Lesson 8 - Gases

Reciprocating compressors also work on the positive displacement principle. There are a variety of reciprocating compressors, but the basis of operation is the same for all of them.

A rotating crankshaft is used to operate a piston. Air is sucked in on the suction stroke and discharged on the discharge stroke. The figure below shows a single-acting air compressor cycle.

Figure: Single-acting air compressor cycle

A piston can also be double-acting, meaning that there are two chambers per piston and each chamber has its own inlet and outlet valves. Thus, when one chamber is discharging the other one is compressing. This means the compressor is always doing useful compression work and not only half the time.

Figure: Double-acting reciprocating compressor piston assembly

Irrespective of whether the pistons are single or double acting, reciprocating compressors can also be single stage or double stage. Double stage means that the output from one piston assembly becomes the input to the next piston assembly. Thus, the air pressure can be increased in stages. It is possible to cool the air down between the stages.

Advantages and disadvantages of reciprocating compressors

Reciprocating compressors offer some advantages:

• They are cheaper than screw compressors in terms of capital cost.

• Their maintenance is simple and can often be done in-house.

The disadvantages are:

• Due to the heat generated during use, reciprocating compressors have a duty cycle. As we saw earlier, that means they are only allowed to operate for a certain time before they need to cool off. Consequently, they either require a large air receiver or more than one air compressor to overcome this.

• They have more wearing parts that need replacement, and over time even the pistons may need replacement due to wear.