EVAPORATOR APPROACH

EVAPORATOR APPROACH

Evaporator Approach is the difference between the saturated temperature of the suction pressure and the leaving chilled water temperature.

Normally, in a high efficiency chiller with a flooded type evaporator, the evaporator approach should be less than 2 degrees C. (3.6 degrees F.).

When historical information on a unit is available, it is always a good idea to compare historical evaporator approach readings with the current reading.

High evaporator approach results in a loss of efficiency and capacity in the chiller.

Potential Causes of High Evaporator Approach:

Loss of refrigerant charge in air cooled chiller – if the chiller experiences high inverter temperature alarms or faults and the evaporator approach is higher than normal, there is an excellent chance the chiller has lost charge.  In this case, the actual liquid level in the evaporator will be less than the liquid level setpoint.

On a water cooled condenser chiller, condenser level is being controlled to a setpoint.  Loss of refrigerant charge will result in lower liquid level in the evaporator.  Verify visually the liquid level in the evaporator.  If liquid level not visible, try lowering the condenser liquid level setpoint, which should raise the liquid level in the evaporator...... This also should result in some reduction of evaporator approach.  Caution must be taken to not lower the condenser liquid level setpoint to the point where there is a loss of liquid ¨seal¨ on the compressor cooling connection.  This will result in bubbles in the motor cooling line sightglass and increasing compressor inverter temperatures.

Fouling of the waterside of the evaporator tubes – This is not very likely that the evaporator tube watersides have fouling, but this is Always a possibility that must be considered.

Problem with the compressor suction pressure/temperature sensor – A small ¨drift¨ in the suction pressure sensor reading can dramatically affect the performance of the chiller.  This can easily be verified by connecting a pressure manifold to the system and verifying the suction sensor pressure reading.  I have actually seen this problem on several different installations.

Problem with the leaving chilled water temperature sensor – This can be verified by measuring chilled water temperature with a meter.  If no meter available, turn of chiller and verify that entering and leaving chilled water readings are the same.