Question

Why do we measure temperatures at the evaporator rather than the suction pressure?

Answer 1

The evaporation process is sensitive and potentially unstable. Minor changes in performance have major effects on the system performance. A one-degree change in the evaporation temperature changes the COP by approximately 3%. An unstable process could also cause the evaporation temperature to fluctuate, with a potential risk of freezing in the evaporator.

The critical parameter for the system performance is the saturated pressure at the compressor inlet.

Check the evaporation temperature at the compressor inlet. Use compressor data to compare the cooling capacity with the design data.Compare the evaporation pressure at the outlet with the evaporating pressure at the compressor inlet. This difference should be kept to a minimum to avoid unnecessary losses in cooling capacity. There is often only one place to measure the pressure on the suction side of the compressor.

Check the positioning of the expansion valve sensor bulb If the bulb is positioned too close to the evaporator, the evaporation temperature may fluctuate. The bulb should never be placed on the bottom of the suction line, because refrigerant droplets may evaporate at this point, affecting the regulation and causing locally low temperatures.

This all reasons leads to measuring the temperature at the evaporator rather than the suction pressure.