Why you need to protect your chiller from surge conditions.
As the true work horses of a district cooling system, chillers are very good at extracting the heat from buildings in very large quantities. If the overall heat transfer process variables venture outside the chiller design limits, the chillers can experience a dangerous condition called surge which can cause extensive damage to the chiller.
What is Chiller Surge?
Chillers are designed to meet specific system requirements and operating conditions. With proper maintenance and operation within design limits, a properly sized and selected chiller will not surge. Generally, two pressures exist in a chiller system: the evaporating (low) pressure and the condensing (high) pressure. The chiller compressor suctions the refrigerant from the low-pressure evaporator and discharges it into the high-pressure condenser. The difference between the compressor suction and discharge pressures is called lift. Chiller surge is a condition in which the compressor pressure differential, or lift, is greater than design and refrigerant flows in reverse from the condenser back to the compressor, which can lead to severe chiller damage. Surge is easily identifiable by a loud, distinctive squealing sound, as well as a fluctuation in compressor amp draw.
What causes too much lift?
A number of things can cause too much compressor lift and chiller surge. Surge can often be attributed to common maintenance issues including fouled tubes, low refrigerant charge, or non-condensables in the refrigerant. Fouled condenser tubes are a common problem and will decrease the chiller’s ability to get rid of the heat, causing condenser pressure to increase. This condition will always result in inefficient chiller operation, and severe tube fouling can cause chiller surge.
Not enough load, or low chilled water return temperature, will cause the evaporator pressure to fall below design limits since the compressor is still pulling suction on the refrigerant vapor. If condenser pressure remains constant, the low evaporator pressure will also cause too much lift and surge will occur.
Another cause of surge can be from the chiller itself. A low refrigerant level will cause low evaporator pressure while too much non-compressible gas in the refrigerant will cause high condenser pressure, creating high lift and possible surge conditions.
How do you prevent chiller surge?
The way to protect a chiller from surge is to decrease the compressor lift being required. Chiller tube fouling can be prevented by utilizing automatic tube cleaning systems and water treatment programs to prevent chiller tube fouling and ensure heat transfer is optimized.
Low load issues (surge issues) can be avoided or corrected with variable frequency drives VFDs utilized on the compressor motor to modulate compressor speed during low-load conditions.
Proper refrigerant system maintenance to ensure refrigerant charge is within design limits and incondensable gases are not accumulating in the system will also help prevent surge conditions.
