SURGE - WHAT CAUSES IT - HOW TO PREVENT

PREVENTING SURGE:

Surge typically occurs when the compressor is operating with a high pressure ratio (PR), or when either the suction or discharge pressure change more rapidly than the compressor controls can adjust the compressor speed (RPM´s) and/ or inlet guide vane (IGV) position.

Pressure Ratio (PR) is a number that is calculated based on the relationship between the discharge pressure and the suction pressure. Pressure Ratio for water cooled chillers typically will be on the order of 2.0 to 2.5. Due to the higher discharge pressure experienced by Air Cooled units, Pressure Ratio for Air cooled chillers will typically be on the order of 2.5 to 3.0. Higher condenser water temperature or higher ambient temperatures will result in higher Pressure Ratio.

It is important to recognize that the compressor is experiencing surge and take action to prevent this from occurring. This action may be the repair of a defective component of the chiller or an adjustment to the chiller control loop algorithm parameters.

Routine review of the chiller controller logs and the compressor histories is important to assist in recognizing that the compressor is entering surge.

Anytime there is a Front Radial Bearing Displacement Fault, the cause must be investigated. Particularly if this Fault is displayed repeatedly.

As an overall rule, the compressor should not cycle more than three to four times per hour under normal operating conditions.

There are many possible causes of surge, however, following are some of the more common ones encountered:

Chilled Water Flow Switch inoperable in the closed position or jumped out – This condition is particularly dangerous for the compressor. If the compressor starts with no chilled water flow or the chilled water flow is reduced/ eliminated during compressor operation, the suction pressure will rapidly drop. The compressor controls can´t adjust rapidly enough to prevent the compressor entering into a surge condition. Eventually, the compressor shaft displacement will exceed the safety limits of the controls due to the surge and the compressor will shut down, typically with a Front Radial Bearing Displacement fault.

The continued operation of the chiller under this condition, with repeated Front Radial Bearing Displacement faults has a high probability of eventually resulting in damage to the touchdown bearing.

Lack of Water Flow proof or inoperative flow proof on condenser of water cooled unit – If there is no water flow proof installed to protect the chiller against operating with low water flow, a condenser pump becoming inoperative will force the compressor into a surge condition.

High Ambient Conditions in combination with lower chilled water setpoint – In cases where the client is operating the chiller at low load with a chilled water setpoint below 4.5 degrees C (40 degrees F.), during high ambient conditions, it is imperative that proper adjustments be made to the chiller control parameters to prevent the compressor entering in surge.

Under these conditions, the compressor will likely be operating close to the surge rpm limit. As the the chiller controls attempt to reduce compressor capacity to meet the low load at the same time maintaining enough compressor rpm to overcome the difference in suction and discharge pressures, surge is a risk.

High Evaporator Approach – High evaporator approach (more than 8 degrees C or 14.4 degrees F.) will result in a lower than normal suction pressure. This, in combination with higher ambient conditions can also result in the compressor operating close to and occassionally entering into surge condition.

Failure of Inverter Temperature Sensor or Motor Cavity Sensor – If either of these sensors fails such that it is giving a higher than normal reading, the compressor controls will automatically reduce compressor rpm in order to limit the amount of heat being generated by the inverter or the compressor motor itself and forcing the chiller into a surge condition.

Chiller Operation with rapidly changing load – In cases where the chilled water loop is extremely small or the load on the chilled water loop suddenly drops off, the compressor will tend to cycle frequently and has risk of entering surge as the load disappears and the compressor attempts to reduce compressor capacity in response to the changing load.

In this case, the compressor may enter surge before the chiller control algorithm reaches the point where the compressor will be cycled off. Repeatedly cycling the compressor under these conditions can result in surge and compressor damage.

Failure of the Compressor Suction Pressure Sensor – The compressor is equipped with a pressure/ temperature sensor on the compressor suction and discharge. If the suction pressure sensor begins to read high, the compressor controls can drive the compressor into surge repeatedly due to the fact that the compressor does not really know where it is operating on it internal compressor map.

It is important to routinely verify the suction pressure sensor accuracy. There is no calibration possible on this sensor. Sensors reading improperly must be replaced.