Document aB4MGEEZgMJe15eZGx6Ja9ZeN

570 CHAPTER 39 1959 Guide Rg. 3 .... Variation of Coefficient of Performance with Temperature 4. Determine the condenser capacity from Q, = Q, + 3413 P. - (1) where Qe = condenser capacity, Btu per hour. Q, n compressor refrigeration effect (evaporator capacity), Btu per hour. Qt. = heat loss from compressor, Btu per hour. (Q< may have to be approximated if unavailable from com pressor manufacturer. For many compressors Qca is negligible.) 5. Plot Q. obtained from Equation 1 on a chart similar to C of Fig. 4. (See Fig. 5.) 6. Select other condensing temperatures in combination with the original evaporator temperature from Step 1 and repeat Steps 2 to 5 as necessary to determine the condenser capacity at which the system balances. Points A and B on Fig. 5 repre of Chapter 38 defines the coefficient of performance (CP) of a Carnot cycle heat pump. Applying the relationships given with evaporator and condenser temperatures illus trated in Fig. 2 yields the coefficient of performance (CP) for a Carnot cycle heat pump shown in Fig. 3. An actual heat pump will, as a rough approximation, be about 50 per cent as efficient as a Carnot cycle heat pump and will have a coefficient of performance dependent on temperatures, as illustrated in Fig. 3. System Balance The performance characteristics of a heat-pump system can be predicted by evaluating and combining the per formance characteristics of its individual components.*' Data available from component manufacturers may be portrayed as shown in Fig. 4. The conditions of system balance can be established by the following procedure: 1. Choose a combination of evaporator refrigerant tempera ture tr and condensing temperature t. 2. Determine the compresor refrigerating effect from per formance curves nrailar to A of Fig. 4. 3. Determine the compressor power input P*, in kilowatts, from curves similar to B of Fig. 4. CVAA TtUf.,X,,r 4A COMPRESSOR CAPACITY CVAA TCMP-.tr.f 46 COMPRESSOR POWER (COMO. TE**1-(*CT.AIR TEMP.) 4C CONOCMSER CAPACITY (MEAT SOURCE TEMPIMINUS (EVAP. TEMP.) V.-V.E . 40 EVAPORATOR CAPACITY Rg. 4 .... Performance Characteristics of Heat-Pump Components sent the results of these calculations. Two points will nor mally be sufficient to determine the balancing Q. . 7. Select other evaporator temperatures and repeat Steps 1-8 (See Fig. 5.) 8. For each evaporator temperature find the corresponding heat source temperature t. from chart similar to D of Fig. 4- With the conditions of system balance found from the foregoing procedure, it is a relatively simple matter to es tablish the heating performance characteristics. The net heating effect may consist only of the condenser heat, or, depending upon the system design, may also include heat losses from the compressor and motors, and the heating ef fect of a refrigerant subcooler coil. The cooling performance can be determined in a similar manner. For a heat pump which employs a constant temperature heat source, a few computations will generally establish the balancing conditions for tr and tc. For a heat pump having source temperature subject to considerable variation (eg. air source), tbe balancing conditions for a wide range of tr will be necessary. Fig. 6 shows the performance characteristics of a typical heat pump determined either from actual system tests or The Heat Pump Unmodulated Heat Pump from an analytical procedure such as already described. Also shown are the heating and cooling loads for a typical resi dence. The temperature at which the heat-pump capacity and the structure heat requirement are equal is referred to as the balance point. If the balance point is above the heat ing design temperature, td, then supplemental heat will be required as denoted by tbe shaded area. Seasonal Performance Fig. 6 illustrates some of the basic factors important in evaluating seasonal power consumption. Following the prin ciples given in Chapter 37 the heating power consumption for any period of time may be estimated from F 2AHD 3413(1. - tiXPF) where P heating kilowatt hours H - structure design heat loss, Btu/hr. D -- degree days for specified period. " outdoor temperature at which structure heat loss is aero (approximate indoor temperature when heat gain from other sources is small), Fahrenheit. td -- outdoor design temperature, Fahrenheit. (PF) " performance factor for specified period. For a heat pump with a constant heat-source temperature, the heating capacity will be essentially constant aDd the performance factor (PF) can be taken as the Heating co efficient of performance (CP)*. For an air-source heat pump, with its varying heat source temperature, it is necessary to either break down the period of operation into relatively narrow temperature spans and to compute the power consumption in each, or, alternatively, to find the performance factor (PF) by weighted average of the different coefficient of performance (CP) and op erating hours that apply at various temperatures. In either 571 case it is necessary to have data on the time distribution of outdoor-air temperatures. The number of hours that wifi apply within a given temperature range may be based on past weather data for a specific location. For example, such data are available for a number of cities broken down by month and time of day." However, where the required weather data are not available and when it is not necessary to break the heating season down into shorter periods, gen eralized data such as portrayed on Fig. 7 may be employed. Fig. 7 is based on an analysis of ] 14 weather stations in the United States from Reference 38, in which the heatingseason days were correlated with the TAC design tempera tures. The TAC design temperatures shown in 1948 to 1958 editions of The Guide were suggested by the ASHAE Tech nical Advisory Committee on Weather Design Conditions. They were temperatures which were equalled or exceeded during 9714 percent of the hours in December, January, February, and March chiefly for airport stations. Maximum deviation for any single location from the averages portrayed is on the order of 5 deg at 70 F out door temperature and is due mainly to variations in summer climate. Since curves must pass through points representing the winter TAC design temperatures, their accuracy is best in the low-temperature range. The vertical scale also gives the outdoor design temperatures for heating, as given in Chapter 12, which take into account the probabilities of oc currence. Reference 1 of Chapter 12 provides the correla tion of these design temperatures with the TAC tempera tures. T3