Correction Of Vacuum Pump Capacity For Altitude
All Vacuum Pump performance curves and technical data are referenced to barometric pressure at
Sea level. When operating at higher altitudes, the barometric pressure is always lower, therefore
some calculations are required to correct for this pressure variation. Caution must be taken when specifying a pump at higher altitudes; the amount of vacuum desired cannot exceed the barometric pressure at that altitude.
Since the pump performance of a positive displacement pump is a function of pressure ratio:
P1/P2 at sea level = P1/P2 at altitude
(P2) at sea level = (P1) at sea level (P2) at altitude/(P1) at altitude
Example: Select a vacuum pump for 200 CFM to be installed at an altitude of 2743 meters (approximately 9,000feet), and to operate at a vacuum of 15 in. Hg at that altitude using 60f.
water as service liquid.
Barometric pressure at 2743 meters is 551.5 mm Hg Abs, or 21.7 inch Hg absolute. Therefore an operating pressure of 15 inches at this altitude means an absolute pressure of 21.7 - 15.0 = 6.7 in. Hg Abs
The actual pressure ratio of the unit is:
P1/P2 at altitude = 21.7/6.7 = 3.238
Because pump performance basically is a function of pressure ratio, the selection should be made at the similar pressure ratio at sea level.
29.92/3.238= 9.24" Hg absolute
29.92 - 9.24= 20.68in. Hg vacuum or 9.24/29.92 x 760 = 235mm