Tài liệu HVAC Systems Design Handbook part 4 pdf

81 Chapter 4 Design Procedures: Part 2 General Concepts for Equipment Selection 4.1 Introduction The purpose of this chapter is to outline the criteria used in the HVAC system and equipment selection process, to describe some of the sys- tems and equipment available, and to develop some of the underlying philosophy and background related to system selection. Details of specific systems and items of equipment are discussed in later chapters. 4.2 Criteria for System and Equipment Selection The problem-solving process requires some criteria that can be applied in describing and evaluating alternatives. In the selection of HVAC systems, the following criteria (Table 4.1) are used—consciously or unconsciously—because only rarely is the problem-solving process for- mally applied. 1. Requirements of comfort or process. These requirements include temperature, always; humidity, ventilation, and pressurization, some- times; and zoning for better control, if needed. In theory at least, the comfort requirement should have a high priority. In practice, this cri- terion is sometimes subordinated to first cost or to the desires of some- one in authority. This is happening less often as building occupants become more sophisticated in their expectations. Process requirements are more difficult and require a thorough inquiry by the HVAC de- signer into the process and its needs. Until the process is fully under- 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. 82 Chapter Four TABLE 4.1 Criteria for HVAC System and Equipment Selection 1. Demands of comfort or process 2. Energy conservation, code requirements 3. First cost versus life-cycle cost 4. Desires of owner, architect, and/ or design office 5. Space limitations 6. Maintainability/ reliability 7. Central plant versus distributed systems 8. Simplicity and controllability stood, the designer cannot provide an adequate HVAC system. Most often, different parts of the process have different temperature, hu- midity, pressure, and cleanliness requirements; the most extreme of these can penalize the entire HVAC system. 2. Energy conservation. This is usually a code requirement and not optional. State and local building codes almost invariably include requirements constraining the use of new, nonrenewable energy. Non- renewable refers primarily to fossil-fuel sources. Renewable sources include solar power, wind, water, geothermal, waste processing, heat reclaim, and the like. The strictest codes prohibit any form of reheat (except from reclaimed or renewable sources) unless humidity control is essential. This restriction eliminates such popular systems as ter- minal reheat, two-deck multizone, multizone, and constant volume dual-duct systems, although the two-fan dual-duct system is still pos- sible and the three-duct multizone system is acceptable (see Chap. 11). Most HVAC systems for process environments have opportunities for heat reclaim and other ingenious ways of conserving energy. Off-peak thermal storage systems are becoming popular for energy cost savings, although these systems may actually consume more energy than con- ventional systems. 1 Thermal storage is a variation on the age-old prac- tice of cutting and storing ice from the lake in winter, for later use in the summer. 3. First cost and life-cycle cost. The first cost reflects only the ini- tial price, installed and ready to operate. The first cost ignores such factors as expected life, ease of maintenance, and even, to some extent, efficiency, although most energy codes require some minimum effi- ciency rating. The life-cycle cost includes all cost factors (first cost, operation, maintenance, replacement, and estimated energy use) and can be used to evaluate the total cost of the system over a period of years. A common method of comparing the life-cycle costs of two or more systems is to convert all costs to present-worth values. Typically, first cost governs in buildings being built for speculation or short-term investment. Life-cycle costs are most often used by institutional Design Procedures: Part 2 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. Design Procedures: Part 2 83 builders—schools, hospitals, government—and owners who expect to occupy the building for an indefinite extended period. Life-cycle cost analysis requires the assumption of an interest, or discount, rate and may also include anticipated inflation. 4. Desires of owner, architect, or design office. Very often, someone in authority lays down guidelines which must be followed by the de- signer. This is particularly true for institutional owners and major retailers. Here the designer’s job is to follow the criteria of the em- ployer or the client unless it is obvious that some requirements are unsuitable in an unusual environment. Examples of such environ- mental conditions are extremely high or low outside-air humidity, high altitude (which affects the AHU and air-cooled condenser capacity), and contaminated outside air (which may require special filtration and treatment). 5. Space limitations. Architects can influence the HVAC system selection by the space they make available in a new building. In re- trofit situations, designers must work with existing space. Sometimes in existing buildings it is necessary to take additional space to provide a suitable HVAC system. For example, in adding air conditioning to a school, it is often necessary to convert a classroom to an equipment room. Rooftop systems are another alternative where space is limited, if the building structure will support such systems. In new buildings, if space is too restricted, it is desirable to discuss the implications of the space limitations in terms of equipment efficiency and maintain- ability with the architect. There are ways of providing a functional HVAC system in very little space, such as individual room units and rooftop units, but these systems often have a high life-cycle cost. 6. Maintainability. This criterion includes equipment quality (the mean time between failures is commonly used); ease of maintenance (are high-maintenance items readily accessible in the unit?); and ac- cessibility (Is the unit readily accessible? Is there adequate space around it for removing and replacing items?). Rooftop units may be readily accessible if an inside stair and a roof penthouse exist; but if an outside ladder must be climbed, the adjective readily must be de- leted. Many equipment rooms are easy to get to but are too small for adequate access or maintenance. This criterion is critical in the life- cycle cost analysis and in the long-term satisfaction of the building owner and occupants. 7. Central plant versus distributed systems. Central plants may in- clude only a chilled water source, both heating and chilled water, an intermediate temperature water supply for individual room heat pumps, or even a large, central air-handling system. Many buildings have no central plant. This decision is, in part, influenced by previ- ously cited criteria and is itself a factor in the life-cycle cost analysis. Design Procedures: Part 2 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. 84 Chapter Four In general, central plant equipment has a longer life than packaged equipment and can be operated more efficiently. The disadvantages include the cost of pumping and piping or, for the central AHU, longer duct systems and more fan horsepower. There is no simple answer to this choice. Each building must be evaluated separately. 8. Simplicity and controllability. Although listed last, this is the most important criterion in terms of how the system will really work. There is an accepted truism that operators will soon reduce the HVAC system and controls to their level of understanding. This is not to criticize the operator, who may have had little or no instruction about the system. It is simply a fact of life. The designer who wants or needs to use a complex system must provide for adequate training—and retraining—for operators. The best rule is: Never add an unnecessary complication to the system or its controls. 4.3 Options in System and Equipment Selection Many of the various systems and equipment available are described in later chapters. They are briefly listed here to summarize the options available to the designer. 4.3.1 Air-handling units Air-handling units (AHUs) include factory-assembled package units and field-erected, built-up units (see Chap. 11). The common compo- nents are a fan or fans, cooling and/or heating coils, and air filters. Most units also include a mixing chamber with outside and return air connections with dampers. The size range is from small fan-coil units with as little as 100 ft 3 /min capacity to built-up systems handling over 100,000 ft 3 /min. When a package unit includes a cooling source, such as a refrigeration compressor and condenser, or a heating source, such as a gas-fired heater or electric heating coil, or both, then the unit is said to be self-contained. This classification includes heat pumps. Many systems for rooftop mounting are self-contained, with capacities as great as 100 tons or more of cooling and a comparable amount of heating. Some room units for wall or window installation have capac- ities as small as 0.5 or 0.75 ton. Split-system packages are also avail- able, with the heating and /or cooling source section matching the fan- coil section but installed outdoors. The two sections are connected by piping. Cooling coils may use chilled water, brine, or refrigerant (direct expansion). Heating coils may use steam or high- or low-temperature water; or ‘‘direct-fired’’ heating may be used, usually gas or electric resistance. Heat reclaim systems of various types are employed. Hu- midification equipment includes the steam grid, evaporative, and Design Procedures: Part 2 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. Design Procedures: Part 2 85 slinger/atomizer types. Dehumidification equipment includes the de- humidifying effect of most cooling coils as well as absorption-type de- humidifiers. Thus, the designer has a wide range of equipment to choose from. Although generalizations are dangerous, some general rules may be applied, but the designer must also develop, through experience, an understanding of the best and worst choices. There are some criteria which are useful: 1. Packaged equipment should be tested, rated, and certified in ac- cordance with standards of American Society of Heating, Refrigera- tion, and Air Conditioning Engineers (ASHRAE), Air Conditioning and Refrigeration Institute (ARI), Association of Home Appliance Manufacturers (AHAM), Air Movement and Control Association (AMCA), and/or others as applicable. 2. Minimum unit efficiencies or effectiveness should be in accord- ance with codes or higher. 3. In general, packaged equipment has a lower first cost and a shorter life than equipment used in built-up systems. This is not al- ways true, and comparisons must be made for the specific application. 4. In general, packaged equipment is designed to be as small as possible for a given capacity. This may create problems of access for maintenance. Also the supplier should show that capacity ratings were determined for the package as assembled and not just for the separate components. See particularly the discussion on the effects of geometry on fan performance in Chap. 5. 5. In hotel guest rooms, motels and apartments, individual room units should be used to give occupants maximum control of their en- vironment. Where many people share the same space, central systems are preferable, with controls which cannot be reset by occupants. 6. Noise is a factor in almost any HVAC installation, yet noise is often neglected in equipment selection and installation. Noise ratings are available for all types of HVAC equipment and should be used in design and specifications (see Chap. 20). 4.3.2 Radiant and convective heating and cooling Convector radiators, using steam or hot water, are one of the oldest heating methods and are still in common use. Modern systems are more compact than the old cast-iron radiators and depend more on natural convection than on radiation. Rating methods are standard- ized by the Hydronics Institute. Radiant heating by means of floor, wall, or ceiling panels is common. Hot-water piping or electric resistance heating tape is used. Maximum Design Procedures: Part 2 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. 86 Chapter Four temperatures of the surface must be limited, and there are some con- trol problems, particularly in floor panels, due to the mass of the panel. Radiant cooling by means of wall or ceiling panels may also be used. Surface temperatures must be kept above the dew point; therefore, any dehumidification required must be accomplished by other means. In modern practice, radiant and/or convective heating or cooling is usually a supplement to the air system and is used primarily to offset exterior wall, roof, and radiant floor heat gains or losses. 4.3.3 Refrigeration equipment Source cooling equipment includes refrigeration compressors of re- ciprocating, centrifugal, and screw types; absorption chillers using steam, hot water, or direct fuel firing; water chiller heat exchangers; condensers cooled by air, water, and evaporation; cooling towers; and evaporative coolers, including spray, slinger, and drip types (see Chap. 9). Self-contained package AHUs typically use direct-expansion cooling with reciprocating or rotary compressors. Other AHUs may use direct expansion, chilled water, or brine cooling, with the cooling medium provided by a separate, centralized, refrigeration system (see Chap. 6). Evaporative cooling is used primarily in climates with low design ambient wet-bulb temperatures, although it may be used in almost any climate to achieve some cooling. Evaporative cooler efficiencies are highest for the spray type and lowest for the drip type. Centrifugal and screw-type compressors and absorption refrigeration are used al- most entirely in large central-station water or brine chillers. Absorp- tion refrigeration may be uneconomical unless there is an adequate source of waste heat or solar energy. Air-cooled condensers are less costly to purchase and maintain than cooling towers or evaporative condensers, but they result in higher peak condensing temperatures at design conditions and may result in lower overall efficiency in the cooling system. The selection of the source cooling equipment is influenced primarily by the selection of the AHU equipment and systems. Often both are selected at the same time. The use of individual room units does not preclude the use of central-station chillers; this combination may be preferable in many situations. For off-peak cooling with storage, a cen- tral chilling plant is an essential item. 4.3.4 Heating equipment Source heating equipment includes central plant boilers for steam and high-, medium-, and low-temperature hot water; heat pumps, both Design Procedures: Part 2 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. Design Procedures: Part 2 87 Figure 4.1 Single-zone draw-through air-handling unit. central and unitary; direct-fired heaters; solar equipment, including solar-assisted heat pumps; and geothermal and heat reclaim. Fuels include coal, oil, natural and manufactured gas, and peat. Waste prod- ucts such as refuse-derived fuel (RDF) and sawdust are also being used in limited ways. Electricity for resistance heating is not a fuel in the combustion sense but is a heat source. Heat reclaim takes many forms, some of which are discussed in Chap. 10. Self-contained package AHUs use direct-fired heaters—usually gas or electric—or heat pumps. For other systems, some kind of central plant equipment is needed. The type of equipment and fuel used is determined on the basis of the owner’s criteria, local availability and comparative cost of fuels, and, to some extent, the expertise of the designer. Large central plants for high-pressure steam or high- temperature hot water, may present safety problems, are regulated by codes and require special expertise on the part of the designer, con- tractor, and operator. New buildings connected to existing central plants will require the use of heat exchangers, secondary pumping or condensate return pumping, and an understanding of limitations im- posed by the existing plant, such as limitations on the pressure and temperature of returned water or condensate. 4.4 The Psychrometric Chart When the system type has been selected and a summary completed, showing design CFM, temperature difference (TD), and latent load, it is time to complete the psychrometric chart. For a detailed discussion of psychrometrics, see Chap. 19. Consider a single-zone, draw-through air-handling system, as in Fig. 4.1. In summer the return air is mixed with some minimum amount of outside air; is pulled through the filter, the coils, and the fan; and is supplied to the space. Cooling is provided by the cooling coil through the use of chilled water (as in this example) Design Procedures: Part 2 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. 88 Chapter Four Figure 4.2 Psychrometric chart, cooling cycle example. or by direct refrigerant expansion. On the psychrometric chart (Fig. 4.2), the designer first plots the inside and outside design conditions—say 75ЊF and 50 percent RH inside and 95ЊF dry bulb (db), 75ЊF wet bulb (wb) outside. The return air to the system will usually be at a higher temperature than the space due to heat gains in return- air plenums. (This does not hold for direct-return units.) This heat gain can be estimated or calculated from the geometry of the building, the wattage of recessed lighting, etc. For this example, assume a 3ЊF rise. Then the return air is at 78ЊF with the same humidity ratio w as the space. A straight line between this point and the outside air state point represents the mixing process. The mixed-air state point lies on this line at a distance from the return-air point equal to the design minimum percentage of outside air—for this example, 20 per- cent. Then the mixed-air condition is 81.4ЊF db and 66.2ЊF wb, with an enthalpy (h) of 30.8 (Btu /lb). The design condition of the supply air is calculated as described in Chap. 3 and for this example is as- sumed to be 56ЊF db with a humidity ratio (w) equal to 0.0086 lbw / lba. Because this is a draw-through system, there is some heating ef- fect due to fan work. If the fan horsepower and efficiency are known, this can be calculated. For preliminary purposes, a temperature rise of 0.5ЊF per inch of pressure rise across the fan can be assumed—for this example, 3-in static pressure or 1.5ЊF. Then the air must leave the coil at 54.5ЊF db, with w equal to 0.0086 as above. The resulting point has an h value of 22.5. Now the cooling (coil) process can be represented by a straight line from the mixed-air point through the ‘‘leaving coil’’ point and can be extended to the saturation curve. The intersection with the saturation curve is called the apparatus dew Design Procedures: Part 2 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. Design Procedures: Part 2 89 Figure 4.3 Psychrometric chart, heating cycle example. point (ADP) and is the coil surface temperature required to obtain the design process (here about 53ЊF). The total cooling load in Btu /h is determined from the difference in h values multiplied by the total CFM, 60 min /h, and the air density in pounds per cubic foot (0.075 for standard air). Thus q ϭ CFM ϫ (h Ϫ h ) ϫ 60 ϫ 0.075 (4.1) cmc where q c ϭ total cooling, Btu/h h m ϭ enthalpy of mixed air entering cooling coil, Btu/lb h c ϭ enthalpy of air leaving the cooling coil, Btu /lb The cooling load thus calculated includes the sensible and latent space load plus the load due to outside air, fan work, and any return-air ‘‘pickup.’’ The cycle at winter design conditions can also be plotted, as shown in Fig. 4.3. The mixed air is controlled at the low-limit condition, say 60ЊF, although this may be reset upward as the outside temperature decreases for energy conservation. Return air will be about 3ЊF above the 72ЊF space temperature, or 75ЊF. For this example, the outside design temperature is assumed to be 32ЊF and 50 percent RH. Heating will be provided as required to maintain the space conditions (some design heating temperature difference will be calculated). If space hu- midity is uncontrolled, the cycle will automatically fall into a position such that the humidity ratio difference between supply air and space will be the same as that for cooling. This will typically result in a lower space humidity in winter. Most designers use the psychrometric chart only for the design cool- ing cycle, or for both heating and cooling if humidity control is pro- vided. It is sometimes useful to look at intermediate conditions such Design Procedures: Part 2 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. 90 Chapter Four Figure 4.4 Psychrometric chart, intermediate cycle example. as in Fig. 4.4. Here an outside temperature of 60ЊF is assumed, and 100 percent outside air is used by the economy cycle. The inside hu- midity will depend on the outside humidity, as discussed before. No mechanical cooling and little or no heating should be needed. Other intermediate conditions can be examined in similar ways. 4.5 Effects of Altitude and Temperature Air density varies directly and linearly with temperature, and inverse/exponentially with altitude. See Fig. 4.5. Standard conditions are defined as 0.075 lb/ft 3 at 59ЊF. All HVAC airside calculations are recognized as being inexact, where seasonal ambient temperatures may vary from 0 to 100ЊF (a 10 percent effect), and local barometric pressures may fluctuate plus or minus 2 percent, depending on weather. But changes in density related to altitude or related to heat- ing or cooling processes may compound all other effects and should not be taken lightly. 4.5.1 Changes due to altitude Atmospheric pressure and related air density decreases as altitude or elevation above sea level increases. Inversely, atmospheric pressure increases for those elevations below sea level or some other reference point. At elevations up to 6000 ft where the altitude/density variation is nearly linear, the rate of density change is approximately 3 to 4 percent per 1000 ft of elevation change. This corresponds to furnace manufacturer’s counsel to derate natural draft burner equipment at 4 percent per 1000 ft elevation. Altitude effects are often ignored below Design Procedures: Part 2 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. [...]... Copyright © 20 04 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 2 Design Procedures: Part 2 93 The computerized selections will always assume the maximum optimum performance of the standard-size coils checked The design conditions specified will always be altered somewhat to compensate for this The designer must be.. .Design Procedures: Part 2 Design Procedures: Part 2 91 Figure 4. 5 Pressure and density versus altitude 2000 ft elevation where more than 90 percent of U.S commerce is involved, but negative effects at altitudes above 2500 ft of 10 percent and more can threaten the success of an otherwise competent design Most of the basic effects of altitude density... factory Usually, a printout returns with several alternative selections For example, the designer may need a cooling coil to provide 120,000 Btu/h total, with a sensible/total ratio of 0.8; 3210 ft3 /min of entering air at 81 .4 F db, 66.2ЊF wb; leaving air at 54. 5ЊF db, 53.8ЊF wb; and 24 gal/min of chilled water supplied at 45 ЊF There may also be some kind of coil face velocity and air and water pressure-drop... devices 4. 5.2 Changes due to temperature Air density varies linearly with absolute temperature Minor variation from the 60ЊF standard condition is seldom a problem The change begins to be significant below 0ЊF and above about 130ЊF In HVAC design, commonly encountered problems are high-temperature exhaust involving temperatures of 200ЊF or more or low-temperature ‘‘sharp-freeze’’ air-cooling systems. .. Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 20 04 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 2 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 20 04 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms... change viscosity as temperatures decline This has an effect on pump performance and pump motor selection 4. 6 Use of Computers Many manufacturers now provide computerized selection of their equipment Some manufacturers provide working software to the design office as they would a catalog For others, the designer provides to the sales engineer a summary of the pertinent criteria; the information is either... schedules to allow for these variations if competition is to be maintained Similar situations arise with other types of equipment, with package airhandling systems having the greatest variations 4. 7 Summary In this chapter the general philosophy and concept of HVAC equipment selection was discussed The details of various equipment items are covered in subsequent chapters References 1 R W Haines, ‘‘Energy Cost... reduced, which determines the acDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 20 04 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 2 92 Chapter Four tual steam temperature Zero lb/in2 gauge steam is 212ЊF in Seattle or Miami, but only 202ЊF in Denver or Albuquerque,... temperatures of 200ЊF or more or low-temperature ‘‘sharp-freeze’’ air-cooling systems using air at 20 to 50ЊF below zero Equipment for these applications must be specially designed with fan bearings and other materials suitable for the design temperatures Motor horsepower must be adjusted to the nonstandard density In high-temperature applications, the required horsepower will be decreased, but the motor . thorough inquiry by the HVAC de- signer into the process and its needs. Until the process is fully under- Source: HVAC Systems Design Handbook Downloaded from. 81 Chapter 4 Design Procedures: Part 2 General Concepts for Equipment Selection 4. 1 Introduction The purpose of this chapter