2000 Hook-up Book

Compressed Air Systems

Table 20A: Pumped Circulation Water Storage Tanks

Compressor Capacity, cfm free air

25 50

50

100 180

150 270

200 440

300 550

450

600

800

SYSTEM DESIGN Tank Capacity, gallons

100

850 1000 1200

Table 20B: Ratio of Compression

Gauge Pressure psi Ratio of Compression Gauge Pressure psi Ratio of Compression

10

20

30

40

50

60

70

80

1•68

2•36

3•04

3•72

4•40

5•08

5•76

6•44

90

100 7•8

110

120

130

140

150 11•2

200

7•12

8•48

9•16

9•84

10•52

14•6

Table 21: Cooling Water Flow Rates

Water Flow per 100 cfm free air

Compressor operating at 100 psi

Single Stage

1.2 gpm 4.8 gpm 2.4 gpm

Single Stage with Aftercooler

Two Stage

Two Stage with Aftercooler

6 gpm

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 dis charged to waste, and require the use of recirculation systems. When single pass cooling is used, temperature controls will help ensure consumption is mini mized. 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 manufactur ers suggest that the temperature of the water leaving the cylinder jack ets 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 manu facturer where possible. The supply of cooling water can sometimes be taken from the soft ened boiler feed water storage tank. The warmed outlet water then becomes a source of pre-heated makeup water for the boiler.

Compressor Cooling Air cooled compressors, formerly available only in the smaller sizes, are now found with capacities up to 750 cfm, and rated for pres sures up to 200 psi. The cylinders are finned and extra cooling is pro vided 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 con fined space where ambient air temperatures may rise and pre vent adequate cooling. Water cooled compressors have water jackets around the cylinders, and cooling water is cir culated through the jackets. Overcooling is wasteful and cost ly, and can lead to corrosion and wear within the compressor. Temperature control of the cool Larger single-staged compres sors 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. ing water is important. Pumped Circulation

An air velocity of 20 to 30 ft/sec ond or 1200 to 1800 ft/minute, is sufficiently low to avoid excessive pressure loss and to prevent re entrainment 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, esti mate the total volume of system from the receiver stop valve to the tools, including all branches, sep arators, 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

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