2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
62
Compressed Air Systems
Air Compressors
Heat is released when air or any
gas is compressed. The compressor
must be cooled to avoid
overheating, usually by circuating
water through the jackets.
Cooling is an important function
which must be controlled to
ensure maximum efficiency.
Overcooling wastes water and
leads to condensation within the
cylinders, with deterioration of the
lubricating oils. Undercooling
reduces compressor capacity
and can result in serious damage
to the compressor. Automatic
temperature control of cooling
water flow ensures maximum efficiency.
The atmosphere is a mixture
of air and water vapor. Free air
has a greater volume, and moisture
holding capacity, than
compressed air at the same temperature.
As the compressed air
is cooled after leaving the compressor,
or between stages,
some of the water is precipitated.
This water must be drained from
the system to avoid damage to
pneumatic valves and tools.
Choice Of Drainer Trap
The quantities of water which
must be drained from the air are
relatively small, even on quite
large installations, providing they
are dealt with continuously. It is
unusual to need air traps in sizes
larger than 1/2". Except where a
worn compressor is allowing
lubricating oils to be discharged
with the compressed air, float
operated drainers are the best
choice.
Where the presence in the
system of water/oil emulsions
interferes with the operation of float
drainers, the thermodynamic TD
trap is used. As the TD trap needs
an operating pressure of at least
50 psi when used as an air drainer,
care must be taken when it is used
on small systems. Preferably, the
TD’s should be valved off at startup
until the system pressure is up
to 50 psi or more.
Sizing Compressed Air Traps
The amount of water which is to be discharged is determined from
steam table saturated vapor density or estimated with the help of a
graph, Fig. 63 and compression ratio table. An example shows how this
is used.
Example:
How much water will precipitate from 150 cfm of free air at 70°F and 90%
relative humidity when compressed to 100 psig and cooled to 80°F?
Air flow = 150 cfm X 60 = 9000 cu. ft/hour.
From Fig. 63, at 70°F water in air drawn in will be
1.15 X 9000 X 90% = 9.32 lb/h
1000
Determine excess moisture due to compression by dividing hourly air
flow by factor from Compession Ratio Table 20B (page 64), and convert
for (absolute) temperature.
Compression ratio at 100 psig = 7.8
Air volume after compression = 9000 X (460 + 80) = 1175 cu. ft./h
7.8 (460 + 70)
From Fig. 63, 1000 cu. ft. at 80°F can carry 1.6 lb. of water.
1175 cu. ft. will carry 1175 X 1.6 = 1.88 lb/h
1000
So, (9.32 lb. – 1.88 lb.) = 7.44 lb/h of water will separate out.
Figure 63: Moisture Holding Capacity of Air at Varying Temperatures
Pounds Water Vapor per 1,000 cubic ft. at Saturation
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
.8
.6
.4
.2
0
-20 -10 0 10 20 30 40 50
Air Temp °F
60 70 80 90 100