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Building a compressed air system is a delicate process, where multiple factors must be taken into consideration. In order to create a system that really provides the optimal amount of air and can meet demands requires having a team who truly knows compressed air. That knowledge comes from years of experience and practical applications….and tried and true compressed air formulas.
Since we know how important these formulas are both to our own operations and to your system, we’ve decided to compile some of the most useful and important formulas in one easy-to-find spot: our blog. Right here. That’s right! You’ve found it! All you have to do is keep reading.
Of course, if you have any questions beyond these formulas, our knowledgeable staff is availability to help. Call our main number at 1-800-728-5500 and ask to speak with one of our engineers. We’ll get you to the right person and the answer you need.
Without further ado, here is a list of important compressed air formulas.
Convert scfm to acfm:
acfm = scfm X (𝑃𝑠/𝑃𝑎) X [(𝑇𝑎+460)/(𝑇𝑠+460)]
Where:
- Ps = Standard pressure, psia (CAGI & ISO use 14.5 psia)
- Pa = Atmospheric pressure, psia
- Ta = Atmospheric Temperature, °F
- Ts = Standard Temperature, °F (CAGI and ISO use 68 °F)
Cost of Compressed Air Formula (At full load)
Cost = (𝑚𝑜𝑡𝑜𝑟𝑏ℎ𝑝 𝑋 0.746 𝑋 𝐻𝑟 𝑜𝑓 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑋 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑟𝑎𝑡𝑒) /𝑀𝑜𝑡𝑜𝑟 𝐸𝑓𝑓𝑖𝑐𝑖𝑐𝑒𝑛𝑦
Where:
- Cost = dollars per year
- Hours of Operation = one year of use
- Electric Rate = dollars/ kwh
Note: Motor BHP May be Higher than the Name plate HP (Generally 110%)
Belt Drive Equations:
Compressor RPM = (Motor Pulley Pitch Diameter X Motor RPM)/ Compressor Pulley Pitch Diameter
Motor Pulley Pitch Diameter = compressor pulley pitch diameter x compressor RPM / motor RPM
Compressor Pulley Pitch diameter = motor pulley pitch diameter x motor RPM / compressor RPM
Motor RPM = compressor pulley pitch diameter x compressor RPM / motor pulley pitch diameter
Note: Pitch diameter is slightly smaller than the pulley outside diameter (about where the middle of the belt rides). To measure the pitch diameter, subtract from OD:
| Belt Section | Approx. Pitch Diameter |
| A | OD – 0.25” |
| B | OD – 0.35” |
| C | OD – 0.40” |
| D | OD – 0.60” |
| E | OD – 0.80” |
Raising System Pressure:
C2 = C1 X (P2 + 14.7) / (P1 + 14.7)
Where:
- C2 = Required CFM to raise system pressure
- C1 = Existing or known CFM
- P2 = Desired Pressure
- P1 = Known Pressure
Pressure Drop:
dp = (7.57q^1.85L 10^4) / (d^5p)
Where:
- dp = Pressure Drop (kg/cm^2)
- q = Air Volume Flow at atmospheric conditions (m^3/min)
- L = Length of pipe (m)
- d = Inside Diameter of pipe (mm)
- p = Initial Pressure – gauge (kg/cm^2)
Unit conversion may be required. The following units are all equivalent values:
- 1 kg/cm^2
- 98068 Pa
- 0.98 bar
- 763 mm Hg
- 10000 mm H2O
- 2050 psf
- 14.2 psi
- 29 in Hg
- 394 in H2O
- 32.8 ft H2O
Air Storage in a Tank (Standard Cubic Feet):
𝑆𝐶𝐹 = [(𝑃2−𝑃1)𝑥𝐺𝑎𝑙] / [7.48 𝑥 14.5]
Where:
- P2 = final gauge pressure
- P1 = Initial Gauge Pressure
- Gal = tank size in gallons
Rule of thumb: For every 1 PSIG reduction in discharge air pressure, Compressor BHP (Brake horsepower) goes down 0.5%.
Definitions: PSIG means pounds per square inch, GAGE pressure. Gage pressure is the absolute pressure of something, with the atmospheric pressure subtracted. In practice, when someone gives a pressure in just “psi” they probably mean gage pressure. If they mean absolute, they should be using “psia.”
Note: This information is provided as a quick reference resource and is not intended to serve as a substitute for qualified engineering assistance. While every effort has been made to ensure the accuracy of this information, errors can occur. It is not the responsibility of CASCO USA should damage, injury, or misapplication occur as a result of using this reference guide
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