Lowering Refrigeration System Operating Costs
Energy SavingsThe largest potential cost savings can be found in electrical energy. While most are not feeling the electrical energy crunch to the same degree as the people of California, if you ask me, it's just a matter of time before you do. The most logical place to look for energy conservation is in the equipment room, where you can use Mother Nature to help lower your head pressure (condensing pressure).
This can be a touchy subject. Many operators will say, "You can't run a plant below 150 or 160 psig." Why? Some might reply, "That's the way I was told to do it." Another operator might say, "I tried it and the coils in the freezer won't defrost!" Still another operator might state, "My refrigerant injection-cooled screw compressor won't operate below that pressure." In the March/April 2001 edition Chill Factor, I suggested some solutions to the defrost problem. Now I'll look at the others.
When you look at Table 1, it's pretty clear why all ammonia refrigeration systems that employ evaporative condensers should operate at the lowest possible discharge pressure; down to 100 psig or even lower if it is within the compressor's operating range.
Note that the BHP/TR on typical high stage compressors drops dramatically from 185 psig down to 100 psig discharge pressure. That means a 300 TR system would have an electrical savings of around 50% under these conditions. And that means your nonpeak season electrical costs would drop dramatically if you allowed your plant discharge pressure to follow the ambient conditions.
I'll break this down into dollars and cents. Assume you operate a pretty typical facility with a 300 TR system. You pay $0.06/kWh and your plant operates about 2,000 hr/yr. If you operate half that time (1,000 hr) at 100 psig instead of 155 psig, you should save about $5,300 each year in power bills. You'll save even more if your peak kilowatt electrical demand is a substantial part of your energy bill because using this technique lowers your peak kilowatt usage.
It's not all that difficult to control evaporative condensers. It can be done fairly easily with a properly programmed PC, saving evaporative condenser fan BHP in addition to compressor BHP.
Predictive MaintenancePredictive maintenance is something many of us have learned about over the past several years. It's a viable approach to extending the life of mechanical equipment, particularly screw compressors and motors. Good maintenance saves energy by keeping refrigeration equipment operating at top efficiency.
For many years, most operators depended on their instincts and ears to predict when a screw compressor's thrust bearings or other parts were about to fail. Of course, this depended upon an operator's experience, which means an expensive equipment breakdown (or two!) must happen before the operator can reliably associate various sounds with impending failure.
Advances in electronic technology have changed all this. Vibration analysis (mechanical analysis) equipment can provide data that, when analyzed by an experienced operator, can predict a fault or potential failure in a screw compressor-motor train. These tests don't take long: About 30 min per compressor/motor are required for a detailed analysis. Best of all, the vibration analysis is made while the equipment is operating -- no need to shut down.
This type of predictive maintenance program works best for screw compressor/motor trains when an initial "footprint" or set of readings are taken and recorded on startup. You may find that the initial readings indicate a problem shipped to you direct from the factory. It's a good idea to set up a program to check the equipment every six months to compare to that original footprint. A trained vibration analysis operator can use this information to predict the potential of component failure before it happens. Repairing equipment before it craters is much less expensive than repairing basket cases.