2000 Hook-up Book

Compressed Air Systems

Closed Circuit Cooling Especially with large compres sors, economies are obtained when the cooling water is recy cled in a closed circuit. This also minimizes any scaling in the jack ets and coolers. The heat may be dissipated at a cooling tower or a mechanical cooler, or sometimes used for space heating in adja cent areas. Usually with closed circuit cool ing, it is preferable to use three-way temperature controls. Where cooling towers are used, freeze protection of the tower sump may be needed in winter conditions. Often a steam heating coil is installed in the sump with a temperature control set to open when the water temperature falls to say 35°F. A three-way temper ature control diverts water direct to the sump instead of to the top of the tower in low temperature conditions. Heat loss from the sump itself then provides suffi cient cooling.

Lubricant Coolers On large reciprocating compres sors, and especially on rotary vane compressors, the lubricat ing oil is usually cooled by passing it through a heat exchanger. Here it gives up heat to cooling water and again the coolant flow should be tempera ture controlled. See Fig. 65.

Compressor Equipment Guide Approx. CFM Air = Compressor HP X 5 GPM Cooling Water =

SYSTEM DESIGN

42.5 X HP/Cylinder 8.33 X Temp. Rise of Cooling Water

Sensor

Bypass if necessary

Strainer

Reverse Acting Temperature Control

Figure 65 Temperature Control of Water to Oil Cooler

Case in Action: Air System Moisture Separation/Drainage

Solution Over 30 separators with drain traps were installed in prob lem areas providing proper drainage of equipment. Benefits • Continuous operation of desiccant dryers, assuring uninterrupted air supply. • Working with dry air, instrument accuracy is more con sistent. • Damage to gauges and other instruments, caused by entrained water, is prevented, reducing maintenance cost. • Air hose stations deliver dry air immediately eliminating delay/inconvenience of having to manually drain water from the hoses before use.

Air is a vital utility for all process plants, primarily to power control valves, measurement devices and to drive tools, pumps, machinery, etc. Outdoor facilities (i.e. refineries and chemical plants) are faced with continual problems related to water accumulation in the air system. Free and compressed air carries varying volumes of water at different pressures and temperatures. This affects the amount of entrained water that must be drained in dif ferent parts of the system for effective operation. Desiccant dryers critical for removing water from com pressed air distributed to instruments and controls become water-logged due to excess moisture entrained in the free air supplied. When this occurs, the dryers shut down, curtailing air for distribution. The heat transfer across the wall of distribution piping creates additional condensing. Moisture, entrained in the air flow, exceeds the capacity of the coalescent filters installed at the point of use. The air-using equipment is then flooded with water, affecting proper operation. This can ruin gauges and instruments and affect control accuracy.

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