HVAC Systems Design Handbook part 9

287 Chapter 9 Equipment: Part 1 Cooling 9.1 Introduction Cooling means the removal of heat. In HVAC, a cooling process is usually identified as one which lowers the temperature or humidity (or both) of the ambient air. The effective temperature includes not only the temperature and humidity of the ambient air but also radiant effects and air movement. Some adiabatic cooling processes, i.e., evap- orative cooling, do not actually remove any heat, but create a sensa- tion of cooling by lowering the sensible temperature of the air. 9.2 Refrigeration Cycles A refrigeration cycle is a means of transferring heat from some place where it is not wanted (heat source) to another place where it can be used or disposed of (heat sink). The necessary components are (1) two or more heat exchangers (one each at source and sink), (2) a refrig- erant, (3) a conduit for conveying the refrigerant, (4) mechanical and/or heat energy to move the refrigerant through the system, and (5) devices to control the rate of flow, to control temperature and pres- sure gradients, and to prevent damage to the system. There are several basic refrigeration cycles. The two most common—two-phase (vapor compression) mechanical, and single- and double-effect absorption—are discussed below. Steam-jet refrigeration has historical importance but is not used in modern practice. Noncon- densing (one-phase) mechanical cycles are used primarily in aircraft where light weight and simplicity are important. Thermoelectric re- Source: HVAC Systems Design Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 288 Chapter Nine Figure 9.1 Mechanical two-phase refrigeration cycle. frigeration utilizes thermocouples working in reverse: When an elec- tric current is impressed on a thermocouple, a cooling effect is ob- tained. These are small systems for specialized applications and are comparatively expensive to install and operate. 9.2.1 The mechanical two-phase vapor compression refrigeration cycle The most common cooling source in HVAC is mechanical two-phase vapor compression refrigeration. In this cycle (Fig. 9.1), a compressor is used to raise the pressure of a refrigerant gas. Work energy (Q W )is required, usually provided by an electric motor or steam turbine or fuel-fired engine. The compression process raises the temperature of the gas. The high-pressure gas flows through piping to a condenser where heat is removed by transfer to a heat sink, usually water or air. The refrigerant is selected with properties which allow it to condense (liquefy) at the temperature and pressure in the condenser. The high- pressure liquid is passed through a pressure-reducing device to the evaporator. At the lower pressure, the liquid tends to evaporate, re- moving the heat of vaporization (Q C ) from its surroundings (the Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Equipment: Part 1 289 evaporator—technically, the heat source). The cold, low-pressure va- por is then returned to the compressor to be recycled. Note that the heat removed in the condenser is equal to the sum Q C ϩ Q W . One index of refrigeration cycle effectiveness is its coefficient of performance (COP) COP ϭ Q /Q (9.1) CW The refrigeration cycle can also be shown on a graph of the prop- erties of a specific refrigerant. The graph in Fig. 9.2 is a pressure- enthalpy or p-h diagram with the basic coordinates of pressure and enthalpy. The four stages of the cycle include compression (with a rise in temperature and enthalpy due to work done), condensing (cooling and liquefying at constant pressure), expansion (at constant enthalpy) and vaporization at constant pressure. The use of the p-h diagram allows the selection of the most effective refrigerant for the pressures and temperatures appropriate to the process. To minimize the work energy required, the temperature difference between the heat source and heat sink should be minimized. 9.2.2 Absorption refrigeration cycle An absorption refrigeration cycle involves a refrigerant-absorbent pair, where the refrigerant is moved from the low-pressure evaporator re- gion to the high-pressure condensing region as an ‘‘absorbed’’ gas on the back of the absorbent. Common refrigerant-absorbent pairs in- clude ammonia-water and water–lithium bromide. In each case there is a strong affinity of each compound for the other. Energy is given up in the absorption process and energy is required to separate the pair. As in vapor compression refrigeration, the beneficial cooling effect is obtained from evaporation of the refrigerant in the low-pressure re- gion of the system. Figure 9.3 is a schematic diagram of a two-shell lithium bromide water chiller using steam as the heat source. The saturated ‘‘strong’’ solution in the generator is heated to drive off water in vapor form. The resulting unsaturated ‘‘weak’’ solution flows by gravity to the ab- sorber, where the solution absorbs water vapor from the evaporator and is then pumped back to the generator. The water driven off in the generator is condensed in the condenser, flows by gravity to the evap- orator and is evaporated there, with the heat of vaporization being extracted from the chilled water. The condensing water is used first to cool the solution in the absorber and then to condense the refrigerant water. The evaporation and regeneration processes also create a pres- sure differential between the upper and lower shell, and restrictors Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 290 Figure 9.2 Refrigeration cycle on pressure-enthalpy diagram for refrigerant-12. Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Equipment: Part 1 291 Figure 9.3 Two-shell absorption refrigeration cycle. (not shown) are used in the pipelines to help maintain this pressure gradient. Heat rejection to the condensing water is roughly twice the refrigeration effect. In addition to the heat supplied to the generator, the chemical process of absorption creates some heat. An absorption refrigeration system has a low coefficient of perform- ance compared to a mechanical refrigeration system. Normally ab- sorption can be justified economically only when plenty of compara- tively low-cost or ‘‘waste’’ heat is available. Solar heat has been used, although in most areas the cost of its collection makes solar energy too expensive to compete with conventional fuels. 9.3 Compressors In the refrigeration cycle, a compressor is a pump, providing the work energy to move the refrigerant from the low-pressure region to the Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 292 Chapter Nine Figure 9.4 Reciprocating compressor. high-pressure region through the system. Compressors come in two general types: positive displacement and centrifugal. Positive displace- ment compressors include reciprocating, rotary, scroll, and helical ro- tary (screw) types. 9.3.1 Reciprocating compressors Reciprocating compressors are usually the single-acting piston type. Figure 9.4 shows one possible arrangement. The volume swept by the piston is the displacement. The remaining volume under the cylinder head is the clearance. The theoretical volumetric efficiency is a func- tion of the ratio of these two volumes together with the compression ratio of the system. Higher compression ratios result in lower volu- metric efficiencies. The actual volumetric efficiency will be somewhat less due to pressure drops across valves and other inefficiencies. The capacity of the compressor is a function of the volumetric efficiency, the properties of the refrigerant, and the operating pressures. A compressor is always designed for a specific refrigerant at some narrowly defined range of operating pressure. A compressor may have from 1 to 12 cylinders. Some older machines had as many as 24 cylinders. Most compressors for comfort cooling are direct-driven by electric motors. Historically, compressors were belt- driven, often by steam engines and at slow speeds. Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Equipment: Part 1 293 Figure 9.5 Rolling piston-type rotary compressor. ( SOURCE : Copyright 2000, American Soci- ety of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org. Reprinted by permission from ASHRAE Handbook, 2000 HVAC Systems and Equipment, Chap. 34, Fig. 4.) Sizes range up to as high as 200 tons or more in one compressor, although units over 100 tons are rare. If larger capacities are needed, two or more compressors are used in parallel. Small compressors—to 5or7 1 ⁄ 2 tons—are capacity-controlled by cycling the unit on and off. Larger units usually have unloaders on all but one or two cylinders. The unloader is activated electrically or pneumatically to lift the suc- tion valve off its seat so that no compression takes place. Unloaders may be activated in stages so that two or more steps of capacity control may be obtained. Hot gas bypass is sometimes used with reciprocating compressors to maintain stable operation at reduced loads. Hot gas bypass does incur a power cost penalty. Reciprocating compressors require from slightly less than 1 hp to as much as 1.5 hp/ton of actual refrigeration capacity at the maximum design temperatures and pressures typical of comfort-cooling pro- cesses. The horsepower per ton increases as the suction pressure and the temperature decrease. Therefore, it is more energy-efficient to op- erate at the highest suction pressure compatible with the needs of the application. Compressors are lubricated by force-feed pumps or, in small units, by splash distribution of oil from the sump. Lubricating oils are se- lected to be miscible with the refrigerant and are carried throughout the piping system, which must be designed to ensure return of the oil to the compressor. 9.3.2 Rotary compressors Rotary compressors are characterized by continuous circular or rotary motion. The two common types are shown in Figs. 9.5 and 9.6. In the Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 294 Chapter Nine Figure 9.6 Rotating-vane type of rotary compressor. ( SOURCE : Copyright 2000, American Soci- ety of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org. Reprinted by permission from ASHRAE Handbook, 2000 HVAC Systems and Equipment, Chap. 34, Fig. 7.) rolling-piston type, the rotor turns on an eccentric shaft, continuously sweeping a volume of space around the cylinder. Suction and discharge ports are separated by a vane which slides in and out against the cylinder wall. In the rotating-vane type of compressor, two sliding vanes are mounted in the rotor to form a compression chamber. The performances of the two types are similar. In HVAC work, rotary compressors are seldom used in other than small sizes, up to about 10-hp capacity. Unloaders are not used on these small machines. A special kind of rotary compressor has become prominent in the marketplace—the scroll compressor. The compression element consists of two interlocking spiral vanes, one stationary and the other rotating. The vanes are arranged so that low-pressure gas enters at the periph- ery and is compressed toward the center, where the gas flows out of an annular discharge port. Some scroll compressor designs are able to tolerate some liquid ‘‘slugging,’’ in contrast to positive displacement compressors. 9.3.3 Helical rotary compressors Helical rotary or screw compressors are made in single-screw and twin-screw types. The single-screw compressor (Fig. 9.7) consists of a helical main rotor with two star wheels. The enclosure of the main rotor has two slots through which the star wheel teeth pass; these teeth, together with the rotor and its enclosure, provide the bounda- ries of the compression chambers. The twin-screw compressor (Fig. 9.8) has two meshing helical gears and works much as a gear pump, with the helical shape forcing the gas to move in a direction parallel to the rotor shaft. These machines typically are direct-driven at 3600 r/min and are usually oil-flooded for lubrication and to seal leakage paths. Capacity control is obtained by means of a sliding or rotating slotted valve. Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Equipment: Part 1 295 Figure 9.7 Principle of operation of a single-screw compressor. ( SOURCE : Copyright 1988, Amer- ican Society of Heating, Refrig- erating and Air Conditioning Engineers, Inc., www.ashrae. org. Reprinted by permission from ASHRAE Handbook, 1988 Equipment, Chap. 12, Fig. 11.) Figure 9.8 Helical rotary twin- screw compressor. ( SOURCE : Copyright 2000, American Soci- ety of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org. Reprinted by permission from ASHRAE Handbook, 2000 HVAC Systems and Equipment, Chap. 34, Fig. 24.) 9.3.4 Centrifugal compressors Centrifugal compressors belong to the family of turbomachines, which includes fans and centrifugal pumps. Pressures and flows result from rotational forces. In HVAC work, these compressors are used primarily in package chillers where the compressors provide large capacities. Typical driven speed is 3600 r/min or more, using electric motor en- gines or steam or gas turbines. Standard centrifugal chillers range in capacity from 100 to 2000 tons, although some special units have been built with capacities as great as 8500 tons. Capacity control is ob- tained by varying the driven speed or by means of inlet vanes, similar to those used on centrifugal fans. Noncondensing air-cycle systems, such as used on commercial aircraft, use high-speed gas-turbine drives at up to 90,000 r / min. 9.3.5 Hermetic compressors Compressors may be built in either hermetic or open configurations. A hermetic unit has a casing which encloses both the compressor and the drive motor, minimizing the possibility of refrigerant leakage. Mo- tors are specially constructed and are normally cooled with suction gas or with liquid refrigerant. In an open machine, the drive motor or turbine is separate from the compressor. Shaft seals must be provided to prevent refrigerant leakage. Standard drives—direct, gear, or Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 296 Chapter Nine Figure 9.9 Flooded liquid chiller. belt—may be used. Semihermetic units have separate casings for the compressor and the motor with matching flanges for gas-tight assem- bly. Open drives have the advantage of removing the motor heat from the refrigeration cycle, thereby improving chiller performance. This advantage is lost if the motor heat is picked back up into the cooling load indirectly. 9.4 Chillers The term chiller is used in connection with a complete chiller package—which includes the compressor, condenser, evaporator, in- ternal piping, and controls—or for a liquid chiller (evaporator) only, where the water or brine is cooled. Liquid chillers come in two general types: flooded and direct- expansion. There are several different configurations: shell-and-tube, double-tube, shell-and-coil, Baudelot (plate-type), and tank with race- way. For HVAC applications, the shell-and-tube configuration is most common. 9.4.1 Flooded chillers A typical flooded shell-and-tube liquid chiller is shown in Fig. 9.9. Refrigerant flow to the shell is controlled by a high- or low-side float valve or by a restrictor. The water flow rate through the tubes is de- fined by the manufacturer but it generally ranges from 6 to 12 ft/s. Tubes may be plain (bare) or have a finned surface. The two-pass ar- rangement shown is most common, although one to four passes are available. The chiller must be arranged with removable water boxes so that the tubes may be cleaned at regular intervals, because even a Equipment: Part 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... Equipment: Part 1 Equipment: Part 1 299 Air out Screen Condenser coil Hot gas in Liquid out Air in Figure 9. 11 Air-cooled condenser coil face area, fan airflow rate, desired condensing temperature, and design ambient dry-bulb (db) temperature Note that at reduced ambient temperatures, performance of air-cooled condensers improves; sometimes approaching the seasonal performance of water-cooled systems 9. 5.2... website Equipment: Part 1 302 Chapter Nine Air out Water eliminator Water in Water sprays Tower fill Air in Water out Figure 9. 14 Forced-draft cooling tower difference between the leaving cooling water temperature and the design ambient wet-bulb (wb) temperature This is usually between 6 and 12ЊF, with 7 to 10ЊF being typical 9. 6.1 Open-circuit cooling towers In Figs 9. 14, 9. 15, and 9. 16, there is only... analysis of the cooling and dehumidifying process, (2) the effect on the heat trans- Figure 9. 20 Direct-expansion evaporation coil (SOURCE: Copyright 199 4 American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org Reprinted by permission from ASHRAE Handbook, 199 4 Refrigeration Systems and Application, Chap 2, Fig 24.) Downloaded from Digital Engineering Library @ McGraw-Hill... Age,’’ Heating / Piping / Air Conditioning, July 198 7, p 67 2 Cooling-Tower Fundamentals, The Marley Cooling Tower Co., 199 8, Mission, KS 3 ASHRAE Handbook, 2001 Fundamentals, Chap 19, ‘‘Refrigerants,’’ and Chap 20, ‘‘Thermophysical Properties of Refrigerants.’’ 4 ASHRAE Handbook, 199 8 Refrigeration, Chap 5, ‘‘Refrigerant System Chemistry.’’ Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)... selection of a point outside Figure 9. 21 Cooling and dehumidifying process on the psychrometric chart Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Equipment: Part 1 Equipment: Part 1 3 09 Figure 9. 22 Partial section through a finned... the Terms of Use as given at the website Equipment: Part 1 Equipment: Part 1 3 19 Figure 9. 31 Psychrometric chart for two-stage evaporative cooling system tional protocols which have set schedules for the elimination of damaging refrigerants from commerce, other replacements have been and are being developed Part of the challenge is technical and part economic: first to find a fluid that has optimal characteristics... given at the website Equipment: Part 1 306 Chapter Nine Figure 9. 18 Dimensional notation for coils Figure 9. 19 Coil circuits Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Equipment: Part 1 Equipment: Part 1 307 tubes are arranged... notation is shown in Fig 9. 18 From 1 to 12 rows is standard, but custom-made coils with any number of rows can be obtained With an odd number of passes, the supply and return headers are on opposite ends In a full-circuit coil (Fig 9. 19A), the number of passes (one pass means water flow from end to end of the coil) is equal to the number of rows fed In a double-circuit coil (Fig 9. 19B), the Downloaded from... at the website Equipment: Part 1 Equipment: Part 1 311 Figure 9. 24 Counterflow, parallel flow, and MED nomograph Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Equipment: Part 1 312 Chapter Nine Figure 9. 25 Temperatures for counterflow... the website Equipment: Part 1 300 Chapter Nine Figure 9. 12 Shell-and-tube, water-cooled condenser Figure 9. 13 Evaporative condenser Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Equipment: Part 1 Equipment: Part 1 301 circulating . 287 Chapter 9 Equipment: Part 1 Cooling 9. 1 Introduction Cooling means the removal of heat. In HVAC, a cooling process is usually. light weight and simplicity are important. Thermoelectric re- Source: HVAC Systems Design Handbook Downloaded from Digital Engineering Library @ McGraw-Hill