RULES OF THUMB
► One horsepower costs $1.00 per 24 hour period (based on 5¢/kWh).
► One brake horsepower drawn will cost approximately $300/year in energy (based on 5¢/kWh).
► Each 2 psi of pressure drop increases the compressor’s horsepower load by 1 percent. Each 2% of pressure drop saves 1% in energy cost.
► On average, first year energy costs exceed the purchase price of an air compressor.
► At 80°F ambient and 80 % relative humidity, a 100 hp compressor will take in 100 gallons of water vapor per day.
► Rotary screw compressors with an after cooler discharge compressed air at approximately 100°F above ambient temperature.
► Single stage reciprocating compressors put out 100 psig air at about 500°F. Add 2° F for each additional psig adds 1°F.
► Two-stage reciprocating compressors are cooler; discharge temperatures are about 250°F and each additional psig adds 1°F.
► Most water-cooled after coolers use 3 gallons of cooling water per minute to cool 100 scfm of air at 100 psig. A well designed after cooler removes 70% of the moisture and reduces the temp to 15° – 20° above ambient.
► Leaks are expensive: in a 100 psig system, a 1/16” hole costs $600 per year, a 1/8” hole costs $3,000 per year, ¼” hole adds $12,000 and a 3/8” hole adds $26,000 to your cost of compression!
► The average air compressor produces 4-5 cfm for every horsepower consumed.
► For every 20° F you decrease the temperature of saturated air, you will cut the moisture content in half.
► A 200 HP compressor will generate 2.3 gallons per hour, 55 gallons per day, with an inlet air temperature of 60 degrees F
and 40 percent humidity. The same compressor will produce 11.2 gallons per hour, 263 gallons per day, with an inlet air
temperature of 90 degrees F and 70 percent humidity.
► Every 2 psi pressure drop costs 1% of compressor horsepower in losses.
► Size air receivers for about 1 gallon of capacity for each CFM of compressor capacity.
► Compressor discharge temperatures are a key indicator of compression efficiency. Typical values before after cooling are: Screw (175°F), Single Stage Recip (350°F), Two Stage recip (250°F).
► Water-cooled after coolers require about 3 GPM per 100 CFM, and will produce about 20 gallons of condensate per day.
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