2000 Hook-up Book

Compressed Air Systems

Air Compressors Heat is released when air or any gas is compressed. The com pressor 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 effi ciency. The atmosphere is a mixture of air and water vapor. Free air has a greater volume, and mois ture holding capacity, than compressed air at the same tem perature. As the compressed air is cooled after leaving the com pressor, 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 start up 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.

SYSTEM DESIGN

Figure 63: Moisture Holding Capacity of Air at Varying Temperatures

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

.8

.6

Pounds Water Vapor per 1,000 cubic ft. at Saturation

.4

.2

0

-20 -10 0 10 20 30 40 50

60 70 80 90 100

Air Temp °F

62