Chiller Systems – Head Pressure Control Information

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This article covers the basics of head pressure controls in chiller systems.

Attached article summary:

Condenser capacities are based, in part, on TD (temper-ature difference) between the ambient and the refriger-ant condensing temperature. As the ambient falls, and the TD increases, the condenser capacity will increase.For example, a condenser rated at 150,000 Btu/h at a110�F and 10�F TD, would have a capacity of 750,000Btu/h with a 50�F TD. In laymen’s terms, it has nowbecome five times larger than it needs to be. An over-sized condenser means lower head pressure, andreduced electrical consumption.

When the actual ambient is below the design ambient, we can take advantage of the now greater condenser capacity, allow the head pressure to fall, and start reaping thebenefits—to a point. Too much of any good thing can become problematic, and reducing head pressure is no exception.

If the head pressure is allowed to fall below certain minimums,system performancecan be adversely affected in the following areas:

1. Underfeeding TEVs (thermostatic expansionvalves) and starving evaporators.

2. Oil logging.

3. Reduced compressor efficiency and higher dis-charge temperatures.

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