The difference between reciprocating and rotary screw compressors

There are several types of compressor available and each has its advantages and disadvantages – as well as applications it is more suited to than others. Here is a brief rundown of two of the most common kinds of compressor – reciprocating and rotary screw.

Reciprocating compressors – how do they work?

Reciprocating compressors use a series of crankshaft-driven pistons to take in air and push it out at a higher pressure. A crosshead is used to convert the rotary movement created by the crankshaft into a linear motion to move the piston.

In fact, they work in a very similar way to a combustion engine, but in reciprocating compressors, the gas is not ignited as it would be in an engine. Normally, they will be powered by an electric motor, but it is possible to run them on steam turbines or diesel or natural gas-powered engines.

Rotary screw compressors – how do they work?

Rotary screw compressors have a much simpler design. They are comprised of two helical screws – or rotors – that fit into one another. As the rotors move, air is sucked into the compressor at one end and forced out of it at the other at a higher pressure.

Essentially, the air will flow between the grooves of the screws, becoming compressed as it does so. Once it reaches the discharge valve at the end of the rotors, it is expelled at this higher pressure.

Cost comparisons

Rotary screw compressors can be more expensive to buy than their reciprocating counterparts, but they tend to cost less in maintenance. However, a well-maintained reciprocating compressor can last considerably longer, which means they can be a more cost-effective investment in the long term.

If you are hiring, rather than buying, a compressor for a one-off job, you will be in a better position to choose a model based on its merits for the task in hand, rather than having to factor the initial purchase price and maintenance charges into your decision.

Applications

Both reciprocating and rotary screw compressors can be used to power pneumatic tools, such as the jackhammers and pneumatic drills often seen on construction sites.

Oil-injected rotary screw compressors can be considerably quieter; however, they are designed for continuous operation and are therefore unsuitable for use in situations where they will have long shutdown periods. This sort of compressor is best suited to workshops and factories, where it will be used frequently and can remain powered up.

Most portable compressors used for road works and other temporary construction projects therefore tend to be reciprocating compressors, which are able to operate effectively after periods of not being used.

When choosing a compressor, you need to make sure it can generate sufficient pressure for the job. The majority of applications that require a compressor with a power of more than 30 horse power (HP) and at pressures between 100 and 150 pound force per sq inch gauge (psig) use rotary screw models.

Reciprocating compressors tend to be preferred for jobs needing less than 30 HP and can deliver pressure of up to 175 psig. It is, however, possible to utilise double-acting units if you require a pressure of 250 psig or higher. There is a growing market for rotary compressors in the 10 to 30 HP range, though, with one advantage of this type of model that they are designed to operate at cooler temperatures than their reciprocating counterparts of the same size.