HVAC Systems Design Handbook part 11

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HVAC Systems Design Handbook part 11

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367 Chapter 11 Equipment: Part 3 Air-Handling Systems 11.1 Introduction By definition, air conditioning involves control of the air temperature, humidity, cleanliness, and distribution. It follows that an air-handling unit (AHU) of some kind is an essential part of an air conditioning system, though not necessarily of a heating-only system. The function of the AHU is to provide air at a quantity, temperature, and humidity to offset the sensible and latent heat gains to the space (in the cooling mode) and the heat losses (in the heating mode), while maintaining the required temperature and humidity in the space. This can be most clearly shown on a psychrometric chart (Fig. 11.1). A typical cooling design room condition is 78ЊF dry-bulb (db) tempera- ture and 50 percent RH. For illustration, a load of 120,000 Btu/h sen- sible and 30,000 Btu/h latent cooling is assumed. Then, for an as- sumed 20ЊF temperature difference between the room and supply air temperatures (58ЊF supply air), the design flow rate of air, designated CFM, in cubic feet per minute (cfm) will be 120,000 3 CFM ϭϭ5555 ft /min (cfm) (11.1) 20 ϫ 1.08 where 1.08 is the air factor in Btu/h, cfm, ЊF. The change in specific humidity ⌬w may be calculated as follows: 3 1min 1ft 1h 1lb w ⌬w ϭ 30,000 Btu/h ϫϫ ϫϫ 3 5555 ft 0.075 lb 60 min 1059 Btu a ϭ 0.0011 lb /lb (11.2) wa 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. 368 Chapter Eleven Figure 11.1 Psychrometric chart for draw-through air conditioning process. The point defined by these two differential values can be plotted on the chart, as shown. The ‘‘validity’’ of this point must be verified, based on the cooling coil capability and the AHU arrangement, as discussed in Sec. 3.6. For a draw-through arrangement (i.e., with the supply fan downstream of the cooling coil), the supply air temperature will be greater than the coil leaving temperature because of heat added by fan work. For this example, if 5 hp is required, the temperature dif- ference (TD) will be 2545 Btu 1 TD ϭ 5hpϫϫ 3 1hp⅐ h 5555 ft /min 3 1h⅐ (ft / min) ⅐ ЊF ϫϭ2.1ЊF (11.3) 1.08 Btu Then a coil leaving condition of 55.9ЊF db and 55.5ЊF wb can be plot- ted, and this will probably be valid. For a blow-through arrangement, the fan work causes an increase in the mixed-air temperature before the air goes through the cooling coil, and the process will be as shown in Fig. 11.2. In this case, it will be necessary to increase the supply air TD to 22ЊF to get a valid coil leaving condition. Equipment: Part 3 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 3 369 Figure 11.2 Psychrometric chart for blow-through air conditioning process. Humidity control is not always required, but some upper limit will be inherent in any refrigeration-type cooling process—chilled water, brine, or direct expansion. Supply air-handling equipment may be classified in several different ways: 1. Type or arrangement. The five basic arrangements are single- zone, multi-zone, double-duct, variable air volume (VAV), and in- duction. 2. Package versus built-up. Package equipment is factory-assembled, and when it is installed, it requires only connections for utilities and ductwork. The term built-up implies that most of or all the components are field-assembled and installed. 3. Self-contained. A self-contained system includes internal thermal energy generation. 4. Central station and terminal units. Central station equipment is remote from and delivers air through ductwork to the conditioned space. Terminal units are installed in or adjacent to the conditioned space. Terminal units are used in conjunction with central station equipment. Equipment: Part 3 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. 370 Chapter Eleven Figure 11.3 Single-zone AHU. Exhaust systems may serve a single space or multiple spaces, and may include heat recovery, special filtration, and other special equip- ment. 11.2 AHU System Arrangements Air conditioning practice includes only five basic AHU arrangements, although there are many variations on these basic concepts. Single- zone and VAV systems have similar, even identical, physical arrange- ments but use different control strategies. Multizone and double-duct systems are similar in arrangement and concept but are different enough to be considered separately. Induction systems are unique. 11.2.1 Single-zone AHU A single-zone AHU is intended to serve only one room, or a group of rooms which are contiguous and which have similar load and exposure characteristics. The maximum area served by a single-zone AHU should not exceed 10,000 ft 2 . The typical single-zone AHU arrangement is shown in Fig. 11.3. This is a draw-through system, with the heating coil in the preheat position to protect the cooling coil from freezing air. The system is controlled as explained in Sec. 8.5.2. It is important to sequence the operation of the control valves to avoid simultaneous heating and cool- ing. When one or more of the rooms served by a single-zone AHU has a load characteristic different from the other rooms, zone reheat must be provided by means of coils in the zone branch ducts (Fig. 11.4), by radiation, or by fan-coil units. Because reheat is potentially energy- wasteful, it may be preferable to use a different type of AHU, as de- scribed below. Equipment: Part 3 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 3 371 Figure 11.4 Zone reheat coil. A single-zone unit may be used to control humidity in the room. The unit is arranged as shown in Fig. 11.5. The cooling coil precedes the heating coil, which is therefore in the reheat position. Humidity con- trol always requires additional energy—as reheat or in other ways. The cooling coil valve is controlled by either the space temperature or the space humidity, whichever creates the greater demand. If humid- ity controls, the temperature will tend to fall and the space thermostat will control the heating coil valve to provide reheat. The humidifier is used when required. 11.2.2 Multizone AHU The typical multizone (MZ) AHU arrangement is shown in Fig. 11.6. Side-by-side hot and cold airstreams are provided. Each zone is pro- vided with dampers to mix hot and cold air to satisfy the requirements of the zone. In this way, one zone may be heated while simultaneously another is cooled. The mixing dampers are located at the unit, with a separate duct run to each zone. Thus, economics and practicality limit the size of the typical MZ unit. The great majority of such units are the package type. From an environmental control standpoint, the conventional MZ unit is less than ideal. Because the control is achieved by reheat, it is an energy waster. The three-duct MZ unit (Fig. 11.7) retains the con- trol benefits while eliminating the energy waste, by adding a bypass duct (plenum). The sequence of control is described in Sec. 8.5.3. Equipment: Part 3 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. 372 Figure 11.5 Single-zone AHU with humidity control. Equipment: Part 3 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. 373 Figure 11.6 Traditional arrangement for multizone AHU. Equipment: Part 3 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. 374 Figure 11.7 Three-duct arrangement for multizone AHU. Equipment: Part 3 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 3 375 11.2.3 Double-duct (dual-duct) AHU The double-duct (DD) AHU uses the same principle of operation as the MZ unit. However, the hot and cold ducts are extended through the building, with a mixing box provided for each zone. Thus, the dou- ble-duct AHU can be as large or as small as desired. The conventional system (Fig. 11.8) has the same advantages and disadvantages as the multizone AHU. Many of the older systems installed in the 1950s and 1960s were designed with high-velocity/high-pressure duct systems to minimize the space occupied by the ducts. Electric energy was rela- tively inexpensive at that time, so the additional fan work was of little concern. Five to six inches of total pressure across the fan was com- mon, and 9 to 10 inches was not unusual. At today’s energy prices, such a system may cost more for fan energy than for thermal energy on an annual basis. Many of these older systems are being retrofitted to variable air volume by changing the heating coil to cooling, removing the mixing boxes, and using both heating and cooling ducts, in parallel, with new VAV boxes. In this way, the duct air velocity is reduced by about 50 percent with a significant saving in fan energy. Some reheat must be added for exterior zones. The ideal dual-duct system is, perhaps, the two-fan system shown in Fig. 11.9 and described in detail in Sec. 8.5.4. 11.2.4 Variable-volume AHU Unlike the AHU systems previously discussed, a VAV system supplies air at constant, or nearly constant, temperature and humidity. Capac- ity is controlled to match cooling load by varying the volume of air supplied to a zone. A VAV box is provided at each zone. The box in- cludes a motorized damper (controlled by the zone thermostat) and usually some means of compensating for changes in static pressure in the supply duct. Such changes can affect the accuracy of control. The compensating device may be mechanical, e.g., a spring-loaded damper, or it may be a flow-sensing controller which is reset by the zone ther- mostat. The latter is given the anomalous description constant vari- able-volume controller. Pressure independent is another term used to describe this type of VAV box control. While the zone supply volume could theoretically go to zero, it is usual to provide a low limit of 35 to 40 percent of design airflow to maintain a minimum air distribution and ventilation rate. Supplemental heating—reheat coils, radiation, fan-coil units—is required in zones with exterior exposure. VAV systems were developed in response to the 1973 ‘‘energy crisis.’’ The concept is based on the fan law which states that the fan horse- power (fan work energy) varies as the cube of the airflow, denoted by Equipment: Part 3 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. 376 Figure 11.8 Traditional arrangement for double-duct AHU. Equipment: Part 3 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. [...]... hoods and exhaust systems Laboratory and industrial fume hoods require large exhaust air quantities Several standards cover the design of some types of exhaust systems. 1,2 11. 11 Smoke Control The use of environmental air-handling systems for primary or supplementary smoke control is neither simple nor economical There is a lack of good data on the design of adequate smoke control systems, and code... Equipment: Part 3 Equipment: Part 3 389 Figure 11. 19 Fan-coil unit (floor-mounted) Figure 11. 20 Unit ventilator 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 3 390 Chapter Eleven Many unit-ventilator system designs... discussion in Sec 8.3.3.3) Figure 11. 10 Volume damper for duct pressure control 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 3 Equipment: Part 3 379 Figure 11. 11 Fan and system curves for Fig 11. 10 If no fan volume control... removable roof sections, or corridors to the outside Note that many of the concerns for field-erected systems also apply to factory-built package equipment The designer or specifier may reject some equipment sources for inadequately handling the design issues mentioned 11. 5 Terminal Units A terminal unit is a part of a larger air-handling system—double-duct, VAV, or induction The terminal unit is installed... coils, and induction units 11. 5.1 Mixing boxes Mixing boxes for double-duct systems are described in Sec 8.5.4 The conventional box includes a constant-volume device to compensate for variations in static pressure in the hot and cold ducts 11. 5.2 VAV boxes The conventional VAV box is described in Sec 11. 2.4 Two other VAV boxes are used One is the fan-powered mixing box (Fig 11. 17) The fan-powered box...Figure 11. 9 Two-fan double-duct AHU Equipment: Part 3 377 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 3 378 Chapter Eleven CFM A reduction to 50 percent of the design CFM could result in a... are common Split systems are usually small, ranging up to 15- or 20-ton capacity Figure 11. 15 Rooftop AHU (Courtesy of Mammoth.) 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 3 384 Chapter Eleven 11. 3.3 Package AHU... 11. 17) The fan-powered box is a small fan-and-damper unit designed to circulate 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 3 Equipment: Part 3 387 Figure 11. 17 Fan-powered mixing box air at constant volume to the... described 11. 5.3 Induction units These units are described in Sec 11. 2.5 Figure 11. 18 Bypass VAV box 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 3 388 11. 5.4 Chapter Eleven Terminal reheat Reheat coils may be installed... humidity control is desired Fan-assisted HEPA filters can be incorporated in a system 11. 4 Built-up (Field-Assembled) AHU The built-up AHU is field-assembled from individual components selected by the designer This allows the designer complete flexibility of size and arrangement The built-up AHU tends to have a higher first Figure 11. 16 Computer room AHU installation Downloaded from Digital Engineering Library . Btu/h ϫϫ ϫϫ 3 5555 ft 0.075 lb 60 min 1059 Btu a ϭ 0.0 011 lb /lb (11. 2) wa Source: HVAC Systems Design Handbook Downloaded from Digital Engineering Library. 367 Chapter 11 Equipment: Part 3 Air-Handling Systems 11. 1 Introduction By definition, air conditioning involves

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