2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
64
Compressed Air Systems
Table 20A: Pumped Circulation Water Storage Tanks
Compressor Capacity, cfm free air 25 50 100 150 200 300 450 600 800
Tank Capacity, gallons 50 100 180 270 440 550 850 1000 1200
Table 20B: Ratio of Compression
Gauge Pressure psi 10 20 30 40 50 60 70 80
Ratio of Compression 1•68 2•36 3•04 3•72 4•40 5•08 5•76 6•44
Gauge Pressure psi 90 100 110 120 130 140 150 200
Ratio of Compression 7•12 7•8 8•48 9•16 9•84 10•52 11•2 14•6
Table 21: Cooling Water Flow Rates
Compressor operating at 100 psi
Water Flow per
100 cfm free air
Single Stage 1.2 gpm
Single Stage with Aftercooler 4.8 gpm
Two Stage 2.4 gpm
Two Stage with Aftercooler 6 gpm
An air velocity of 20 to 30 ft/second
or 1200 to 1800 ft/minute, is
sufficiently low to avoid excessive
pressure loss and to prevent reentrainment
of precipitated
moisture. In short branches to the
air-using equipment, volocities up
to 60-80 ft/second or 3600-4800
ft/minute are often acceptable.
Checking Leakage Losses
Air line leaks both waste valuable
air and also contribute to pressure
loss in mains by adding useless
load to compressors and mains.
Hand operated drain valves are a
common source of leakage that
can be stopped by using reliable
automatic drain traps. Here is a
simple way of making a rough
check of leakage loss. First, estimate
the total volume of system
from the receiver stop valve to the
tools, including all branches, separators,
etc. Then with no
equipment in use, close the stop
valve and with a stop watch note
the time taken for the pressure in
the system to drop by 15 psi. The
leakage loss per minute is:
Cu. ft. of Free Air
Loss per Minute
=
Volume of System Cu. Ft.
Time in Minutes to Drop
Pressure 15 psi
Compressor Cooling
Air cooled compressors, formerly
available only in the smaller sizes,
are now found with capacities up
to 750 cfm, and rated for pressures
up to 200 psi. The cylinders
are finned and extra cooling is provided
by arranging the flywheel or
a fan to direct a stream of air on to
the cylinder. Such compressors
should not be located in a confined
space where ambient air
temperatures may rise and prevent
adequate cooling.
Water cooled compressors
have water jackets around the
cylinders, and cooling water is circulated
through the jackets.
Overcooling is wasteful and costly,
and can lead to corrosion and
wear within the compressor.
Temperature control of the cooling
water is important.
Pumped Circulation
Larger single-staged compressors
may require a pump to
increase the water velocity when
thermo-siphon circulation is too
slow. The size of the water tank
should be discussed with the
compressor manufacturer, but in
the absence of information, Table
20A can be used as a guide for
compressors operating at up to
100 psi.
Single Pass Cooling
The hook-up shown in Fig. II-103
(page 135) is used where water
from the local supply is passed
directly through the compressor
to be cooled. With increasing
demands on limited water
resources, many water supply
authorities do not permit use of
water in this way, especially
where the warmed water is discharged
to waste, and require the
use of recirculation systems.
When single pass cooling is
used, temperature controls will
help ensure consumption is minimized.
To avoid the sensor control
being in a dead pocket if the control
valve ever closes, a small
bleed valve is arranged to bypass
the control valve. This ensures a
small flow past the sensor at all
times.
Many compressor manufacturers
suggest that the temperature of
the water leaving the cylinder jackets
should be in the range of
95-120°F. Typical water flow rates
needed for compressors are shown
in Table 21, but again, these should
be checked with the manufacturer
where possible.
The supply of cooling water can
sometimes be taken from the softened
boiler feed water storage
tank. The warmed outlet water then
becomes a source of pre-heated
makeup water for the boiler.