Rules of Thumb in Compressed Air Land

Some rules of thumb to make quick estimates of compressed air related stuff.

It's always handy to know some rules of thumb - it allows you to quickly estimate the size of a compressor or the energy that it will cost to run it, for example.

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!