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574 CHAPTER 39 1959 Guide A refrigerant receiver Is commonly used, as in other types of refrigeration systems, to provide a storage place for liquid refrigerant. It is particularly useful in a beat pump in order to take eare of the unequal refrigerant requirements be tween heating and 'cooling. Control Components Heat pump control systems and components are of the gamp, general type used in other types of heating and cooling equipment. Reference should be made to Chapter 43. Practically all heat pumps for residential beating and cooling are controlled automatically from the temperature of the conditioned space. Room air thermostats are usually of the type that combine the control of both the.heating and cooling function, the selection of function either being accomplished automatically in response to air temperature or manually by a selector switch. When supplementary heat ers are employed, usually electric resistance heaters, it is common to control them with the second stage of a twostage hearing control. This restricts their usage to makeup of the difference between the beating requirement and the heat-pump capacity (see Fig. 6). To avoid unnecessary usage and excessive electrical demand, particularly when the thermostat setting is raised suddenly, it is customary to use one or more outdoor thermostats to limit the amount of resistance heat used at the higher outdoor temperatures. For information on electric resistance beaters see Chapter 17. In residential heat pump applications, night-set-back, common with fuel-fired systems, is not ordinarily employed because substantial excess capacity is required for warm-up and because possible operating cost savings, if any, are deemed to be small. On the larger type systems, a proportional action type of control is sometimes used for controlling compressor and supplementary stages in steps. A variety of defrosting control schemes have been used to sense the need for defrosting air-source heat pumps and to initiate and terminate the defrost cycle. A timer is sometimes used and set to cause defrosting at predetermined intervals for instance, about every two hours. After initiation of the defrost cycle defrosting can be ter minated either by the use of a control sensing the coil pres sure ora thermostat located so as to measure the tempera ture of the liquid refrigerant in the outdoor coil. When the temperature (or corresponding saturation pressure) of the liquid leaving the outdoor coil rises to about 40 F the com pletion of defrosting is assured. Termination of defrosting may also be obtained by use of a second time interval con trol. Another method of starting the defrost cycle is to use a pressure control which reacts to the air pressure drop across the coil. Under conditions of frost accumulation the air' flow will be reduced and the increased pressure drop across the coil will initiate the defrost cycle. Again the preferred method of terminating the defrost cycle is to use a refriger ant temperature control measuring the temperature of the liquid refrigerant in the coil. A third method for defrosting involves a temperature dif ferential control in which two temperature sensing elements are used; one responsive to the outdoor-air temperature and the other responsive to the temperature of the refriger ant in the coil. As frost accumulates, the differential between outdoor temperature and refrigerant temperature will in crease, causing a defrost cycle to be initiated. The system will be restored to operation when the refrigerant tempera ture in the coil reaches a specified temperature indicating that defrosting has been completed. When the outdoor-air temperature decreases, the differential between outdoor-air temperature and refrigerant temperature decreases, and the defrost cycle is initiated sooner, Other Components Supplementary resistance heaters, commonly used with many unitary type heat pumps may be incorporated either within the unit or external to it. They may take any of the forms discussed in Chapter 17. When installed in the dis tribution ductwork, they are frequently controlled to tem per the air during defrosting operation on units employing reverse-cycle defrost. APPLICATION Criterio for Feasibility Factors to be considered in determining the practicability of employing a heat pump include the following: 1. Presence of a cooling load. 2. Relative amounts of heating and cooling. 3. Availability of suitable heat source and rink. 4. Equipment requirements and installed cost. 5. Operating cost. These criteria may be applied whether the equipment to be considered is of the factory-built, unitary type or the fieldassembled type. In fact, an analysis of these factors, in addi tion to determining feasibility of the heat pump for a par ticular application, should also be useful for choosing the equipment type. Unless there is a need for cooling, heating with a heat pump is ordinarily not economically feasible. Under certain circumstances this may not be true, but usually a heat-pump system will have a higher first cost than a fuel-fired heating system. With the almost universal need for comfort cooling, par ticularly in commercial establishments, but also to an in creasing 'extent in residences, the feasibility of a heat pump becomes more favorable. Only a portion of the required equipment will then have to compete with a separate heat-' ing system. In fact, in large buildings, the existence of si multaneous heating and cooling loads in different zones for a large portion of the time is often uniquely suited to a heatpump system. The relative amounts of heating and cooling required may greatly influence-the first cost and the suitability of a heat pump. In the usual case the size of the heating load is the predominating factor, particularly with structures in colder climates having low internal heat gain. The application of insulation and adequate spiling against unwanted air infil tration reduces the ratio of the heating to cooling loads. The availability of a statable heat source and sink gener ally involves making the optimum selection from the stand point of overall economics. Factors to be considered were presented earlier in this chapter in the section Heat Sources and Sinks. The heat source selected will usually also serve as a heat sink. It should be recognized that in applications requiring heating and cooling in different zones simultaneously, the heat source or sink need only handle the net heat input or output of the system. The Heat Pump To estimate the equipment requirements and installed cost it is necessary to moke at least a tentative equipment selec tion. Before this can be done it will, first be necessary to de termine the heating and cooling loads. For small structures this generally entails only the calculation of the peak heat ing and cooling loads. For larger buildings, a more detailed analysis may be required in order to take into account the loads at other than the design conditions, the time periods at which loads occur, ventilation schedule, variations in loads due to internal heat gains from lights, equipment and occu pancy, and simultaneous heating and cooling loads in dif ferent zones. It is implicit in a feasibility study of this kind that the objective is to obtain a comparison between the cost of a heat-pump system and a combination air-conditioning and fuel-fired heating system. Therefore, in order to estimate the first cost of both systems, a preliminary design and selection of each must be made. It does not necessarily follow that the system with the lowest first cost is the optimum choice. An increase of initial cost for any system may be justified by savings in operating cost or by other benefits derived. The principal factors to be taken into account in esti mating operating cost are given in the section Performance Characteristics. The application or feasibility study of a heat-pump system with regard to operating cost involves the comparison between it and alternative types of systems con sidered. Operating cost of fuel-fired systems may be esti mated by the procedures given in Chapter 37. Heating and Cooling Distribution The distribution of the heating and cooiing'effect from a heat-pump system is an important application consideration, particularly because of the relatively low temperatures in volved as compared to fuel-fired heating systems. In a commercial-type structure, any system of distribution which is satisfactory for cooling will generally be satisfactory for heating with a heat pump. Conversely, any system satis factory for heat distribution with a heat pump will be satisfactory for cooling, except, however, that panel sys tems will seldom have sufficient capacity to handle normal cooling loads satisfactorily. Heat distribution in the space can be accomplished by forced air at high, medium, or low pressures; primary air with induction units; individual'fan and coil units; a split system composed of forced air and panels; or panels alone. All these methods of heating, except panels alone, are as applicable to cooling as to heating. The distribution from the beat pump to space units be by use of .water or air. Quantities circulated, pipe or duct sizes, transfer units, and other characteristics of the dis tribution system will resemble very closely standard prac tice for cooling systems. Heating and cooling distribution in residential heat-pump systems generally follows the same principles as for sum mer cooling. Refer to Chapter 46. It has been found with residential air distribution systems, comfort may be achieved with the air volumes and temperatures encountered with heat pumps when proper attention is given to the number and location of air outlets." Design and Selection Irrespective of the size and type of building and whether unitary or field-erected equipment is being considered, the first steps that should be performed are those described 575 under Criteria for Feasibility. In applying unitary heat pumps, the manufacturers of these units ordinarily provide sufficient data and instructions for proper application. The typical procedure followed for both unitary and field-erected heat pumps is outlined below: 1. Determine heating and cooling loads. It is desirable also to determine the amount that loads may be reduced by addi tional insulation, sealing, and shading. Savings effected in equipment due to distribution and to operating costs may more jhun offset the added cost involved. In larger buildings, loads should be determined for the different conditions of operation including simultaneous heating arid cooling. 2. Wjrfjthlinh heat source and sink. See previous section on this subject. 3. Determine type of distribution system. 4. Choose major components: compressors, heat exchangers, and air handlers. For unitary equipment, establish equipment size, type, and make. 5. Establish design of refrigeration system. 6. Choose auxiliary equipment: fans, pumps, valves, etc. 7. Determine equipment location. Since the heat pump operates without fuel and combustion products, considerable flexibility is permitted in its location. A single-package unit, may be located in & basement, crawl space, attic, roof, garage, or carport, in the conditioned space, or out-of-doors. For remote-type, or split units, one section is generally located out doors and the other in a convenient indoor location. Consider able flexibility in locating equipment of a field-erected heat pump in one or more areas is possible. Factors to be considered in equipment location are: (a) Ac cessibility to heat source'and gink, (6) Convenient location with respect to indoor distribution system, usually duct work, (c) Availability of condensate drain, (d) Required accessibility and sufficient space for servicing, (e) Convenience to electric power wiring, (/) Suitability of structure for supporting weight of equipment, (?) Effect of noise and vibration transmission, and (A) Appearance. 8. Determine control system and components. 9. Estimate first cost and operating cost. Illustrative Systems for Larger Buildings Fig. 9 shows a system employing well water as a heat source and sink. In this particular case a heat exchanger is used between the well water circuit and the fluid circuit to transmit heat to the heat pump, heat exchangers, and the indoor conditioning coils. Although this requires, an extra step of heat exchange, it avoids the necessity for pumping well water throughout the building and permits a high rate of flow in the building water circuit with resulting improve ment in heat transfer. The use of a water preconditioning coil as shown to preheat, or precool, ventilation air often reduces the size of refrigeration capacity required as well as the cost of operation. It should be noted that both heated and cooled water can be furnished simultaneously to indoorair-handling or remote-room units as may be required by the existence of heating and cooling loads in different zones at the same time. Fig. 10 illustrates an air-to-air heat pump system which is similarly suited to a structure having simultaneous heat ing and cooling loads. The refrigeration subcooling surface shown is used to preheat ventilation air and thereby effect substantia! economies in equipment size and operating cost. HEAT STORAGE The use of thermal storage in a heat-pump system can improve its performance characteristics. Installations of heat pumps with thermal storage have been made principally in a few large building systems.13' " " All materials possess the property of thermal storage in