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Rules of Thumb




Power and installation:

  • For every horsepower, a compressor delivers 4-5 cfm (m3/hr), at 100 psi (7 bar) pressure .
  • A 50 HP air compressor can produce up to 12 gallons / 45 liters of water per day (with only 8 running hours daily).
  • Air receivers should be sized about 4 gallon capacity for each CFM of compressor capacity. That's 9 liters per m3/h in metric units.

For example, a small 20 cfm compressor needs an 80 gallon air receiver.

Costs and energy savings:

  • On average, compressed air costs about 30 cents per 1000 cfm (1700 m3/hr). This depends on the size of the compressor and includes electricity, purchase price of compressor and maintenance costs.
  • A 50 HP air compressor will cost you about 25.000 dollars in electricity cost per year (at 6.000 running hours per year).
  • Every 2 psi (0.15 bar) of pressure drop in your system will cost you 1% extra in energy cost (this adds up quickly!!!)
Pressure - Rule of Thumb

  • Lowering Compressor Inlet Air Temperature 10° F (5.5 °C) will result in a 2% energy savings.
  • A two stage reciprocating air compressor is about 15% more efficient compared to a single stage unit.


Temperatures:

  • The typical discharge temperature of a rotary screw compressor (before aftercooler) is: 175°F or 80°C
  • The typical discharge temperature of a single stage reciprocating compressor is: 350°F or 180°C
  • The typical discharge temperature of a two stage reciprocating compressor is: 250°F or 120°C
  • In rotary screw compressors, every 18 °F (or 10 °C) above 200 °F (or 95 °C) reduces the compressor oil life by 50%!


The one rule of thumbs that amazes people the most is the amount of water that an air compressor produces!

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