Having a compressed air audit done can open your eyes to things that have gone on under your nose for a long time. Basic to a compressed air study is calculating the annual operating cost of the system by taking the measurements collected by system instrumentation (temporary or permanent) and the site utility costs and multiplying by the kilowatt hours consumed.
System costs vary greatly between systems, depending mostly on hours of operation and the electricity rate. A rough rule of thumb would be to take the nameplate horsepower of your compressor and multiply this by 0.85 to get kW consumption when running fully loaded.
Take the result and multiply by 0.35 to get kW when the compressor is running unloaded. Next, find the number of hours the compressor is loaded and unloaded, this is often recorded by a compressor hour meter. Now take kW x hours x electricity rate for each compressor condition to calculate annual electricity cost.
An example: a 25-hp compressor might consume 25 x 0.85 = 21 kW fully loaded. Unloaded power consumption might be 21 x 0.35 = 7.4 kW. If this compressor runs 1,500 hours loaded and 2,700 unloaded per year it would consume 31,500 kWh (21 kW x 1,500 hours) while loaded and 20,000 kWh (7.4 x 2,700 hours) while unloaded for a total of about 51,480 kWh. At 10 cents per kWh, this would cost about $5,100 per year in electricity costs to run.
Typical energy assessments might find between 20% and 50% savings in an average system, so if the system contained one lone 25-hp compressor, the potential savings might be between $1,000 and $2,500 per year on this small system in getting the compressor controlled properly and reducing system waste.
But energy savings are not the most important improvement typically found. Often times, pressure fluctuations, poor air quality, and unreliable compressor operation negatively affect plant production. It is in improving these areas where the biggest savings lie.
By the way, the compressor consumption calculation done for the example shows a compressor control problem where the compressor is consuming about 34 kW per 100 cfm. Normal is about 22 kW per 100 cfm. Simply doing the calculations shows potential savings of up to 38% in correcting this problem. Further savings could be gained by reducing inappropriate uses of compressed air and fixing leaks.
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