From workhorse to thoroughbred
The evolution of the screw compressor

For more than 30 years, screw compressors have been a vital part of modern and efficient compressed air systems. As such, they are widely employed in a vast range of applications throughout industry. Yet despite the fact that it is today almost indispensable, the screw compressor has its origins more than a century ago.
Jürgen Hütter
A patent filed as early as 1878 is considered the beginning of the modern screw compressor. In those days, however, it was impossible to manufacture the air-end to the required tolerances. As a result, the first useful screw compressors were built in the early 1930s, and even these were fairly primitive. Such machines were mainly used in plant construction facilities in large drive applications, and operated at no more than 3 bar.

The first commercial successes came in the 1940s and 1950s, when screw compressors gradually gained importance in the process gas sector. Their great advantage was that they could compress and transport gases containing gluey particles, such as the tars found in coke oven gases.

Originally, most screw compressors were non-lubricated “dry runners”. They were not, however, strictly dry: to wash out impurities, water was injected into the compression chamber, which in turn necessitated the use of expensive stainless steel components. Further development resulted in the development of oil-injected compressors as we know them today. Ironically, dry-running compressors still have an important place in our industry, especially for generating oil-free compressed air.

The advantage of these early screw compressors lay in their easy maintenance and potentially lower life cycle costs compared to traditional piston compressors. Unfortunately their great disadvantage was their lack of efficiency. Great precision is required in cutting the rotors, reducing gap sizes and metal porosity in order to minimize performance losses. Initially this was almost impossible to achieve, so screw compressors were less energy-efficient than traditional piston compressors.

A breakthrough in rotor design

Increased performance efficiency of screw compressors went hand in hand with improvements in precision manufacturing. In this respect the geometry of the screw rotor is fundamental. Until the 1970s, screws were based on a symmetric circular arch profile that was easy to design and manufacture. Then, concurrently with the development of special CNC grinding machines for the manufacture of rotors, new asymmetric profiles were developed that increased efficiency enormously. This was the breakthrough that turned screw compressors into thoroughbreds able to replace the workhorse piston compressor.

It was in the early 1970s that Boge began to manufacture its own line of screw compressors. These were refined through the 1980s, and then in 1993 the company launched its landmark S series. These machines were characterized by their horizontal oil separators and the patented GM drive system, which provides optimal belt tension under all conditions and eliminates the need for manual adjustment. An updated S series, introduced to the market in 2003 with motor sizes up to nearly 400 kW, is even more energy-efficient.

Despite the success of medium-sized and large screw compressors, however, piston compressors still dominated the market for machines rated at just a few kW. Around 2000, therefore, the company started engineering a compact range known as the C series that integrates all the main components into a single block. The C series has proved to be highly efficient and ideally suited for compressors down to 2.2 kW.

Control is crucial

Another key difference between pistons and screws lies in control. When screw compressors were first introduced, they were controlled by simple relays. As a result, they wasted a great deal of energy through unnecessary run-on and idling. By the early 1990s, more intelligent control units were available to increase efficiency.

Key to effective control was the introduction of variable-speed drives, without which it is impossible to eliminate idling (no-load operation). With frequency-controlled variable-speed drives, a compressor can now work just as hard as demand requires, maintaining constant pressure but without wasting energy. The latest units, such as Boge’s Basic, Focus and Prime controllers, avoid fixed run-on times and all but eliminate expensive idling through the use of variable speed and proportional control. As a result, they contribute a large part of the energy efficiency of modern screw compressors.

Throughout the development of screw compressor technology, energy efficiency has always been a major goal. Once screw compressors began to equal the efficiency of piston compressors, the latter began to disappear from the industrial market. Commercial dominance does not mean the end of technical development, however, and the quest to develop increasingly efficient compressors will continue. Existing systems will be improved while new systems will be developed. New control systems will become even more powerful and flexible, taking over functions which currently require separate components. Screw compressors have evolved from unsophisticated devices to intelligent, flexible machines that can adapt effectively to any specific compressed air requirements.n