The screw compressor

This is a positive displacement machine used with refrigerants at pressures above atmospheric and available in single or twin rotor form. It is applied in air conditioning for chilling water and is usually, but not always, water-cooled.

(a) The single screw compressor (see Figure 12.10)

The machine comprises a single screw (or main rotor) having six, carefully machined, helical grooves mounted in a cast iron, cylindrical casing with suction pressure at one end and discharge pressure at the other. Two planet wheels (otherwise termed gaterotors), each having eleven teeth and made from a special plastic material, engage in the helical grooves on each side of the main rotor through slots on opposite sides of the rotor casing. Refrigerant gas in the suction chamber is able to enter all six grooves in the screw as far as the tooth of the planet wheel engaging with a groove, on opposite sides of the rotor. Gas cannot pass along the grooves that are not fully engaged with the teeth of the planet wheel because a plate at the discharge end of the casing prevents this. There are, however, two ports in the plate that will permit gas to be discharged from two of the grooves as the rotation of the screw brings them opposite the port openings.

Hot gas

The screw compressor

Gas

Fig. 12.10 Simplified diagram of a single screw compressor.

In the first, suction stage of the operation, the screw rotates and gas from the suction chamber fills part of one of the grooves. As rotation proceeds this volume of suction gas moves along the groove until the engagement of the next tooth on the planet wheel closes off the groove from the suction chamber. This volume of gas in the groove is still at the suction pressure and is enclosed by: the engaging tooth from the planet wheel, the surfaces of the helical groove itself, the cylindrical casing of the screw and the plate at the discharge end of the screw. In the next phase of the operation, further rotation of the screw causes the tooth to move along the helical groove, reducing the volume of the trapped gas and compressing it. Finally, as the other end of the groove comes opposite the opening in the plate at the discharge end of the casing, the trapped gas is delivered at high pressure into a discharge manifold. This process takes place in the helical grooves on both sides of the screw and the manifold receives hot, high pressure gas from both discharge ports. From the above it is seen that the planet wheels separate the high and low pressure regions of the casing as they mesh with the screw.

The screw is driven by an electric motor in a hermetic or semi-hermetic arrangement. The planet wheels idle and virtually no power is lost to them from the screw, except a very small amount by friction. Frictional losses are small because of the natural lubricating property of the plastic material of the planet wheel teeth, their compliance, and the close machining tolerances adopted. A further advantage of the design is that bearing constraints are small and operating lives can be as much as 200 000 hours.

The screw can tolerate some liquid feedback.

A version having only one planet wheel is also used for some applications. Operation is similar.

Lubrication can be by oil injection, its function being to lubricate, cool and seal between high and low pressures, and actuate the capacity unloading mechanism. An oil separator is then needed in the high pressure refrigerant and an oil cooling system is also required.

A favoured alternative is to use liquid refrigerant that is virtually free from oil. (There may be some small amount of oil present because of the need for oil in the bearings.) This is injected into the compressor casing. The advantages are that no oil separator is required and no external oil cooler is needed. Since the frictional losses are very small where the planet wheel teeth engage the screw, the only functions of the injected refrigerant are to seal and to cool.

Economisers are often fitted and volumetric and isentropic efficiencies are high (see Figure 12.11).

The screw compressor

Compression ratio

Fig. 12.11 Comparative, typical volumetric efficiencies for scroll, reciprocating, single screw and

Twin screw compressors.

Capacity control is by means of a slide valve that changes the place where compression begins, resulting in variable compressor displacement. Actuation is from a piston driven by oil pressure or, if an oil-free system is used, by an electric motor. Unloading may be proportional by continuous modulation of the slide valve position, or in steps.

Both water-cooled and air-cooled machines are available and the range of cooling capacities is from about 15 kW to 4500 kW, depending on the operating conditions, whether or not oil is used for lubrication, and the use of water or air for condenser cooling. Low noise and vibration levels are likely because of the absence of valves and the small variations in torque.

(b) The twin screw compressor

This is similar to the single screw but comprises a pair of intermeshing rotors or screws: the male screw has four or five helical lobes and the female screw six or seven helical grooves, in 4 + 6, 5 + 6 or 5 + 7, male-female combinations. As with the single screw machine the process is in three stages: suction, compression and discharge.

The two screws are contained in a compression chamber having a suction opening at one end and a discharge opening at the other. As the screws rotate the space between one pair of male lobes and female grooves is presented to the suction opening. Suction gas flows into the lobe/groove space until the screws have rotated enough for the exposed end of this space to pass beyond the suction opening. The entire length of the lobe/groove space is then full of gas at the suction pressure.

As the rotation of the screws continues the male lobe reduces the volume of the trapped gas in the female groove which, being contained by the plate at the discharge end of the compression chamber, is compressed.

The third phase of the process occurs as rotation continues and the end of the lobe/ groove space starts to pass the discharge port, allowing the hot gas to flow into the discharge manifold. Multiple lobe/groove spaces pass across the suction and discharge openings in rapid sequence and the flow of compressed gas is smooth and continuous, for all practical purposes.

The most common arrangement is for the male rotor to be driven from a two-pole electric motor. The female screw is then driven by the male rotor, being separated from it by a film of oil. Alternatively, the female rotor is driven instead of the male rotor. A less popular method is to drive the female screw through synchronised timing gears from the male screw but this has proved very noisy in early installations.

Rolling contact between the meshing screws minimises frictional wear and volumetric efficiencies are high (see Figure 12.11).

If both rotors are driven, using synchronised timing gears, no oil is needed since there is no contact between the two screws. However, with other arrangements oil is needed for lubrication, sealing and cooling. The oil takes up much of the heat of compression and discharge temperatures are less than 88°C. An oil separator is necessary in the discharge hot gas line and cooling is provided for the oil, either by an external heat exchanger or by the injection of liquid refrigerant into the compression process.

Economisers are sometimes used to improve performance, a secondary suction port introducing gas between the main suction and discharge ports.

Capacity control is by a sliding valve which, as it opens, allows suction gas to escape to the suction side of the screws, without being compressed. This can give smooth proportional control over capacity down to 10 per cent of design duty but some machines are controlled in steps, 100 per cent, 75 per cent, 50 per cent, 25 per cent and 0 per cent being typical. The electrical power absorbed is usually proportional to the refrigeration duty down to about 30 per cent load but thereafter efficiency is poorer, about 20 per cent power being absorbed at 10 per cent duty.

The machines are mostly water-cooled but air-cooling is possible and the range of refrigeration capacities available is from about 120 kW up to more than 2000 kW.

Like the single screw, the twin screw can tolerate some liquid floodback.

Machines are hermetic, semi-hermetic or open and the motors in hermetic or semi­hermetic compressors are best cooled by suction gas. However, one make of machine uses discharge gas for this purpose, thus avoiding the need for an oil separator at discharge. A hermetic machine should not then be used with high condensing temperatures to give heat pumping or heat reclaim. Instead, an open compressor should be used, where the driving motor is air-cooled in a conventional manner.

Posted in Air Conditioning Engineering


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