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Figure 2-12 outside air evaporator outside air condensor water out water in fan compressor (main) TXV thermal expansion valve Where: EER = Energy Efficiency Ratio Qc = cooling capacity (Btu/hr) We = electrical power input to heat pump (watts) The higher the EER of a heat pump, the lower the electrical power required to produce a given rate of cooling. Like COP, the EER of an air-to-water heat pump depends on the source and sink temperature. The warmer the source media temperature is compared to the sink media temperature, the higher the heat pump’s EER. Figure 2-11 shows how the outdoor air temperature and leaving chilled- water temperature affect the EER of a specific air-to-water heat pump. To maximize EER, designers of chilled-water cooling systems using either air-to-water or water-to-water heat pumps should use the highest possible chilled-water temperature that still allows adequate dehumidification. EER is also slightly influenced by flow rates. Higher flow rates of either the source media or the sink media produce small increases in EER. This is the result of increased convection on both the air-side and water-side heat exchangers. WHAT DOES “TONS” MEAN? In North America, the heating and cooling capacity of a heat pump is often stated in “tons.” In this context, a ton describes a rate of heat flow. More specifically, 1 ton equals 12,000 Btu/hr. Thus, a “4-ton” heat pump implies a nominal heating or cooling capacity of 4 x 12,000 or 48,000 Btu/ hr. The tonnage of a heat pump has nothing to do with the heat pump’s weight. The unit of “ton” originated during the transition from stored natural ice as a means of cooling to mechanical refrigeration. It represents the average heat transfer rate associated with melting one ton of ice over a 24-hour period. A description of a heat pump heating or cooling capacity based on tons is usually a nominal rating at some specific set of operating conditions. Thus, a “3-ton” rated heat pump could yield a heat output rate significantly higher than 3 tons when operated under more favorable conditions, and significantly less than 3 tons when operated under unfavorable conditions. ENHANCED VAPOR INJECTION One of the developments that has significantly improved the ability of air-source heat pumps to operate at low outside air temperature is called enhanced vapor injection (EVI). This refers to a modified refrigeration circuit that lowers the temperature of liquid refrigerant entering the outdoor evaporator when the heat pump is operating in heating mode. The lower the liquid refrigerant temperature entering the evaporator, the lower the air temperature at which the heat pump can operate. EVI also increases the refrigerant mass flow through the compressor, which helps in maintaining heating capacity at low outdoor air temperatures. To understand EVI, it is helpful to consider a basic refrigeration circuit of a heating-only air-to-water heat system, as shown in Figure 2-12. The temperature and liquid/vapor proportions of the refrigerant leaving the condenser, in part, determine the extent to which the thermal expansion valve can lower the refrigerant temperature entering the outdoor evaporator. This, in turn, limits the low ambient heating capacity and COP of the heat pump. Figure 2-13 shows how the basic refrigeration circuit of Figure 2-12 is modified to allow EVI functionality. EVI works by routing the refrigerant leaving the condenser through an intermediate heat exchanger called a “sub- cooler.” A portion of the refrigerant passes directly through one side of the sub-cooler. The other portion passes through an electronic expansion valve that lowers the refrigerant’s pressure and temperature prior to flowing through the other side of the sub-cooler. This portion of the refrigerant evaporates in the sub-cooler, absorbing heat from the other portion. This reduces the temperature of the liquid entering the thermal expansion valve, and thus, the temperature entering the evaporator. The lower the 16PDF Image | Heat Pump Systems 2020
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