CINCINNATI -- Energy conservation efforts focused on refrigeration have helped Kroger Co. here save more than $500,000 in electricity costs at approximately 80 stores in the Dallas Kroger Marketing Area during the first half of 1997.
Lowering of head pressures on refrigeration compressors has been the key element in reducing electricity costs, said Keith Oliver, manager and facility engineer for the Dallas KMA, based in Keller, Texas. Refrigeration typically accounts for 50% of electricity costs in stores, Oliver added.
In addition, stepped-up routine maintenance efforts and use of engineering management to audit stores on a periodic basis helped the Kroger stores achieve savings, said Oliver.
Oliver explained that the compressor is the part of the refrigeration system that compresses low pressure refrigerant gas into a high pressure gas, a process that adds heat to the gas. This very hot, high pressure gas is fed into a heat exchanger or condenser, where enough heat is removed to turn the gas into a liquid that is still under high pressure.
When this high pressure liquid is metered into another heat exchanger called an evaporator, pressure on it is dropped sufficiently to change it from a liquid to a saturated liquid/vapor. This liquid/vapor refrigerant boils in the evaporator by taking heat from refrigerated product, ultimately returning to the compressor as a superheated vapor.
"The fact that lowering head pressures reduces electricity consumption is a fairly well-known fact, but it's a process that needs to be managed," said Oliver.
"If head pressures are lowered too far, it can create additional maintenance problems," he explained. "For example, units can short-cycle, where the compressor doesn't run long enough to generate the velocity needed to return oil to the compressor. As a result, the compressor will turn off due to an oil failure."
Other potential problems with lowering head pressures include an imbalance created if outside temperatures drop too low, because outside air is used to condense refrigerant through a condensing unit or coil. Another issue with lower head pressures is problems feeding refrigerant in its liquid state through the system's expansion valve.
"The key is to put head pressures as low as possible without having an inordinate amount of problems. However, our experience has shown that the savings in money and energy will outweigh what would be spent on maintenance and service calls," said Oliver.
He added that a 70-degree condensing temperature "is very achievable, and a good starting place for people who want to take advantage of lower head pressures."