Arenas > Floating Head Pressure Control

Existing Conditions

The arena refrigeration plant presently operates to maintain relatively high condenser ("head") pressure at all times. High head pressure results in increased energy consumption at the compressors, since they have to work hard to overcome the greater pressure difference between the suction and discharge sides. Any method to reduce head pressure will save electrical consumption. Condenser fans run more under reduced head pressure control, but this increase is small compared to the compressor savings.

Although it uses more energy, high head pressure operation has its benefits:

1.	Refrigerant is prevented from flashing prematurely into gas, to maintain system design capacity.

2.	Higher head pressure results in a higher condenser temperature. This is desirable in very cold weather to prevent freezing of the evaporative condenser, since ice build-up can reduce heat rejection capacity.

3.	A higher head pressure creates more refrigerant superheat, which is recovered at this facility to heat domestic hot water for flooding and/or showers.

In reference to the the third point above, an analysis of the refrigeration cycle shows that head pressure should not be kept high just for flood water heating. Where adequate supplementary heat exists, the added electrical cost at the compressors is approximately double the cost of heating the same water using natural gas.

Thus the control objective should be to hold the minimum head pressure required to satisfy the load, while avoiding condenser freezing.

Retrofit Conditions

We recommend installing a small unitary controller programmed to automatically adjust head pressure settings based on brine loop, ice surface and outside air temperatures. Head pressure should be reset based on outside air temperature and should vary between 120 psig and 170 psig. We also recommend installing an infrared ice temperature sensor to fine tune the control of the brine temperature.

Commissioning of the controls is critical to ensure that the reset schedule is set as low as possible without causing condenser freeze-up or other operational problems. If the reset schedule is too high, the estimated savings will not be achieved. If the reset schedule is too low, the arena operators will likely bypass the system, and no savings will be achieved.

The controls should be set such that the condenser fan operates as the first stage in very cold weather, to minimize the freeze-up risk. If freezing should begin, the head pressure setting can be increased slightly to avoid further ice buildup. In summer, the condenser water pump should operate as the first stage to minimize scale buildup on the heat exchange surfaces.

This measure will provide another benefit: compressors operating at a lower discharge pressure will run cooler and require less maintenance.

Further Benefits

Application Details

Issues and Concerns

"Flash gas" is incapable of absorbing heat and becomes detrimental to the refrigeration process. For example, if a saturated liquid refrigerant at 100ºF were comprised of just 2% flash gas by weight, the flash gas would account for 27% of its volume indicating a 27% loss in cooling capacity. Therefore it is important to maintain sufficient pressure to keep the refrigerant in liquid form until after it passes through the thermal expansion valve (TXV).

References

Analysis

This is applicable only to ammonia systems. It does not apply where an expansion valve is used for refrigerant metering. For Freon refrigerants, consider "Arena liquid pressure amplifier". Pricing of this measure: approx $8000 per refrigeration plant, including installation.