Inlet Volume Calculations

There is a great deal of confusion about the terms SCFM and ACFM.

SCFM is measured at standard conditions (68°F, 29.92” Hg or 14.7 psia).

ACFM is measured at actual inlet conditions.

Conversion from SCFM to ACFM and vice versa, is derived from the Ideal Gas Laws specifically Boyle’s Law.

Boyle’s Law states that the volume and pressure of a gas will change in inverse proportion to one another, i.e., if the pressure in a system decreases (higher vacuum) then the volume the gas occupies will increase proportionally according to the following formula:

P1 V1 = P2 V2

In other words, the product of the initial pressure and volume equals the product of the final pressure and volume. When we use this formula in our calculations, the values must be in absolute terms, i.e., “Hg absolute or Torr.

Example 1:

Convert 20 SCFM of air to ACFM at a vacuum level of 25” Hg at sea level.

We first convert 25” Hg gauge to “Hg absolute: P2 = 29.92 - 25 = 4.92” HgA or 125 Torr.

We now use the above formula and fill in the numbers:

29.92 x 20 SCFM = 4.92 x V2 ACFM

V2 = (29.92/4.92) x 20 = 121.6 ACFM

 

Example 2:

The customer has a 200 ACFM pump installed which holds a vacuum level of 22” Hg and they want to increase the vacuum level to 26” Hg.

To calculate the pump capacity required to bring the vacuum to a higher level, we convert the vacuum levels quoted to absolute terms:

  • P1 = 29.92 - 22 = 7.92” HgA
  • P2 = 29.92 - 26 = 3.92” HgA

We use the formula:

7.92 x 200 ACFM = 3.92 x V2 ACFM

Therefore, to increase the vacuum level to 26” Hg the customer would have to double the pump capacity from 200 ACFM to 400 ACFM.

 

Example 3:

This shows the effect of pressure loss in inlet piping and filters on pump capacity. The customer requires a total capacity of 100 SCFM at a vacuum level of 24” Hg at sea level. Inlet line losses, including inlet filters, are 2” Hg. To calculate the actual pump capacity required, we first make the conversion to ACFM based on 24” Hg. P2 = 29.92 - 24 = 5.92” HgA.

29.92 x 100 SCFM = 5.92 x V2 ACFM

V2 = (29.92/5.92) x 100 = 505 ACFM

Without line losses, we would require a vacuum pump sized for 505 ACFM at 24” Hg.

If we look at the effect of the inlet line and filter losses on the required pump capacity, the required capacity increases substantially. First the conversion: P2 = 29.92 - 24 - 2 = 3.92” HgA.

29.92 x 100 SCFM = 3.92 x V2 ACFM

V2 = (29.92/3.92) x 100 = 763 ACFM

This shows that the pump capacity needs to be 33% higher to overcome the 2” pressure drop at 24” Hg.

Note: The above calculations are all based on constant temperatures. If the temperature varies substantially from one condition to another, a correction needs to be made. In this case, contact our factory for more details.

The table below shows the effect of undersized inlet piping and dirty inlet filters on the capacity of a pump at four different inlet pressure drops at a vacuum range from 15-28" Hg at sea level.

Inlet_Volume_Calculations_Graph

 

 

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