THE HISTORY OF SCREW COMPRESSORS
What is a screw compressor and how does it work?
A screw compressor is a positive displacement machine that uses a pair of intermeshing rotors instead of a piston to produce compression. The rotors comprise of helical lobes affixed to a shaft. One rotor is called the male rotor and it will typically have four bulbous lobes*. The other rotor is the female rotor and this has valleys machined into it that match the curvature of the male lobes. Typically the female rotor will have six valleys*. This means that for one revolution of the male rotor, the female rotor will only turn through 240 deg. For the female rotor to complete one cycle, the male rotor will have to rotate 11/2 times.
*The number of lobes on the male and female rotor will vary from one compressor manufacturer to another. However, the female rotor will always have numerically more valleys that the male rotor has lobes. The male rotor is driven by either an electric motor or an engine. 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.
Where did the screw compressor come from?
The principle of the screw compressor was first patented by Heinrich Krigar in Germany on 24th March 1878, patent number 4121. He modified and improved his designs later that year and lodged a second patent (number 7116) on 16th August 1878. Both of these patents are amongst the earliest on record, as the German Patent Office had only been formed just one year earlier.
Heinrich Krigar resided in Hannover and his illustrations clearly show a two lobe rotor assembly, each rotor having the same profile as each other. In fact the rotor configuration resembles the Roots design, exhibited in Europe during 1867, with the exception that the rotors in the Krigar screw compressor twist through an angle of 180 deg along their length.
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 the pivotal in the development of the modern screw compressor. At that time, Lysholm was looking into light weight compressors for gas and steam turbine use.
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.
One of the largest screw air-end manufacturers in Europe is Gutehoffnungshutte, better known as GHH. They currently supply rotary screw air-ends to a variety of well know compressor 'packagers'.
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.