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Refrigeration system components performances: reciprocating compressors

Refrigeration system components performances: reciprocating compressors

Selele Mashilo looks at the compressor piston movement and its effect on the compressor as an important part of the refrigeration cycle.

Pressure losses in the refrigerant piping, condenser and evaporator must be considered when drawing the p H diagram. We first consider the compressor as the source of pressure for circulating the refrigerant in the system. The reciprocating compressor is exemplum of understanding compressor performance for the purpose of this article.

Compression cycle
We are going to look at the compressor piston movement in the cylinder for the performance of the total compressor. From my previous article, we know that the compressor comprises the following components: cylinder, piston, big end bearings, small end bearings, and intake and discharge valves. Unlike a car engine where the valves are mechanically operated, refrigerant compressor valves are opened and closed by pressure in the cylinder. Figure 1 indicates the pressure against cylinder volume as the piston moves from its bottom dead centre to top dead centre.


figure 1
Figure 1: Piston in the cylinder.

This figure indicates piston movement between the bottom end and the top end, with its clearance space above the top dead centre. On a downward movement, suction stroke and low pressure vapour refrigerant will be drawn in, filling the cylinder until it reaches the bottom dead centre. The inlet valve will be open and the discharge valve closed. The piston will reach its bottom centre and start an upward stroke, compressing the vapour in the cylinder. The temperature and the pressure will increase. The piston will reach its top centre and, with only clearance space remaining at a very high pressure, the high pressure will be responsible for opening the discharge valve. The suction is already closed when the piston starts an upward movement.

Figure 2 shows the relationship between pressure and volume when the piston moves inside the cylinder. It is assumed that there are no pressure losses in the inlet and outlet valves.

figure 2
Figure 2: Theoretical compression cycle.

We start at point A on figure 2, with the piston starting to compress the refrigerant. At point A, the volume of the cylinder is at its maximum (100%). Point B is where the discharge starts and all the refrigerant is compressed in the clearance volume. Line B C is the discharge process of the refrigerant from the cylinder. The suction valve is still closed, with the discharge valve open. At point C, the piston starts moving down and the pressure decreases, closing the discharge valve.

In the clearance space, the remaining refrigerant starts re expanding. This represents a loss, because not all the refrigerant has been discharged into the system. Line C D represents the expansion. Only at point D — and back at point A where compression starts — does the refrigerant start entering the cylinder.

Reference: ASHRAE

• Catch the rest of Selele Mashilo’s series on refrigeration components in the RACA Journal every month or online at www.hvacronline.co.za

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