API 614 mandates twin coolers. For a compressor in critical service or, even in some cases, an unspared compressor, it is important to be able to switch coolers without requiring a main unit shutdown in the case of a problem. For shell and tube coolers using water, a good design outlet temperature is 120°F. The normal expected input from the compressor is approximately 140°F. A removable bundle design is generally recommended above 5 square feet, though this may vary from plant to plant. The classic API tube size has been 5/8 inch outside diameter with a minimum of 18 BWG tube wall thickness. Again, plant standards may require a larger and heavier tube, which is no problem as the cooler size increases, but causes problems with the smaller sizes. In fact, the API minimum requirement may cause procurement problems and is not generally used on the smaller standard compressor lube systems. Most coolers are of a size which brings them under the ASME pressure vessel code rules. If so, the cooler should bear the ASME pressure vessel code stamp. For all the noise made about it, the extra cost to the vendor is not that severe. Cooler shells, channels, and covers should be steel. Tubes should be inhibited admiralty and the tube sheets should be naval brass, unless the plant standards call for different material, in which case, the vendor must be informed by way of the specification for the equipment. On some packaged standard compressors, the materials of construction are not an option. The materials used should be reviewed to ensure the compatibility with plant operations. If an incompatibility is noted and cannot be changed, the cooler may require changing once the unit is received. The vendor should be fully aware of the plan so as not to void his warranty on the balance of the equipment. If any of this becomes a problem, another vendor should be considered or, if the balance of the equipment is so attractive, it should be owner warranted.

If cooling water is not available, air-cooled exchangers can be furnished, even in relatively small sizes. The cost is higher than a shell and tube-water-cooled exchanger. Also the outlet oil temperature will be higher than that from the water-cooled exchanger. This is no particular problem if the compressor designer is aware of the higher temperature. More oil will have to be circulated to make up for the loss of the temperature differential.

Twin coolers should include a pre-piped vent and orificed fill line to permit filling the idle cooler prior to being put into service. A drain valve, located at the low point of the cooler, should be furnished for both the oil and the water side (if water is used).


While there may be reasons to only use one cooler, filters should be dual on all but the smallest standard compressors. The filter should remove particles to a nominal size of 10 microns. A filtration to a smaller particle size should be considered; however, getting the system clean to that level will take more patience. The transfer between filters will occur frequently during the initial operation period and the dual filters will be found quite handy.

The filter elements should be replaceable and should be corrosion resistant. The filter should not contain any type of internal relief valve that would permit the bypassing of the dirty oil.

The filters should be located downstream of the coolers and should be equipped with a vent and orificed fill line to permit air removal prior to being put into service to prevent shocking the system. External lifts for the filter covers should be furnished if the covers are too heavy for an operator to safely handle. A suggested weight is 35 pounds, deferring however to operator safety. An adequate valved drain should be furnished for each filter body to permit easy removal of dirty oil and sludge.

API mandates that the pressure drop across a clean filter element be no more than 15% of the allowable pressure drop when dirty. An upper limit of 5 psi drop is set for clean filters. This is a reasonable criterion. If a little arithmetic is performed, the head rise for a centrifugal pump may be calculated. The specifications on both of these items must be coordinated and made compatible.

As with the coolers, the larger filter bodies come under the jurisdiction of the AS ME unfired pressure vessel code. If they do, the filter should receive the code stamp. The filter bodies and the heads should be con -structed of steel.

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