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:
A higher head pressure creates more refrigerant superheat, which is recovered at this facility to heat domestic hot water for flooding and/or showers. 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.
Refrigerant is prevented from flashing prematurely into gas, to maintain system design capacity.
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.
Thus the control objective should be to hold the minimum head pressure required to satisfy the load, while delivering a stream of pure liquid to the TXV and avoiding condenser freezing.
A liquid pressure amplifier (LPA) is a small refrigerant pump which can be placed into the liquid line after the condenser, to boost pressure by an amount sufficient to prevent flash gas formation. With an LPA, the existing controls can be adjusted to operate at a lower head pressure...the TXV will still receive a pure liquid refrigerant stream. The arena operator should then be able to reduce refrigerant head pressure from 165 psig to 120 psig, however the condenser should be inspected regularly in very cold weather to ensure that it does not freeze up. If freezing should begin, the head pressure can be increased slightly to melt the ice and prevent further freeze-up.
As an added benefit, compressors operating at lower discharge pressure will run cooler and require less maintenance.
This is not applicable to ammonia systems. It applies only where an expansion valve is used for refrigerant metering...ammonia systems use a float device similar to a steam trap. For ammonia, consider 'Arena floating head pressure control'.
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).
Pricing of this measure: approx $150 per compressor hp, including all installation and profit.