The principle of the screw compressor was first patented by Heinrich Krigar in Germany on 24th March 1878.  At that time it was not possible to develop the idea any further because of the lack of manufacturing technology.
Half a century later, a Swedish steam turbine manufacturer called Ljungstroms Angturbin AB appointed a new Chief Engineer, his name was Alf Lysholm and he was to be pivotal in the development of the modern screw compressor.  By this time the original patent rights had expired. Lysholm developed the profile of the screw compressor and tested various configurations and rotor lobe combinations. Not only was the shape of the rotors important, he solved the problem and patented the method for accurately machining the rotors. The 1935 patent clearly shows his asymmetric 5 female – 4 male lobe rotor design, although the shapes have been ‘fine tuned’ over the years, the screw compressor had at last been born.
Ljungstroms Angturbin AB changed its name to Svenska Rotor Maskiner AB in 1951.  This company is well known throughout the world as SRM, and has issued manufacturing licences to almost every screw compressor manufacturer that exists today.
Several compressor manufacturers have since invested in their own R&D programmes to produce new ‘in-house’ profiles, all based upon an initial concept now over 120 years old.
Very soon the screw compressor acquired the reputation of a machine with reliable performance. However, the break-through did not occur until 1960, in conjunction with intensive tests to inject oil into the compression spaces. Injection technology led to the development of the screw compressor without synchronous transmission but with oil injection cooling, where the rotors could counter rotate in a non-contacting manner because of the lubrication provided by the injected oil.
At the present time, the oil injected screw compressor is the most popular representative of compressors in the world market and industry can no longer be imagined without it.
 Screw compressors are equipped with two spindle-shaped mutually-engaging rotors. The number of lobes (or valleys) on the rotors will vary from one compressor manufacturer to another.  As a rule, the main (male) rotor is made with four; the auxiliary (female) rotor with six. The main rotor is driven by either an electric motor or an engine and transforms about 85-90% of the energy received at the coupling into pressure and heat energy.
Because of the number of male lobes, there are four compression cycles per revolution which means that the resulting compressed air has small pulsation’s compared to a reciprocating compressor. By the time that the compressed air leaves the package, it is to all intents and purposes pulsation free.
It is the sole task of the auxiliary rotors to seal the working space between the suction and pressure side. In the course of rotation, main and auxiliary rotors generate a v-shaped space for the air drawn in, which becomes smaller and smaller right up to the end, between the rotor lobes and the cylinder walls.
In the course of compression, the continuing rotation of the rotors closes the air inlet opening and shrinks the volume at increasing pressure.  Simultaneously with this process, oil is injected into the system.  With the outflow, compression is completed and simultaneously the final pressure is reached. The heat energy caused by this is absorbed by the injected oil and conducted to the oil cooler. The additional tasks of the injected oil consist of entraining the non-driven rotor via a lubricating film and through its sealing effect, preventing internal leakages.


Screw compressors draw in air via a suction filter (1) and contamination indicator (2).  After passing through the intake regulator (3), the air reaches the compressor stage (4) and is compressed.

Oil at about 55°C is continuously injected into the compression space in controlled quantities.  This oil has the functions: Cooling, Sealing and Lubricating.  The oil absorbs the heat of compression and reaches about 85°C.  However, the oil also seals the gap between the pairs of runners and the housing and it lubricates bearings and screw flanks, as well as the transmission.

In the combined compressed air/oil reservoir (5), oil and compressed air are mechanically separated using flow diversions.  An oil separator (6) completes the remaining separation.  


Via a minimum pressure non return valve (7), the compressed air then reaches the after-cooler (8) and is cooled down to a temperature of 10-15°C above inlet temperature.  It is subsequently fed into the compressed air system via the shut-off valve (9).

The oil, separated out in the oil separator, is cooled down from 85°C to 55°C in an oil cooler (10) and is returned through an oil filter (12) in order to protect the compression stage from contaminants.  The oil circuit, in addition, contains a thermostatic oil regulation valve (item 11).

The rotary screw air compressor has become the most popular source of compressed air for industrial applications. A major reason is its simple compression concept.
Air enters a sealed chamber where it is trapped between two contra-rotating rotors. As the rotors intermesh, they reduce the volume of trapped air and deliver it compressed to the proper pressure level.  This simple compression concept, with continuous contact cooling, allows the rotary screw air compressor to operate with temperatures approximately one half that generated by a reciprocating compressor. This lower temperature enables the rotary screw air compressor to operate in a “fully loaded” continuous duty cycle 24 hours per day, 365 days per year, if necessary.