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A Closer Look
Vacuum Application:
Pump Down of Closed Systems
In general, there are two types of vacuum system applications – “cyclic” and “continuous vacuum”.
An application involving the repeated pump-down of a closed system from atmospheric pressure
(or some other initial pressure) to a specified vacuum over a course of time is called a cyclic application.
In contrast, the type of vacuum application where there is a steady flow of air coming in from the surrounding
atmospheric environment is called a continuous vacuum application. The focus of this Closer Look is to
explain how to calculate the vacuum requirements for closed loop, cyclic applications.
Some examples of cyclic applications include vacuum forming of plastics, vacuum heat-treating of metallic parts,
and vacuum filing containers.
When selecting vacuum pumps for cyclic applications, one of the most important parameters is to consider the
effects of time. For example, to evacuate a closed system with a volume of 300 ft3 to 25 torr in one minute
would require a vacuum system sized for approximately 75 H.P. To evacuate the same system to the same pressure
in 10 seconds would require about 400 H.P. worth of vacuum pumps. Not only are the first costs for the latter
design considerably more expensive, the long-term power costs are also over 4 times greater.
Conversely, to convert the same system to the same pressure in 5 minutes would require only a 5 H.P. vacuum pump.
It is apparent that understanding how to design cyclic vacuum applications can have a significant impact
on bottom line profits.
The formula to determine vacuum pump sizing for a cyclic application is:
S = 2.3 x (V / T) x log (P1 / P2), where:
S = Pumping Speed (capacity in ACFM)
V = Volume of the system
T = Pump-down time
P1 / P2 = Initial pressure and target pressure.
Or
T = 2.3 x (V / S) x log (P1 / P2) (solve for time)
Example:
A closed system, with a volume of 3000 gallons needs to be evacuated from atmospheric pressure to 28.5” HgV in
20 seconds. What size vacuum pump is needed to do the job?
Solving the above equation for S, the inputs are:
V = 3000 gallons = 401 ft3
T = 20 seconds = 0.333 minutes
P1 = 14.7 PSIA = 29.92”HgA
P2 = 29.92 – 28.5 HgV = 1.42” HgA(1)
(1) – The term HgV means inches of mercury vacuum. The “g” stands for gage pressure.
This calculation must be done in absolute pressure. Even at 28.5”HgV, there is still a positive
pressure in the system of 1.42”HgA.
The calculation follows:
S = 2.3 x (401 / 0.333) x log (29.92 / 1.42)
S = 3,657 ACFM
The final answer of 3,657 ACFM (Actual Cubic Feet per Minute) requires a vacuum pump of
approximately 250 H.P., which is a rather large pump. To reduce first cost and operating expenses,
the end user may want to consider extending the pump down time to 1 minute, instead of 20 seconds.
The new pumping speed is now 1215 ACFM, which requires about a 75 H.P. vacuum pump,
instead of a 250 H.P. unit!
It is apparent that proper understanding of vacuum system technology and design can have a significant
impact on bottom line profits. Contact your Fluid Energy representative before embarking on a vacuum
system project, or if you would like to investigate energy saving opportunities with your current system.
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