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Service Application Manual SAM Chapter 650-002 Section 3A

PRACTICAL APPLICATIONS OF

by Harlan Krepcik, CMS Associate Professor of HVACR, Tidewater Community College, Portsmouth, VA

Energy used to change air is defined as BASIC PRINCIPLES sensible . Psychrometrics is the science of the physical properties of air and its associated vapor. All The next most common physical property of air HVACR professionals must have a good command described by customers is relative . of the basic principles that support this science. We Complaints about humidity range from air being too select, install, and service mechanical systems that damp to air being too dry. Relative humidity (RH) is are designed to manipulate the physical properties a value used to communicate the ratio of water vapor of air for the purpose of controlling its temperature, in an air sample to the maximum amount of water humidity level, and cleanliness. This is done to vapor that the air sample can hold at a given dry- provide improved comfort within a conditioned space, bulb temperature. The vapor of the air as well as for other reasons—to protect construction sample is compared to the of that materials used in the building, for example, or to same air sample when saturated with water vapor. preserve food products. The following paragraphs The resulting ratio is converted to a percentage that review eight physical properties of air: we call relative humidity.

þ dry-bulb temperature For example, air at a dry-bulb (db) temperature of þ wet-bulb temperature 75°F has a vapor pressure equal to 0.88 inches of þ relative humidity mercury (in. Hg) when saturated with water vapor— þ specific humidity that is, when its relative humidity is 100%. The same þ air sample would have a relative humidity value of þ 50% when the water vapor content of the air creates þ vapor pressure vapor pressure equal to 0.44 in. Hg: þ .

The physical property of air most commonly described by customers is dry-bulb temperature. The temperature value is measured in degrees, Air at 50% RH can accept more water vapor. Any either or . Heat intensity, not additional water vapor added to air that is already heat quantity, is measured in units of temperature. completely saturated will fail to dissolve into the air, Warmer molecules vibrate more rapidly than do and fog will appear. cooler molecules, and this rate of vibration is called heat intensity. However, a temperature value does The relative humidity of air is important not only not tell you the quantity of heat present in a system. to comfort, but also to providing a healthy indoor Heat quantity is measured in British thermal units, environment. Flu viruses, for example, thrive in air or Btu. The temperature value of an air sample is that is too dry. On the other hand, dust mites cannot only one measurement of several that must be taken survive in dry air. These tiny creatures lack the to calculate the heat quantity present in the sample. ability to drink water. They must absorb water from

© 2010 by the Service Engineers Society, Des Plaines, IL Supplement to the Refrigeration Service Engineers Society. 1 the surrounding air to live. Dust mites cannot absorb This smaller and denser “container” cannot hold as water from air when its relative humidity value is less much water in solution as the larger “container” than 50%, and as a result they dehydrate and die. at 75°F. At the lower temperature of 55.15°F, the The job of HVACR professionals is to provide indoor existing 64.9 grains of water in each pound of air environments that are both comfortable and healthy. (specific humidity) results in a relative humidity To do so, mechanical systems must be able to control equal to 100%. all physical properties of air. The reverse occurs when air is heated—the density Specific humidity (also referred to as humidity ratio) of the air decreases and its specific is the term used to describe a third physical property increases. As the “container” or volume of air of air. This value is a measure of the actual mass of expands, it can hold more water in solution. Consider water dissolved into an air sample. It is measured in the same air sample at 75°F db and 50% RH. When either pounds of water or grains of water that have heated to a temperature of 100°F db, the sample can been evaporated into one pound of air (7,000 grains hold 302.5 grains of water in each pound of air at equals 1 lb of water). As a reference of scale, one saturation (100% RH). At this new higher pint of water has a mass of 1.04 lbm (pounds mass). temperature, the of the sample used to remove water mass from an air sample increases to 15.083 ft3/lb of air. The “ball” of air or to add water mass to an air sample is measured in increases to a diameter equal to 36.79 in. The units defined as . The quantity of heat existing 64.9 grains of water create only enough (Btu) required to change the moisture content of air vapor pressure to result in a relative humidity equal varies with the temperature of the air sample. For to 22.6%. This physical property explains why winter example, at a temperature equal to 212°F, heat air that infiltrates into a home and is heated to room quantity equal to 970 Btu is required to change the temperature has a lower relative humidity. of 1 lb of water from liquid to vapor, or vice versa. However, when the temperature of standard Clearly, as is done on an air sample by a air is cooled to 60°F by an , heat quantity mechanical system, changes in the physical equal to approximately 1,060 Btu is required to properties of that air are significant. An HVACR change the phase of 1 lb of water. professional must be able to measure and calculate the impact of work done on an air sample for the When air is cooled, its specific volume decreases, purpose of predicting the acceptability of an airstream as does its ability to hold water in solution. This is for a given application. The psychrometric chart is a due to the increase in the density of air as it cools. useful tool that allows you to make those calculations Specific volume is the reciprocal of density—in without the need for higher-level mathematics. other words, the two properties are inversely related. Consider an air sample at 75°F db and 50% RH. Another term with which technicians must be The density of the air equals 0.073126 lb/ft3 and its familiar is wet-bulb temperature. The wet-bulb (wb) specific volume equals 13.675 ft3/lb (1 ÷ 0.073126). temperature is measured using a thermometer with Try to visualize this volume by picturing a ball of air its bulb covered with a water-saturated wick. Air 35.6 in. in diameter. At 50% RH, 1 lb of the air is caused to flow over the wick at approximately sample holds 64.9 grains of water in solution. When 900 ft/min, resulting in of water from saturated, 75°F air can hold 131.8 grains. Notice that the wick. The evaporation process draws heat from relative humidity is a ratio of the vapor of the bulb of the thermometer, causing a decrease in the two air samples, not a ratio of the water mass temperature. When the temperature equalizes, the dissolved into each air sample. When this air sample wet-bulb temperature of the air has been measured. (50% RH at 75°F) is cooled to 55.15°F, its density This temperature is subtracted from the dry-bulb increases to 0.0759 lb/ft3. At this point, its specific temperature of the air, and the difference is called volume decreases to 13.175 ft3/lb. The “ball” of air the wet-bulb depression. A large wet-bulb depression in question now has a smaller diameter of 35.16 in. indicates that the air is dry and readily accepts water

2 from the wick, thus accelerating the evaporation rate. The last physical property of air to be considered A small wet-bulb depression indicates that the air here is enthalpy. Enthalpy is a measure of the total is damp and lacks ability to accept water from the heat content (measured in Btu/lb) of an air sample. wick, thus reducing the evaporation rate. Psychrometric charts do not use enthalpy values referenced to (–460°F). Values used on Electronic instruments typically are used to measure the chart are referenced to the heat content of liquid the physical properties of an air sample, since they water at 32°F and dry air at 0°F. This method is afford greater accuracy than sling psychrometers. similar to that used on pressure gauges. Pressure A manually operated sling psychrometer that measurements are referenced to atmospheric includes a slide rule calculator is shown in Figure 1. pressure. A gauge reading of 0 psig does not mean An example of a digital psychrometer used more that the space is under no pressure. Rather, a reading commonly today is pictured in Figure 2. of 0 psig indicates that the space is at , which is 14.696 psi at sea level. Therefore, The dew point temperature of an air sample is also pressures below 0 psig are referred to as vacuum important. When air is cooled to its dew point readings. They are merely measurements of pressure temperature, water vapor in the sample cannot be below atmospheric pressure. As with pressure held in solution and will condense out of the air. gauges, some psychrometric charts indicate negative An evaporator coil must be at a temperature below enthalpy values for working with frigid air. In this the dew point of an airstream to reduce its moisture discussion, we will utilize an ASHRAE psychrometric content. If the coil temperature is above the air’s dew chart formatted for using point, the dry-bulb temperature of the airstream will above the 0°F and 32°F reference points. Values decrease without a change in its moisture content. calculated will be positive—that is, the enthalpy This results in poor control of relative humidity values calculated will be in Btu per pound of air within the conditioned space. There is another above the enthalpy of air at 0°F and liquid water important issue that relates to dew at 32°F. T . I

point temperature. The dew point F O / S C P X S of the outdoor ambient air is often The HVACR industry uses standard air T C N O E higher than the dry-bulb equations rather than mass flow rate R M P U O

R temperatures of the indoor calculations to evaluate R T A S T N I

atmosphere and construction the work done on an air I O & N

S materials used to form the building sample. This practice L O

O walls. Water vapor in the infiltrating simplifies the evaluation T L

A air can condense inside cool walls, process. Careful use of R E

N causing damage to construction standard air equations E G materials and the formation of produces results that are contaminants that are unhealthy for typically within 2% of occupants. Therefore, a good vapor mass flow rate calculations barrier and adequate insulation are for the same work process important to the success of a performed on a given climate-controlled structure. airstream.

Figure 1 Figure 2

3 Standard air equations are based on the physical the needed physical properties. From the state point properties of air at sea level. “Standard air” is plotted on the chart, the remaining physical defined as having a dry-bulb temperature equal to properties of the air sample can be determined. 70°F at a pressure equal to 14.696 psi and a density equal to 0.075 lb/ft3 of air. At these conditions, the SOLUTION SCHEMATICS enthalpy of dry air is equal to 16.8 Btu/lb and its specific heat is 0.24 Btu/lb. The standard air It is important for you to be able to calculate the equations are stated as follows: projected heat load on a cooling coil accurately, so that the system chosen for an application has × × þ equation: QS = cfm 1.08 ∆T adequate sensible and latent capacities. A system × × þ Latent heat equation: QL = cfm 0.68 ∆WGR designer can determine these capacity values by × × þ Total heat equation: QT = cfm 4.5 ∆h using higher-level mathematics or by utilizing a psychrometric chart. A solution schematic drawn where: on a psychrometric chart helps a designer visualize Q = Btuh and calculate the work done on an airstream without cfm = cubic feet of air per minute the need for advanced mathematics. T = dry-bulb temperature (°F)

WGR = grains of water per pound of air Typically, a technician will use a sling psychrometer h = enthalpy. or an electronic psychrometer to measure the dry- bulb and wet-bulb temperatures of the air. These two The numeric values shown in the three equations values are used to position a state point on the chart. above (1.08, 0.68, 4.5) are not constants—they are Figure 3 illustrates how the state point is established factors that have been derived by using the known for an air sample that measures 80°F db and 67°F wb physical properties of standard air and dimensional temperatures. This state point is the AHRI entering analysis. The factors serve the purpose of converting air test point used to establish cooling performance all the units of measurement used in the equations to for HVAC systems. Notice that dry-bulb temperature Btuh. The mathematical calculations used to derive values are provided on the horizontal axis at the all three factors are shown in the shaded box below. bottom of the chart. The dry-bulb value lines are drawn vertically upward, almost perpendicular to the Standard air equations are useful tools that can be horizontal axis. Wet-bulb values are provided on the used to solve complex psychrometric problems. Two saturation curve located on the left side of the chart. of the physical properties of an air sample must be The wet-bulb value lines are drawn descending from known in order to establish state points and generate the saturation curve downward and to the right. a solution schematic on a psychrometric chart. Arrows have been superimposed on the chart to Relatively common instruments are used to measure illustrate how these value lines are used to position the state point for this air sample.

When sensible work is performed on an air sample, the state point shifts horizontally along the dew point line on the psychrometric chart. Dew point and humidity ratio values share horizontal lines on the chart. The state

4 ASHRAE

Figure 3

5 point will shift to the right when sensible heat is difference equal to 20°F. The known values are now added to the air sample. It will shift to the left when inserted into the sensible heat equation, which is sensible heat is removed. When latent work is solved as follows: performed on an air sample, the state point moves × × vertically. It will shift upward along the dry-bulb QS = cfm 1.08 ∆T temperature line when water vapor is added and = 8,000 cfm × 1.08 × 20°F = 172,800 Btuh downward when water vapor is removed from the air sample. Change in the specific humidity of an airstream must be known in order to calculate the quantity of latent In practical applications, cooling processes most heat removed from the air. These values are taken often include both sensible heat and latent heat from the psychrometric chart, as shown in Figure 5 transfer. This shifts the airstream’s state point on page 8. Specific humidity values (also known as diagonally, depending on whether heat energy is humidity ratio values) are shown on the vertical axis being added or removed. Condition lines are drawn located on the right side of the chart. Trace a line between the initial and final state points to provide from each state point horizontally to the right and a visual representation of the work done on the read the specific humidity on the humidity ratio airstream. The angle of the condition line indicates scale. Note that these values are provided in pounds the ratio of sensible to total heat of water per pound of air on the ASHRAE chart transfer. Steep condition lines depict work processes shown in Figure 5. They must be converted to grains that have small sensible heat ratios. Condition lines of water per pound of air for use in the standard air with shallower slopes depict work processes that equation. The initial humidity ratio is 0.0112 pounds have larger sensible heat ratios (meaning that more of water per pound of air and the final value is sensible heat than latent heat was transferred). 0.0088 pounds of water per pound of air, which yields a difference equal to 0.0024 pounds of water Figure 4 illustrates an airstream that has been cooled per pound of air. Since one pound of water equals from 80°F db, 67°F wb to 60°F db, 56.3°F wb. The 7,000 grains, the change in water vapor content of diagonal condition line illustrates that both the air the air is 16.8 grains of water per pound of air temperature and the water vapor content have been (0.0024 × 7,000). This converted value is now changed. As sensible heat was removed from the inserted into the latent heat equation, which is airstream, the state point shifted to the left. The solved as follows (the answer is rounded off): downward slope of the condition line indicates that × × water vapor was removed during the process. The QL = cfm 0.68 ∆WGR × × relatively shallow angle of the line indicates that = 8,000 cfm 0.68 16.8 WGR = 91,390 Btuh more sensible work than latent work was done on the airstream. Standard air equations provided earlier In order to calculate the amount of water mass can be used to calculate the quantity of sensible and removed by the cooling process, you need to utilize latent work performed. To utilize these equations, the the thermodynamic properties of water at saturation. air flow volume rate in cubic feet of air per minute These values can be found in the ASHRAE (cfm) must be known. Methods used to determine Fundamentals Handbook (Table 3 of Chapter 1 in the air flow are outside the scope of this discussion. 2009 edition). The specific enthalpy for evaporation For now, assume that blower data provided by the at the cooling coil temperature in this example is manufacturer were used to verify an air flow volume 1,060 Btu per pound of water. You can use this value rate of 8,000 cfm. to show that approximately 86.2 lb (91,390 ÷ 1,060) or about 10.3 gallons (86.2 ÷ 8.33) of water per hour The change in dry-bulb temperature is used to of operation will be condensed from the airstream. calculate the sensible heat removed from the airstream. In this example, the dry-bulb temperature The sum of sensible heat and latent heat values decreased from 80 to 60°F, for a temperature equals the total cooling work done on the air, which

6 ASHRAE

Figure 4

7 ASHRAE

Figure 5

8 ASHRAE

Figure 6

9 × × in this example is 264,190 Btuh (172,800 + 91,390). QT = cfm 4.5 ∆h The ratio of sensible heat transferred is calculated = 8,000 cfm × 4.5 × 7.5 ∆h = 270,000 Btuh by dividing the sensible work performed by the total work performed. In this case, the sensible heat ratio The sum of the sensible heat and latent heat values of the cooling process is 0.654 (172,800 ÷ 264,190), from earlier calculations was 264,190 Btuh. This indicating that more sensible heat than latent heat varies from the solution to the enthalpy equation was transferred. Recall that we were able to predict above by 5,810 Btuh (270,000 – 264,190), or 2.15% that outcome from the slope of the condition line in [100 × (5,810 ÷ 270,000)]. This variation is easily Figure 4. By plugging values taken from the solution accounted for by the rounding off that is done in schematic into the standard air equations, you have deriving the conversion multipliers used in the now determined that 65.4% of the total work done on standard air equations. When the total heat value the air utilized sensible heat energy, which means yielded by the standard air equation for enthalpy is that 34.6% utilized latent heat energy. within 2 or 3% of the total heat value yielded by summing sensible and latent heat values, your work You can check your work by using enthalpy values is reasonably accurate. The two total heat values will taken from the solution schematic to calculate the rarely be equal—the comparison merely serves as a total heat energy transferred during this cooling check of the calculations performed. Remember, your process. Enthalpy values are plotted on a scale that work must be accurate, which means that it must be in reality wraps around the entire perimeter of the error-free. Measurement instruments used in the field psychrometric chart. Look at Figure 6 on page 9. and the resolution of a psychrometric chart do not In order to keep the drawing from becoming too allow your work to be precise, which means exact. congested, only a few of the relevant enthalpy value lines are continued through the main body of the Next, you will apply what you have learned about chart—see the diagonal lines extending below the developing a solution schematic to HVAC systems horizontal axis (between 20 and 25 Btu/lb) and that mix outdoor ventilation air with air recirculated beyond the vertical axis on the far right-hand side from the conditioned space. Developing this skill of the chart (between 30 and 35 Btu/lb). Notice that allows a system designer to calculate accurately the these enthalpy value lines descend from left to right heat loads that will be imposed on a cooling coil and on the chart, as do the wet-bulb temperature lines. make better equipment selections. The two sets of lines are almost, but not quite, parallel. Some system designers simply extend the MIXED-AIR PROCESSES wet-bulb temperature line from a state point upward to the left to intersect the enthalpy scale, and in this In many residential applications and in all commercial way attempt to determine the total heat content of an applications, outdoor air is mixed with recirculated air sample. This practice yields an enthalpy value air to provide supply air that satisfies ventilation that approximates the correct value—however, a slight requirements. The quantity of outside air that must error is introduced into the calculation. Therefore, it be brought into an occupied space varies with the is a better practice to trace a line through each state activity of the occupants. A portion of Table 403.3 point connecting equal enthalpy values on the two taken from the 2009 International Mechanical Code® sides of the scale, as illustrated in Figure 6. is reproduced in Figure 7. It lists the outdoor air ventilation requirements for various kinds of In this example, the initial heat content of the commercial spaces. The requirement of air per entering air is 31.5 Btu/lb and the heat content of person is added to the requirement of air per square the leaving air is very close to 24 Btu/lb. The change foot of floor area to arrive at the total ventilation air in total heat content caused by the cooling process required. The introduction of outdoor air adds heat therefore equals 7.5 Btu per pound of dry air, and load to a cooling coil during warm periods in this enthalpy value is entered into the standard air summer. Additional heating capacity is needed to equation for total heat as follows: temper winter ventilation air. In this section, you will

10 2 0 0

People outdoor Area outdoor 9 I air flow rate in air flow rate in N T E

breathing zone, breathing zone, Default occupant Exhaust air flow R

2 2 2 N

Occupancy classification cfm/person cfm/ft density, lb/1000 ft rate, cfm/ft A T I O N

Offices A L

Conference rooms 5 0.06 50 — M E

Office spaces 5 0.06 5 — C H

Reception areas 5 0.06 30 — A N I

Telephone/data entry 5 0.06 60 — C A

Main entry lobbies 5 0.06 10 — L C O

Private dwellings, single and multiple D E Garages, common for multiple units — — — 0.75 Garages, separate for each dwelling — — — 100 cfm per car Kitchens — — — 25/100 Living areas 0.35 ACH but — Based on number of — not less than bedrooms. First 15 cfm/person bedroom, 2. Each additional bedroom, 1. Toilet rooms and bathrooms — — — 20/50

Public spaces Corridors — 0.06 — — Elevator car — — — 1.0 Shower room (per shower head) — — — 50/20 Smoking lounges 60 — 70 — Toilet rooms—public — — — 50/70 Places of religious worship 5 0.06 120 — Courtrooms 5 0.06 70 — Legislative chambers 5 0.06 50 — Libraries 5 0.12 10 — Museums (children’s) 7.5 0.12 40 — Museums/galleries 7.5 0.06 40 —

Retail stores, sales floors, and showroom floors Sales (except as below) 7.5 0.12 15 — Dressing rooms — — — 0.25 Mall common areas 7.5 0.06 40 — Shipping and receiving — 0.12 — — Smoking lounges 60 — 70 — Storage rooms — 0.12 — — Warehouses (see storage) — — — —

Specialty shops Automotive motor- dispensing stations — — — 1.5 Barber 7.5 0.06 25 0.5 Beauty and nail salons 20 0.12 25 0.6 Embalming room — — — 2.0 Pet shops (animal areas) 7.5 0.18 10 0.9 Supermarkets 7.5 0.06 8 —

Sports and amusement Disco/dance floors 20 0.06 100 — Bowling alleys (seating areas) 10 0.12 40 — Game arcades 7.5 0.18 20 — Ice arenas without — 0.30 — 0.5 Gym, stadium, arena (play area) — 0.30 — — Spectator areas 7.5 0.06 150 — Swimming pools (pool and deck area) — 0.48 — — Health club/aerobics room 20 0.06 40 — Health club/weight room 20 0.06 10 —

Figure 7. Excerpt from Table 403.3: Minimum ventilation rates

11 ASHRAE r i a r o o d t u O t n i o p t e s n g i s e d m o o R

Figure 8

12 study a draw-through equipment design in which the A psychrometric chart can be used to calculate the mixed air travels first through the cooling coil before heat loads that must be handled by the cooling coil. passing through the indoor blower. energy will The physical properties of the mixed airstream must provide reheat and slightly longer “ON” cycles for be known to make that calculation. To determine improved humidity control in the space. For the sake those properties, a system designer must plot state of clarity, conditioned air leaving the cooling coil is points for both the air recirculated from the referred to here as primary air, and air entering the conditioned space and the outdoor air used for conditioned space through registers or is ventilation. In this example, the indoor air will be referred to as supply air. The temperatures of primary maintained at a 75°F db temperature and at 45% air and supply air differ according to the amount of relative humidity. On design day, the ventilation air heat gained by the airstream from the fan motor and will be 91°F db and 80°F wb. The state points through the supply ductwork. This example consists representing these two air samples are plotted as of packaged rooftop equipment with the supply shown in Figure 8. ductwork installed in the plenum cavity above a drop ceiling. The plenum cavity has been sealed for use as A room sensible heat ratio (RSHR) line must be traced a return-air pathway. on the chart in order to visualize how the conditioned space gains sensible and latent heat. This ratio is A system designer must begin the process by derived from the heat load calculation. For this completing a heat gain/heat loss load calculation for example, let’s say that the room sensible heat gain the conditioned space. The outcome of this exercise on design day is 153,600 Btuh and the room latent will describe how the space gains (or loses) heat heat gain is 20,950 Btuh. Total room heat gain equals energy. In commercial buildings, heat transfer through 174,550 Btuh, or 14.54 tons (174,550 ÷ 12,000). exterior walls affects interior spaces that are within The RSHR, therefore, is 0.88 (153,600 ÷ 174,550). 10 ft of those walls. For core areas of the building, This ratio is first traced onto the protractor located in heat gain is almost entirely internally generated. the upper left corner of the ASHRAE psychrometric In many regions, core areas experience a demand chart. Sensible heat ratio (SHR) values are printed for cooling 12 months out of the year. The magnitude on the inside of the arc. A reference line is traced of internal heat gain can vary widely, depending on from the origin of the protractor through the arc at lighting, machines, equipment, the number of people 0.88 SHR. The slope of this line represents the working within the space, and the type of work being manner in which the space experiences heat gain. done. Only heat gain appearing within the space is The shallow angle of the SHR line indicates that used to determine the volume of supply air needed more sensible heat than latent heat is gained on to cool the conditioned area. This includes heat design day. Next, an RSHR line parallel to the SHR transmission from outdoors as well as internally reference line on the protractor is traced from the room generated heat. The heat load provided to the cooling setpoint condition downward and to the left until it coil by ventilation air taken from outside is not intersects the saturation curve. In this example, the factored into the equation used to determine the RSHR line intersects the saturation curve at 49.5°F, needed indoor blower air volume rate. Several load as illustrated in Figure 9 on page 14. calculation procedures are available for commercial applications. The ACCA Manual N procedure and Supply air with physical properties that yield a state the ASHRAE load calculation procedure are two point positioned anywhere on the RSHR line will commonly used by industry professionals. Equipment maintain room setpoint conditions. However, the manufacturers such as Carrier and Trane also have dry-bulb temperature of the supply air determines procedures available for this purpose. A load the amount of supply air needed to condition a space. calculation for an interior space does not reflect the The air flow needed decreases as the temperature of total heat load that appears at the cooling coil in the supply air decreases. As the temperature of the mixed-air applications. The heat load imposed by the supply air increases, the air flow required to maintain outdoor ventilation air must be accounted for also. design conditions also increases.

13 ASHRAE e n i l R H S R e n i l e c n e r e f e r R H S

Figure 9

14 ASHRAE

Figure 10

15 A designer must consider another factor. Supply air rests directly on the RSHR line, indicating that the entering the conditioned space acts like a “sponge,” physical properties of the supply air are sufficient to absorbing sensible heat and humidity from the room maintain room design conditions. As the supply air air. As it does so, the physical properties of the supply gains sensible and latent heat from the room air, its air shift in a direction parallel to the RSHR line. state point will be altered in a manner that tracks the Supply air represented by a state point located above RSHR line to reflect the ratio of heat sources in the the RSHR line can maintain the room’s dry-bulb space. The supply air entering the room will absorb temperature—however, the resulting room relative sensible heat and latent heat in a ratio that causes its humidity will be higher than the specified value of state point to ascend to the right until it reaches the 45%. This is illustrated in Figure 10 on page 15, in room’s design setpoint. which supply-air state point A is situated above the RSHR line. As the supply air gains temperature and The supply air volume needed to maintain room humidity, it shifts to state point A', yielding a room design conditions is calculated from the data plotted temperature equal to 75°F and a relative humidity on the chart up to this point. The supply air dry-bulb above 45%. Supply air represented by a state point temperature is 57°F, which is 18°F cooler than located below the RSHR line can maintain the room the room design temperature. This temperature dry-bulb temperature, but the room relative humidity difference, together with the room’s sensible heat will be lower than 45%. This is also illustrated in gain, will be used in the sensible heat equation to Figure 10, which shows state point B shifting to state determine the supply air flow rate needed for the point B'. space. In the equation shown below, “q” equals cubic feet of air per minute (cfm). Again, the answer has The position of the primary-air state point varies with been rounded off: the type of cooling equipment used. Direct expansion (DX) cooling coils typically yield primary air with relative humidity between 75 and 90%. coils can yield primary air with relative humidity as high as 100% and dry-bulb temperatures lower than At this point, the designer must calculate the physical those produced by DX equipment. A designer must properties of the air entering the cooling coil. This be familiar with the performance of the equipment airstream will be a mixture of air recirculated from being installed to make a good judgment call when the space and outdoor air used for ventilation. For plotting the primary-air state point. In this example, this example, let’s assume that the design calls for we will plan for a DX coil that provides primary air with a relative humidity of 85%. On the solution schematic, the primary-air state point will be positioned at 85% RH, approximately 2°F to the left of a horizontal intersection with the RSHR line (to allow for the heat rise resulting from fan motor heat and supply gain). This heat gain will cause Primary air the primary-air state point to move horizontally to the right. Supply air entering the room will therefore be about 2°F warmer than the primary air leaving the cooling coil in this example. State Supply air points representing the primary air and the supply air are shown in Figure 11. Note that the supply-air state point now Figure 11

16 Packaged rooftop unit yields a return-air dry-bulb temperature equal to 78°F (75 + 3). Return air with these new properties is then mixed l i with ventilation air inside the HVAC o r c e t l g equipment. Ventilation air from outside i f n i l r i

o is supplied through a manual outdoor A o

C air or an , as illustrated in Figure 12.

Figure 13 on page 18 shows the return-air state point located 3°F to the right of the room-air state point on the RSHR line. Next a line is drawn Figure 12 on the psychrometric chart connecting the state points of the outdoor air and the return air. A new state point “neutral” space pressure—which means that the representing the air mixture entering the cooling exhaust air volume equals the volume of ventilation coil then must be positioned somewhere on this air being introduced from outdoors. Regulating codes connecting line. A convenient method for positioning must be followed when choosing ventilation air for an the mixed-air state point on the chart is first to application. Say that ventilation air equal to 1,200 cfm calculate the dry-bulb temperature of the mixed is required. Then 6,700 cfm (7,900 – 1,200) of room airstream. This is done by using the air volumes and air will be recirculated, while 1,200 cfm of room air dry-bulb temperatures of the returning room air and will be exhausted. Air returning from the conditioned the ventilation air. The return air (6,700 cfm) reaches space can increase in temperature while moving the HVAC unit at a dry-bulb temperature of 78°F, through the return plenum or ductwork. and the ventilation air (1,200 cfm) has a dry-bulb temperature of 91°F. These values are used to find Applications with ducted returns that have been the dry-bulb temperature of the mixed air as follows: correctly sealed, insulated, and installed in unconditioned interior spaces are subject to less heat gain than ductwork installed outdoors. Any heat gain through the return-air duct will cause an increase in air temperature, which must be accounted for on the solution schematic that you create on a psychrometric chart.

In a plenum return application, the return-air temperature will increase due to heat emitted by recessed ceiling lights, as well as heat gain that takes A state point representing the mixed air now can be place through the roof, exterior walls, and partitions. positioned on the line that connects the outdoor-air Partitions are walls separating the plenum cavity and return-air state points. The mixed-air state point, from unconditioned interior spaces. This rise must shown fixed on that line at 80°F db temperature in be accounted for by moving the room state point Figure 13, represents primary air entering the cooling horizontally to the right by an amount equal to the coil. The curved condition line drawn on the chart temperature gain. In this example, assume that a illustrates how the physical properties of the primary calculation made to determine gain in the plenum air change as work is done on the airstream by the finds the rise to be equal to 3°F. The room-air state cooling coil. Notice that the initial work done on the point is shifted to the right by that amount. This airstream is the transfer of sensible heat, causing

17 ASHRAE r i a r o o d t u O r i a g y n r i r r a i e a t m i n r n r E p u t e R r i a m o o R e n i l n o i t i d n o C r i a y l p p u S r i g a n i v y r a a e L m i r p

Figure 13

18 only a change in dry-bulb temperature. As the air (1,200 cfm ÷ 10.44 tons) in this application. airstream approaches its dew point temperature, Certainly, the cost of an energy recovery ventilator moisture begins to condense from the air, causing a (ERV) can be justified to reduce the size of the cooling downward shift in the condition line. The entering equipment needed to condition the environment in and leaving primary-air state points are now used to this space. An ERV that reduces the load imposed by calculate the work that must be done on the airstream the ventilation air by 50% (5.2 tons) would reduce by the cooling coil. the capacity needed for the HVAC cooling equipment from 25 nominal tons to 20 nominal tons, and lower The sensible capacity that must be provided by the operating costs significantly. DX equipment can be calculated once you know the change in temperature of the primary air from As a general guide, DX equipment can handle the entering point (80°F db) to the leaving point additional load from outdoor air when the ventilation (55°F db). In this case, a temperature difference air makes up no more than 20 to 25% of the primary of 25°F is plugged into the sensible heat equation, air. This value varies with the equipment selected. together with the air flow volume (7,900 cfm), to When more outdoor air is needed, a designer must select the sensible equipment capacity needed: shed load from the outdoor air using ERV equipment or select a chilled water system for the application × × QS = cfm 1.08 ∆T (or both). = 7,900 cfm × 1.08 × 25°F = 213,300 Btuh Enthalpy values taken from the solution schematic The needed latent equipment capacity is calculated shown in Figure 14 can be used to check the above by finding the change in the humidity ratio of the calculations. The enthalpy of the entering and primary air, from the entering point (0.010 pounds leaving airstreams are approximately 30.2 Btu/lb and of water per pound of air) to the leaving point 21.7 Btu/lb, respectively, for a total change in heat (0.0077 pounds of water per pound of air), as taken content equal to 8.5 Btu per pound of dry air. The from the psychrometric chart shown in Figure 14 on change in enthalpy and the air flow volume rate are page 20. The difference (0.0023 pounds of water per used as shown below to calculate the total heat pound of air) must be converted to change in grains transferred in this cooling process: of water by multiplying it times the conversion factor × × of 7,000 grains per pound of water, yielding a change QT = cfm 4.5 ∆h in specific humidity equal to 16.1 grains of moisture = 7,900 cfm × 4.5 × 8.5 ∆h = 302,175 Btuh per pound of dry air (7,000 × 0.0023). The air flow volume rate and the change in humidity ratio can This value is within 2,385 Btuh of the total cooling then be used in the latent heat equation as shown capacity calculated earlier (302,175 – 299,790) and below: indicates that calculations made using the solution schematic are reasonably accurate. Variation from the × × QL = cfm 0.68 ∆WGR enthalpy calculation amounts to a discrepancy of less × × × = 7,900 cfm 0.68 16.1 WGR = 86,490 Btuh than 1% [100 (2,385 ÷ 299,790) = 0.79%].

Adding the sensible heat and latent heat values EVAPORATIVE COOLING PROCESSES yields the needed total cooling capacity that must be delivered by the HVAC equipment selected, which in In arid regions, it is not necessary to remove this example equals 299,790 Btuh (213,300 + 86,490), humidity from the conditioned space during the or 24.98 tons (299,790 ÷ 12,000). It has been shown cooling cycle. The following paragraphs describe that a nominal 25-ton unit must be selected to handle applications in which water may be used in a direct the space load (14.54 tons) plus the load imposed by evaporative process to provide comfort cooling in the ventilation air (10.44 tons). This translates to one such regions of the world. These systems do not ton of load on the cooling coil per 115 cfm of outdoor recirculate indoor air—instead, they utilize 100%

19 ASHRAE

Figure 14

20 ASHRAE A B 3 . 8 2

Figure 15

21 outdoor air to cool a space. A system designer must of the entering air, which is point A on the chart. be aware that the entering airstream’s wet-bulb A line is traced upward to the left along the enthalpy temperature limits direct evaporative cooling. line that intersects the entering-air state point. In this However, design wet-bulb temperatures are rarely example, the enthalpy is shown to be approximately higher than 78°F, making direct evaporative cooling equal to 28.3 Btu/lb. (Since wet-bulb lines nearly economical in arid regions. parallel enthalpy lines, it is the practice of some system designers to trace their line along the wet- Direct evaporative cooling is an adiabatic exchange of bulb line that intersects the entering-air state point, heat. By definition, no heat is added to, or extracted although this introduces a very slight error.) The state from, an . The initial and final air point representing the physical properties of the conditions fall on a line of constant total heat leaving air is positioned on the traced line at its (enthalpy), which nearly coincides with a line of intersection with the 70°F dry-bulb line, which is constant wet-bulb temperature. Heat must be added point B on the chart in Figure 15. to evaporate water. In the case of direct evaporative cooling, the heat is supplied by the air into which In the adiabatic , only part of the water is evaporated. The dry-bulb temperature of recirculated water evaporates and the water supply the air is lowered, and sensible cooling results. is recirculated. The recirculated water will reach an The amount of heat removed from the air equals equilibrium temperature approximately equal to the the amount of heat absorbed by the water evaporated. wet-bulb temperature of the entering air. When water is recirculated in the direct evaporative cooling apparatus, the water temperature in the A heat gain load calculation is consulted to reservoir will approach the wet-bulb temperature determine the air flow volume rate needed to of the air entering the process. maintain the desired space temperature. For this example, assume that the sensible room gain equals The maximum possible reduction in the dry-bulb air 42,000 Btuh, and that the room design setpoint temperature is the difference between the entering temperature is 80°F. The temperature difference air dry-bulb and wet-bulb temperatures. If the air between the room setpoint and the air leaving the could be cooled to the wet-bulb temperature, the direct evaporative cooler is 10°F (80 – 70). The process would be 100% effective and the leaving air sensible heat equation then can be used to calculate would be saturated. Effectiveness is defined using a the air flow volume rate (q) required to maintain the mathematical calculation in which the depression of setpoint, as shown below. The answer is rounded off: the dry-bulb temperature of the leaving air is divided by the difference between the dry-bulb and wet-bulb temperatures of the entering air. Theoretically, adiabatic direct evaporative cooling is less than 100% effective. However, evaporative coolers can If air is not exhausted freely, the increased static be 95% (or more) effective and are a practical choice pressure within the space will reduce air flow through for comfort cooling in arid regions. the evaporative cooler. The result is an increase in the moisture and heat absorbed by the air leaving the By way of example, consider entering air at 96°F db evaporative cooler. Reduced air flow also reduces the and 63°F wb. The resulting initial difference is 33°F air velocity in the room. These effects combine to (96 – 63). If the effectiveness of the evaporative reduce the comfort level for the occupants. Properly cooler is 80%, the depression in air temperature designed systems should have a minimum of 2 ft2 will be approximately 26°F (33 × 0.80). The dry-bulb of exhaust area for every 1,000 cfm of supply air. temperature of the air leaving the cooler then would If the exhaust area is not sufficient, a powered be 70°F (96 – 26). This process is illustrated on the exhaust should be used. The amount of powered psychrometric chart shown in Figure 15 on page 21. exhaust depends on the total air flow and the amount First, a state point is plotted to represent the condition of free or gravity exhaust. Some applications require

22 powered exhaust capacity equal to the cooler cfm before is injected into the airstream. Steam output. added to the cold ventilation air will condense inside the ductwork. When a direct evaporative cooling unit alone cannot provide the desired conditions, several alternatives The sensible heat and latent heat required to can be employed to satisfy application requirements condition the ventilation air can be determined by and still be energy-efficient. One possible option is developing a solution schematic on a psychrometric to increase the volume of the recirculating water chart. The injected steam will provide a portion of supplying the direct evaporative cooling, while the sensible heat that must be added to the airstream. chilling it by mechanical refrigeration. This process To generate the solution, the system designer must provides lower leaving wet-bulb and dry-bulb first consult Table 3 of the chapter on psychrometrics temperatures. Compared to the cost of using in the ASHRAE Fundamentals Handbook to extract mechanical refrigeration only, this strategy can the specific enthalpy value for the saturated steam reduce operating costs by as much as 25 to 40%. being added to the ventilation air. A portion of that table is reproduced in Figure 17 on page 25. As you can see, the specific enthalpy of 230°F steam is STEAM HUMIDIFICATION approximately 1,157 Btu per pound of water. Now It is a common practice to inject steam or liquid look again at Figure 16. On the protractor printed water into an airstream to raise its humidity. If in the upper left corner of the psychrometric chart, mixing is adiabatic, the final state point of the moist a line is traced from the origin of the protractor air lies on a straight line in the direction fixed by the through the point on the curve corresponding to specific enthalpy of the injected water drawn through 1,157 Btu/lb. Specific enthalpy on the protractor the initial state point of the air. A typical example is is the change in enthalpy divided by the change in the addition of humidity to dry winter air used for humidity ratio (∆h/∆W ). Those values are shown on ventilation. the outside of the protractor curve.

For example, consider winter air at 35°F db and A state point for the final desired air condition has 24.8°F wb (see point A in Figure 16 on page 24). already been plotted on the chart as (point B). Next, A horizontal line can be traced to the right to a sloped line is drawn parallel to the line traced on determine the humidity ratio of the ventilation air. the protractor downward from point B to intersect the The specific humidity (humidity ratio) of this air is horizontal line drawn from point A. The intersection 0.0007 pounds of water per pound of air, or point is labeled point C on the solution schematic in approximately 4.9 grains of water per pound of air Figure 16. The slope of this line is used to reveal the (0.0007 × 7,000). The relative humidity of the air temperature gain from sensible heat added by the is 16.6%. When brought into an indoor space and steam. In this example, temperature gain from the heated to 70°F db, its resulting relative humidity will steam is approximately 1.5°F. be a mere 4.6%. Humidity must be added to improve the quality of the air for use at a comfortable indoor With this information, the sensible heat that must be atmosphere.To achieve a 70°F db temperature with added to the air can be determined for a known air 45% relative humidity (see point B in Figure 16), a flow volume rate. In this example, assume that the final specific humidity of 0.007 pounds of water per needed ventilation air from outside equals 3,000 cfm. pound of air, or 49.0 grains of water per pound of air Sensible heat must be added from a new energy (0.007 × 7,000) must be attained. This means that source to warm the air from 35°F to 68.5°F, for a total 44.1 grains (49.0 – 4.9) of moisture must be added temperature difference of 33.5°F. The sensible heat to every pound of ventilation air. In this example, equation is used to make the calculation as follows: humidification will be accomplished by adiabatic × × injection of saturated steam at 230°F. It is important QS = cfm 1.08 ∆T to note that sensible heat must be added to the air = 3,000 cfm × 1.08 × 33.5°F= 108,540 Btuh

23 ASHRAE B C 7 5 1 , 1 A

Figure 16

24 The sensible heat added by the injected steam also of air. The latent heat added by the steam therefore can be determined using the sensible heat equation, can be calculated using the appropriate standard air as shown below: equation, as shown below. As is customary, the answer has been rounded off: × × QS = cfm 1.08 ∆T × × × × = 3,000 cfm 1.08 1.5°F= 4,860 Btuh QL = cfm 0.68 ∆WGR × × = 3,000 cfm 0.68 44.1 WGR = 89,960 Btuh Dividing by the conversion factor of 3,413 Btu/kWh, you can determine that 108,540 Btuh = 31.8 kW, and The specific volume of the entering ventilation air is 4,860 Btuh = 1.42 kW. 12.48 ft3/lb. This means that approximately 240.38 lb of ventilation air is introduced into the space every Earlier in this discussion, it was shown that the minute, or about 14,423 lb per hour. Dimensional quantity of water that must be injected into the analysis is used to make this calculation, as follows: airstream equals 44.1 grains of moisture per pound air flow of 3,000 cfm × 1 lb of air per 12.48 ft3 of A S

3 H

Absolute Specific volume, ft /lb Specific enthalpy, Btu/lb Specific , Btu/lb/°F R A

Temp, pressure, E °F psia Sat. solid Evap. Sat. vapor Sat. solid Evap. Sat. vapor Sat. solid Evap. Sat. vapor

190 9.3497 0.01657 40.901 40.918 158.05 983.78 1141.83 0.2787 1.5143 1.7930 191 9.5515 0.01658 40.092 40.108 159.05 983.17 1142.22 0.2802 1.5110 1.7912 192 9.7570 0.01658 39.301 39.317 160.06 982.55 1142.61 0.2818 1.5077 1.7895 193 9.9662 0.01659 38.528 38.545 161.06 981.94 1143.00 0.2833 1.5045 1.7878 194 10.1791 0.01659 37.773 37.790 162.07 981.32 1143.39 0.2849 1.5012 1.7861 195 10.3958 0.01660 37.036 37.053 163.07 980.71 1143.78 0.2864 1.4980 1.7844 196 10.6163 0.01661 36.315 36.332 164.08 980.09 1144.17 0.2879 1.4948 1.7827 197 10.8407 0.01661 35.611 35.628 165.08 979.47 1144.56 0.2895 1.4916 1.7810 198 11.0690 0.01662 34.924 34.940 166.09 978.86 1144.94 0.2910 1.4884 1.7793 199 11.3013 0.01663 34.251 34.268 167.09 978.24 1145.33 0.2925 1.4852 1.7777

200 11.5376 0.01663 33.594 33.611 168.10 977.62 1145.71 0.2940 1.4820 1.7760 201 11.7781 0.01664 32.952 32.968 169.10 976.99 1146.10 0.2956 1.4788 1.7743 202 12.0227 0.01665 32.324 32.341 170.11 976.37 1146.48 0.2971 1.4756 1.7727 203 12.2715 0.01665 31.710 31.727 171.12 975.75 1146.87 0.2986 1.4724 1.7710 204 12.5246 0.01666 31.110 31.127 172.12 975.13 1147.25 0.3001 1.4693 1.7694 205 12.7819 0.01667 30.524 30.540 173.13 974.50 1147.63 0.3016 1.4661 1.7678 206 13.0437 0.01667 29.950 29.967 174.14 973.88 1148.01 0.3031 1.4630 1.7661 207 13.3099 0.01668 29.389 29.406 175.14 973.25 1148.40 0.3047 1.4599 1.7645 208 13.5806 0.01669 28.840 28.857 176.15 972.62 1148.78 0.3062 1.4567 1.7629 209 13.8558 0.01669 28.304 28.321 177.16 972.00 1149.15 0.3077 1.4536 1.7613

210 14.1357 0.01670 27.779 27.796 178.17 971.37 1149.53 0.3092 1.4505 1.7597 212 14.7094 0.01671 26.764 26.781 180.18 970.11 1150.29 0.3122 1.4443 1.7565 214 15.3023 0.01673 25.792 25.809 182.20 968.85 1151.04 0.3152 1.4382 1.7533 216 15.9149 0.01674 24.862 24.879 184.21 967.58 1151.79 0.3182 1.4320 1.7502 218 16.5475 0.01676 23.971 23.988 186.23 966.31 1152.54 0.3211 1.4259 1.7471 220 17.2008 0.01677 23.118 23.135 188.25 965.03 1153.28 0.3241 1.4198 1.7440 222 17.8753 0.01679 22.301 22.317 190.27 963.75 1154.02 0.3271 1.4138 1.7409 224 18.5714 0.01680 21.517 21.534 192.29 962.47 1154.76 0.3300 1.4078 1.7378 226 19.2896 0.01681 20.766 20.783 194.31 961.19 1155.49 0.3330 1.4018 1.7348 228 20.0307 0.01683 20.046 20.063 196.33 959.89 1156.22 0.3359 1.3959 1.7318

230 20.7949 0.01684 19.356 19.373 198.35 958.60 1156.95 0.3388 1.3899 1.7288 232 21.5830 0.01686 18.693 18.710 200.37 957.30 1157.68 0.3418 1.3840 1.7258 234 22.3955 0.01687 18.057 18.074 202.40 956.00 1158.40 0.3447 1.3782 1.7229

Figure 17. Excerpt from Table 3: Thermodynamic properties of water at saturation

25 ASHRAE

Figure 18

26 air × 60 minutes per hour. The math is easier to a condition line that crosses through this region of visualize if it is written as follows: the chart, fog will appear in the ductwork sometime during the mixing process.

To ensure that fog does not develop in the ductwork, sensible heat must be added to the ventilation air before it is mixed with the recirculated indoor air in this example. The sensible heat added must shift Each pound of ventilation air must be injected the state point for the ventilation air to the right until with 44.1 grains of moisture, or 0.0063 lb of water the condition line does not invade the region of fog. (44.1 ÷ 7,000) to achieve the needed humidity level In this example, heating the ventilation air to 40°F in the conditioned space. This means that a steam (point A' in Figure 18) will shift the condition line must deliver 91.2 lb, or 10.9 gallons, of out of the region of fog with a comfortable margin water every hour to the airstream in this example. of safety. The solution for the needed water is calculated as follows: 14,423 lb of air per hour × 0,00632 lb of SUMMARY water per pound of air × 1 gallon of water per 8.33 lb of water, or: This discussion is not a definitive exploration of the many uses for a psychrometric chart. All HVAC professionals must have good working knowledge of psychrometrics and be able to create solution schematics proficiently. Solution schematics are excellent visualization tools that communicate the effect of heat transfer, both sensible and latent, It is important to note that fog can appear in ductwork imposed on an airstream. The use of standard air when two airstreams are mixed. This can result in equations simplifies the mathematics needed to condensate forming on the duct walls, leading to the calculate accurately the work done on an air sample. growth of unhealthy contaminants. The psychrometric The output from these calculations can be used to chart can be used as a forensic tool to predict when indicate the load that will be imposed on a cooling such an episode might occur. Consider the mixing of coil. Evaporative cooling processes can be easily cold winter ventilation air with warm, moist air from modeled on a psychrometric chart and used to an industrial area. Figure 18 shows the mixing of calculate the needed air flow volume rate. ventilation air at 35°F db and 90% RH (point A) Humidification calculations can be made quickly with indoor air at 85°F db and 70% RH (point B). and effectively without the need for higher-level The mixed air will fall somewhere on the condition mathematics. Forensic observations can predict line connecting these two airstreams. the formation of fog in the ductwork of mixed-air systems, which leads to the growth of unwanted Note that the condition line invades the “region of contaminants on duct walls. For all of these reasons, fog” on the chart, which is that area to the left of the the psychrometric chart should be a familiar friend saturation curve. When a solution schematic yields to anyone working with HVAC systems.

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