Compressed air driven devices are always given a specification
for the compressed air flow at a certain pressure. For example, an
EXAIR model 1101 Super Air Nozzle has a
specified flow of 14 SCFM at 80 PSIG. This means that when this nozzle
is operated at 80 PSIG, it will require 14 SCFM of compressed air flow.
But what if the force from the nozzle is too high when operated at 80 PSIG
and a lower operating pressure is needed?
Thankfully, we can calculate the compressed air flow at a
different pressure using the absolute
pressure ratio. The absolute pressure ratio says that for
any given change in absolute operating pressure, there will be a proportional
change in the air consumption of a device. So, what is an absolute
pressure?
Put simply, an absolute pressure is the value which you would
measure on pressure gauge plus
the atmospheric pressure (PSIA, or Pounds per Square Inch Atmospheric).
So, our 80 PSIG operating pressure mentioned above is an absolute pressure of
94.5 PSI (80PSIG + 14.5 PSIA). Similarly, if we wanted to determine the
compressed air flow at an operating pressure of 60 PSIG, our absolute
pressure would be 74.5 PSI (60 PSIG + 14.5 PSIA).
The absolute pressure ratio is a ratio of the new absolute
operating pressure (new PSIG + PSIA) compared to the known absolute operating
pressure (known PSIG + PSIA). For example, when comparing an operating
pressure of 60 PSIG to an operating pressure of 80 PSIG, we will end up with
the following ratio:
This means that our absolute pressure ratio in this case is
0.7884. To determine the compressed air flow for the model 1101 Super
Air Nozzle at 60 PSIG, we will take this ratio value and multiply it by the
known flow value at 80 PSIG. This will yield the following:
Utilizing this formula allows us to truly compare a compressed
air powered device at different operating pressures. If we did not use
the absolute pressures when comparing compressed air devices at differing
pressures, our values would be erroneously low, which could yield to improper
compressed air system planning and performance. And, using the absolute
pressure ratio allows anyone to make a true comparison of compressed air
device performance. If specifications are given at different pressures,
performance data can be misleading. But, by using the absolute pressure
ratio we can make a more exact evaluation of device operation.
Lee Evans
Application Engineer
EXAIR - Distributor in Portugal : Tecmer,Lda Contact : José Garcia Tel:+351 252317314 Mobile:+351 932 308 640 Email: geral@tecmer.pt /j.araujo@tecmer.pt www.tecmer.pt |
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