VACUUM TERMINOLOGY
Atmospheric Pressure: 30 inches column of mercury = 14.7 pounds per square inch = 760 millimeters of absolute pressure.
Torr: Millimeters of mercury absolute. There are 25.4 millimeters to 1 inch.
Micron (MilliTorr): 0.001 of a millimeter. 1/1000th of a millimeter.
Cubic Feet Per Minute (CFM) 1 CFM = 1.7 Cubic Meters per Hour (M3H) = 283 liters per minute (L/M).
Blank Off Pressure: Lowest pressure attainable with a pump having its inlet dosed off connected only to a test gauge.
Factors Determining Type and Size of Pump Required.
 Operating Pressure Required
 Pump down time from initial pressure to final desired pressure.
 Volume of system to be evacuated.
 What is the gas load. Type of gases that will evolve from the process.
The HIGHER the vacuum the LOWER the pressure. Low pressure equals high vacuum. Low vacuum equals high pressure.
Atmospheric Pressure: The pressure of the atmosphere at a specified time and place.
Vacuum Pumps are described by two factors: Pumping Speed and End Vacuum.
Pumping Speed can also be referred to as capacity. This is how much gas can be pumped through the pump. The CRV24 has a pumping speed of 15.2 Cubic Feet per Minute, 24 Cubic Meters per Hour, and 432 liters per minute.
End Vacuum can also be referred to as End Pressure, Ultimate Vacuum, and Blank Off. This is normally measured in Microns (MilliTorr) or Torr. The CRV24 has an ultimate vacuum of .5 Microns, which is .0005 Torr or 5 x 104 Torr.
VACUUM PUMP SELECTION
P1 = Initial Pressure
P2= Final Desired Pressure
T= Maximum Time to reach P2
S= Pumping Speed required to achieve P2 in desired time
V = Volume to be evacuated
SF = System Factor.
Since it is a well known fact that in practice the pumpdown time for a given system deviates from the basic calculations, it is necessary to use what is called a System Factor.
We assign the following factors:
1.2 from 76010 Torr
13 from 101 Torr
1.4 from 10.1 Torr
1.5 from 0.1 0.01 Torr
S = V x Log P1 x SF
T P2
Example 1 Sizing
What model and size Cacejen vacuum pump is required to evacuate a 100 cubic foot chamber from atmosphere to a 1 Torr in a maximum of 20 minutes?
S = 100 cubic feet x log 760 x 13
20 .1
S= 5 x 8.9 x 1.3 = 58 CFM
We select the Cacejen model CRV90 with a pumping speed of 63 CFM.
EFFECT OF TIME AND PRESSURE ON PUMP SELECTION
We know from previous examples that the pumping speed required to evacuate a 100 cubic foot chamber from atmosphere to 0.1 Torr in 20 minutes is 58 CFM.
If the final desired pressure is reduced to ao1 Torr, we show the following:
S = 100 cubic feet x log 760 x 135
20 .01
S = 5 x 11.2x 1.35 = 76 CFN
This lower pressure results in a 31% percent increase in the pumping speed required.
If we can accept a 30 minute pumpdown time of the 100 cubic foot volume to 0.01 Torr, we show the following:
S = 100 cubic feet x log 760 x 135
30 .01
S = 333 x 1L2 x 135 = 50 CFM
This longer allowable pumpdown time results in a 34% reduction in the pumping speed required
CONCLUSION: It is very important to specify only the real desired end pressure and the maximum allowable time. There will be a significant increases or decreases in initial cost and operating costs for even slight changes in pressure and time. Specify only what is really required!
