Often, because compressors are located in a far off corner of the building, air compressors are left to run all night and on weekends. If there are no production activities during these hours, the air compressor(s) will continue to feed leaks and other miscellaneous loads, such as air consuming equipment left on, or air dryer purge. Depending on how many cubic feet per minute these consume, the compressor operating costs can be significant.
If we are talking about a facility that has only one production shift that requires the compressor to be on 10 hours per day, 5 days per week the non-production hours (2,550 hours) represent only about 30% of the total 8,760 hours in a year. And if the facility leakage level is about average, about 30% of total production flow, the annual energy consumption for leaks might be substantial.
Let’s say, for example, about 15 cubic feet per minute for a 25 hp (18 kW) system, this would equate to about 28,000 kWh annually. At $0.10 per kWh this would cost about $2,800 per year in extra energy (larger compressed air systems would cost proportionally more). The extra 6,200 operating hours per year would also add maintenance costs. In systems such as this the off hours production usually equates to 40% to 50% of the total annual compressor energy consumption.
Turning off the compressor during non-productive hours can save these additional costs and help your system last longer. Here are some tips:
- Realize that power during non-productive hours is often less costly than during the daytime, therefore the savings are sometimes not as much as you expect.
- If you have a large storage receiver the pressurization of the plant can take quite a while. Time can often be saved by allowing the receiver to remain pressurized overnight (simply turn off the valve to the plant and allow the compressor to keep it pressurized during off hours).
- Turning the air dryer off during off hours will save additional power if it is a refrigerated style.
- If you have some critical loads that must be pressurized overnight, like HVAC systems or dry charged fire systems, often using a small reciprocating compressor to feed these exclusively can save energy.
- Make sure you keep on top of your leak level, measuring the leakage flow at the end of shift or in early morning is the easiest time.
Another option is a system that does a blowdown of the air distribution system at quitting time, along with then switching off both the compressor and dryer, and closing the valve between accumulator and the distribution system. This saves the energy used to fill that big accumulator and allows an instant startup in the morning.