Compressed air is one of the most expensive uses of energy in a manufacturing plant. About eight horsepower of electricity is used to generate one horsepower of compressed air. The following example represents a typical small industrial air compressor installation. A facility operates a 100 HP air compressor 4,160 hours annually. It runs fully loaded, at 94.5 percent efficiency, 85 percent of the time. It runs unloaded at 25 percent of full load at 90 percent efficiency, 15 percent of the time. The electric rate is $ 0.10 per kWh, including energy and demand costs. The cost per year to power the air compressor will be as follows.
• Fully Loaded $ 27,913
• Unloaded $ 1,293
• Total $ 29,206
Total compressed air generated for one year is 76,377,600 cubic feet. Cost per 1000 cubic feet is $.38.
A common energy conservation measure is 1 PSIG of excess operating pressure increases power consumption by 1/2%.
As an example if your required minimum compressed air operating pressure for your facility is 90 psig, then operating your compressors at 10 psig about that minimum required pressure would be a good starting point for your compressor room. The compressors must produce air at a pressure high enough to overcome pressure losses in the supply system and still meet the minimum operating pressure of the end use equipment. Pressure loss in a properly designed system will be less than 10% of the compressor’s discharge pressure—found on a gauge on the outlet of the compressor.
If you find that you have to operate your plant air pressure at 110 psig to offset issues in your compressed air system your power bill would be 5% higher than required or in our example an added $ 1,460 per year.
Consider installing flow meters
to understand your plant air usage and demand patterns. Flow meters include orifice plates, vortex meters, and thermal mass flow meters. We recommend thermal mass flow meters which do not require pressure or temperature compensation and run about 50% the cost of other types of air flow meters.
Carrying excess pressures can be corrected by examining the following:
• Leaks which can rob large amounts of air. Consider doing a leak test on your system
• Air Drain Traps
which do not operate properly or in the case of timer drains waste large amounts of compressed air.
• Dryers and filters
which are clogged and not maintained at proper intervals.
• Look for inappropriate uses of compressed air at your facility. Instead of using compressed air, use air conditioning or fans to cool electrical cabinets
; use blowers to agitate, aspirate, cool, mix, and inflate packaging; and use low-pressure air for blow guns and air lances. Disconnect the compressed air source from unused equipment.
• If your compressed air system does not have an air receiver tank, add one to buffer short-term demand changes and reduce on/off cycling of the compressor. The tank is sized to the power of the compressor. For example, a 50 HP air compressor needs approximately a 50-gallon air receiver tank.
• Artificial demand is created when an end use is supplied air pressure higher than required for the application. If an application requires 50 psi but is supplied 90 psi, excess compressed air is used. Use pressure regulators
at the end use to minimize artificial demand.
Increasing the efficiency of your compressed air system pays many monetary and environmental benefits.
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