3.3.3 - Screw Compressors
The operating principle of screw compressors has been known for more than 100 years. However, it took nearly 75 years until the wide ranging opportunities for this type of compressor were recognised, but this time was also needed to create the technological preconditions of its industrial manufacture. Today, the use of screw compressors is widespread and displaces to an increasing extent the established piston, rotary and other compressor systems. The first prototype of a screw compressor was built in 1943. This was an oil-free compressor with counter- rotating rotors not touching each other. The principal runner drove the auxiliary one via a synchronous transmission in this instance.
Figure 3.3.3.1 The symmetrical circular profile was developed by SRM in 1946. This has been improved several times up to the asymmetrical profile version and is still known as the SRM profile today. In conjunction with the Holroyd company, the first screw compressors with economic application potential were manufactured in March 1955. Very soon the screw compressor acquired the reputation reputation of a machine with reliable reliable performance. However, the break-through did not occur until 1960, in conjunction with intensive tests to inject oil and other liquids into the compression spaces in order to be able to cope with the compression of impure and explosive gases. Injection technology led to the development of the screw compressor without synchronous transmission but with oil injection cooling, where the rotors could counter rotate in a non-contacting manner because of the lubrication provided by the injected oil. Screw compressors are equipped with two spindle shaped mutually engaging ro tors. As a rule, the main rotor is made with four, the auxiliary auxiliary rotor with six lobes, where the main rotor rotor transforms about 85-90% of the energy received at the coupling from the drive motor into pressure and heat energy. It is the sole task of the auxiliary rotors to seal the working space between suction and pressure side. In the course of rotation, main and auxiliary rotors generate a v-shaped space for the air drawn in, which becomes smaller and smaller right up to the end, between the rotor lobes and the cylinder walls. Before the leading chamber s eals the outlet edge, the following chamber reaches this outlet edge.
Figure 3.3.3.2 Fig. 3.3.3.2 shows air intake by suction. The air enters through the inlet opening into the screw grooves of the rotor which are open towards the suction side.
Figure 3.3.3.3 In the course of compression, the continuing rotation of the rotors closes the air inlet opening and shrinks the volume at increasing pressure. Simultaneously with this process, oil is injected into the system.
Figure 3.3.3.4 In this manner, all chambers compress the air to the outlet pressure almost entirely without pulsation. The rapid sequence of filling and evacuating the working spaces create the impression of a pulsation free flow.
Figure 3.3.3.5 With the outflow, compression is completed and simultaneously the final pressure is r eached. The heat energy caused by this is absorbed by the injected oil and conducted to the oil cooler. The additional tasks of the injected oil consist of entraining the non driven rotor via a lubricating film and, through its sealing effect, preventing internal leakages. In single stage form, these screw compressors with oil injection cooling ar e used in the pressure range of up to 13 bar gauge. These compressors are built as compact packages with closed loop oil circulation. During the early years, separating oil from the compressed air presented problems, as the quantity of injected oil was considerable. After many separation experiments with various separator designs, the breakthrough occurred through coarse separation via sufficiently large settling chambers and supplementary fine separation through glassfibre cartridges. Screw compressors as in Fig. 3.3.3.6 draw in air via a suction filter (item 1) and contamination indicator (item 2). After passing through the intake regulator (item 3), the air reaches the compressor stage (item 4) and is compressed. Oil at about 55°C is continuously injected into the compression space in controlled quantities. This oil has the functions: Cooling, sealing and lubricating. The o il absorbs the heat of compression and reaches about 85°C. However, the oil also seals the gap between the pairs of runners and the housing, it lubricates bearings and screw flanks as well as the transmission. In the combined compressed air/oil reservoir (item 5 ), oil and compressed air ar e mechanically separated with a separation efficiency of more than 95%, using flow diversions. An oil separator (item 6) completes the remaining separation. With this system of compression, residual oil values are reduced to about 3-15 mg/m3. Via a minimum pressure non return valve (item 7), the compressed air then reaches the aftercooler (item 8) and is cooled down to a temperature of 10-15°C above inlet temperature. It is subsequently fed into.the compressed air system via the shut-off valve (item 9). The oil separated out in the oil separator is cooled down from 85°C to 55°C in an oil cooler (item 10) and returned. It passes through an oil filter (item 12) in order to protect the compression stage from contaminants. The oil circuit, in addition, contains a thermostatic oil regulation valve (item 11).
Figure 3.3.3.6 At the present time, the oil injected screw compressor is the most popular representative of the compressor species in the world market and industry can no longer be imagined without it. The few moving parts of the screw compressor, which, moreover, solely rotate, have formed the basis for the reputation as a machine of utmost operational reliability. That is not yet all, if one reverses the direction of rotation, the screw machine is an expansion motor of remarkable efficiency. All these factors have contributed to making the screw system a firm part of compressed air generation. Screw compressors are manufactured from small up to medium output volume ranges and thus overlap market areas which, until recently, were still reserved for turbo- compressors. The smaller ranges of screw compressors are supplied in air cooled form whereas the larger ones are constructed either in air cooled or in water cooled versions. All compressor units appearing on the market today, consist of: A drive with the coupled transmission for adaptation to the compressor rotational speed. Regulation and safety circuits in order to automate pressure monitoring. Intercoolers between individual compression stages in the case of multi-stage compression. An additional aftercooler for reducing the final temperature which will include a condensate separator and condensate drain in order to remove the precipitated condensate. In addition, the compressors are mounted on a basic frame and fitted into a sound attenuating box. Today, systems are supplied complete with heat r ecirculating modules arranged in kit form, ready to connect.
Diagram 3.3.3.1 Diagram 3.3.3.1 shows the relation between shaft power input, operating pressure and output quantity on the basis of a variety of performance curves from two national manufacturers of screw compressors systems.
