BUILDING SERVICES III Unit I - Introduction Introduction to A/C conditions, basic of refrigeration systems, components of refrigeration system, compressor, condenser, control devices, evaporator and cooling times. Vapour compression cycle. Concepts of cooling load, calculation of cooling load, concepts of zoning.
Glossary of terms 1. Absolute zero: The zero point on the absolute temperature scale, i.e.459.67 degrees below the zero of the Farenheit scale or 273.17 degrees below the zero on the Celsius scale.
2. Air changes: A method of expressing the amount of air leakage in to or out of a building or room in terms of the number of building volumes or room volumes exchanged.
3. Air conditioning: The process of treating air so as to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space.
4. Brine: Any liquid cooled by the refrigerant and used for the transmission of heat without a change in its state.
5. Capacity of refrigerating plant: The amount of heat extracted in the evaporator by the refrigerant. This is measured by the quantity of refrigerant flow and the difference in total heat content of refrigerant entering and leaving the evaporator expressed in kilocalories per hour or tons of refrigeration or in kilowatts.
6. Direct or dry expansion system: A system of refrigeration in which liquid refrigerant enters the evaporator, usually consisting of a tube of sufficient length so that the refrigerant leaves in the vapour form.
7. Dry ice: A term used to describe solid carbon do oxide. The word dry is used on account of its sublimation in to gas without intermediate liquefaction.
8. Duct: A passageway made of sheet metal or other suitable material not necessarily leak tight, used for conveying air or other gas at low pressure.
9. Evaporative cooling: Cooling caused by the adiabatic exchange of heat between air and a water spray or wetted surface, as is done in air coolers popularly known as desert coolers.
1 10. Evaporator: That part of a refrigerating system in which refrigerant is vaporized to produce refrigeration.
11. Filter: A device used to remove solid material from a fluid.
12. Grille: A lattice or grating for delivery or intake opening of an air passage.
13. Humidity: Water vapour within a given space.
14. Absolute humidity: The weight of water vapour per unit volume, as grams of water per litre of air.
15. Percentage humidity (saturation ratio): The ratio of the weight of water vapour associated with a kilogram of dry air to the weight of water vapour associated with a kilogram of dry air saturated at the same temperature. Also called as degree of saturation.
16. Relative humidity: The ratio of the mol fraction of water vapour present in the air, to the fraction of water vapour present in saturated air at the same temperature and pressure; approximately it equals the ratio of the partial pressure or density of water vapour in the air, to the saturation pressure or density, respectively of water vapour at the same temperature.
17. Latent heat: Change of enthalpy during a change of state, usually expressed in kilocalories/ kilogram.
18. Psychrometer: An instrument for measuring relative humidities by means of wet and dry bulb temperature.
19. Sensible heat: Heat which is associated with a change in temperature; specific heat exchange in temperature, in contrast to a heat interchange in which a change of state occurs.
20. Absolute temperature: Temperature expressed in degrees above absolute zero.
21. Dry bulb temperature: Temperature by ordinary thermometer.
22. Dew point temperature: Temperature at which condensation starts if moist air is cooled at constant pressure without loss or gain of moisture during the cooling process.
2 23. Wet bulb temperature: Thermodynamic wet bulb temperature is the temperature at which liquid or solid water, by evaporating in to air, may bring the air to saturation adiabatically at the same temperature.
24. Thermal conductivity: A characteristic property of a material, depending upon its density, porosity and temperature, which determine the quantity of heat passing per unit time through unit area of a slab of unit thickness, when unit difference of temperature is established between its faces.
25. Vapour barrier: a waterproof sink employed to prevent moisture absorption by an insulant, by capillary attraction or difference of vapour pressure.
26. Ton of refrigeration: The quantity of heat absorbed in the melting of one ton of water ice per 24 hours, that is, 72755 kcal/day or 3024 Kcal/hour or 12000Btu/ 24 hours.
27. Water cooling tower: An enclosed device for evaporatively cooling water by contact with air.
Basic Refrigeration Principles Refrigeration engineering deals almost entirely with the transfer of heat. This seeming paradox is one of the most fundamental concepts that must be grasped to understand the working of a refrigeration system.
Thermodynamics: It is that branch of science dealing with the mechanical action of heat. There are certain fundamental principles of nature, often called laws of thermodynamics, which govern our existence here on earth, several of which are basic, in the study of refrigeration. • Energy can neither be created nor destroyed, but can be converted from one form to another. • Heat is often defined as energy in transfer, for it is never content to stand still, but is always moving from warm body to a colder body. • Much of the heat on the earth is derived from radiation from the sun. • Heat exists at any temperature above absolute zero, even though it may be in small quantities. ( Absolute zero is the term used by scientists to describe the lowest theoretical temperature possible, the temperature at which no heat exists which is approx. 450 degrees below zero Farenheit.
Temperature: Temperature is the scale used to measure the intensity of heat, the indicator that determines which way the heat energy will move.
3 Units - Temperature
Fahrenheit scale Centigrade scale
Freezing point of water – 32 F Freezing point of water – 0 C Boiling point of water – 212 F Boiling point of water – 100 C Number of divisions - 180 Number of divisions - 100
Relation between Fahrenheit and Centigrade scales can be established by the foll. Formulae: Farenheit = (9/5 Centigrade) + 32 degrees Centigrade = 5/9 (Farenheit – 32 degrees)
Heat measurement: The measurement of temperature has no relation to the quantity of heat. The basic unit of heat measurement is British Thermal Unit commonly expressed as Btu. A Btu is defined as the amount of heat necessary to raise one pound of water through one degree Fahrenheit.
1 Btu = 1.055 Kj ( Relationship between Btu and Kilo joule) Heat transfer: Second law of thermodynamics: Heat always travels from a warm object to a colder one. The rate of heat travel is in direct proportion to the temperature difference between the two bodies.
4 Three ways in which heat travels
Radiation: Transfer of heat by waves. Ex. Sun’s energy is transferred to the Earth by radiation. Sun – major factor in the refrigeration load.
Conduction: Flow of heat through a substance Actual physical contact is required for heat transfer An efficient means of heat transfer
Convection: Flow of heat by means of a fluid medium, either gasor liquid, normally air or water
Change of state: Most common substances can exist as a solid, liquid or a vapour, depending on their temperature and the pressure they are exposed. Heat can change their temperature, and also can change their state. Heat is absorbed even though no temperature change takes place when a solid changes to a liquid or when a liquid changes to a vapour. The same amount of heat is given off when the vapour changes back to a liquid and when liquid is changed in to a solid.
5 Heat is absorbed
Melts at 0 deg C Boils at 100 deg C
Ice Water Steam (solid state) (liquid state) (gaseous state)
Heat is released
ooling process anged in certain patterns to form different patterns. cules re arranged themselves, changing the ice in to water & the water in to steam. When the steam condenses back in to water, the same mole
Sensible heat: It is defined as the heat involved in a change of temperature of a substance.
Example: • When the temperature of water is raised from 32 F to 212 F, an increase in sensible heat content is taking place the Btu’s required to raise the temperature of one pound of substance through 1F is termed its specific heat.
6 • By definition, the specific heat of water is 1, but the amount of required to raise the temperature of different substance through a given temperature will vary. • It requires only 64 Btu to raise the temperature of one pound of butter 1F and only 0.22 Btu is required to raise the temperature of one pound of aliminium 1F. Hence the specific heat of these two substances are 0.64 & 0.22 respectively.
Latent heat of fusion or latent heat of melting or latent heat of freezing: A change of substance from a solid to a liquid or from a liquid to a solid involves the latent heat of fusion.
Absorbs 144 Btu at a constant temp of 32 F
1 pound of ice 1 pound of water
144 Btu at a constant temp of 32 F to be removed
Latent heat of evaporation or latent heat of boiling or latent heat of sublimation: A change of a substance from a liquid to a vapour or from a vapour back to a liquid involves latent heat of evaporation.
7 Absorbs 970 Btu at a constant temperature of 212 F
1 pound of water 1 pound of steam
970 Btu at a constant temp of 212F to be removed
ndensation, heat transfer can be very efficient during this process. at different temperatures and pressures. e of that heat by condensing the vapour is the key stone to the whole mechanical refrigeration process and the movement of the latent heat invo
Latent heat of sublimation: A change in state directly from a solid to a vapour without going through the liquid phase can occur in some substances. Example: Solid carbon di oxide or dry ice
Latent heat of sublimation = latent heat of fusion + latent heat of evaporation
Saturation temperature: The condition of temperature and pressure at which both liquid and vapour can exist simultaneously is termed saturation. • A saturated liquid or vapour is one at its boiling point, and for water sea level, the saturation temperature is 212 F.
8 • At higher pressures, the saturation temperature increases, and with a decrease in pressure, the saturation temperature decreases.
Superheated vapour: After a liquid has changed to a vapour, any further heat added to the vapour raises its temperature so long as the pressure to which it is exposed remains constant. The term superheated vapour is used to describe a gas whose temperature is above its boiling or saturation point.
Super cooled liquid: Any liquid which has a temperature lower than the saturation temperature corresponding to its pressure is said to be subcooled. Water at any temperature less than its boiling point temperature (212 F) is subcooled.
Pressure-temperature relationships, liquids The temperature at which a liquid boils is dependent on the pressure exerted on it. The vapour pressure of the liquid, which is the pressure being exerted by the tiny molecules seeking to escape the liquid and become vapour, increase with an increase in temperature until at the point where vapour equals the external pressure, boiling occurs. • Water at sea level boils at 212 F or 100 degrees but at 5000’ elevation elevation it boils at 203 F or due to atmospheric pressure. • Since all liquids react in the same fashion, although at different temperature and pressures, pressure provides a means of regulating a refrigerating temperature.
Pressure – temperature relationships, Gases One of the basic fundamentals of thermodynamics is called the “perfect gas law”. This describes the relationship of three basic factors controlling the behaviour of a gas – pressure, volume and temperature. For all practical purposes, air and highly superheated refrigerant gases may be considered perfect gases, and their behaviour follows the following relation:
Pressure 1 X Volume 1 Pressure 2 X Volume 2 ------= ------Temperature 1 Temperature 2
Although the “perfect gas” relation is not exact, it provides a basis for approximating the effect on a gas of a change in one of the three factors. In this relation pressure and temperature must be expressed in absolute values, pressure in psia, and temperature in degree Rankine or degrees Fahrenheit above absolute zero • A practical solution for disposing of heat is achieved by raising the pressure of the gas so that the saturation or condensing temperature will be sufficiently above the temperature of the available cooling medium to ensure efficient heat transfer.
9 Refrigerants Large quantities of heat can be absorbed by a substance through an increase in sensible heat involving either a big temperature difference or a large weight. In mechanical refrigeration a process is required that can transfer large quantities of heat economically and efficiently and can be repeated continuously. The process of evaporation and condensation of a liquid are therefore the logical steps in the refrigeration process.
A refrigerant must satisfy two main requirements: • It must readily absorb heat at the temperature required by the product load. • For economy and continuous cooling, the system must use the same refrigerant over and over again.
Simple compression refrigeration cycle:
Two pressures existing in a compression system
Evaporating or low pressure side Condensing or high pressure side
• Refrigerant acts as a transporting medium to move heat from the evaporator to the condenser where it is given off to the ambient air or in a water cooled system to the cooling tower. • A change of state from liquid to vapour and back to liquid allows the refrigerant to absorb and discharge large quantities of heat efficiently.
The basic cycle operates as follows: • High pressure liquid refrigerant is fed from the receiver through the liquid line and through the filter drier to the metering device separating high pressure side of the system from the low pressure evaporator. • Various types of control devices may be used, but thermostatic expansion valve will be considered.
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