The scroll compressor

This is a positive displacement machine that achieves compression in a way that is different from either the reciprocating or screw compressors. The process of compression is accomplished by a pair of scrolls as illustrated in Figure 12.12. Referring to the figure, the scroll having an open end on the left-hand side of sub-figure b is fixed and cannot move. The other scroll has its right-hand end fixed but the remainder is driven by an electric motor to orbit about the centre-line of the fixed scroll. Suction gas enters the two open slots formed by the fixed ends of each of the scrolls (sub-figure b) and, as orbiting commences, a pair of trapped gas volumes is formed (sub-figure c). As the orbiting proceeds these two trapped volumes move radially towards the centre of the scrolls and are compressed. Meanwhile, the open ends of the scroll are again able to admit another pair of volumes of suction gas. For clarity of illustration, Figure 12.12 only shows the radial progress of one pair of trapped gas volumes from suction at the outer slots to compression at the middle and discharge at one end of the centre of the scrolls. It can be seen in the figure that, at the middle of the scrolls, the end of the fixed scroll does not move but the end of the orbiting scroll does, to effect compression. As orbiting continues, a series of pulses of hot gas is discharged at the centre of the scrolls, virtually as a continuous stream of high pressure gas.

There are no valves and fewer moving parts than in the reciprocating compressor. Frictional and other losses are reduced because of the close manufacturing tolerances and the isentropic efficiency (equation (9.19)) is improved. The compressor has a higher volumetric efficiency (see Figure 12.11) and a better coefficient of performance. It is quieter and vibration is less.

Scroll compressors are available in the range of 3 to 50 kW of refrigeration, with air — cooled condensers. Application has been world-wide in residential and commercial air conditioning. Capacity control is usually through an inverter giving variable compressor speeds from 15 to 50 Hz with corresponding variable flowrates of refrigerant. A benefit of this is a soft start for the electric driving motors.

In comparison with air-cooled reciprocating machines, the scroll condensing set has a flatter characteristic performance curve of cooling capacity versus outside air temperature. Most are hermetic and there is virtually no maintenance.

Virtually all split systems now use scroll compressors with variable refrigerant flow through inverter control of the driving motor.

Posted in Air Conditioning Engineering

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