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Time for Pump to Reach Rated Vacuum

Fluids Systems Design and Engineering

Time for Pump to Reach Rated Vacuum

The time a given pump will take to reach its rated vacuum pressure depends on the volume of the system in cubic feet (cubic meters) and the capacity of the pump in scfm (sL/s) at the vacuum-rated pressure. However, simply dividing the system volume by the capacity of the pump does not produce an accurate answer because the vacuum pump does not pump the same quantity of air at different pressures.

A logarithmic relationship can be approximated by the following formula:

T = ( V / Q) N




T = Time, minutes
V = Volume of system, ft3 (m3)
Q = Flow capacity of pump, scfm (sL/s)
N = Natural log constant:

  • For vacuum up to 10 inches of mercury (33.9 kPa),
    N = 1
  • For vacuum up to 22.5 inches of mercury (76.2 kPa),
    N = 2
  • For vacuum up to 26 inches of mercury (88.0 kPa),
    N = 3
  • For vacuum up to 28 inches of mercury (94.8 kPa),
    N = 4

To obtain the most accurate answer, you should obtain pump curves from the manufacturer and substitute the N value for each scfm (sL/s) capacity at increments of 5 inches of mercury (16.9 kPa) and then add the N values to find the total time. Selection of the value for N depends on the highest level of system vacuum pressure and is constant for several calculations.

Reference: ASPE

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