9.4 Evaporator underperformance

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. Some checkpoints for determining the reason for underperformance are discussed below.
Check the installation. The exchanger should be mounted upright, with the refrigerant circuit connected to the left side when the arrow sticker is pointing upwards. The refrigerant and water circuits should be connected in counter-current mode. For further information, see chapter 8.2.

  • Check the evaporation temperature at the compressor inlet. Use compressor data to compare the cooling capacity with the design data. Check the evaporation pressure at the evaporator outlet if possible. 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. Measuring the temperature into the evaporator and knowing the calculated pressure drop over the BPHE can give a good indication of the actual pressure drop.
  • Check the inlet/outlet water temperatures. Use the cooling capacity (above) to calculate the water flow rate, and compare it with the design data.
  • Check the amount of superheat. Adjust the superheat (normally 5-6 K) at a large pressure difference and check the stability. A high level of superheating could indicate that the expansion valve is too small.
  • Check the expansion valve. Make sure that the valve is able to regulate. If not, a larger valve should be installed. An expansion valve with external pressure equalization should be used. For further information, see chapter 4.2.
  • 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.
  • Check the external equalization line. The external equalization line should be close to the surrounding temperature. If this line is at evaporation temperature, it indicates a leak in the valve, i.e. refrigerant is flowing from the expansion valve to the suction line without passing the evaporator. This will cause the valve to close if the bulb is positioned after the connection for the equalization line, decreasing the evaporation temperature. If the bulb is positioned before the connection, the valve will not close and liquid refrigerant will be injected into the compressor.
  • Check the sight glass in the liquid line. There should be no bubbles in the liquid line. If there are, adjust the refrigerant charge or the operation of the condenser to establish sufficient sub-cooling. If there are bubbles, measure the liquid temperature. If bubbles occur even when there is sub-cooling, check whether there is a temperature change over any components in the liquid line, indicating a pressure drop.
  • Check the pressure drop over the secondary side of the evaporator. If the pressure drop is higher than the design conditions, fouling could be the reason. Make sure that the pressure drop is read at the design flow rate, because the pressure drop is proportional to the flow rate squared.

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