There are many ways of detecting air leaks in a compressed-air system.
Perhaps the simplest way of detecting a large air leak is to listen for it. The
high velocity of the air escaping the line produces a hissing soundthat is dif-
ficult not to notice except in environments with a high noise level. Another
way of detecting air leaks, especially small ones, is to test the suspected area
with soap waterand to watch for soap bubbles. This method is obviously not
practical for a large system with many connections. A modern way of check-
ing for air leaks is to use an acoustic leak detector, which consists of a direc-
tional microphone, amplifiers, audio filters, and digital indicators.
A practical way of quantifying the air leaks in a production facility in its
entirety is to conduct a pressure drop test. The test is conducted by stopping
all the operations that use compressed air and by shutting down the compres-
sors and closing the pressure relief valve, which relieves pressure automati-
cally if the compressor is equipped with one. This way, any pressure drop in
the compressed-air lines is due to the cumulative effects of air leaks. The drop
in pressure in the system with time is observed, and the test is conducted until
the pressure drops by an amount that can be measured accurately, usually 0.5
atm. The time it takes for the pressure to drop by this amount is measured,
and the decay of pressure as a function of time is recorded. The total volume
of the compressed-air system, including the compressed-air tanks, the head-
ers, accumulators, and the primary compressed-air lines, is calculated. Ignor-
ing the small lines will make the job easier and will cause the result to be
more conservative. The rate of air leak can be determined using the ideal gas
equation of state.
The amount of mechanical energy wastedas a unit mass of air escapes
through the leaks is equivalent to the actual amount of energy it takes to
compress it, and is determined from Eq. 7–57, modified as (Fig. 7–74)
(7–89)where nis the polytropic compression exponent (n1.4 when the compres-
sion is isentropic and 1 n1.4 when there is intercooling) and hcompis
the compressor efficiency, whose value usually ranges between 0.7 and 0.9.
Using compressible-flow theory (see Chap. 17), it can be shown that
whenever the line pressure is above 2 atm, which is usually the case, the
velocity of air at the leak site must be equal to the local speed of sound.
wcomp,inwreversible comp,in
hcompnRT 1
hcomp 1 n 12¬caP 2
P 1b1 n 1 2>n
1 dChapter 7 | 393Air inlet
1 atm
Air leak (20%)
0.2 m
24 kWmAir
CompressorMotor
120 kW
FIGURE 7–74
The energy wasted as compressed air
escapes through the leaks is equivalent
to the energy it takes to compress it.