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Technology, Energy Efficiency

Speaking the Same Industrial Language: Understanding a Compressor’s Common Units of Measurement

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Working with air compressors requires understanding its units of measurement – from sizing your compressor to calculating your energy costs and everything in between. Today on Speaking the Same Industrial Language, we’ll get back to the basics and break down some of the most common units of measurement and explain what exactly they mean.

Unit of Measurement What It Means
kW (kilowatt) A measure of power equal to 1,000 watts
kWh (kilowatt hour) A measure of energy equal to the power consumption of 1,000 watts over 1 hour
CFM (cubic feet per minute) The rate at which a certain volume of air is delivered at a certain pressure valve
SCFM (standard cubic feet per minute) Determines weight of air based on different fixed reference conditions:
  • 14.7 Pounds per Square Inch (psi)
  • 60 Degrees Fahrenheit (°F)
  • 0% Relative Humidity (RH)

OR

  • 14.7 Pounds per Square Inch (psi)
  • 68 Degrees Fahrenheit (°F)
  • 0% Relative Humidity (RH)

OR

  • 14.7 Pounds per Square Inch (psi)
  • 68 Degrees Fahrenheit (°F)
  • 36% Relative Humidity (RH)
ACFM (actual cubic feet per minute) Compressors are normally rated in ACFM, which is measured at the delivery point of prevailing ambient temperatures
HP (horsepower) A measure of the amount of power the compressor’s motor can supply
BHP (brake horsepower) A measure of the motor’s power without any losses from the auxiliary engine components
PSI (pounds per square inch) A measure of pressure resulting from a one-pound force applied to one square inch
PSIA (pounds per square inch absolute) A measure of ambient barometric pressure as it varies with altitude and weather
PSIG (pounds per square inch gauge) A measure of gauge pressure relative to ambient or atmospheric pressure

 

Now that we’re familiar with an air compressor’s common units of measurement, we can see how they’re used to calculate things like the cost of annual energy consumption.

Calculating the cost of annual energy consumption:

Cost ($) =       (bhp) x (0.746) x (operating hours) x ($/kWh) x (% time) x (% full load bhp)                                                                                                                                    Motor Efficiency

Where:

  • bhp = compressor shaft horsepower
  • Operating hours = number of hours your compressor operates
  • 1 hp = 0.746 kW
  • $/kWh = electricity rate
  • percent time = percentage of time running at this operating level
  • motor efficiency = motor efficiency at this operating level

Let’s take a look at a look at an example offered by the Department of Energy:

A typical manufacturing facility has a 200 hp compressor (which requires 215 bhp) that operates for 6800 hours annually. It is fully loaded 85% of the time (motor efficiency = 95%) and unloaded the rest of the time (25% full-load bhp and motor efficiency = 90%). The aggregate electric rate is $0.05/kWh.

Cost when fully loaded = 

[(215 bhp) x (0.746) x (6800 hrs) x ($0.05/kWh) x (0.85) x (1.0)] / 0.95 = $48,792

Cost when partially loaded = 

[(215 bhp) x (0.746) x (6800 hrs) x ($0.05/kWh) x (0.15) x (0.25)] / 0.90 = $2,272                        

Annual energy cost = $48,792 + $2,272 = $51,064 

Keep in mind that in order to meet optimal efficiency and conserve energy, only the minimum number and size of compressors should be used to meet the required capacity and pressure of your applications.

 

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