Document 2qmoKN0nyN4w7Ya1a58Q3NNn7

948 CHAPTER 66 1965 Guide And'Data Book volume) of about 5 percent; A decrease'in capacity at low evaporator temperatures corresponding to low heat-source temperatures is also evident, as well as a decrease in power at low evaporator and condensing temperatures.. ' If the compressor has low clearance volume, e.g., 2| per cent,' then it is more suitable for low-temperature operation and will provide, for example,'about 15 percent greater re frigerating capacity at an evaporator temperature of;0 F and.a condenser temperature of 110 F. However, this com pressor hss somewhat more-power demand under maximum cooling load conditions-than does one of medium clearance. It is obvious that more total'heat capacity can be obtained at low!outdoor temperatures by deliberatelyoversixmg ,the compressor. When this is done, it may be necessary to provide some type of capacity reduction by means of two-speed motor drives, cylinder cutouts, or other methods. . The disadvantage of; this arrangement is that, the greater number of operating hours that occur at the higher suction' temperatures, must be served.with the compressor in: the unloaded condition which, generally causes dower efficiency. Therefore, the annual operating cost will tend to' rise. It-is also true that the additional initial cost of the oversized compressor must be economically justified by the grun in heat ing capacity. One method proposed for increasing the heating' output* at' low temperatures involves the use of staged com1 pression, in which one compressor may pump from -- 20 Fsuc tion temperature-to 40 F condensing temperature, and a sec ond compressor compress the vapor from 40 F to 120 F.`;Iri such an arrangement, it is possible to interconnect any/ two compressors so that they are-in parallel,' both pumping from approximately 45 F to 120 F at the normal eooling rating point, while at some.predetermined outdoor temperature on heating they, are reconnected so that they pump in-staged relationship.-' flg,7 shows the performance of such a pair ofoomprestore for compressors of both medium and low clearance volume; It is apparent that at low suction temperatures! the .recon-! nection- into a staged-relationship does provide some added capacity: It should also be-understood that.tire'motor selec tion.involved must be based upon the maximum loading con-' ditions- for summer operation,' even though;the low stage compressor has a greatly reduced power requirement under, the heating condition. The coefficient of, performance will.be approximately the same whether the compressors are coupled in parallel or com pound-staged at any set of operating conditions, depending somewhat on the motor characteristics when lightly loaded;- A rotary.. compressor has characteristics similar to"a re ciprocating compressor except that it has low rWimraj and high volumetric efficiency! From this standpoint it is'-well suited to' heat-pump service, providing about 30 percent greater capacity at O F -- 110 F lift than a medium-clearance reciprocating compressor. However, this characteristic! also tends to increase the power demand at the maximum <wdmg load conditions. At-present, reciprocating compressors are most widely used for heat pumps,, with rotary compresors being restricted to the first-stage of a staged-compression system and not used during the cooling cycle.- - Heat Transfer Components .. Refrigerant-to-air and refrigerant-to-water heat exchang ers,; as previously described in the section Heat Sources and Sinks, are similar to heat exchangers used in current air- conditioning practice. A refrigerant subcooler -coil may be employed in conjunction with an indoor air coil, or; on systems with a ventilation air supply,'"to preheat ventilation air. A substantial gain in capacity and coefficient of performance can result.' ~ ` ......... Refrigeration Components Refrigerant piping, receivers, expansion devices, and re frigeration accessories in-heat pumps are usually the same as those used in other types of refrigeration mid airTCohditidning systems. ( A reversing valve is.used to change.the system from the cooling to the heating operation. This change-over'requires the use'of a: valve, or`valves, in the refrigerant circuit,1 except where the'chahge is .accomplished in'fluid circuits external to the refrigerant circuit (see Table 1),'Reversing valves are.usu ally pilot-operated by means of solenoid valves which admit head and suction pressures to move the operating'elements; Expansion deoicesjor, controlling,,the.refrigerant flow are normally.tiiermbstatic expansion valves as described in Chap^ ter 45."Ifithe circuiting is arranged so that tile refrigerant line upon which the control bulb is placed can beeome the' com^ pressor discharge line, the resulting pressure developed in the power element of the valve may be excesive, requiring the use .ofa special ocmtrol'charge or pressure-limiting element. When a thermostatic expansion valve is applied to an outdoor air coil, a special cross-charge is desirable to Emit the superheat at low temperatures and thereby obtain better utilisation of the coiL .t ; When an expansion valve is attached to a coil that is operated as- a-condenser, a-bypass, with a .check valve is normally provided as indicated in.Table 1. ' l/._j On..emalTfactory-built systems,- capillary tubes are.nor mally used as expansion devices. While a single capillary tube on both heating and. cooling is-sometimes employed, better efficiency and`performance can be obtained by H^irig a: more restrictive capillary tube' for heating than for coolingJ This may be-accomplished .by using two capillary tubes in either a series or parallel arrangement with a check valve to bypass onej for cooiling or to block one for heating,-respectively. On'an air-source beat pump that must operate over a wide range of evaporating temperatures, a capillary! tube tend to pass refrigerant at an excessive-rate at low .pressures, causing liquid floodback to the compressor. In some' cases suction line accumulators or charge-control devices are employed to minimize this effect. A refrigerant receiver,-which is,commonly used to provide a storage place for liquid refrigerant, is particularly useful in a heat pomp^totake care of.the unejyial-refrigerant .require ments of heating andcooling.! v Heat Pumps 949 Control Components j. Heat pump control systems and components are of the same general type used in other types of heating and cooling equipment. (See Chapter 13.) Practically all heat pump3 for residential heating 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 beating and cooling function, the selection of function being a<v complished either automatically in response to air tempera ture or manually by a selector switch. When supplementary heaters usually electric resistance heaters, are employed, it is to control them with' the'second' stage of a twostage-heating control This restricts their usage to makeup of the difference between the heating requirement and the heat-pump capacity (see Fig. 6). To minimize the effect of the differential between stages, outdoor anticipation is sometimes used. 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. Another method of controlling the heat pump and supple mentary heaters is to use a room thermostat in conjunction with a sequencing controller which operates as a function of thermostat load. . 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 any possible savings in operating cost would be gmaU 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 Hpnsft the need for defrosting air-eource heat pumps, and'to initiate and terminate the defrost cycle. A timer, is sometimes used and set to cause defrosting at predetermined intervals, eg., 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 or a 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 a second time interval control. Another iwnn< of starting the defrost cycle is to use a pres sure 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 refrigerant tempera ture oontrol 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, bulging 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, causing the defrost cycle to be initiated sooner, unless compensation is provided. ! '-- 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 discused in Chapter 21 of the 1064 Guide And Data Book. When ingtallprf in the distribution ductwork, they are frequently controlled to temper the air during defrosting operation on units employing reverse-cycle defrost.. ......... 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 principally, been made in, a few large building systems.7'0'*1 All possess the property of.thermal storage to a; greater or lesser degree. In the case of a building, the struo-. tural materials.are almost always in the.process of either absorbing heat from or delivering.heat to the interior space; This effect is more pronounced in' cooling operation' where greater air temperature variation is,tolerated. Storage tends, to reduce the rate of temperature change and helps in some measure to reduce the peak equipment.requirements. In.this, atn< every heating and cooling system can be said to in volve heat storage in some degree. Many attempts have been made, particularly in recent years, to increase the heat-storage effect by using special, heat-storage materials as part of the heating or cooling sys tem. The result has been a reduction in the size of the.heating, or cooling equipment necessary to take care of peak demands. > In the '`ft**1 of the heat pump, a provision for.heat storage can serve not only to reduce the size of the heat pump neces-'. sary for a given load, but also to, provide a more desirable electrical load by shifting part of the load to the time of day when the cost of power is least. The off-peak electric hot water heater is a, common example of such a heat-storage application. ` In general, there are two types of heat-storage systems that have been employed, (1) sensible heat-storage systems and (2) latent heat-storage systems. Usually, the latter is actually a combination of the two, making use of some sensi ble heat storage in addition to the latent effect. Heat storage in a heat-pump system may be utilized on the high side, when heat is available at a temperature suitable for direct hating, or on the low side as an intermittent heat source at temperatures lower than the heated space. REFERENCES 1 Philip Spom, E. R. Ambrose, and Theodore Bannister: Heat Pumps (John Wiley A Boas, New York, 1947). * E. N. Kemler and Sabert Oglesby, Jr.: Heat Pump Appli cation* (McGraw-Hill Book Co., New York, 1950). * Bibliography of the Heat Pump through 1951 (Edison Elec tric Institute Publication No. 53-4). * Heat Pump Bibliography (compiled by Southern Research Institute for Southeastern Electric Exchange). * J. D. Kroeker and R. C. Chewning: Heat pump in an office Km'Ming (ASHVE Tbansactions, VoL 54, 1948, p. 221). J. D. Kroeker, J. H. Bonebrake, and J. A. Melvin: Heat pump application to a newspaper plant (ASHVE Tuansactiok3, VoL 57, 1951, p. 467). i Philip Spom and E. R. Ambrose: Two-year performance of a heat pmp system furnishing year-round air conditioning in modem building (ASHVE TEiNSAcnoxs, VoL 57, 1951, p. 4S3). * J. D. Kroeker and R. C. Chewning: Costs of operating the heat pump in the Equitable building (ASHVE Tkansachokb, Vol. 60. 1964, p. 157).