Reciprocating compressors are categorised as positive displacement compressors, due to the way they deal with the gas to be compressed. They are equipped with a compression chamber designed to deal with specific amounts of gas, the volume of which is then mechanically reduced during the compression process, increasing the overall pressure inside the chamber. So how do they work? Reciprocators function in a relatively simple manner: the gas first enters through the suction manifold, where it is then passed into the compression cylinder. This is where the compression actually takes place – the gas is compressed by a piston driven in a reciprocating motion by a crankshaft, hence the name reciprocating compressor, and is then discharged.

Like most compressors, reciprocating models have their advantages and disadvantages, and will be suited to a particular set of requirements. Reciprocating compressors are cheap to acquire, install and set up, and have the ability to produce very high pressure – most will on average produce more than 500hp in power and 400 bar pressure. Due to their specific mechanics, they also don’t encounter the issue of oil carry over. However, they do have their limitations. Reciprocators are expensive to maintain and their functioning can lead to unwanted vibration. In addition, they contain a lot of moving parts that carry the potential of breaking in use.

Screw compressors, sometimes also called rotary screw compressors, use a rotary type positive displacement mechanism. They are used primarily for industrial purposes, and are commonly installed to replace piston compressors that are unable to handle large volumes of high pressure in industrial applications. In contrast to piston compressors, which are known for causing high levels of vibration, rotary screw compressors use a continuous sweeping motion, which means they cause very little pulsation when in use.

Screw compressors contain a complex series of mechanisms that are used throughout the compression process. Essentially, they make use of two meshing helical screws, or rotors, which compress the gas. There are two main types that function in different ways: the first, dry running rotary screw compressors, use timing gears to ensure that the male and female elements maintain their precise alignment inside the machine; the second type are oil-flooded rotary screw compressors, and instead use oil as a lubricant to bridge the gap between the male and female rotors. This lubricant provides a hydraulic seal and also transfers mechanical energy between the driving and the driven rotor. The machine relies on the precise alignment of and space between the two rotors, and also the gap between the rotors and the chamber for sealing of the compression cavities. They function by taking in gas at the suction side of the machine, which then moves through the screws as they rotate and is compressed. The meshing rotors force the gas or air through the compressor, which then exits at the end of the screws.

Despite being initially more expensive than some alternative compressors, which is one of their only disadvantages, the screw compressor has a lot to offer. They are known for long life spans when they are well maintained, are designed as quiet machines compared to the vibration-causing reciprocating compressors above, and they’re more efficient. In addition, they can reach very high levels of pressure that cannot be reached by piston compressors.

Scroll compressors are primarily used in air conditioning equipment, however they can also be used for alternative purposes, please ask for more details. Many residential central heat pump and air conditioning systems, as well as some automotive air conditioning systems, employ a scroll compressor instead of the more traditional rotary, reciprocating, and wobble-plate compressors. When used in reverse, these compressors are known an scroll-expanders.

A scroll compressor consists of two scrolls; one moving and one fixed. The fixed scroll is attached to the body of the compressor, and marks out the orbit that the moving scroll must take. The moving scroll is fixed to a crankshaft, and as it moves causes gas pockets to form between the two scrolls. These pockets suck in the gas or air and move it into the centre of the machine, where it is eventually discharged. These internal pockets gradually decrease in size, which causes an increase of pressure within the machine.

Scroll compressors come with plenty of advantages, not least that the absence of pistons enables them to reach 100% volumetric efficiency, which leads to reduced energy costs. In addition, their weight and footprint are significantly smaller than other types of air compressor, including the other two types mentioned in this article. This means they function with much less vibration, which also comes as a consequence of their minimised gas pulsation and their mechanical make-up. Due to the lack of several moving parts that are employed in other types of compressor, this also means they run quieter than most other machines. Scroll-type compressors are also somewhat technically superior to other machines, most notably in the fact that re-expansion losses, a typical feature of each piston stroke encountered in reciprocating models, are eliminated. In addition, due to the lack of suction and discharge valves (ports), losses from these are eliminated altogether.

They do, however, have their disadvantages, most notably that they are not tolerant on rotating both directions in the same way that a reciprocating compressor would be. Moreover, their hermetic design means they cannot be disassembled for maintenance in the event of a breakdown, and when using several together, you may experience problems with incremental capacity control.