After my article on the service factor used in Screw air-compressors, it is confirmed that there is a lot of misunderstanding and confusion on the topic of Service Factor. Service Factor when it is used and why it is used.
Therefor, I like to explain by the history of the Screw air-compressor being introduced from the 60ies up to now.
Before the 60ies, the Reciprocating air-compressor was the standard. The most efficient compressor type you can find. But in the sixties, it was not a very reliable and high maintenance demanding machine.
You could compare with your car from those days, they were not to be compared with your current engine in your new limousine you bought last year ! Development, innovation and new materials have made the gasoline and diesel engines super efficient and highly reliable.
A Swedish professor and inventor, Alf Lysholm born 1893, designed the first screw compressor design. He was an inventor working on Steam turbines, gas turbines, jet engines. He had cooperations with Elliott for marine gas turbines, with Siemens, ABB … The screw compressor was initially designed to improve the components of the turbines.
The screw design for air-compressor was licensed to air-compressor manufacturers and
it has formed the basis of many screw compressor organisations such as SRM, Atlas Copco, Howden,…
In historic literature(*) is mentioned : A. Lysholm was an extremely elegant designer who went directly to the essentials. He was often a pioneer of ideas that came through long after.
The Screw Air-compressor, Oil flooded, was a more reliable design than the Piston compressor in that time and was a solution to many factory owners to become more efficient with the production process. Less need to stop production because the compressor had a break down. Less production stops results in higher production efficiency.
Oil-flooded screw manufacturers, however had the challenge to meet the high efficiency of the piston compressors. The volumetric efficiency is lower.
As the Air-compressors are sold on Flow and not on efficiency, the Screw compressor had to meet the flow requirements for a nominal power rating. The Screw air-compressor is a volumetric machine and can therefore be sized to a higher air-flow by running faster. The real power consumed will increase proportional.
In order to hide the lower volumetric efficiency, the Service Factor was used of the electrical motor. By using the SF of the motor, the factory owner would not realise that the screw compressor was less efficient as they show similar or higher flows .
In the beginning of the introduction of Screw air-compressors, most factory service technicians, were very reluctant to switch from piston to screw air-compressors. The technicians understood completely how a Piston compressors works and were able to do maintenance themselves.
The benefits for the factory owners were no production stops anymore and for the compressor manufacturers higher revenues as the screw compressor is cheaper to produce and would generate more aftermarket activities on the screws.
Would you have switched to screw compressor ?
(*)https://sok.riksarkivet.se/SBL/Presentation.aspx?id=9978