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3.4 Compressor performance

A change in the evaporation or the condensing temperature influences the operating conditions for the compressor. Any change in temperature affects the density of the refrigerant, which alters the compression ratio between the low-pressure and high-pressure sides. The influence of changes in the evaporation and condensation temperatures on compressor performance is discussed in this section.

High temperature in the evaporator is eq​​uivalent to high pressure and high vapor density. This means that 1 kg high-pressure vapor occupies less volume than 1 kg low-pressure vapor. In a refrigerant system, the mass flow of high-pressure vapor into the compressor is therefore larger at each displacement than the mass flow of low-pressure vapor. To maintain a specific suction pressure, i.e. to maintain a specific evaporating temperature, the evaporator must be designed to vaporize the same mass of refrigerant as is compressed in the compressor.

If the entering water temperature, EWT, and the leaving water temperature, LWT, increase e.g. 1 K from 12°C and 7°C, respectively, to 13°C and 8°C, respectively, the mean temperature difference, MTD, will increase (see Figure 3.5). Hence, a larger amount of refrigerant than before will evaporate in the evaporator. However, the compressor still removes the same amount of vapor as before the change in water temperature. Excess gas that is not removed by the compressor therefore remains inside the evaporator. The accumulation of excess vapor in the evaporator leads to higher pressure and temperature on the refrigerant side. The increased vapor pressure means the vapor density also increases. Consequently, a larger mass of refrigerant becomes compressed on every compressor stroke, i.e. the compressor capacity will increase if EWT and LWT increase 1 K. However, the evaporator and the compressor will subsequently find a new operating point where equal masses of refrigerant vapor are produced by the evaporator and removed by the compressor. Thus, when the conditions in a refrigerant system change, the compressor and the evaporator together will find a new operating point.

Figure 3.6 shows three operating lines for a compressor at different evaporation temperatures but a constant condensation temperature for each compressor line. The compressor suction performance corresponds to a certain cooling capacity (THA) at each pressure ratio. Increasing the evaporation temperature at a constant condensing temperature leads to increasing compressor performance.

Just as every compressor has its own characteristic operating line, every evaporator has its own characteristic operating line. Figure 3.7 shows that the evaporator performance decreases when the evaporation temperature increases. One of the compressor lines (Tcond = 40°C) in Figure 3.6 is also plotted in Figure 3.7. The compressor operating line crosses every BPHE operating line only once. The intersection point, marked with a circle in Figure 3.7, determines the evaporation temperature and thus the cooling performance of the specific compressor/BPHE combination. This highlights the importance of matching the compressor and the BPHE correctly to achieve the desired operating conditions.

The condensing temperature can also fluctuate for various reasons. One reason is that suction pressure differences can affect the pressure ratio of the compressor, which leads to an altered condensation pressure, i.e. a different condensation temperature. Other reasons could be changes in the flow or temperature of the cooling water to the condenser.

Figure 3.8 is similar to Figure 3.6, with the differences that the characteristic operating line is plotted for constant evaporation temperatures and varying condensation temperatures. Note that the effect of increased condensing temperature on the compressor heating capacity (THR) is less than that of increased evaporation temperature on the compressor cooling capacity (THA) (see Figure 3.6). The compressor heating capacity decreases only slightly when the condenser temperature increases.

Figure 3.9 shows the operating points for three different SWEP condensers. It can be concluded from Figures 3.6, 3.7, 3.8 and 3.9 that a change in the evaporation temperature affects the cooling/condensing capacity more than a change in the condensation temperature. Thus, to maintain the designed total system capacity, it is more important to maintain the designed evaporation temperature than the designed condensation temperature.

Table 3.3. Impact from changes in evaporation and condensation temperatures on cooling capacity (Q2), or total heat of absorption (THA), condenser heating capacity (Q1), or total heat of rejection (THR), and compressor power input (W). Also shown are the coefficients of performance for a chiller (COPREF) and a heat pump (COPHP). The table is valid for TEVAP=2°C and TCOND=40°C.​​​​​​​​​​​​​​​​​​​​​​​

The values in Table 3.3 have been calculated in design software for compressors.​​​​​​​​

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