Rotary Screw Compressors
Figure 9 shows the distribution of screw units.
Undoubtedly the most common form of rotary positive displacement compressor is the helical screw compressor, developed primarily by the Swedish companies SRM and Atlas Copco but now available from a number of manufacturers. The compressive cycle is illustrated in Figure 10.
Rotary screw compressors employ two intermeshing rotors with helical lobes. As the rotors revolve, the space between the unmeshing rotors increases, into which inlet air is admitted. On completion of the filling operation the inlet faces of the two rotors pass the inlet port and the air is sealed in the casing. With continued rotation the flute volume between the two rotors decreases, compressing the air. Compression increases with continued rotation until the built-in pressure ratio is reached and the air is discharged through the delivery port.
Screw compressors can be either oil-flooded or oil-free. Oil-flooded types rely on a substantial volume of oil injected into the compression space. This oil serves a number of purposes: it seals the clearance gaps between the screws and the casing; it lubricates the drive between the male and female screws; and it assists in extraction of the heat of compression.
Dry oil-free types cannot permit a drive to take place between the screws so there has to be external gearing. Some designs incorporate water injection. All-metal screws cannot be adequately lubricated by water so these types also require external gearing. There is an interesting new development in water-lubricated screws in which the screws them selves are made of a ceramic material fastened to a steel shaft. These screws have to be made very accurately in a special mould because only a limited amount of machining after manufacture is possible. They are finally run together in sacrificial bearings to achieve the final shape.
Whether the screws are water or oil-flooded, it will be apparent that in order to minimise leakage losses, the clearance between the screws has to be kept to a minimum at all rotational positions. The generation of the mathematical forms necessary to produce this minimum clearance and the subsequent manufacturing techniques are the secrets of efficient screw compressor technology.
A characteristic of rotary screws is that the power absorbed by the male rotor is about 95% of the total with the remaining 5% absorbed by the female rotor. The female rotors can be looked upon as being primarily rotary valves. Because of this power distribution between the rotors, it is customary to connect the male rotor (through gearing if necessary) to the prime mover. The screws are not designed as driving gears, but 5% is well within their capacity.
Oil-flooded and dry types each have their own advantages and disadvantages. Dry screw units can only have a limited pressure increase per stage (a maximum of about 3:1), otherwise the temperature increase would be excessive. Oil-flooded screws on the other hand can accommodate a normal pressure ratio of 8:1, and often as high as 13:1.
The total leakage flow path is larger than in a corresponding reciprocating compressor. In addition to the clearance between the rotors themselves and between the rotors and the casing, there is also the “blow hole”, which is the triangular hole formed at the intersection of the two rotors and the casing cusp (Figure 11 ). It is not possible to have anything like a piston ring to seal the gaps. The leakage flow is the same whatever the operating frequency (it depends only on the pressure difference) so in order to keep the percentage loss through leakage to a minimum, the rotational frequency has to be high. This is true for both oil-flooded and dry types, but as there is no assistance from the oil in sealing the dry screw, its optimum frequency has to be much higher. The churning of the oil inside the compression chamber represents a power loss and a resultant fall in efficiency. There is an optimum operating speed at which such losses are balanced by losses through leakage. For oil-flooded units, the optimum linear peripheral speed of the screw is about 30 m/s and for dry screws about 100 m/s. For a typical oil-flooded air-end see Figure 12.
When driven from an internal combustion energy or an electric motor, a step-up gear drive (of about 3:1 in the smaller sizes of the oil-flooded types and about 10:1 in the dry types) is required. This is usually separate from the gearing provided to keep the two rotors apart. The latter has to be very carefully manufactured and set: the backlash in the timing gears has to be much less than that between the rotors, which itself has to be kept small to reduce leakage. A technique frequently adopted is to coat the rotors with a thin layer of plastic (PTFE or similar), so that on initial assembly the rotors are tight; after a careful running-in period the coating rubs off locally to form a good seal.
Much effort has been put into the careful design of the rotor profile. Originally rotors were part-circular in shape, but now much more mathematically complex shapes are favoured. Figure 13 shows a widely adopted shape. Note that the lobes are asymmetrical in cross section. This ensures efficient compression with a limited leakage area. Many attempts have been made over the years to produce more efficient meshing profiles. The rewards likely to be obtained through the exercise of such ingenuity are likely to be small, although one of the main motives for producing alternative designs is an attempt to circumvent the strong existing patents.
One feature of the oil-flooded compressor is the necessity for removing the oil after compression. This is done by a combination of mechanical impingement and filtration. The residual oil left in the air after such treatment is very small (of the order of 5 parts per million) – good for most purposes but not for process equipment or breathing. See Figure 14 for a typical system. For further information see the chapter on Compressor Lubrication.