Function and types
Cylinders comprise the working chambers , the compressor valves and the cylinder cooling.
The design of the cylinder is adapted to the specific pumping duty.
The number and size of the working chambers is defined by the thermodynamic function of the compressor. Several working chambers can be fitted within one cylinder.
The arrangement of the cylinder axes influence the design of the crank shaft and the mechanical stresses of the compressor. In a compressor with in line cylinders the axes of the cylinders are parallel and are at right angles to the crankshaft. In general the axes are in a vertical plane, therefore also called a vertical piston arrangement ( fig. a)
If the cylinders are arranged in two opposite groups the compressor is of the boxer type (balanced opposed piston). The axis of the cylinders are then mostly in a horizontal plane ( fig. b).
If the cylinders are arranged in two planes which are at an angle to each other it is called a V design ( fig c).
A V- angle of 90 degrees is favourable for the balance of the mass forces . A special case is the arrangement of both vertical and horizontal cylinder axis (vertical / horizontal design fig. d).
Examples of design
The design of the cylinder depends to a large extent on the valve arrangement. Also the cooling, lubrication and the cylinder pressure are influencing factors.
Example 1: Low pressure compressors
The water cooled cylinder (1) has double walls. The inner cylinder wall carries the liner (2). On its top it is fitted with the cylinder head, the lower part is equipped with a pressure packing (7), which seals the piston rod passage.
The circumference of the inner cylinder wall holds the valve chests (4), into which the valves are fitted. The slots (3) connect the valve chests to the working chamber. The outer cylinder walls have openings, through which the valves can be fitted. The suction and delivery valves of both working chambers have common outlet chambers (5,8).
Example 2 : Highest pressure compressor
The cylinders are manufactured in several layers from impact resistant alloy steel. The innermost layer works as a wear resistant liner. The middle and the outer section are shrunk together. This shrinking results in compression stresses , which counteract the tensional stresses caused by the high working chamber pressure [Nickol, Hefele].
The cylinder lubrication on cross head machines is carried out by lubricators, where the lubrication rate can be adjusted (via adjustable pistons),on plunger type piston compressors via splash lubrication. The amount of lubrication is thereby difficult to control.