Most industrial reciprocating compressors are built with two, three, or four banks of cylinders, each bank incorporating either two or four compressors side-by-side m the axial orientation. This arrangement allows a choice of compressor with 4 6, 8, 12, or 16 cylinders. As the cutaway view in Figure 4.18 shows, the pistons are equipped with rings that are lubricated with oil forced through passages in the connecting rods. Spring-loaded safety heads permit the heads to relieve the extreme pressure that might be caused by liquid becoming trapped at the top of the piston. Two bearings support the crankshaft of the compressor of Figure 4.18, while 12- and 16-cylinder compressors are provided with a center bearing as well. The oil pump is driven off the shaft and delivers oil through a filter and oil cooler that is usually water-cooled, but the option is available on some compressors to cool the oil with refrigerant. Both the suction and discharge valves are of the spring-loaded plate type.