Document KRzjbKBV3eqojGrJ4x2g9ey42

Water service* {continued) estimating heating load (continued) fixtures, 817, 818 ess input requirements, 819 kitchen fixtures, 820 probable maximum demand, 818 water requirements, 818 friction lose, 810-813 copper tube, Sli fairly rough pipe, 812 rough pipe,' 813 heat pumps, 829.830 typical arrangement, 829 heat transfer, 825,826 computation, 825,826 heating water, methods, 821-825 direct-fired, 821-823 electric, 823, 824 indirect, 824, 825 ASHRAE Guide And Data Book Water services (continued) haatlrig water, methods (continued) monthly costs, 822, 824 hot water supply piping, 826,827 check-valves, 826 circulation tin**, 826, 827 water pressure, 827 safety devices, 827,823 pressure-relief valves, 828 temperature-relief valves, 828 siring cold water systems, 814 ning piping, 809-817. HprnanH load, 809,810 fixture units, 809,810 rate of flow, 809,810 solar water heaters, 828,829 design data. 829 geographic limitations, 828 tank connections, 828 Water services (continued) temperature control, 827 immersion thermostat, 827 Wine making, 396 chill proofing wine, 399 cooling must, 400 refrigeration required, 400 fermentation, 396 calculations, 396, 397 moling methods, 397 cooling requirements, 396 heat transfer, 398 water required, 398 holding wine temperature, 399 heat load estimate, 399 tartrate removal, 398 temperature required, 398 wine classification, 396 xxfv CHAPTER 1 CENTRAL STATION SYSTEMS FOR AIR CONDITIONING Determination of System Requirements, Control Requirements, Simplified Psychromefnc and Control Processes, Selection of Refrigeration Equipment, Selection of Central Station Equipment, Equipment Arrangement CENTRAL station systems are systems in which air and 35,000 cfm capacity, a carefuly analysis should be made. handling equipment, including fans, coils, and filters, is There are many times when, because of long equipment life, located at a central point to serve a number of rooms or amaintenance and accessibility requirements, it is better to use large area by means of a duct system. This chapter will de built-up equipment rather t-h*n factory assembled central scribe various arrangements of central air handling equipment station units. that are in common use. For central station equipment with less than 10,000 cfm, The refrigeration system will not be considered as a part of factory assembled unite usually are used unless the premium the central station equipment in the discussions in this chap in initial cost can be justified. ter. The refrigeration, system may be located in the same The degree of air filtering required is another factor that apparatus room or at a remote location from the central sta might influence the selection of equipment. Normally, factory tion equipment with the chilled water piping or refrigerant assembled equipment is available only with throw-away or piping connecting to the dehumidifier or cooling coils. permanent filters. If electrostatic or automatic self-cleaning A central station system is the only type that logically filters are required, a built-up unit would be favored. could be considered for certain air conditioning applications. Architectural considerations and the use of the conditioned CONTROL REQUIREMENTS space frequently require remotely located air treatment equip ment. Certain control requirements, such as constant hu midity, may make it impracticable to use the unit system of air conditioning in which fans, coils and filters are located within the space to be conditioned. Even when it is practi cable to use the unit system of air conditioning, the advan tages of a central station system over a unit system should be It is important to analyze the control requirements of the system before attempting to select or lay out equipment. Generally for comfort air conditioning, one of the following control processes will meet the system requirements. 1. Simple cooling and heating control. No definite control of humidity level, either in summer or .winter. 2. Cooling and heatiQg control with controlled high limit for carefully considered. Central station equipment generally is considered to have longer life, to be easier and less expensive to maintain, to occupy leas expensive building.space, and to reduce maintenance requirements in the occupied parts of the building to a minimum. humidity in summer. No humidification in winter. 3. Cooling and heating control with humidity maintained at a fixed level during the entire year. 4. Cooling ana heating control with humidity maintained at one fixed level in summer and at a lower level in winter, to pre vent condensation on windows. The various components of a central system are discussed In Chapters 34, 38, 39, 40, 48, and 49 of the 1961 Guide And Data Book. - Central station-equipment also is used in dual-duct, induc tion unit, fan-coil unit, and panel-air systems. For specific arrangements required for these particular types of systems, see Chapters 2 ami 3 of this volume. Other control factors which will affect the equipment ar rangement and should be determined, are as follows: 1. Zoning requirements of central station equipment. Decide whether final room temperature control is to be determined by air discharge from central station equipment, or by rnwna of reheat coils or mixing box units. 2. Fixed minimum amount of outdoor air or variable amount with economy cycle using outdoor air to conserve refrigeration. DETERMINATION OF SYSTEM REQUIREMENTS Before attempting to design and select equipment for a cen tral station air conditioning system, it is necessary to analyte the requirements of the system including first cost, operating and maintenance cost, degree of year around control, and equipment space allocation. Simple cooling and heating with out humidity control may be all that an owner desires, or can afford from a first cost standpoint. In this case, subject to total air supply capacity limitations, a factory assembled airconditioning unit with a filter section, heating and cooling coils, and fan, in a factory built casing would meet the job requirements. Due to duct space limitations and toning re quirements, it might be desirable to provide several central station units rather than one large system with an extensive duct system. Fot large central systems (over 35,000 cfm), it is desirable m almost all cases to use built-up or field assembled central station equipment. For central systems with between 10,000 The latter two factors might be considered with any one of the four basic control processes given in the preceding section. SIMPLIFIED PSYCHROMETRIC AND CONTROL PROCESSES It is usually beneficial and many times absolutely neces sary, before selecting equipment, to sketch the air treatment process on a psychrometric chart. Space cooling load calcula tions alone cannot determine the cooling coil or dehumidifier load requirements since additional load might be imposed by reheat, outdoor air miring and other specific requirements. Before presenting the skeleton psychrometric process for the four control processes listed in the section Control Require ments, the mixing and bypass principles will be be discussed briefly. For detailed information, see Chapter 3 of the 1961 Guide And Data Book. For the miring of two air streams such as return air with outdoor air, the following equations are derived from the laws 1