature tc (°C) Temperature difference Dt (K) Water temperature tw (°C) Wet bulb temperature twb (°C) Velocity v (m/s) 3 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Volume V (m )

Note: The symbol q is often used for temperature (including in CIBSE Guides) but the decision has been made to use the more familiar symbol t in this publication.

Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Part 1: Psychrometry and the psychrometric chart Psychrometry

1 Introduction Air is made up of a mixture of dry air and water vapour and Our atmosphere, the air all around us, is something that we take for granted. psychrometry is the science which studies the physical properties and We expect it to contain adequate oxygen, to be clean and fresh, and provide behaviour of this humid air. us with a comfortable environment; yet in many situations the air will need Psychrometry is used in many to be treated, cleaned, heated or cooled to provide the required comfort industries in addition to building conditions within buildings. Knowledge of the properties and behaviour services engineering, e.g. aeronautical engineering, of air is therefore essential for those involved in achieving comfortable agriculture, industrial drying of environments for occupants and in designing and operating the systems that crops and pharmaceuticals, food technology, meteorology and help to realise this, such as air conditioning. others.

This publication provides a practical introduction to psychrometry for engineers, students and anyone who wants an insight into the subject. It is not intended to provide a comprehensive theoretical approach to psychrometry but to give the newcomer to the topic enough information to understand Psychrometric chart how the psychrometric chart is constructed and how the information it provides can be used. It will show how psychrometry, the psychrometric The psychrometric chart is a chart that shows the properties of humid chart and psychrometric processes are used in the practice of building air graphically. services engineering.

The nature of atmospheric air and its properties is examined, together with the quantities by which it is measured and the processes by which it can be modified. The parameters commonly used to provide specifications for An engineer’s comment designs, and for measuring air conditions are considered. “The psychrometric chart is a very useful tool when investigating the Having explained the construction of the psychrometric chart the various condensation risk for residential psychrometric processes and the equipment used to provide them are developments.” explored. These include the changes which air undergoes as it passes through an occupied space and the engineered processes such as heating, cooling, humidification, dehumidification and mixing.

With the basics and processes covered there is a brief overview explaining how the natural external environment can be transformed into a controlled internal environment by using items of equipment that can vary the Psychrometry and building services temperature and humidity of air within air conditioning systems. This A knowledge of psychrometry, the section is limited to describing some typical examples of sample systems. psychrometric chart and psychrometric processes is It is intended that the reader should understand the basic operation of the essential to understand the way air systems but it does not address the design process in detail. behaves in building services systems and to appreciate the required system components, control needs and system The publication assumes knowledge of the general Gas Laws and some basic performance criteria needed to scientific notation and symbols. This information can be found in a number of achieve comfortable internal environments. general science publications as well as the ones given in the bibliography.

Practical psychrometry 1 An engineer’s comment 1.1 Why you need to know about the psychrometric chart

“When looking at system choices The psychrometric chart is very useful during system design as a modelling for an office design in Hong Kong the psychrometric chart was a design tool to enable options to be explored and to investigate system simple way to show the impact of performance under different conditions. Because the chart shows visually the designing with very hot and humid external conditions.” changes that air goes through as it is heated, cooled, mixed, humidified or dehumidified, it is easy to explore changes and control requirements.

For example, by using the chart to explore changing load requirements (the ‘what if?’ questions), the response of a proposed system can be quickly investigated to see if it can cope with the changes or the control elements, and sequencing can start to be developed. Once the system processes

Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 have been plotted on the chart, the engineer/building professional can ask Psychrometric chart questions such as:

The psychrometric chart is a very — ‘What if there is a sudden cold snap?’ The chart may be used to see useful tool to explore the ‘what if?’ what happens if the external temperature suddenly falls, and how the questions and look at system response and control under system will adapt. changing conditions. — ‘What if there is a sudden increase in internal gains?’ Suppose fifty people enter the room for a meeting/lecture; what happens to the internal conditions? What sensors are needed? What needs to change?

In its simplest form the psychrometric chart can show what processes are needed, what equipment is required to carry them out, and how to get from a design external condition to a desired room condition.

An engineer’s comment Equally, processes drawn on the chart can be built-up to give a snapshot of how a system will respond. By looking at the psychrometric process for “We can use the chart for going beyond our base design conditions each item of equipment (pre-heater, heater, cooler, humidifier etc.), these to look at scenarios that worry us processes can be linked to show an overall picture of what happens as the air as engineers.” moves through the plant.

2 Psychrometric fundamentals

2.1 Humid air

The atmosphere is made up of a mixture of gases, known as ‘dry air’, and varying amounts of water vapour. The combination of the two is known as An engineer’s comment ‘humid air’.

“Knowledge of psychrometry is essential for air conditioning design, 2.2 Dry air and its composition and the psychrometric chart is really useful to help you think about your design and how it will Dry air comprises a mixture of gases of which nitrogen makes up about 78% work (and check that it will and oxygen 21% by volume, with the rest being made up of CO2 and the work!)” inert gases (the ‘noble gases’): argon, helium, xenon, neon and krypton.

2 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

There are also small amounts of other gases called trace gases. With a mixture Dalton’s Law of non-reactive gases, such as these, each gas behaves independently of the Dalton’s law of partial pressures, others and contributes its own ‘partial pressure’ to the total atmospheric also known as Dalton’s Law, states pressure. This is described by Dalton’s law of partial pressures (see box). As that the total pressure of a mixture of gases is equal to the sum of the the proportions of the gases comprising atmospheric air remain more or less partial pressures for each individual constant, air can be regarded as if it were a single gas in its own right. This gas. The partial pressure is the pressure each gas would exert by greatly simplifies psychrometric calculations. itself at the same volume and temperature. At sea level atmospheric pressure is normally within the range of 95–105 kPa (950–1050 mbar) dependent upon weather conditions. The standard atmospheric pressure is taken as a pressure of 101.325 kPa (1013.25 mbar) and this is the pressure used as the basis for the CIBSE psychrometric chart.

2.3 Atmospheric water vapour Standard atmospheric pressure

In addition to the above gases, the atmosphere contains a varying amount Standard atmospheric pressure is of water vapour. Water vapour is in fact a gas but whenever a substance is 101.325 kPa. This is the pressure used as the basis for the standard present in both its liquid and gaseous form the latter is known as vapour. CIBSE psychrometric chart. Water vapour obeys the Gas Laws in the same way as any other gas. The water vapour is completely independent of the other atmospheric gases and its behaviour is not affected by their presence or absence.

Figure 1:

Water Vapour pressure vapour

Vapour pressure

Water liquid

Composition of the atmosphere

Composition of dry air by volume: — nitrogen: 78% — oxygen: 21% — other gases: 1%

Within a liquid the molecules are in motion and thus have kinetic energy. Composition of humid air by volume: This energy allows some of the molecules to escape (i.e. evaporate) as a gas, — dry air: 98–99% which is known as a vapour. As more molecules escape, the amount of vapour — water vapour: 1 to 2% increases, as does the vapour pressure it exerts. This increasing vapour Atmospheric pressure: pressure makes it more and more difficult for water molecules to escape. — 95 to 105 kPa When equilibrium is reached no more water can evaporate and the vapour is said to be ‘saturated’. Note that this does not mean it is wet. Vapour is dry and invisible as it is a gas. This process will occur whether or not air or any other gases are present.

Practical psychrometry 3 With an increase in temperature the molecular activity of the water will increase, as will the kinetic energy. This will enable more molecules to escape as vapour, which increases the vapour pressure until a new equilibrium point is reached at a new saturated condition. This can be illustrated graphically by plotting vapour pressure (pv) at saturation against temperature; the resulting curve is the saturated vapour pressure (ps). Table 1 shows the saturated vapour pressure at various temperatures. CIBSE Guide C (CIBSE, 2007) contains tables giving the saturated vapour pressures for temperatures between –10 °C and +60 °C.

Table 1: Temperature (°C) Saturated vapour pressure (kPa) Saturated vapour pressure 10.0 1.23 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 for water at various temperatures 20.0 2.34 (source: CIBSE Guide C) 30.0 4.24 40.0 7.38 50.0 12.33 60.0 19.92

This relationship forms the basis of the psychrometric chart, see Figure 2, with the saturated vapour pressure curve (i.e. 100% humidity) being the maximum condition line. All psychrometric conditions will occur under this curve. This relationship is a constant for all atmospheric pressures. The difference between the actual vapour pressure and the saturated vapour pressure at the same temperature for any psychrometric condition is known as the drying force.

Figure 2: Basic psychrometric chart

Saturated vapour pressure Vapour pressure Vapour

Temperature of air

3 The psychrometric chart

The psychrometric chart is a means of showing the properties of humid air graphically. Tabular data on the properties of humid air are also available but using graphical information enables psychrometric processes (i.e. the physical

4 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

changes that air goes through as it is heated, cooled etc.) to be shown easily Saturated vapour and quickly in diagrammatic form. This is particularly useful when looking at system design as it allows options to be explored and system performance Saturated vapour occurs when the space above the liquid is saturated under different conditions investigated. with vapour particles — as many molecules are re-entering the liquid as are leaving it through When the chart is used to investigate the behaviour of air as it is treated in, evaporation. The amount of vapour the air can hold depends for example, an air conditioning system, it is useful to show the properties on temperature — warmer air can that are most relevant to that treatment. These include: hold more moisture, as vapour, than colder air. — temperature: to look at comfort requirements, required supply temperatures and temperature changes throughout the system or at different times of year Psychrometric conditions — humidity: to look at comfort requirements and condensation issues All psychrometric conditions — moisture content: to look at the amount of vapour that has to be occur under the saturated vapour pressure curve on the psychro- physically removed or added to the air to achieve the required metric chart. comfort conditions, which will provide information on the required performance of any humidification or de-humidification equipment

— energy: to look at the energy needed to heat or cool the air, for The psychrometric chart example. The first psychrometric chart was devised by Willis H Carrier in 1904 The CIBSE psychrometric chart was originally constructed by plotting vapour — at about the same time as the pressure against dry bulb temperature, with the saturated vapour pressure very first scientifically designed air conditioning system with humidity line, giving the familiar chart shape. Usually the vapour pressure scale is not control — in order to provide an shown, being replaced with the more convenient moisture content scale. This easy way to estimate the heat gain and moisture gain to the air. chart has very slightly curved wet bulb temperature and enthalpy lines. The chart is designed to show the properties of air that are most The current CIBSE psychrometric chart has been designed and constructed useful to the design of air using the two fundamental properties of mass (moisture content) against conditioning and other systems, providing information on the energy (enthalpy) as linear co-ordinates, to give greater accuracy in plotting temperature, humidity and energy the processes that use these key values. However, the chart is constructed of the air. with the enthalpy axis skewed so that the dry bulb temperature scale can still be used as the bottom scale. This gives perfectly straight lines of wet bulb temperature and enthalpy. The dry bulb temperature lines are also straight, but not quite parallel. So the chart is presented with temperature shown on the horizontal (x-) axis against moisture content (g) on the vertical (y-) axis.

In both cases, as the chart is based on moisture content rather than vapour pressure, the chart has to be plotted for a specific atmospheric pressure. The standard chart is for an atmospheric pressure of 101.325 kPa, but charts can be constructed, and are available, for other atmospheric pressures.

Because moisture content and enthalpy are the linear co-ordinates for the chart, lines of constant moisture content are straight and parallel as are the lines of constant enthalpy. All other physical properties are then not precise

Practical psychrometry 5 linear scales although some might appear to be so. The enthalpy scale is given at an angle so that dry bulb temperatures can be shown on the x-axis. The dry bulb temperature scale is based on the construction of the 30 °C dry bulb temperature line at right angles to the horizontal lines of constant moisture content. So, although the lines of dry bulb temperature appear to be vertical, in fact only the 30 °C line is truly vertical with the other temperature lines diverging very slightly to either side.

The full chart can look very complicated on first view, see Figure 3, so the Figure 3: next section describes the key quantities used on the psychrometric chart to The CIBSE psychrometric show how it is built up. The chart is shown on a larger scale as Figure 18. chart Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 115 120 125 130 135 140

110 Percentage saturation / % 90 80 70 60 50 40 30 20 105 0·030 0·90 0·029 100

0·028 140 30 95 0·027 90

0·026 135

85 0·025 Based on a barometric 130 pressure of 101·325 kPa 80 0·024 0·1 0 0·2 0·023 75

0·3 125 0·022 0·4

70 0·5 0·021 65

25 120 0·6 0·020 ) –1 0·7 60 . kg 0·8 0·019 0·9 Sensible/total heat 115 55 0·018 1·0 ratio for water –1 0·9 added at 30 C 3.kg 0·8 0·017 110

0·7 50 Specific volume / m ) –1 0·016 dry air)

0·85 –1 kg

0·6 Specific enthalpy / (kJ .

. 45

0·5 20 0·015 105 40 0·4 0·014

0·3 35 ) 0·2 g 0·013 100 in 0·1 0 sl ( C ° 0·012 30 / re (kJ / enthalpy Specific u 95 t Moisture content / (kg ra 0·011 e 15 p 25 em t 0·010 lb u b t- 90 20 e W 0·009 15 10 0·008 0·80 85 10 0·007

5 0·006

5 80 0·005

0 0 0·004 75

–5 0·003 0·75 –5 0·002 –10 70 –10 0·001

0·000 –10 –5 0 5 10 15 20 25 30 35 40 45 50 55 60 Dry-bulb temperature / C

45 50 55 60 65 20 25 30 35 40 –10 –5 0 5 10 15 Specific enthalpy / (kJ.kg–1)

3.1 Dry bulb temperature, tdb (°C)

One of the key factors when considering comfort (and the heating or cooling of the air to achieve this) is the temperature of the air. To obtain a ‘true’ air temperature a conventional thermometer must be shielded from thermal radiation, such as sunlight, and have a completely dry moisture-free bulb. In psychrometry, this normal everyday temperature is referred to as the ‘dry

6 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

bulb temperature’ (tdb) to distinguish it from the other temperatures that are used. On the CIBSE psychrometric chart, dry bulb temperature is given as a linear scale and shown as the horizontal (x-) axis, with lines of temperature values shown ‘vertically’. Note that only the 30 °C line is actually a true vertical, with the temperature lines either side diverging very slightly from the vertical.

Dry bulb temperature is normally measured with a mercury-in-glass thermometer with the bulb shielded from thermal radiation, or by reading the dry-bulb on a sling psychrometer (see section 3.4). A suitable digital thermometer can also be used, again protected from radiation influences.

Figure 4: Lines of dry bulb temperature plotted on the psychrometric chart

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

3.2 Moisture content, g (kg/kg) The psychrometric chart and air conditioning calculations Moisture content (or, more correctly, ‘vapour content’) is defined as the mass of water vapour associated with one kilogram of dry air in an air–water The psychrometric chart uses moisture content, rather than vapour mixture. (Note that this means it is the mass of water vapour added vapour pressure, as one of the to one kilogram of dry air, not the amount in one kilogram of the mixture.) axes because it is more useful for air conditioning calculations to This vapour mass is called ‘moisture content’ on the CIBSE chart. Moisture know how much moisture (i.e. content (g) is the mass of water vapour (m ) per unit mass of dry air (m ), i.e: how many grams or kilograms) v a need to be added or removed.

mv g = — (1) It uses dry bulb temperature, rather than enthalpy, for the other ma axis because it is more useful in air conditioning design and comfort considerations to consider room The usual units are kg/kg although sometimes, as the amounts are small (e.g. and supply temperatures when air at standard pressure at 20 °C and 50% humidity has a moisture content of looking at the air condition. 0.0074 kg/kg), it is expressed as g/kg (e.g. 7.4 g/kg) in which case extra care must be taken to ensure consistent units in any calculations.

Moisture content is sometimes also referred to as ‘specific humidity’.

In order to plot lines of moisture content on the psychrometric chart moisture content values need to be calculated for the values of dry bulb

Practical psychrometry 7 temperature and vapour pressure. Although related to the amount of water vapour associated with a 1 kg dry air sample, the vapour pressure is not directly proportional to it (although it is nearly so) and so the values for moisture content shown on the chart must be calculated using the combined gas law: p V = m R T (2)

Moisture content where (for each gas) p is the partial pressure of gas (Pa), V is the volume of gas 3 Although the CIBSE psychrometric (m ), m is the mass of gas (kg), R is the gas constant for the gas (J/kg·K) and T chart uses the term ‘moisture is the absolute temperature of the gas (K). content’, it is important to note that humid air contains water vapour and therefore the term ‘moisture’ is slightly misleading. As moisture content is a ratio of the mass of water vapour (mv) to the mass Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Visible drops of moisture in air of dry air (ma) the equation can be applied to both water vapour and dry air cause mist and fog, whereas with humid air the vapour content is and shown as a ratio and, as they both occupy the same volume and are at the invisible, even in very humid air. same temperature:

pv mv Rv — = ——– (3) pa ma Ra

where pv is the vapour pressure of water vapour (kPa), pa is the air pressure (kPa), mv is the mass of water vapour (kg), ma is the mass of dry air (kg), Rv is the gas constant for water vapour (J/kg·K) and Ra is the gas constant for dry air (J/kg·K).

Therefore: An engineer’s comment mv pv Ra “I used psychrometric charts for — = ——– (4) predicting a rough number of m p R hours per year that plumes would a a v form in cooling tower discharge air assuming a condenser water supply set point of 30 °C. This was a i.e: contentious issue on a project I worked on.” pv Ra g = ——– (5) “I’ve also used it to determine p R reheat requirements for air a v handlers in high density occupancy spaces.” where g is the moisture content (kg/kg of dry air).

The values of the gas constants for air and water vapour are 287 J/kg·K and 461 J/kg·K respectively, and the air pressure (pa) is atmospheric pressure (pat) minus vapour pressure (pv). Then, since Ra / Rv = 287/461 = 0.622, the moisture content is:

pv g = 0.622 ———– kg/kg of dry air (6) (pat – pv)

8 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 5: Lines of moisture content on the psychrometric chart

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

Saturated vapour pressure

On the CIBSE psychrometric chart the moisture content is a linear scale and Saturated vapour pressure is shown on the vertical (y-) axis, with lines of moisture content values shown independent of atmospheric pressure, but moisture content at straight, parallel and horizontal (see Figure 5). Note, however, that the 100% saturation is not. moisture content axis is not a true vertical, i.e. it is not at 90° to the x-axis. This is very obvious on a psychrometric chart with a wide range of values, e.g. from 10 to 120 °C as shown below in Figure 6, but less so on the more commonly used chart with the range of –10 to 60 °C as shown in Figure 5 above and Figure 3.

Figure 6: 10–120 °C psychrometric chart

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

The saturation moisture content line, i.e. the moisture content of fully saturated air at that temperature (see Figure 7), is plotted as a curve, which represents 100% saturation and 100% humidity, on a graph of dry bulb temperature against moisture content as the first step in producing a psychrometric chart.

Practical psychrometry 9 Figure 7: 100% saturation line 100% saturation line

Saturation moisture content at t °C

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C) Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 3.3 Percentage saturation, m (%), and relative humidity f (%) Percentage saturation and relative humidity In psychrometry, humidity is usually specified in terms of ‘percentage The difference between percentage saturation and relative saturation’ (m), which is defined as the ratio of the moisture content (g) of the humidity is very small, normally a air to the saturation moisture content (gs) of the air at the same temperature. fraction of 1% over the normal comfort range within the UK, and This is normally expressed as a percentage. in fact the two terms are often treated as being synonymous in g practice. However, at higher m = — Í 100 (7) temperatures such as may occur in gs hotter climates, the values can diverge by 2–7% and it is important to be clear which value Traditionally humidity was specified in terms of ‘relative humidity’ (rh orRH , is being used for design. symbol f), which is defined as the ratio of the actual vapour pressure of the air condition (pv) compared with the saturated vapour pressure (ps) at the same temperature. This is also normally expressed as a percentage:

pv f = — Í 100 (8) ps

Warning! The values for percentage saturation and relative humidity are the same when Although percentage saturation is the air is completely dry or fully saturated, i.e. 0% and 100% respectively. commonly used for psychrometry, The greatest divergence between the two figures is at the midway point, i.e. design conditions given in a design brief can sometimes be given as approximately 50% saturation. The difference between relative humidity percentage saturation and and percentage saturation is very small, normally a fraction of 1% over the sometimes as relative humidity. Depending on the design location normal comfort range and, in fact, the two terms are often treated as being and type, this can make a synonymous in practice. However at higher temperatures the values diverge difference, so may need to be clarified with the client. more and in these cases, for example when looking at industrial drying or Psychrometric data tables in CIBSE grain storage, it is very important to be clear which term is being used in Guide C can be used to find corresponding values of the two order to use the correct value. For example, at 60 °C a percentage saturation parameters. of 50% is equal to a relative humidity of 55.5%, i.e. a 10% difference between the two values. When designing for more extreme external conditions it is also necessary to be precise. For example, temperatures in Dubai or Kuwait can reach over 40 °C. At an external design temperature of

10 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

45 °C and around 35% saturation, as may be experienced in Kuwait City, the relative humidity value is around 37.3%, giving a difference between the two values of over 6%. Therefore a confusion between percentage saturation and relative humidity could result in a 5–10% error at higher temperatures.

Lines of percentage saturation are shown on the chart at 10% intervals. Percentage saturation is commonly used in specifications to define the psychrometric condition required in a space.

80% 100% saturation line Figure 8:

60% Lines of percentage saturation on the 40% psychrometric chart

20%

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

3.4 Wet bulb temperature, twb (°C)

Although, together with dry bulb temperature, percentage saturation defines the psychrometric condition of an air sample, it is very difficult to measure directly. To resolve this problem the concept of wet bulb temperature was devised. It is defined as the temperature measured by a thermometer with a wetted bulb. There are two versions: ‘sling’ and ‘screen’.

3.4.1 Sling wet-bulb Figure 9: This is the reading from a psychrometer containing dry bulb and Sling psychrometer wet bulb thermometers, where the rate of air movement across the thermometer bulbs is controlled either by a fan or whirling it through the air at a prescribed rate. Sling wet-bulb readings are more accurate than screen wet-bulb readings (see below) and are therefore preferred for psychrometric calculations. Figure 9 shows a sling psychrometer containing two thermometers — one covered with a muslin cloth wick that dips into a water reservoir so that the thermometer bulb is kept wet (see Figure 10). To take a reading the psychrometer is spun quickly round like a football rattle, and the values of both the dry bulb and wet bulb thermometers then read immediately on stopping. Although only the wet-bulb reading is needed to give wet bulb temperature, the dry-bulb reading will be needed in order to calculate the value of percentage saturation (see section 3.4.3).

Practical psychrometry 11 Figure 10:

Dry bulb and wet bulb Dry bulb Wet bulb temperatures temperature temperature

Wet-bulb depression

Air

Water vapour Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

The wet-bulb reading will be lower than the dry-bulb reading due to the cooling effect of the water evaporating from the muslin. The dryer the surrounding air (i.e. lower percentage saturation) the more evaporation will take place and the greater the temperature difference. The difference between the dry-bulb and wet-bulb readings is known as the wet-bulb depression. The rate of moisture evaporation from the wet bulb depends on the air speed over the bulb. However, the value becomes independent of the air velocity at speeds above 2 m/s, hence the reading from a sling psychrometer is more accurate than that from a screen psychrometer.

3.4.2 Screen wet-bulb

This is typically obtained when the thermometer is mounted in a louvred box known as a Stevenson screen. This is subject to the vagaries of the wind and is therefore less accurate than the sling readings. It is mainly used for the collection of meteorological data.

Psychrometric equation 3.4.3 The psychrometric equation

The psychrometric equation relates The relationship between vapour pressure and wet bulb temperature for any wet bulb temperature to the corresponding vapour pressure and given psychrometric condition is given by the psychrometric equation: to atmospheric pressure.

pv = ps – A pa (tdb – twb) (9)

where pv is the vapour pressure (kPa), ps is the saturated vapour pressure at the wet bulb temperature (kPa), pa is the atmospheric pressure (kPa), tdb is the dry bulb temperature (°C), twb is the wet bulb temperature (°C) and A is the psychrometric constant (K–1).

The psychrometric constant is 6.66 Í 10–4 K–1 for temperatures above 0 °C and 5.94 Í 10–4 K–1 for temperatures below 0 °C for sling wet-bulb readings.

12 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 11 shows lines of wet bulb temperature as they appear on the psychrometric chart.

Figure 11: Lines of wet bulb temperature (sling) on ) C ° ( the psychrometric chart b t w , re tu ra e p m te b ul b et W

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

3.5 Specific enthalpy, h (kJ/kg)

Specific enthalpy is the total enthalpy of humid air (kJ/kg dry air). Enthalpy is a measure of energy but since the only form of energy relevant to psychrometry is heat, specific enthalpy is the de facto measure of the heat in the humid air. It is calculated above a zero enthalpy at 0 °C and zero moisture content. Enthalpy lines are not shown directly on the CIBSE psychrometric chart; instead two scales are drawn on the chart and to find the specific enthalpy a straight edge has to be laid across the chart such that it reads the same on both scales and passes through the point of interest (see Figure 12).

Figure 12:

Y Specific enthalpy scale on the psychrometric chart

X Specific enthalpy (kJ/kg)

Y Specific enthalpy (kJ/kg) Sensible and latent heat

In psychrometry, ‘sensible heat’ X refers to a change in the enthalpy Specific enthalpy (kJ/kg) (or energy content) of humid air at constant moisture content, and ‘latent heat’ refers to a change in the enthalpy (or energy content) of Enthalpy, or heat, is made up of two components: sensible heat and latent humid air at constant temperature. heat. Changes in enthalpy at constant moisture content are termed ‘sensible’ heat, and changes occurring at constant dry bulb temperature are termed ‘latent’ heat. The difference in enthalpy between two air conditions is often a combination of both sensible and latent heat (see Figure 13).

Practical psychrometry 13 Figure 13: Specific enthalpy change Latent on the psychrometric chart Enthalpy change

Sensible B gB g A A

Moisture content, g (kg/kg)

tA tB

Dry bulb temperature, tdb (°C)

3.6 Specific volume, n (m3/kg) Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Specific volume (denoted by Greek ‘nu’, n) is the volume of unit mass of dry air at a given temperature, normally expressed as m3/kg. The addition of associated vapour has no effect on the volume. Lines of specific volume appear on the chart as steep downward sloping lines (see Figure 14).

Figure 14: Lines of specific volume on Specific volume, ν (m3/kg) the psychrometric chart

Moisture content, g (kg/kg)

Dry bulb temperature, tdb (°C)

The density (r) of air (kg/m3) is of a humid air sample and includes the mass of Specific volume and air density the associated water vapour; thus specific volume is not exactly the reciprocal of density, i.e: Specific volume is defined in terms of unit mass of dry air, whereas air density is defined as the mass of humid air per unit volume. Which r = (1 + g) / n (10) one to use depends on the purpose of the calculation. It is usual to use specific volume for air In practice the difference is very small and negligible for most practical conditioning load calculations and purposes in air conditioning system design. Density values are not shown density when measuring flow rates 3 through pressure drop devices. on the chart and, in practice, a value of 1.2 kg/m is commonly used as an approximation for the density of air.

3.7 Dew point temperature, tdp (°C)

The dew point temperature is not shown directly on the psychrometric chart. It is the temperature at which, if the sample were cooled slowly, it would

14 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

reach saturation point and beyond which any further cooling would result in condensation occurring. This is an important temperature for building professionals as condensation is often a problem in buildings, both on building surfaces and within building services systems.

Figure 15: Dew point temperature

e in l n o ti ra tu g Condensation risk a s % g is the moisture 00 1 content at When analysing condensation risk

t °C (dry-bulb) Moisture content, g (kg/kg) for a building and/or building services system dew point

tdp t temperature is a key parameter.

tdp is the dew point temperature at t °C (dry-bulb)

3.8 Plotting state points on the psychrometric chart

Any air condition plotted on the psychrometric chart is known as a ‘state point’. Any two parameters will suffice to plot a state point and the others can then be read-off as required. In practice, the two most common combinations are dry bulb and wet bulb temperatures, which are usually obtained from measurement, and dry bulb temperature and percentage saturation, obtained from specifications. Figure 16 shows a state point plotted on the psychrometric chart and the values that can then be read from the chart; Figure 17 provides more detail.

µ % sat Figure 16: Simple state point on the psychrometric chart ν m3/kg

h kJ/kg twb °C

g kg/kg

Moisture content, g (kg/kg)

t °C State point

Dry bulb temperature, tdb (°C) Any air condition plotted on the psychrometric chart is known as a A variety of suffices are used in addition to those shown to indicate various ‘state point’ and is fixed by any two psychrometric properties. conditions but these are too numerous to list (e.g. ta is generally used for air

Practical psychrometry 15 temperature but tai would be used for ‘indoor air temperature’ and tao for ‘outdoor air temperature’, etc).

Figure 17: 100% All the psychrometric humidity values that can be read

from the psychrometric Saturated chart vapour pressure Saturated moisture Specific content n volume o ti ra tu sa e g Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Enthalpy Wet bulb ta en temp. rc Pe Moisture

Vapour content Moisture (vapour) content pressure dity umi ive h Relat

Dew point Dry bulb Enthalpy temp. temp.

Dry bulb temperature

State points in practice 3.9 The CIBSE psychrometric chart In practice, a design state point is usually fixed by specifying dry bulb Two CIBSE psychrometric charts have been designed and constructed — one temperature and percentage saturation. However, when for the temperature range –10 °C to 60 °C (shown opposite as Figure 18) and investigating actual conditions in a one for the temperature range +10 °C to 120 °C. Both are shown in chapter space it is more likely to be fixed by finding the dry bulb and wet 1 of CIBSE Guide C (CIBSE, 2007) and are also available as pads of A3-size bulb temperatures in the space as charts. these can be more easily measured (e.g. by using a sling psychrometer). Psychrometric data are also given in the form of tables. CIBSE Guide C provides tables of psychrometric data for dry bulb temperatures at 0.5 °C intervals from –10 °C to 60 °C. An example table for 20 °C is shown below in Table 1.

3.10 Psychrometric calculations

It is not the purpose of this publication to explore all the fundamental psychrometric calculations. Such calculations to establish the thermodynamic properties of humid air are used to establish the psychrometric tables and chart and, in practice, these are the sources used for everyday calculations. Further details can be found in CIBSE Guide C and the references given therein.

16 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 18: The CIBSE psychrometric chart

Specific enthalpy / (kJ / enthalpy Specific kg ) )

. –1 140 140 135 130 125 120 115

110 110 105 100 95 90

85 85 80 75 70

dry air) air) dry

Moisture content / (kg / content Moisture . –1 60 0·030 0·029 © CIBSE London 2003 0·028 0·027 0·026 0·025 0·024 0·023 0·022 0·021 0·020 0·019 0·018 0·017 0·016 0·015 0·014 0·013 0·012 0·011 0·010 0·009 0·008 0·007 0·006 0·005 20 0·004 0·003 0·002 0·001 0·000 60 55 140 55 50 30 135 50 45 40 130 40 45 50 Percentage saturation / % Percentage 125 60 35 40 70 120 80 30 35 ) –1 90 kg .

25 115 115

30 0·90 0·90

30 110 110 105 105

Specific enthalpy / (kJ

Dry-bulb temperature / C 100 100 20

25 95 95 25

15 –1 –1

90 90 . 3

85 85

0·85 0·85 20 80 80

75 75

)

s 70 70

Specific volume / m / volume Specific (

° 15

u 15

m 5

65 65

e 60 60

W 10

) )

–1

55 55 kg 10

50 50 0

45 45

0·80 0·80 Specific enthalpy / (kJ / enthalpy Specific 5

40 40 –5 Based on a barometric pressure of 101·325 kPa 35 35

30 30 0 25 25

–10 20 20 0 0 –5

0·1 0·1

–5 15 15

0·2 0·2 10 10

0·3 0·3

5 5 0·75 0·75 0·4 0·4 –10

Sensible/total heat ratio for water added at 30 C 0 0 –10 0·5 0·5 0·6 0·6

0·7 0·7 0·8 0·9 1·0 0·9 0·8 –5 –10 –10

Practical psychrometry 17 C1 (v2):Template copy 20/11/2012 16:46 Page 37

Properties of humid air 1-37

20 °C DRY-BULB Percentage Relative Value of stated parameter per kg dry air Vapour Dew point Adiabatic Wet bulb temperature saturation, humidity, pressure, temperature, saturation μ / % φ /% Moisture Specific Specific pv / kPa θd / ЊC temperature, Screen, Sling, content, g enthalpy, h volume, θ * / ЊC θ Јsc / ЊC θ Јsl / ЊC / (g·kgϪ1) / (kJ·kgϪ1) / (m3·kgϪ1) 100 100.00 14.75 57.55 0.8497 2.337 20.0 20.0 20.0 20.0 96 96.09 14.16 56.05 0.8489 2.246 19.4 19.6 19.6 19.6 92 92.17 13.57 54.56 0.8481 2.154 18.7 19.1 19.2 19.1 88 88.25 12.98 53.06 0.8473 2.062 18.0 18.7 18.8 18.7 84 84.31 12.39 51.56 0.8466 1.970 17.3 18.2 18.3 18.2 80 80.37 11.80 50.06 0.8458 1.878 16.5 17.7 17.9 17.7 76 76.43 11.21 48.57 0.8450 1.786 15.7 17.2 17.5 17.3 72 72.47 10.62 47.07 0.8442 1.694 14.9 16.7 17.0 16.8 70 70.49 10.33 46.32 0.8438 1.647 14.5 16.5 16.8 16.5 68 68.51 10.03 45.57 0.8434 1.601 14.0 16.2 16.5 16.3 66 66.53 9.736 44.82 0.8431 1.555 13.6 16.0 16.3 16.0 64 64.54 9.441 44.08 0.8427 1.508 13.1 15.7 16.1 15.8 62 62.55 9.146 43.33 0.8423 1.462 12.6 15.5 15.8 15.5 60 60.56 8.851 42.58 0.8419 1.415 12.1 15.2 15.6 15.3 58 58.57 8.556 41.83 0.8415 1.369 11.6 14.9 15.4 15.0 56 56.58 8.260 41.08 0.8411 1.322 11.1 14.7 15.1 14.7 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 54 54.59 7.966 40.33 0.8407 1.276 10.6 14.4 14.9 14.5 52 52.59 7.670 39.58 0.8403 1.229 10.0 14.1 14.6 14.2 50 50.59 7.376 38.84 0.8399 1.182 9.4 13.9 14.4 13.9 48 48.59 7.080 38.09 0.8395 1.136 8.8 13.6 14.1 13.7 46 46.59 6.785 37.34 0.8391 1.089 8.2 13.3 13.9 13.4 44 44.58 6.490 36.59 0.8388 1.042 7.6 13.0 13.6 13.1 42 42.58 6.195 35.84 0.8384 0.9945 6.9 12.7 13.4 12.8 40 40.57 5.900 35.09 0.8380 0.9480 6.2 12.4 13.1 12.5 38 38.56 5.605 34.34 0.8376 0.9011 5.5 12.1 12.8 12.2 36 36.55 5.310 33.60 0.8372 0.8541 4.7 11.8 12.6 12.0 34 34.53 5.015 32.85 0.8368 0.8070 3.9 11.5 12.3 11.7 32 32.52 4.720 32.10 0.8364 0.7600 3.0 11.2 12.0 11.4 30 30.50 4.425 31.35 0.8360 0.7127 2.1 10.9 11.8 11.1 28 28.48 4.130 30.60 0.8356 0.6656 1.2 10.6 11.5 10.7 24 24.43 3.540 29.10 0.8348 0.5710 Ϫ0.8 10.0 10.9 10.1 20 20.38 2.950 27.61 0.8341 0.4763 Ϫ3.0 9.3 10.4 9.5 16 16.32 2.360 26.11 0.8333 0.3814 Ϫ5.6 8.6 9.8 8.8 12 12.25 1.770 24.61 0.8325 0.2863 Ϫ8.9 8.0 9.2 8.2 8 8.17 1.180 23.11 0.8317 0.1910 Ϫ13.4 7.3 8.6 7.5 4 4.09 0.590 21.62 0.8309 0.0956 Ϫ20.8 6.5 8.0 6.8 0 0.00 0.000 20.11 0.8301 0.0000 — 5.8 7.3 6.1

20.5 °C DRY-BULB TablePerce n1:tage Relative Value of stated parameter per kg dry air Vapour Dew point Adiabatic Wet bulb temperature saturation, humidity, pressure, temperature, saturation Exampleμ / % ofφ psychrometric/% Moisture Specific Specific pv / kPa θd / ЊC temperature, Screen, Sling, content, g enthalpy, h volume, θ * / ЊC θ Јsc / ЊC θ Јsl / ЊC table from CIBSE Guide/ (g·kg ϪC1) / (kJHowever·kgϪ1) there/ (m3·k gareϪ1) some basic psychrometric calculations that the engineer 100 100.00 15.22 59.2must7 use in0 .8the518 design,2. 4evaluation10 20 and.5 checking20.5 of air conditioning20.5 systems.20.5 In 96 96.09 14.62 57.73 0.8510 2.316 19.9 20.1 20.1 20.1 92 92.18 14.01 56.1essence8 these0.850 can2 be 2reduced.222 to1 the9.2 following.19.6 19.7 19.6 88 88.25 13.49 54.63 0.8493 2.127 18.5 19.1 19.2 19.2 84 84.32 12.79 53.09 0.8485 2.032 17.8 18.7 18.8 18.7 80 80.39 12.18 51.54 0.8477 1.938 17.0 18.2 18.4 18.2 76 76.44 11.57 50.0The0 sensible0.8 4heat69 equation:1.842 16.2 17.7 17.9 17.7 72 72.49 10.96 48.45 0.8461 1.747 15.4 17.2 17.5 17.2 70 70.51 10.66 47.68 0.8457 1.699 15.0 17.0 17.2 17.0 • 68 68.53 10.35 46.9 0 Fs0 .=84 5m3 a Í cp1. 6Í52 Dt 14.5 16.7 17.0 16.7 (11) 66 66.54 10.05 46.13 0.8449 1.604 14.1 16.4 16.8 16.5 64 64.56 9.744 45.36 0.8445 1.556 13.6 16.2 16.5 16.2 62 62.57 9.440 44.59 0.8441 1.508 13.1 15.9 16.3 16•.0 60 60.58 9.135 43.8where1 Fs is0. 8the437 sensible1.46 0heat added12.6 or removed15.6 from humid16. 0air (kW),1 5m.7a is 58 58.59 8.831 43.04 0.8433 1.412 12.1 15.4 15.8 15.4 56 56.60 8.526 42.2the7 air mass0.8 flow429 rate1 .3(kg/s),64 c p is1 1the.6 specific15 .1heat capacity1 5for.5 air (kJ/kg·K)15.2 54 54.60 8.222 41.49 0.8425 1.316 11.0 14.8 15.3 14.9 52 52.61 7.917 40.7and2 Dt is the0.8 4temperature21 1.268 difference10.5 of air before14.6 and after1 treatment5.0 1 (K).4.6 50 50.61 7.613 39.95 0.8417 1.220 9.9 14.3 14.8 14.4 48 48.61 7.308 39.18 0.8413 1.172 9.3 14.0 14.5 14.1 46 46.61 7.004 38.4The0 specific0.8 40heat9 capacity1.123 c( p) for8 humid.7 air 1varies3.7 slightly depending14.3 on13. 8the 44 44.60 6.699 37.63 0.8405 1.075 8.0 13.4 14.0 13.5 42 42.59 6.395 36.8moisture6 content0.8401 of the1.02 air7 but is 7normally.4 taken13.1 to be about13. 81.02 kJ/kg·K.13.2 40 40.59 6.090 36.08 0.8397 0.9782 6.7 12.8 13.5 12.9 38 38.58 5.786 35.31 0.8393 0.9298 5.9 12.5 13.2 12.6 36 36.56 5.481 34.54 0.8388 0.8813 5.2 12.2 13.0 12.3 34 34.55 5.177 33.7The6 latent 0heat.8384 equation:0.8327 4.3 11.9 12.7 12.0 32 32.53 4.872 32.99 0.8380 0.7841 3.5 11.6 12.4 11.7 30 30.51 4.568 32.22 0.8376 0.7355 2.6 11.3 12.1 11.4 • 28 28.49 4.263 31.4 4 FL0 .8=3 7m2a Í hfg0. 6Í86 8Dg 1.6 11.0 11.8 11.1 (12) 24 24.45 3.654 29.90 0.8364 0.5892 Ϫ0.4 10.3 11.3 10.5 20 20.39 3.045 28.35 0.8356 0.4915 Ϫ2.6 9.6 10.7 9.8 18 Practical16 psychrometry 16.33 2.436 26.81 0.8348 0.3936 Ϫ5.2 9.0 10.1 9.2 12 12.26 1.827 25.26 0.8340 0.2955 Ϫ8.5 8.3 9.5 8.5 8 8.18 1.218 23.71 0.8332 0.1972 Ϫ13.1 7.5 8.9 7.8 4 4.09 0.609 22.17 0.8324 0.0986 Ϫ20.5 6.8 8.2 7.1 0 0.00 0.000 20.62 0.8316 0.0000 — 6.1 7.6 6.3 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

• where FL is the latent heat added or removed from the air (kW), ma is the air mass flow rate (kg/s),D g is the moisture added or removed from the air (kg) and hfg is the latent heat of evaporation for water (kJ/kg).

The latent heat of evaporation (hfg) varies slightly with temperature but is normally taken to be about 2450 kJ/kg.

It is of course necessary in air conditioning to know the volume of air for a given mass flow rate and this can be found from the relationship:

mass flow rate (kg/s) • 3 Volume flow rate (Va) (m /s) = ————————– (13) 3 air density (kg/m )

Air density varies slightly with moisture content and temperature but is normally taken to be about 1.2 kg/m3.

Alternatively, specific volume can be used, i.e:

• 3 Volume flow rate (Va) (m /s) = mass flow rate (kg/s) Í specific volume (m3/kg) (14)

3.11 Using the chart for psychrometric processes

The psychrometric chart is very useful during system design as it effectively acts as a modelling design tool to enable the designer to investigate the changes required and build up the system and equipment elements that can achieve this. Looking at the basic psychrometric processes, it simplifies to adding or removing sensible heat to raise or lower the temperature, or adding or removing latent heat to raise or lower the moisture content.

Effectively, the challenge is to get from one state point, such as an external condition, to another state point, such as a required room condition, using available plant and equipment and the psychrometric processes they can deliver. However, it is not simply a case of joining the dots — careful thought must be given to the process or processes that actually need to happen. . Figure 19 shows what happens by just joining the dots — i.e. it is not possible to get from point A to point B (or B to A) directly. For a start, the joining line goes outside the boundaries of the chart, i.e. over the 100% saturation line, which is impossible. All actual processes must stay within the boundaries of the chart. In this case to get from A to B might involve processes that would look more like those shown on Figure 20.

Practical psychrometry 19 Figure 19: Impossible process line B gB

The psychrometric challenge

The challenge is to get from point A to point B whilst keeping within the boundaries of the chart and using real processes; this usually involves several steps. Moisture content, g (kg/kg) A gA

tA tB

Dry bulb temperature, tdb (°C)

Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Figure 20:

Feasible process lines B gB

Moisture content, g (kg/kg) Keep within the chart g A C A t All actual psychrometric processes A tB

must stay within the boundaries of Dry bulb temperature, tdb (°C) the psychrometric chart, i.e. under the 100% saturation line.

3.11.1 Sensible heating and cooling

Sensible heating and cooling are both shown as horizontal lines on the chart, along a line of constant moisture content. For the sensible heating example

shown in Figure 21, the temperature increases from tA to tB but the moisture content remains the same, i.e. gA = gB. Similarly, for the sensible cooling example shown in Figure 22, the temperature decreases from tA to tB but the moisture content remains the same, i.e. gA = gB.

Figure 21: Sensible heating

A B gA = gB

Moisture content, g (kg/kg)

tA tB

Dry bulb temperature, tdb (°C)

20 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 22: Sensible cooling

B A gA = gB

Moisture content, g (kg/kg)

tB tA

Dry bulb temperature, tdb (°C)

Other humid air properties also change as the air is heated or cooled as shown in Table 2.

Effect on property during stated process Table 2: Property Sensible heating Sensible cooling Effect on psychrometric Dry bulb temperature Increases Decreases properties during sensible Moisture content Constant Constant heating and cooling Percentage saturation Decreases Increases Enthalpy Increases Decreases Specific volume Increases Decreases Wet bulb temperature Increases Decreases Design tip Dew point temperature Constant Constant Vapour pressure Constant Constant Always think about which piece of equipment (heater, cooler, humidifier, de-humidifier, heat recovery device etc) or process (mixing, equipment 3.11.2 Humidification and dehumidification heat gain etc) is appropriate and the psychrometric process that it will achieve. The rest is a matter of When air is humidified the vapour (moisture) content is increased and usually optimisation — sizing, control and part-load performance investigations there is also a small change of temperature as shown in Figure 23. Some (the ‘what if?’ questions). forms of humidification, such as steam humidifiers, will increase the dry bulb temperature (line AC); others, such as spray humidifiers, decrease the temperature (line AB).

Figure 23: Humidification processes B C gB = gC

A gA

Moisture content, g (kg/kg)

tB tA tB

Dry bulb temperature, tdb (°C)

Practical psychrometry 21 When air is dehumidified the vapour (moisture) content is decreased and usually there is also a change of temperature as shown in Figure 24. Most forms of dehumidifier, such as the commonly used cooling coil, will decrease the dry bulb temperature, effectively working by cooling some of the air to such a degree that moisture condenses out of the air thus reducing the moisture content.

Figure 24: Dehumidification process

Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 A gA B gB

Moisture content, g (kg/kg)

tB tA

Dry bulb temperature, tdb (°C)

22 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Part 2: Using the psychrometric chart in practice

4 Design conditions Operative and air temperatures

In well-insulated buildings that are Air conditioning is essentially a means of providing an alternative environment predominantly heated by from the one that exists naturally. In order to achieve this it is necessary to convective means, the dry bulb air temperature and the operative understand the range of atmospheric conditions that will be experienced at temperature can be taken to be the location being considered, and have knowledge of the conditions that are approximately the same. to be achieved in the air conditioned space. In the majority of projects the internal condition required is one that makes the occupants of the space feel comfortable.

4.1 Thermal comfort

Most people think of thermal factors almost exclusively when thinking of comfort but, in fact, a general feeling of well-being is governed by a myriad of other factors. A fuller discussion of thermal comfort is given in CIBSE KS6: Comfort (CIBSE, 2006a) and in CIBSE Guide A: Environmental design (CIBSE, 2006b).

CIBSE recommends the use of operative temperature (tc) as a thermal index for design purposes. This combines the effects of air temperature, radiant temperature and, to some extent, air velocity. A full discussion and definition of operative temperature is given in CIBSE Guide A, but for practical purposes in ‘still’ air (i.e. air speeds of around 0.1 m/s), it can be taken to be equivalent to the average of the air (ta ) and radiant temperatures (tr ), i.e:

tc = ½ ta + ½ tr (15) Main comfort factors

In well-insulated buildings that are predominantly heated by convective With regard to psychrometry, the main comfort factors are dry bulb means, such as tempered ventilation, the difference between the air and temperature and humidity, which the mean radiant temperatures (and hence between the air and operative will usually be measured in terms of dry bulb temperature and wet temperatures) is usually small. bulb temperature, or specified in terms of dry bulb temperature and As temperature increases, the effect of humidity on comfort becomes more percentage saturation. significant. In cold environments it has little effect on the feeling of comfort but at higher temperatures high humidities can severely limit the body’s ability to lose heat.

Therefore, in psychrometric terms, the only comfort parameters involved are dry bulb temperature and humidity. When controlling the condition of the air in an air conditioning or similar system, it is the temperature, humidity and cleanliness that are controlled. Whereas air cleanliness is dealt with by appropriate filters, temperature and humidity are controlled

Practical psychrometry 23 by the psychrometric processes of heating, cooling, humidifying and/or dehumidifying the air, which can be illustrated on the psychrometric chart.

4.2 Internal design conditions

As a general guide, the psychrometric conditions generally accepted to be reasonably comfortable for most people are dry bulb temperatures of between 18 °C and 23 °C, and humidities of between 30% and 70% Comfort envelope percentage saturation. At humidities less than 30%, in addition to the nasal dryness and eye irritation that can occur, there can be problems with the A comfort envelope for a particular application and location can be generation of static electricity that can cause electrical shocks and sometimes shown on the psychrometric chart. damage to electronic equipment. Above 70% there will be a tendency to increased discomfort due to the reduction in the rate of evaporation of Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 perspiration.

There are, of course, industrial applications where comfort is not the only criterion and different internal design conditions may be specifiedfor these situations.

Figure 25 shows a comfort envelope on the CIBSE psychrometric chart, illustrating the range of temperatures and percentage saturation that most people will feel is adequately comfortable.

Figure 25: 115 120 125 130 135 140

110 Percentage saturation / % 90 80 70 60 50 40 30 20 105 0·030 Comfort envelope drawn 0·90 0·029 100

0·028 140 30 on the CIBSE 95 0·027 90

0·026 135

85 psychrometric chart 0·025 Based on a barometric 130 pressure of 101·325 kPa 80 0·024 0·1 0 0·2 0·023 75

0·3 125 0·022 0·4

70 0·5 0·021 65

25 120 0·6 0·020 ) –1 0·7 60 . kg 0·8 0·019 0·9 Sensible/total heat 115 55 0·018 1·0 ratio for water –1 0·9 added at 30 C 3.kg 0·8 0·017 110

0·7 50 Specific volume / m ) –1 0·016 dry air)

0·85 –1 kg

0·6 Specific enthalpy / (kJ .

. 45

0·5 20 0·015 105 40 0·4 0·014

0·3 35 ) 0·2 g 0·013 100 in 0·1 0 sl ( C ° 0·012 30 / re (kJ / enthalpy Specific u 95 t Moisture content / (kg Comfort ra 15 0·011 e p 25 em t 0·010 lb u b

envelope - 90 t 20 e W 0·009 15 10 0·008 0·80 85 10 0·007

5 0·006

5 80 0·005

0 0 0·004 75

–5 0·003 0·75 –5 0·002 –10 70 –10 0·001

0·000 –10 –5 0 5 10 15 20 25 30 35 40 45 50 55 60 Dry-bulb temperature / C

45 50 55 60 65 20 25 30 35 40 –10 –5 0 5 10 15 Specific enthalpy / (kJ.kg–1)

4.3 External design conditions

External design conditions are derived from meteorological data. The results of the analysis of these data are published as recommended design conditions

24 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

for summer and winter in CIBSE Guide A (CIBSE, 2006a), Guide J (CIBSE, 2002) and in other publications, for both UK and overseas locations. Data on minimum and maximum temperatures and humidities are usually provided, Figure 26: together with other weather data. Examples from CIBSE Guide A (CIBSE Example tables from 2006a) are shown below as Figure 26. CIBSE Guide A

CIBSE Guide A Table 2.4 Wintertime dry bulb temperatures and coincident wet bulb temperatures equal to or exceeded for given percentages of hours in the year (approx. 1982–2002) Location Hourly temperature (/ °C) equal to or exceeded for stated percentage of hours in the year 99.6% 99% 98% 95% Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulb Belfast –2.6 –3.1 –1.2 –1.8 –0.2 –0.8 1.3 0.5 Birmingham –5.4 –5.6 –3.4 –3.8 –2.0 –2.4 0.3 –0.4 Cardiff –3.2 –4.0 –1.6 –2.4 –0.4 –1.2 1.5 0.6 Edinburgh –5.4 –5.6 –3.4 –3.7 –1.9 –2.3 0.3 –0.5 Glasgow –5.9 –6.0 –3.9 –4.1 –2.1 –2.6 0.2 –0.5 London –3.3 –4.0 –1.8 –2.5 –0.6 –1.3 1.4 0.5 Manchester –3.6 –4.0 –2.2 –2.7 –0.9 –1.7 0.9 0.0 Plymouth –1.6 –2.6 –0.2 –1.2 0.9 –0.1 2.9 1.9

CIBSE Guide A Table 2.6 Summertime dry bulb temperatures and coincident wet bulb temperatures equal to or exceeded for given percentages of hours in the year (approx. 1982–2002) Location Hourly temperature (/ °C) equal to or exceeded for stated percentage of hours in the year 0.4% 1% 2% 3% Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulb Belfast 22.6 18.0 20.8 17.1 19.3 16.3 17.3 14.9 Birmingham 26.1 19.2 24.1 18.2 22.4 17.3 19.6 15.9 Cardiff 24.6 19.0 22.6 18.0 21.0 17.2 18.6 16.0 Edinburgh 22.2 17.8 20.6 16.8 19.2 15.9 17.2 14.6 Glasgow 23.5 18.2 21.3 17.1 19.7 16.2 17.4 14.7 London 28.0 20.0 26.0 19.1 24.3 18.2 21.5 16.9 Manchester 25.5 18.8 23.4 17.9 21.7 17.0 19.0 15.6 Plymouth 23.5 18.7 21.8 17.9 20.4 17.2 18.5 16.1

Although these are not the extreme conditions they are only exceeded for (at most) 0.4% of the time and are therefore perfectly adequate for designing for comfort purposes.

These conditions are, of course, only two design points and, using meteorological data ‘envelopes’, can be produced to cover all the annual External envelopes psychrometric conditions. These envelopes do not normally show the Different climates have very absolute extremes but cover, say, 90% of the conditions that might normally different external condition be expected. envelopes and the psychrometric chart shows very graphically how these vary — with subsequent Figure 27 shows some typical external envelopes for different climates impact on the necessary system design. compared with the comfort envelope.

Practical psychrometry 25 Tropical (e.g. Singpore) 115 120 125 130 135 140

110 Semi-tropical (e.g. Hong Kong) Percentage saturation / % 90 80 70 60 50 40 30 20 105 0·030 0·90 Temperate (UK) 0·029 100

0·028 140 30 95 0·027 Desert 90

0·026 135

85 0·025 Based on a barometric 130 pressure of 101·325 kPa 80 0·024 0·1 0 0·2 0·023 75

0·3 125 0·022 0·4

70 0·5 0·021 65

25 120 0·6 0·020 ) –1 0·7 60 . kg 0·8 0·019 0·9 Sensible/total heat 115 55 0·018 1·0 ratio for water –1 0·9 added at 30 C 3.kg 0·8 0·017 110

0·7 50 Specific volume / m ) –1 0·016 dry air)

0·85 –1 kg

0·6 Specific enthalpy / (kJ .

. 45

0·5 20 0·015 105 40 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 0·4 0·014

0·3 35 ) 0·2 g 0·013 100 in 0·1 0 sl ( C ° 0·012 30 / re (kJ / enthalpy Specific u 95 Comfort t Moisture content / (kg ra 0·011 e 15 p 25 envelope em t 0·010 lb u b t- 90 20 e W 0·009 15 10 0·008 0·80 85 10 0·007

5 0·006

5 80 0·005

0 0 0·004 75

–5 0·003 0·75 –5 0·002 –10 70 –10 0·001

0·000 –10 –5 0 5 10 15 20 25 30 35 40 45 50 55 60 Dry-bulb temperature / C

45 50 55 60 65 20 25 30 35 40 –10 –5 0 5 10 15 Specific enthalpy / (kJ.kg–1) Figure 27: © CIBSE London 2003 External envelopes for different climates compared with the comfort envelope 5 Psychrometric processes

In order to convert the naturally occurring external environment into a controlled internal environment the air may have to be taken through various processes. This section examines the processes and the plant and equipment used to achieve the required changes.

5.1 The room process

Before looking at the controlled processes, the action of air passing through a room should be examined. Air is supplied to a room at a higher or lower temperature and higher or lower moisture content than the air in the room in order to maintain its psychrometric condition.

Changing the temperature of the air will change the quantity of sensible heat in the air, and the addition or reduction of water vapour (moisture content) will result in a change in the latent heat content of the air.

26 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

A room may experience either a sensible heat gain or a sensible heat loss depending on the circumstances (usually a gain in summer and a loss in winter). Thus the temperature of the air required to be supplied to the room will vary according to whether the air is required to heat or cool the room.

Processes causing a change in latent heat predominately give a gain rather than a loss. Infiltration or ventilation can give rise to a latent gain in summer due to the ingress of outside air with a high moisture content and, in winter, a latent loss due to the ingress of outside air with a low moisture content. Overall though, the conditioned air being supplied to a room will usually have a moisture content less than the room air irrespective of the season of the year.

Room ratio line

See Figure 28. The air supplied to the room (R) is said to be at the supply condition (S) (both summer and winter supply conditions are shown). Note that the condition of the air leaving the room is considered to be at the room condition (R). This is because the jet of supply air entering the room induces surrounding air into the stream and rapidly mixes with the air in the room to form the overall room condition. The line between these two conditions on a psychrometric chart is known as the ‘room ratio line’ and represents the psychrometric process of air passing through the room.

Figure 28: Room ratio line

R gR Summer Winter g S S S

Moisture content, g (kg/kg)

Room ratio line (RRL)

tS tR

Dry bulb temperature, tdb (°C)

The room ratio line can be constructed using the protractor on the psychrometric chart (see Figure 29).

Having assessed the heat loads on the room in terms of the sensible and latent heat gains and losses the sensible/total heat ratio can be calculated. Taking a summer case as an example, where the sensible heat gain is 40 kW and the latent heat gain is 10 kW, then the ratio would be:

Practical psychrometry 27

Sensible heat 40 Room ratio = —————–—————– = ———– = 0.8 (16) ( ) Sensible heat + latent heat 40 + 10

A line is then drawn on the protractor from its axis passing through the value of 0.8 (see Figure 29), making sure that the slope is in the correct direction for a summer condition, i.e. the lower part of the protractor. A parallel line can then be constructed on the chart using simple geometry instruments so that it passes through the room condition (22 °C dry-bulb and 50% saturation in this example). The supply point will then be somewhere on this line, Figure 29: Room ratio line on the depending on the supply temperature selected by the designer. CIBSE psychrometric chart Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

115 120 125 130 135 140

110 Percentage saturation / % 90 80 70 60 50 40 30 20 105 0·030 0·90 0·029 100

0·028 140 30 95 0·027 90

0·026 135

85 0·025 Based on a barometric 130 pressure of 101·325 kPa 80 0·024 0·1 0 0·2 0·023 75

0·3 125 0·022 0·4

70 0·5 0·021 65

25 120 0·6 0·020 ) –1 0·7 60 . kg 0·8 0·019 0·9 Sensible/total heat 115 55 0·018 1·0 ratio for water –1 0·9 added at 30 C 3.kg 0·8 0·017 110

0·7 50 Specific volume / m ) –1 0·016 dry air)

0·85 –1 kg

0·6 Specific enthalpy / (kJ .

. 45

0·5 20 0·015 105 40 0·4 0·014

5 Line drawn through 0·006

5 80 room condition parallel 0·005

0 0 to line drawn through 0·004 0.8 on protractor 75 –5 0·003 0·75 –5 0·002 –10 70 –10 0·001

0·000 –10 –5 0 5 10 15 20 25 30 35 40 45 50 55 60 Dry-bulb temperature / C

45 50 55 60 65 20 25 30 35 40 –10 –5 0 5 10 15 Specific enthalpy / (kJ.kg–1)

5.2 Mixing air streams

When two air streams mix together the result is a mixture point on a straight line between the conditions of the two air streams. The actual condition will vary in proportion to the relative masses of the two air streams.

28 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 30 shows a mixing process between air at condition A and air at Mixing air streams condition B, e.g. between outside and recirculated air to achieve a required supply condition, i.e: If in doubt when positioning the mixture point, remember that the mixed condition will always be m• h = m• h + m• h (17) closest to the condition of the air aM M aA A aB B stream which is of the greatest quantity. • • • maM gM = maA gA + maB gB (18)

• where ma is the air mass flow rate (kg/s),h is the air enthalpy (kJ/kg) and g is the air moisture content (kg/kg). (Suffices ‘M’, ‘A’ and ‘B’ denote the values for air at conditions M, A and B respectively.)

Remember:

• • ma = Va Í r (19)

• • 3 where ma is the mass flow rate (kg/s),V a is the volume flow rate (m /s) and r is the air density (kg/m3).

It is not absolutely correct to apply this relationship to temperatures because of variations in specific heat capacity for humid air. However, it is acceptable for all practical purposes, i.e:

• • • maM tM = maA tA + maB tB (20) where tM, tA and tB are the temperatures of the air at conditions M, A and B respectively.

Figure 30: Psychrometric processes h A for mixing air streams hM

Enthalpy, h (kJ/kg) A gA hB g M M

B gB

Moisture content, g (kg/kg)

tB tM tA

Dry bulb temperature, tdb (°C)

5.3 Air heating

An air heater coil (sometimes referred to as a ‘heater battery’) consists of a number of heating elements, arranged at right angles to the direction of

Practical psychrometry 29 air flow, contained in a sheet metal casing with flanged ends. The heating elements are either plain or finned tubes (carrying water or steam), or electric heating elements. Steam tubes are normally arranged vertically or sloping to facilitate condensate removal; hot water tubes are normally arranged horizontally. Tubes are usually of copper with either copper or aluminium fins. With larger heating loads more than one bank of tubes will be required and each bank of tubes is called a ‘row’. The tubes in each row are usually connected in parallel. For most applications a two-row coil is sufficient. Figure 31: Heating/cooling water coil Heater load (illustration courtesy of Armstrong International Inc.) See Figure 32. To determine the amount of enthalpy (heat) required to

Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 raise the air temperature from A to B, it is possible to use the sensible heat equation:

• Fs = ma cp (tB – tA) (21)

• where Fs is the sensible heat flow (W), ma is the mass flow rate (kg/s), 2 cp is the specific heat capacity (W/m ·K) and tA and tB are the dry bulb temperatures (K) at conditions A and B respectively.

However, it is usually obtained by reading values of enthalpy (h) from the psychrometric chart:

• Fs = ma (hB – hA) (22)

where hA and hB are the enthalpies (kJ/kg) at conditions A and B respectively.

Both calculations will give the same value.

Figure 32: A + B Heating psychrometric process and heater duty

hA Enthalpy, h (kJ/kg) A B g

Moisture content, g (kg/kg)

tA tB

Dry bulb temperature, tdb (°C)

30 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

5.4 Air cooling Figure 33: Refrigerant cooling coil Air cooling coils (or batteries) are similar to air heaters with the provision of a (illustration courtesy of SFI Coils collection tray and drain for condensate. The cooling medium is either chilled Inc.) or cool water or refrigerant; the tubes are normally arranged horizontally. Tubes are usually fed in parallel for each row and the rows fed in series from ‘back to front’ to approximate to contraflow. Refrigerant coils are often referred to as direct expansion or DX (American abbreviation). These coils often comprise a number of refrigerant loops interlaced to obtain a more even surface temperature.

Cooling coils usually have more rows than heater coils as there is a smaller temperature differential between the air and the cooling medium. Typically four to six rows are required, compared with one or two for an air heater battery.

The key to the psychrometric process of a cooling coil, see Figure 34, is the relationship between the dew point temperature of the entering air (td) and the apparatus dew point (tADP or tx). The apparatus dew point (ADP) is the average surface temperature of the cooling surface. When the ADP is above the entering air dew-point, there will be a ‘sensible heat only’ removal process (see Figure 34a) whereas, when the ADP is below the entering air dew-point, there will be a ‘sensible plus latent heat’ removal process (see Figure 34b). The cooling process will be from the entering air condition (A) Figure 34: towards the saturation point at the dew point temperature (X). Cooling psychrometric process and cooler duty; (a) sensible cooling, (b) sensible and latent cooling

A – B (a) (b)

h h A A Latent

h B Coolingduty Enthalpy, h (kJ/kg) Enthalpy,h h (kJ/kg) B Cooling duty Sensible B A A g gA B X gB

Moisture content, g (kg/kg) Moisture content, g (kg/kg)

tdp tx tB tA tx tdp tB tA

Dry bulb temperature, tdb (°C) Dry bulb temperature, tdb (°C)

Practical psychrometry 31 Contact factor

The contact factor is a measure of the effectiveness of a coil. It compares the difference AB with AX in terms of enthalpy, moisture content or temperature. ‘Cooling load’ and ‘room load’ A value of 0.85 would be a typical value for a four-row coil. ‘Effectiveness’ is Care should be taken when using an alternative term and then expressed as a percentage. It is also commonly the term ‘cooling load’. Although it is correct to use it for the demand called ‘efficiency’, which is technically incorrect. from the cooling coil, it is also sometimes wrongly used to mean Cooling load the heat gain to a building or room. This heat gain should be called the ‘room load’. These two loads are not the same. To calculate the enthalpy change required to determine the cooling required it is possible to use the fundamental sensible and latent heat equations: Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 • Fs = ma cp (tA – tB) (23)

• Fl = ma hfg (gA – gB) (24)

where cp is the specific heat capacity of humid air (kJ/kg·K) andh fg is the latent heat of evaporation of the water (kJ/kg).

However, as with heating, it is more usual to use the enthalpy values from the psychrometric chart — it is not normally necessary to determine the sensible and latent loads separately as it is the overall load which is required. i.e:

• F = ma (hA – hB) (25)

5.5 Air humidification

Humidifiers are either water or steam fed. In some instances, particularly industrial applications requiring high humidity, ‘direct humidification’ is used. This can be achieved using water which can be atomised, either by injecting Figure 35: with compressed air through high pressure nozzles, or by mechanical High pressure spray impingement. Alternatively steam can be injected directly into the space. humidifier However, in the majority of cases ‘indirect humidification’ is used whereby a steam or water humidifier is incorporated in an air supply system, which supplies treated air to the space through ductwork.

At one time nearly all humidifiers were of the water type. However steam humidifiers have become increasingly popular due to their compactness and ease of maintenance and, more recently, the fear of Legionnaires’ disease. Water humidifiers are often referred to as ‘washers’ and this name can be confusing in that they are not in any way intended to be air cleaners. A fuller discussion on humidifiers is given in CIBSE KS19:Humidification (CIBSE, 2012).

32 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

5.5.1 Water spray types

Originally water spray humidifiers were adapted from industrial washers Figure 36: used for cleaning flue gases etc. These were relatively bulky constructions Ultrasonic humidifier that required regular maintenance. All of these humidifiers were prone to bacteriological growth and were very maintenance intensive. There are various types, but recirculatory types are not used today because of issues Mist relating to Legionnaires’ disease. Water — High pressure sprays: These are currently the most commonly reservoir used types of water humidifier. Water is injected at high pressure, sometimes aided with compressed air, forming a very fine mist which is completely evaporated, negating the need for any recirculation of the water and hence avoiding the risk of Legionnaires’ disease.

— Ultrasonic: These are low-energy devices using an ultrasonic sound source under the water surface. They require careful water treatment to avoid carry-over of undissolved solids into the air stream and are not widely used. Ultrasonic transducers

5.5.2 Steam humidifiers

The original steam humidifiers were fed directly with steam available on site for other purposes. They were very common in the USA, as was steam heating. They are fitted with a superheater to avoid introducing water into the airstream and are fully modulating. Electric steam humidifiers, generating steam locally, were at first introduced to satisfy small demands and had only on/off control. Their use has become more common and they are now available in a range of sizes with on/off, step or modulating control. The big drawback of electric humidifiers is their demand for high-cost electrical energy. Figure 37: Steam humidifier 5.5.3 Psychrometric processes

The psychrometric process through a humidifier, see Figure 38, depends upon the humidifying medium:

— Water humidifiers: These have a psychrometric process usually somewhere between constant enthalpy (adiabatic) and constant wet bulb temperature, but for practical purposes can be considered to be adiabatic. This results in a gain in moisture content with a reduction in dry bulb temperature. As with cooling coils the term ‘contact factor’ is commonly used.

— Steam humidifiers: these have a psychrometric process that approximates to an isothermal process, i.e. at constant temperature. (There is actually a very small temperature rise as the steam is at a higher temperature than the air; normally Dt < 1 K.)

Practical psychrometry 33

≈ ≈

A B A ≈ B

Water Steam

hA ≈ hB hB

B B gB gB hA A gA

Moisture content, g (kg/kg) A Moisture content, g (kg/kg)

tB tA tA tB Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Dry bulb temperature, tdb (°C) Dry bulb temperature, tdb (°C)

(a) (b)

Figure 38: Humidification psychrometric process; (a) water, (b) steam It should be noted that contact factor (effectiveness) does not apply to direct water injection or steam humidifiers. It is possible to supply too much water or steam unless appropriate precautions are taken.

5.5.4 Humidifier load

Again, this is most commonly determined using the psychrometric chart:

• F = ma (hA – hB) (26)

Note that the water spray process is adiabatic and there is no load as the humidification is achieved by cooling the air to provide the heat to evaporate the water.

5.6 Heat recovery

In spaces requiring ventilation, treated vitiated air has to be replaced with fresh outside air at the prevailing external conditions. The entering air normally has to be heated or cooled and the energy required for this purpose can be significantly reduced by using a heat exchanger to recover energy from the discharged air and use it to treat the incoming ventilation air.

The simplest of heat recovery units (HRUs) basically recover only sensible heat, but sophisticated units can recover both sensible and latent heat. The benefits of theHRU are to some extent offset by the extra fan energy required to move air through the heat exchanger.

34 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

5.6.1 Effectiveness/efficiency

In practice this is usually called ‘efficiency’ although this is not strictly the correct term as it implies a loss of energy, which does not occur. All heat exchangers are ‘100% efficient’ in that all the energy recovered from one fluid (gas or liquid) is passed to the other fluid.

5.6.2 Unit types

(a) Parallel plate heat exchanger

This is the most commonly used type of HRU and comprises an open ended box with a matrix of thin plates of metal, plastic or glass. These form a series of narrow linear passages, alternate rows of which carry the supply air or exhaust air. They essentially recover only sensible heat although some latent heat may also be recovered when the outside temperature is sufficiently low to condense moisture on the exhaust air side of the plates. When latent heat is recovered from the exhaust air it is transferred to the incoming air as sensible heat.

Figure 39 shows a room being heated to 20 °C when the outside air is at 0 °C with air being supplied at 24 °C, comparing a case with a heater coil only and a case where a heater coil is supplemented by a plate heat exchanger with an efficiency of 70%.

Heating Supply Figure 39: coil fan Potential for saving sensible

To heat for a supply and Fresh 0°C supply 24°C air ductwork extract system (a) without heat exchanger, and (b)

From with heat exchanger Exhaust 20°C extract 20°C air ductwork

(a)

Heat exchanger

From Fresh 0°C extract 20°C air ductwork

To Exhaust supply 6°C air 24°C 14°C ductwork

Heating Supply coil fan (b)

Practical psychrometry 35 It can be seen that, without the heat exchanger, the heater coil has to heat the air from 0 to 24 °C whereas with the heat exchanger it only has to be heated from 14 °C to 24 °C. Thus, with the heat exchanger, only 10/24, or 42%, of the energy is required and therefore there will be a saving of about 58% of the energy required in its absence.

Note that although the heat exchanger has an efficiency of 70% the energy saving is only 58%. This is because the heat exchanger operates between the outside air and room temperature, not the outside air and supply temperatures.

Psychrometric processes Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 As previously stated, the plate heat exchanger normally recovers only sensible heat. However, as can be seen from the two psychrometric sketches in Figure 40, if the entering hot air has a dew point temperature above the dew point temperature of the entering cold air, condensation will occur and some latent Figure 40: heat will be recovered in these circumstances. It should be noted that this will Plate heat exchanger be transferred to the incoming cold air as sensible heat and the temperature psychrometric process; (a) rise of the cold air will therefore be greater than the drop in temperature of sensible cooling, (b) the hot air. The difference in enthalpies will, of course, be the same. sensible plus latent cooling

R R’ R R’

Moisture content, g (kg/kg) O O’ O O’ Moisture content, g (kg/kg)

0 10 20 30 0 10 20 30 Dry bulb temperature (°C) Dry bulb temperature (°C) (a) (b)

(b) Run-around system

As with the plate heat exchanger, this system using finned coil heat exchangers essentially recovers only sensible heat and will have the same psychrometric cycles. The exchanger in the exhaust duct picks up and transfers heat to the water or glycol mixture which, in turn, heats the supply air in winter. A typical arrangement is shown in Figure 41, the control of the supply duct temperature being achieved by a thermostat and 3-way valve or variable speed pump.

36 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

The efficiency of heat transfer (effectiveness) is usually 40–60%, which is Figure 41: significantly less than the plate heat exchanger. The main advantage of this Run-around coil system is that it may be used where it is impossible to run the supply and Preheat coil exhaust ducts close together. It will therefore often be the most appropriate Fresh Suppy air air scheme for existing systems. It is also flexible, as a number of coils may be incorporated into the system. There is also no risk of cross-contamination if the coils are separated.

Exhaust Extract (c) Thermal wheel air air

The thermal wheel comprises a framework, like a thick cartwheel, filled with Heat recovery a suitable matrix with a large surface area, see Figure 42. The unit is installed coil between the two counter-flowing airstreams. The wheel is rotated slowly (0–10 rpm) by a small electric motor. The part of the wheel in the exhaust air is warmed up and this in turn heats the incoming air as the wheel revolves.

Very little exhaust air carries over to the supply air provided the correct pressure differentials between the two air streams are observed. To avoid leakage of the exhaust air into supply air stream, the air duct pressure should be positive in the supply with respect to the exhaust. This can be ensured by placing the supply fan on the upstream side of the wheel but an excessive pressure differential will result in a flow of outside air to exhaust and consequently a reduced efficiency. Where air flows from exhaust to supply, Figure 42: efficiency is increased. The risk of the exhaust air contaminating the supply air Thermal wheel can be further reduced by fitting a purge unit. Rotating matrix

Hygroscopic wheels are available that transfer latent heat as well as sensible heat, and these are particularly suitable for use in spaces that have high humidity.

Output is controlled by varying the speed of rotation of the wheel. The main Exhaust advantage of the thermal wheel is that efficiencies remain high at low loads. There are no problems with bacterial growths or frost/ice build-up at sub- Supply zero outside air temperatures on sensible heat exchangers, though frosting Drive can occur on hygroscopic wheels. motor

The psychrometric processes for winter and summer design, with and without a hygroscopic thermal wheel, are shown in Figure 43.

Note that the overall energy savings are much greater in winter than in summer due to the exchanger only operating between the room and outside conditions. This is why heat exchangers are much more attractive economically in cold rather than hot climates.

Practical psychrometry 37 Constant volume All outside air 20 20 Single zone Winter design

15 15 load Humidifier load Humidifier

10 10 R Heaterload R S Heaterload S R’ H

5 Moisture content (g/kg) 5

O’ Moisture content (g/kg) O H O

0 0 0 10 20 30 0 10 20 30 Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Dry bulb temperature (°C) Dry bulb temperature (°C)

(a) (b)

Constant volume All outside air 20 20 Single zone Summer design

15 15

Coolingload O O Coolingload O’ 10 R’ 10 R R X C S X C S

5 Moisture content (g/kg) 5 Moisture content (g/kg)

0 0 10 20 30 0 10 20 30 Dry bulb temperature (°C) Dry bulb temperature (°C)

Figure 43: Psychrometric process with and without hygroscopic thermal wheel; (a) winter design, without wheel, (b) winter design, with wheel, (c) summer design, without wheel, (d) summer design with wheel (Note that moisture content is expressed in g/kg)

38 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

6 Applied psychrometry

This section will examine the uses to which psychrometry can be put. Psychrometry will, of course, play its part in the air conditioning design process but is also used during commissioning and in operational maintenance.

It has to be used with care in the commissioning and maintenance roles as quite small errors in measurement can give very misleading results when plotted on the psychrometric chart. There is a need to apply a degree of interpretation and to take a series of repetitive measurements.

Figure 44 illustrates the relationships between the main categories of air conditioning system.

A/C Figure 44: systems Air conditioning systems

Part Centralised centralised Local

Air/ Heat CAV VAV Unit water pump conditioner

Single Chilled Induction Split duct beam Versatemp system

Dual Fan path coil VRV or VRF

Dual duct/ VAV

Dual duct

Hot/cold deck

The air conditioning system uses a number of psychrometric processes in order to achieve the desired condition within the air conditioned space.

It is beyond the scope of this publication to examine the performance of every type of air conditioning system. The intention is to simply examine sample cases to explore the type of combination of psychrometric process which systems employ. A centralised all-air system and unit systems will be explored.

The air supplied by an air-conditioning system has to fulfil three main functions:

Practical psychrometry 39 (1) To provide sufficient air at a temperature that will enable the space to be heated or cooled to maintain the required space temperature. It may be that the humidity is also controlled.

(2) To provide sufficient ventilation for the occupants of the space.

(3) To maintain a good air flow pattern within the occupied area.

There are many different types of air conditioning systems but they are all the same in their essentials. Broadly speaking systems can be divided into three classifications:

(1) centralised Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 (2) part-centralised or unitary

(3) local.

Figure 45: 6.1 Centralised systems Air handling unit (illustration courtesy of Nuaire With centralised systems all the air is treated in a central plant, an air handling Ltd.) unit (AHU), as shown in Figure 45. The air is then delivered and extracted to and from the air conditioned spaces via ductwork. It is almost always the case that the quantity of air required to heat and cool is considerably more than that required to ventilate, often four or five times greater. It is therefore usual whenever possible to recirculate as much air as possible to minimise the energy demand of the system. There are cases where recirculation is not possible and in this case the system may need to operate in full outside air mode. This is the simplest case and will be explored first.

It is almost always the case that the summer case is more critical than the winter one in designing air conditioning systems as the heat loads tend to be higher and the acceptable/attainable temperature differentials tend to be smaller. Therefore the summer case will be considered first.

6.1.1 All-air systems using all outside air

(a) Summer operation

These days virtually all AHUs are modular with a variety of combinations of modules giving the designer considerable scope in the choice and position of components and types of fans, heaters, coolers etc. Figure 46 shows a typical schematic arrangement for a central air handling unit comprising preheater, filter, cooling coil, heater, humidifier and supply fan. The preheater is simply an anti-frost protection device to prevent frost build-up on the filter and damage in freezing-fog conditions.

40 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Figure 46: Filter Cooling Heating Supply coil coil fan All-air system with 100% O M outside air – C + H

Room Humidifier

There must of course also be provision for extracting air from the room.

The psychrometric diagram in Figure 47 shows the initial design points, where O is the outside condition, R is the room condition and S is the supply condition.

Summer 20 Figure 47: Initial design points (summer) 15 (Note that moisture content is O expressed in g/kg) R 10 S

5 Moisture content (g/kg)

0 10 20 30 Dry bulb temperature (°C)

The values for temperature and moisture content are typical values for the UK to give a sense of scale. The room ratio line is shown connecting the room and supply conditions.

The air conditioning system has to take air at condition O and transform it into condition S if the design room condition is to be achieved. It is not normally possible to do this in one step as the cooling coil is being required to perform two functions (i.e. to cool and to dehumidify), and it can only be controlled to do one of these. If control of both temperature and humidity is required then the first priority will be achieve the required air moisture content to supply to the room (8 g/kg, in this case). With a contact factor of

90% this would result in a required apparatus dew point (tadp) of 10 °C, as shown in Figure 48. The air would be cooled from O to the nearest point on the 10 °C line (X). The resulting temperature leaving the cooling coil would be 12 °C (in this case), which is below the required supply temperature of 14 °C.

Practical psychrometry 41 Figure 48: Summer Temperature and humidity 20 control (summer) (Note that moisture content is 15 expressed in g/kg) O

10 R X C S

5 Moisture content (g/kg)

‘Air conditioning’ and 0 ‘comfort cooling’ 0 10 20 30 Dry bulb temperature (°C) Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 The term ‘air conditioning’ is often incorrectly used for any system that includes cooling. Air Although there might be some slight temperature gain to the supply air due to conditioning systems give control heat gains in the fan and friction in the ductwork, the heater would probably over all aspects of the air condition, i.e. temperature, be required to achieve the designated supply temperature. This is obviously humidity and cleanliness. If the not energy efficient but necessary to obtain the required conditions. system does not provide humidity control then it is called ‘comfort cooling’. In many cases, though humidity control is not employed and although air temperature is controlled, the humidity is allowed to vary, see Figure 49. This is often acceptable as the humidity may not vary beyond the comfort zone. These systems are often termed ‘comfort cooling’ systems.

Figure 49: Constant volume 20 All outside air Temperature control with Single zone humidity allowed to vary Summer design 15 (‘comfort cooling’) C=S (Note that moisture content is 1 O R 10 expressed in g/kg) X 1 R S

5 Moisture content (g/kg)

0 0 10 20 30 Dry bulb temperature (°C)

Figure 49 shows the outside air (O) being cooled to the correct supply

temperature (14 °C) giving rise to a supply condition (S1). The air then follows the slope of the room ratio line to give a room condition (R1), which will be at the correct temperature but above the design room humidity.

In most circumstances, the higher humidity is acceptable but under certain conditions, such as a combination of high external moisture content coupled

42 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

with a low sensible heat gain to the room, very high room humidities may result. This could occur, for example, in the afternoon of a hot humid August afternoon in rooms on the east face of a building where there are low solar gains and hence less cooling, as a higher supply temperature would be required.

(b) Winter operation

See Figure 50. The winter cycle is very simple; a steam humidifier is shown in this case. The temperature of the entering outside air is heated to the required supply temperature and then humidified to obtain the correct supply moisture content.

Winter 20 Figure 50: Winter psychrometric processes 15 (Note that moisture content is expressed in g/kg) 10 R S

5 Moisture content (g/kg)

0 10 20 30 Dry bulb temperature (°C)

Note that it is common, if not usual, with a constant volume system for the temperature difference between room air and supply air to be much smaller in winter than in summer. This is because the heating losses from a building are normally much less than the heat gains.

As explained earlier, these systems using all outside air are not normally used as shown but, most commonly, as the ventilation system component of a part-centralised system where a local unit provides most of the heating and cooling.

This is because the amount of air required to cool the space is several times more than that required to ventilate the space, thus allowing for the majority of the air supplied to the space to be recirculated back through the AHU.

6.1.2 All-air systems with recirculation

Figure 51 shows an air handling unit in which a proportion of room air is recirculated through the system to economise on energy consumption. It can

Practical psychrometry 43 be seen from the psychrometric summer diagram (Figure 52(a)) the incoming outside air (O) will be mixed with a normally larger volume of recirculated air (R) to give a mixed condition (M). Typically the amount of outside air will be in the order of 20% of the total air volume.

Figure 51: Mixing Filter Cooling Heating Supply All-air system with box coil coil fan O recirculation M M – C + H

Room R Humidifier R R

Extract fan Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Figure 52: All-air system with The process in summer then continues in much the same way as with the recirculation; (a) summer all outside air system, except that it is the mixed air (M) that is cooled to the and (b) winter operation off-coil condition (C), which is then heated to the supply condition (S) if both (Note that moisture content is temperature and humidity control is required. expressed in g/kg)

Constant volume Constant volume Single zone 20 Single zone 20 Summer design Winter design

15 15

O M R 10 S X R M C S H

5 Moisture content (g/kg) 5 O Moisture content (g/kg)

0 0 0 10 20 30 0 10 20 30 Dry bulb temperature (°C) Dry bulb temperature (°C)

(a) (b)

It can be seen therefore that the cooling load is significantly reduced as the air has to be cooled only from M to C rather than from O to C. The diagram is for full control of temperature and humidity and the same caveat applies to humidity variations if ‘comfort cooling’ is employed.

Similarly, the winter case shown in Figure 52(b) follows the same basic process as for all outside air except of course that the air entering heater is a mixture of room and outside air which is heated (H) and then humidified to give the supply condition. The reduction in energy demand for both heating and humidifying are reduced dramatically.

44 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

If no humidifier is employed, as is frequently the case then the cycle will appear as shown in Figure 53. The room humidity will probably fall to levels of 30% saturation or less, which might be considered as outside the comfort zone. However this will only occur under more extreme conditions and may not continue for an extended period.

Constant volume 20 Figure 53: Single zone Summer design All-air system with No humidification recirculation; winter 15 operation without humidity (Note that moisture content is 10 expressed in g/kg)

R M 5

O S Moisture content (g/kg)

0 0 10 20 30 Dry bulb temperature (°C)

Each case has to be considered on its individual merits.

6.2 Unitary systems

With unitary systems, locally installed recirculatory units provide the bulk of the heating and cooling required in the space. This saves service space in the building but they are not so economic in year-round operation. Ventilation is provided either via a centralised air handling unit or some local provision, such as air taken-in through the adjacent wall or opening windows. The latter case is usually only applied to small scale installations.

All unitary systems operate the same in basic psychrometric terms but vary in the method by which this is achieved. The diagrams in Figure 54 show some of the more common systems.

Some units are cooling only whilst others provide both cooling and heating facilities. Only the most basic units provide ventilation locally, usually via a through-the-wall duct, the norm being to provide a separate ducted ventilation supply. Units are normally temperature-only controlled, with any humidification control being provided through the ventilation system. The ventilation can be provided either through the unit or separately.

Years ago, units were usually designed to operate on a sensible-only cooling mode as there was no simple way of removing condensate prior to the development of the small condensate pump (this still applies today with chilled ceilings and chilled beams).

Practical psychrometry 45 Figure 54: Examples of unitary

systems Supply Supply fan fan

Room Room

TR TR

+ – Fan coil Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Fan coil Versatemp

Supply Supply fan fan

TR Room Room TR

Passive chilled beam Active chilled beam

As the cooling surface had to be maintained above the room dew point temperature to avoid condensation, units had to be of a larger size to obtain the required cooling capacity.

This is known as ‘dry operation’. Units operating with cooling surface temperatures below the entering air dew point temperature will induce condensation on the coil and this is known as ‘wet operation’.

6.2.1 Dry unit operation

Figure 55 shows the unit process cooling ventilation air being cooled and

dehumidified by the central ventilation plant (O to 2S ).

The unit cooling process is from the room condition R to supply condition S1. If the ventilation air is supplied via the unit the S1 and S2 would mix to give an overall supply condition S. Whether the room is supplied with one condition

(S) or two conditions (S1) and (S2), the effect will be the same.

46 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

Dry unit operation 20 Figure 55: Dry unit operation psychrometric process 15 (Note that moisture content is O expressed in g/kg) 10 S1 R S S 2 5 Moisture content (g/kg)

0 0 10 20 30 Dry bulb temperature (°C)

‘Dry unit’ and ‘wet unit’ operation Although on the chart the ventilation load appears to be the larger it will not be because the mass of air being cooled is small compared with the air being The term ‘dry unit operation’ is used when room based units such handled by the unit. Remember the cooling load is the multiple of the mass of as fan coils, chilled beams etc run air and enthalpy difference. Because of the smaller amount of ventilation air it ‘dry’, with cooling surface temperatures above the air dew can be difficult to prevent high room humidities when external humidities are point temperature. ‘Wet unit high, particularly where room loads are low. operation’ is used when they run wet with cooling surface below air dew point temperature. In this case provision for collection and 6.2.2 Wet unit operation drainage of condensate is essential.

With a wet unit, colder water can be used in the unit which will thus provide a higher cooling capacity for a given size of unit. Figure 56 shows both unit and ventilation air being supplied with chilled water at the same temperature but this does not have to be the case. Supplying low temperature water temperature to the unit can mean it is difficult to avoid low humidities in the room.

Wet unit operation 20 Figure 56: Wet unit operation psychrometric process 15 (Note that moisture content is O expressed in g/kg) 10 S1 R

S S2 5 Moisture content (g/kg)

0 0 10 20 30 Dry bulb temperature (°C)

Practical psychrometry 47 6.3 Local systems

These normally take the form of proprietary units that treat a single space on a one-off basis. They are often used in applications such as small hotels and increasingly in the domestic market. Although some units provide cooling and heating many are simply employed to provide cooling on demand with heating being provided from another source, such as conventional central heating. They provide no humidity control and often there is no provision for ventilation.

7 Further reading

CIBSE (2006a) Lawrence Race G Comfort CIBSE KS6 (London: Chartered Institution of Building Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820 Services Engineers)

CIBSE (2006b) Environmental design CIBSE Guide A (London: Chartered Institution of Building Services Engineers)

CIBSE (2007) Reference data CIBSE Guide C (London: Chartered Institution of Building Services Engineers)

Jones WP (2000) Air Conditioning Engineering (5th. edn.) (chapters 2 and 3) (Oxford: Butterworth- Heinemann)

ASHRAE (2009) Fundamentals Handbook (chapter 1) (Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers)

McQuiston FC, Parker JD and Spitler JD (2004) Heating, Ventilating and Air Conditioning: Analysis and Design (6th. edn.) (chapter 3) (Chichester: Wiley) Kreider JF, Curtiss P and Rabl A (2010) Heating and Cooling of Buildings (chapter 4) (Boulder, CO: Kreider and Associates)

Hall F and Greeno R (2001) Building Services Handbook (2nd. edn.) (Psychrometrics section) (Oxford: Butterworth-Heinemann)

48 Practical psychrometry Colin Campbell, [email protected], 1:23pm 13/12/2013, 3, 44820

CIBSE KNOWLEDGE SERIES Practical psychrometry CONTACT US AT: Practical psychrometry The Chartered Institution of Building Services Engineers 222 Balham High Road, London SW12 9BS

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