Condensing Pressure Drives Energy Use in Refrigeration Plants
A refrigeration plant can "produce" the same amount of cold working at different levels of condensing pressure. While variations in pressure make little difference for the compressor, they make a big difference to the bottom line: a 2 psig decrease in condensing pressure lowers energy use by 1–2%.
This may not seem like much, but while energy was cheap, industrial refrigeration plants were commonly set to high condensing pressure levels to simplify operation. Today — with controls are cheaper and energy becomes more expensive — the condensing pressure can be lowered by 30-80 psig in most plants, cutting energy use by 15-40%.
As a rule of thumb in Ontario, a mid-size industrial refrigeration system consumes CA$600 worth of electricity per 1 hp of its compressor annually. Cutting 40% of energy use in a 200 hp refrigeration plant will bring CA$40,000-50,000 of direct savings to the bottom line every year.
If Product is Lost, Who Cares about Refrigerator Efficiency?
Cutting energy use is valuable, but only so long as the system fully performs its design functions and works within manufacturer-recommended specifications.
Reducing the energy consumption of a refrigeration system is never as simple as dimming or turning off lights. Along with lowering of the condensing pressure, energy use can be cut by floating condensing and suction pressures, optimizing hot gas defrosting procedure, optimizing the operating sequences of compressors, and other measures.
However, none of these measures is simple or straightforward. A deep understanding of a particular's system design and use and of refrigeration technology in general, as well as a diligent implementation of the required changes, are paramount in cutting costs and maintaining reliable operation.
Theory Meets Real Life
Despite numerous studies outlining the benefits optimization, many industrial refrigeration operators are wary of changing decades-old settings. Typical arguments against changes are as follows:
"This is how we've always worked."
"Our refrigeration maintenance contractor deals with this."
"It works now; why touch it?"
"We will not see savings; so many factors affect energy consumption."
These are all important considerations, but they should not be deal-breakers on the way to efficient operation of a refrigeration plant. All these objections can be overcome through education, cooperation with refrigeration contractors and equipment vendors. Clear communication about the expected results and about the change implementation procedure at different levels within the company is also very important.
Savings in real-life conditions are best calculated using the recommendations of the International Performance Measurement and Verification Protocol (IPMVP). This protocol summarises the best practices in the area of efficiency measurement and verification, including the calculation of savings based on comparison of the properly determined "before" and "after" consumption baselines.
Deep Changes and Automation Bring Deep Cost Reduction
In most industrial refrigeration plants, the condensing pressure can be decreased to below 100 psig, provided some minor technical barriers are overcome. Such pressure reduction would lower consumption by 20–40% with only minor capital investments, if any. The proper installation and setting of variable frequency drives in condenser fans, brine pumps, and circulation pumps would reduce consumption by an additional 10–15%. Prior to utility incentives, the implementation of these changes would have a 6- to 18-month payback.
