Refrigeration Systems

Operating at very low evaporation temperatures (e.g. when providing cooling for a deep-freezer and still using ambient air or similar as a heat sink for the condenser) often requires special system solutions. Low-temperature evaporation requires low evaporation pressures while the condensing pressure is at normal levels. It is often beneficial, and in some cases necessary, to separate the evaporation and condensing pressure levels by more than one compressor step. This is because when the pressure ratio over the compressor increases, the discharge temperature out of the compressor will also increase. Simultaneously, the compressor efficiency decreases, which increases operating costs. High discharge temperatures may cause both the refrigerant and the lubrication oil to decompose. This in turn will shorten the life of the compressor. Figure 10.15, point (a1), shows the higher discharge temperature of a single-step refrigeration system with low evaporation temperature.

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