Rules of Thumb

4-5 CFM per horsepower

 

For every horsepower, a compressor delivers 4-5 cfm, at 100 psi pressure.

In other words – a 1 horsepower compressor will output around 4 to 5 cfm at 100 psi pressure. A 10 HP unit will output around 40 to 50 cfm at 100 psi.

If we need a higher pressure, say 125 psi, this rule of thumb doesn’t work anymore – we will need more HP to compressed to 125 psi with the same output. In other words, per HP, the compressor output will be smaller at 125 psi.

If we convert this rule of thumb to SI units, we get something like:

For every 1 kW power, a compressor delivers 2.5 to 3.2l/s

 

1 gallon water per day per compressor horsepower

 

For every horsepower, a compressor generates 1 gallon of water per day (worst case).

A compressor makes compressed air – but it also produces lot’s of water! The water comes from the ambient intake air.

How much water is generated in a compressed air system depends mainly on the relative humidity and temperature of the ambient air.

In this rule of thumb, a day is 24 running hours, and this is really worst-case, like running your compressor in a tropical rain forest for example.

A more typical example:

Ambient temperature: 20 degrees Celsius

Relative humidity:50%

A 45 HP compressor

Runs 24 hours a day

A 45 HP takes in around 7 m3 per minute. At 20 degrees Celsius and a relative humidity of 50%, a quick calculation gives us that this compressor takes in around 23 gallons of water per day.

Not all of that water in condense into liquid. Typically, in a system without air dryer, 1/4 of that will condense into liquid in the after cooler. 2/4 will condense in the piping system (if given time to cool down) and the last 1/4 will stay in the compressed air as water vapor.

In SI units, this rule of thumb would be:

5 liters per day per kW compressor power

Air receivers should be 4 gallons per

CFM compressor capacity

Actually, I would like to say ‘the bigger the better’. A bigger air receiver is ALWAYS better than a smaller one.

A big air receiver helps to keep the compressed air system stable. It acts like a buffer for sudden air consumption spikes. It keeps the pressure more stable, because with a bigger air receiver, it requires more air flow out of the receiver for the same amount of pressure drop.

This makes the compressors start and stop less, which is great for their health. It also helps in removing oil and water from the compressed air (condensation in the air receiver).

But let’s use this rule of thumb as a bare minimum, ok 

Converted to SI units, this rule of thumb becomes:

Air receivers should be 30 liters for every 1 l/s compressor capacity.

or for bigger systems:

Air receivers should be 500 liters for every 1 m3/h compressor capacity.

 

Electricity cost per year is about $0.50 per HP compressor power * 1000

 

This rule of thumb is for a compressor that runs around 6000 hours per year (2 full shifts per day).

For example, lets take a 45 HP /  33 kW air compressor.

If we run it for 6000 hours per year, at 10 cents per kWh, we pay:

33 * 6000 * 0.10 = $ 19,800  in electricity cost per year!

Close enough for a rule of thumb 

Of course, this heavily depends on the compressor size, cost of electricity and of course running hours.

 

2 psi pressure drop costs 1% extra energy

Pressure drop, every compressed air system worst enemy!

Pressure drop is created by resistance in the system between the compressor and the air consumer. It means we have less pressure at the consumer. So we must create a higher pressure at the compressor, to account for the pressure drop.

This costs use extra energy, and therefore money.

As a rule of thumb, 2 psi pressure drop costs 1% extra energy.

In SI units, this rule of thumb would be: every 0.1 bar pressure costs 0.7% extra energy. Not as catchy, I know 

 

Typical discharge temperatures

 

Not really a rule of thumb, but here are some typical discharge air temperatures of industrial air compressors:

  • Oil-injected rotary screw: 175°F or 80°C

  • Oil-free rotary screw: 350°F or 180°C

  • Single-stage piston compressor: 350°F or 180°C

  • Two-stage piston compressor:  250°F or 120°C

Of course, temperatures will vary depending on running conditions like cleanliness of coolers, ambient temperatures, loading times, etc.

 

Oil degradation at high temperature

Oil in rotary screw compressors will degrade quicker when the compressor runs at high temperatures. (read: change your oil more often!).

As a rule of thumb, for every 18 °F above 200 °F, the compressor oil life is reduced by 50%.

In SI units, this is: for every 10 °C above 95 °C the compressor oil life is reduced by 50%.

In other words: if your screw compressor runs at or above 220 °F or 105 °C, you should change your oil at half of the normal running hours!

Here are the TOP TEN Compressed Air "Rules of Thumb" – designed to respond to the questions that industrial air compressor users ask about most.


 

1. CFM delivery per Horse Power at 100 PSIG:

For "home owner" type of air compressors---2 to 2.5 CFM per HP

• For Industrial Air-cooled 2-stage air compressors----3.5 CFM per HP

• For Small Vane & Screw air compressors (25 HP or less) 4 CFM per HP

• For large Piston, Screw & Centrifugal air compressors--4.5 to 5 CFM per HP

 

NOTE: THE MORE CFM PER HP - THE LESS ENERGY USED.

 

2. Air Receiver Size needed for these types of inlet control:

• Modulating Control---------------0 to 1 gallon per CFM

• On-Line/Off-Line-----------------3 to 4 gallons per CFM

• Stop-Start/ Variable Speed-------4 to 6 gallons per CFM

 

NOTE: THE MORE AIR STORAGE - THE LESS ENERGY USED

 

3. Amperage per Horse Power:

115 Volts------- 1 phase-------- 10 amps per horse power

230 Volts------- 1 phase-------- 5 amps per horse power

208 Volts------- 3 phase-------- 3 amps per horse power

230 Volts------- 3 phase-------- 2.5 amps per horse power

460 Volts------- 3 phase-------- 1.25 amps per horse power

574 Volts------- 3 phase-------- 1 amp per horse power

 

NOTE: THE MORE ENERGY EFFICIENT THE MOTOR - THE LESS ENERGY IS USED.

 

4. Air Piping Size by CFM and Pressure Drop:

• Compressor Room Header---0.25 PSIG pressure drop per 100 feet of piping

• Main Line-----------------0.5 PSIG pressure drop per 100 feet of piping

• Loop Line-----------------1 PSIG pressure drop per 100 feet of piping

• Branch Line---------------2 PSIG pressure drop per 100 feet of piping

 

NOTE: THE LESS AIR PRESSURE DROP - THE LESS ENERGY USED

 

5. Size Compressed Air Line Filters to be twice (2x) your compressor CFM flow rate.

• This will lower your pressure drop 2-3 PSIG and save an additional 1% on electrical energy costs.

• Elements will last twice (2x) as long and this can save on your maintenance costs.

 

6. Lowering Compressor Pressure settings 2 PSIG will result in a 1% energy savings.

 

7. Lowering Compressor Inlet Air Temperature 10° F will result in a 2% energy savings.

 

8. The average energy cost to operate an air compressor is approximately $0.10 per horse

power per hour.

 

9. Compressed Air system leaks totaling the size of a 1/4" orifice, at 100 PSIG, running 24 hours a day will waste approximately $15,000 worth of electrical energy a year.

 

10. Using Synthetic Compressor Lubricants can save you up to 9% of the energy cost of operating your compressor as compared to using a non-synthetic lubricant.


 

COMPRESSED AIR DEFINITIONS:

  • CFM= Cubic Feet per Minute  - Volumetric air flow rate.

  • Inlet Pressure = The actual pressure at the inlet flange of the compressor.

  • PSIG= Pounds Per Square Inch Gauge

  • AMPS=Abbreviation of the plural for Ampere, a unit of electrical current

  • Pressure = Force per unit area, measured in pounds per square inch (psi).

  • Pressure Dew Point = For a given pressure, the temperature at which water will begin to condense out of air.

  • Inlet Temperature = The total temperature at the inlet connection of the compressor.