Many variations and combinations of cylinder types and arrangements are available from the compressor manufacturers. The compressor manufacturer will generally make its selection based on the most economical combination it has available.
Figure 11-7 is a cut-away of a steeple cylinder. This cylinder design is actually two single-acting cylinders coupled together with different-size pistons on the same piston rod. This arrangement allows two stages of compression on the same compressor throw and is usually used in low capacity, low rod load applications.
Another variation is the tandem cylinder. The tandem cylinder arrangement again allows two stages of compression on the same compressor throw but uses two double-acting cylinders separated by a second
distance piece. This arrangement is usually used in low rod load applications where higher capacity is required.
Figure 11-8 is a cut-away of the latest innovation in compressor cylinder design. In this design, the two suction valves and the two discharge valves are installed inside the compressor cylinder bore. The suction valves are stationary and located at each end of the cylinder. The discharge valves are connected to the piston rod to form the piston; thus the name valve-in-piston design. This design offers the advantages of lower clearances (thus higher efficiencies), reduced sources of fugitive emissions, fewer replacement parts, simpler maintenance procedures, and reduced weight.