SINGLE ACTING RECIPROCATING COMPRESSORS

SINGLE ACTING RECIPROCATING COMPRESSORS

In industry, reciprocating compressors are of many sizes and designs. Their operation is similar to the bicycle pump described above.
An industrial reciprocating compressor is constructed of metal and has the following main parts :-

1. THE CYLINDER
This is a metal tube-shaped casing (or body), which is generally fitted with a metal lining called a 'cylinder liner'. The liner is replaceable when it becomes worn and inefficient. The cylinder is also fitted with suction and discharge ports which contain special spring loaded valves to allow liquid to flow in one direction only - similar to check valves.

2. THE PISTON
The piston consists of a metal drive rod connected to the piston head which is located inside the cylinder. The piston head is fitted with piston rings to give a seal against the cylinder lining and minimise internal leakage. The other end of the drive rod extends to the outside of the cylinder and is connected to the driver. Modern industry generally used high power electric motors and gearing to convert the rotating motion into a reciprocating action.
In a single acting compressor, the backward stroke of the piston causes a suction which pulls in gas (or air) through the inlet valve. (The same suction action keeps the discharge valve closed). On the forward stroke, the positive pressure generated by the piston, closes the inlet valve and opens the discharge valve. The liquid is displaced into the discharge system. Because the action is positive displacement, a piston compressor can generate very high pressure and therefore MUST NEVER be operated against a closed discharge system valve unless it is fitted with a safety relief system in order to prevent damage to the compressor and/or the driver and/or other downstream equipment.
( See Figure : 14 )


Figure : 14 - Single Acting, Reciprocating Compressor (Simplified)

In the old days of piston pumps, the driver used to be (and still is in some cases), high pressure steam which was fed to drive double acting cylinders by a system of valves in a steam chest – like the driver of an old steam engine. The reciprocating action is converted to rotation to drive the engine wheels.
(See photograph below).

Figure : 15 – Conversion of Rotation to Reciprocation