While a form of helical-rotor compressor was invented in Germany in 1878 [1], the helical-lobe compressor as used today is credited to Alf Lysholm, the chief engineer of Svenska Rotor Maskiner AB (SRM). Mr. Lysholm conceived the idea in 1934 as part of gas turbine development at SRM. The original compressor was an oil-free design using timing gears to synchronize the rotors. Three male and four female rotor lobes were used, and a steeper helix angle permitted higher built-in compression ratios and improved operation at higher pressures. The pressures were in the 20 to 30 psig range. Unfortunately, the profile created a trapped pocket where the gas was overcompressed prior to being released. This led to lower efficiency and high noise levels. Despite the disadvantages, the compressor was licensed and used in varying applications. Because of Mr. Lysholm's contributions, the helical-lobe compressor is sometimes referred to as the Lysholm compressor. It also goes by the name of SRM. the company controlling the development and licensing.

Hans Nilson became chief engineer of SRM in the late 1940s and later became president of the company. He made numerous contributions to the technical and commercial growth of the compressor, such as the circular profile invented in 1952. The profile used the four male lobe and six female lobe rotors. The design eliminated the trapped pocket, permitting a steeper helix angle. The resulting higher, built-in pressure ratios also improved efficiency.

The next significant event in the evolution of the SRM compressor was the application of a Holroyd rotor cutting machine to the production of the rotors. Prior to this event, producing the rotors was both slow and costly. In 1952, the first special Holroyd machine was delivered to How-den Company, a licensee in the U.K., who later contributed to the development of the oil-flooded compressor.

The slide valve was invented in the early 1950s, giving the SRM compressor a new dimension by providing a means of flow control. Capacity control had been a limiting factor for applications needing a range of flows. The slide valve provided infinite capacity control while still retaining built-in compression during flow reduction. The slide valve became widely used with the advent of the oil-flooded compressor.

The development and subsequent patent of the oil-flooded compressor was the result of a joint effort on the part of Howden and SRM. The oil-

injected prototype was run at SRM on July 4, 1954 and proved to be 8 to 10% belter in performance than the dry compressor with timing gears. Performance at low speeds was improved, permitting the use of direct drive motors. The flooding provided both cooling, which permitted high er pressure ratios and lubrication allowing the elimination of the timing gears. The male rotor drives the female rotor through the oil film. The first commercial application was introduced by Atlas Copco in 1957 for air compression. The slide valve was incorporated into the flooded design in the 1960s and was originally used in refrigerant service. More recently, it has also been incorporated into gas compressor service.

Lars Schibbye became chief engineer of SRM in 1950 and contributed to the technical advancement of the compressor. Most significant was the invention of the asymmetric rotor profile, which was introduced commercially by Sullair in the U.S. in 1969. The asymmetric rotor profile reduces the leakage path area and sealing line length resulting in increased efficiency.

The more recent developments have been in the area of manufacture. Rune Nilson of SRM worked with the machine tool manufacturers to develop precision carbide form cutting tools. These permit the rotors to be cut in two to three passes with high accuracy. The machining time has been reduced significantly.

The development of the helical-lobe compressor is quite unique in that it has been controlled primarily by one company, with the cooperation of licensees who, in turn, provide application expertise [2],

Operating Principles

Another name for the helical-lobe compressor is the screw compressor. This is probably the most common, even though all the names are used quite interchangeably. While the screw compressor originally fit the area between the centrifugal and the reciprocating compressor, the application areas have expanded. The larger, screw type machines now range to 40,000 cfm, and definitely cross into the centrifugal area. The smaller ones, particularly the oil-flooded type, are being considered for automotive air conditioning service, therefore, completely overlapping the reciprocating compressor in volume. The dry variety generally stops in the 50 cfm area.

Compression is achieved by the intermeshing of the male and female rotor. Power is applied to the male rotor and as a lobe of the male rotor starts to move out of mesh with the female rotor a void is created and gas is taken in at the inlet port (see Figure 4-2). As the rotor continues to turn, the intermesh space is increased and gas continues to flow into the




Figure 4-2. The compression cycle of a helical-lobe compressor.


Figure 4-2. The compression cycle of a helical-lobe compressor.

compressor until the entire interlobe space is filled. Continued rotation brings a male lobe into the interlobe space compressing and moving the gas in the direction of the discharge port. The volume of the gas is progressively reduced, increasing the pressure. Further rotation uncovers the discharge port, and the compressed gas starts to flow out of the compressor. Rotation then moves the balance of the trapped gas out while a new charge is drawn into the suction of the unmeshing of a new pair of lobes as the compression cycle begins.

The compressor porting is physically arranged to match the application pressure ratio. To maintain the best efficiency, it is important that the matching be as close as possible.

Figure 4-3 includes four diagrams to show two cases, (A) a low-ratio compressor and (B) a high-ratio compressor. The lower diagram (C) demonstrates a low-volume ratio compressor in a higher-than-design application. Because the gas arriving at the discharge port has not been sufficiently compressed, the resulting negative ratio across the discharge port causes a backflow and resulting loss. Diagram (D) shows a compressor with too high a volume ratio for the process. Here the gas is compressed higher than needed to match the pressure of the gas on the outlet side of the discharge port, resulting in energy waste.

When the design condition matches operational conditions.

When the design condition matches operational conditions.

A VI = Low B VS - High

When the design condition does not match operational conditions

When the design condition does not match operational conditions

Figure 4-3. Effects of low- and high-volume ratios on the cycle of a screw compressor. {Courtesy of Mayekawa Manufacturing Company, Ltd.)

The terms pressure ratio and volume ratio are used interchangeably in the literature on these machines. To prevent confusion, volume ratio rv, is defined as the volume of the trapped gas at the start of the compression cycle divided by the volume of the gas just prior to the opening of the discharge port. Pressure ratio is defined, in Equation 2.64, as the discharge pressure divided by the suction pressure. Their relationship is given in the following equation.

where rp = pressure ratio k = isentropic exponent rv = volume ratio


The displacement of the screw compressor is a function of the interlobe volume and speed. The interlobe volume is a function of rotor profile, diameter, and length. Table 4-1 provides some typical rotor diameters and corresponding L/d ratios. The interlobe volume can be expressed by the following equation.

Qr = displacement per revolution d = rotor diameter L = rotor length

C = typical profile constant, for 4 -I- 6 rotor arrangement C = 2.231 circular profile = 2.055 asymmetric profile [3]


Table 4-1 Rotor Diameters with Available L/d Ratios

Rotor Diameter Inches



Available sizes (X)

Data for table courtesy ofA-C Compressor.

t43l where

Qd - displacement N - compressor speed

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