6.7 Operating point

The compressor is the "heart" of the refrigerant system, and is the component that limits the capacity of a refrigerant system. The task of the evaporator and condenser is to utilize the available energy potential fully, thereby maximizing the efficiency and economy of the useful power that can be extracted from the system. The effect of selecting the evaporator for a chosen compressor is discussed in this section.

Matching the evaporator with a compressor

As discussed in chapter 3.4, compressor performance depends on the evaporating and condensing pressures. The saturation pressure of the evaporator determines the density of the refrigerant gas at the inlet of the compressor, thus affecting the refrigerant mass flow per compressor revolution. The total pressure difference between the evaporating and condensing sides also affects the required compressor power consumption.

The evaporator and condenser affect these pressure levels though their ability to transfer energy between the refrigerant and the secondary fluids. The evaporator operating point is the equilibrium point at which the performance of the evaporator matches the performance of the compressor. The refrigerant mass flow is determined by the compressor, and at the operating point the refrigerant is evaporated at a stable saturation temperature.

The compressor curve in Figure 6.24 has a positive slope, indicating that the available cooling capacity increases with higher evaporation temperature. The evaporator performance curve has a negative slope, indicating that a smaller temperature difference between the refrigerant and the secondary fluid leads to less heat transfer per area. The intersection of these two curves signifies the operating point.

A compressor curve is defined for a certain level of superheating and a certain condensation temperature. Increasing the values of these parameters will decrease the capacity of the compressor, i.e. shift the compressor curve downwards. The evaporator performance should be calculated for the same level of superheating in order to remain comparable with the compressor line. The performance and size of the evaporator influence the total cooling capacity and performance of the system, because the operating points will differ. Figure 6.25 shows the performance curves of three different evaporators and a compressor curve.

Below are some examples of what happens if the heat exchanger is over- or undersurfaced. In this case, a brazed plate heat exchanger with 50 plates has 0% oversurface.

Table 6.1. Operating points of three different evaporators.

Although the active surface area is reduced from 50 to 40 plates, the reduction in cooling capacity is only 4%. The system performance (COP) falls by 3%.

It should be clear from this example that the refrigerant mass flow, controlled by the compressor, is the single most important parameter for determining the system performance. An underdimensioned evaporator will reduce the evaporation temperature but not alter the system capacity very much.

Oversurfacing the evaporator (in this case from 50 to 60 plates, corresponding to a 20% increase in the heat transfer surface), will not give a performance increase of the same magnitude since the compressor controls the system, as mentioned in the previous paragraph. Despite the large increase in heat transfer area, the cooling capacity increases by only 2%. The increased number of plates will also reduce the turbulence that is essential for stable heat transfer and the brazed plate heat exchangers self-cleaning ability.

When determining or analyzing the operating point, it is very important to remember that the refrigeration system is dynamic. The compressor performance curve depends on the condenser performance and the level of superheating. If the conditions of the evaporator are changed, the slope and position of the evaporator curve are also changed, which will alter the operating point. Always make sure that the input data for the compressor and evaporator curve correspond in order to obtain realistic operating points.

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