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THE COMPRESSION CYCLE
Vapor-compressors used in refrigeration are of three principle types: (1) reciprocating, (2) rotary, and (3) centrifugal. Of the three, the reciprocating compressor is by far the one most frequently used.
A compressor, with the piston shown at four points in its travel in the cylinder, is illustrated in Fig. 7.1. As the piston moves downward on the suction stroke, low-pressure vapor from the suction line is drawn into the cylinder through the suction valves. On the upstroke of the piston, the low-pressure vapor is first compressed and then discharged as a high-pressure vapor through the discharge valves into the head of the compressor.
a) b) c) d)
Fig. 7.1. (a) Piston at top dead center. (b) Suction valves open. (c) Piston at bottom dead center. (d) Discharge valves open.
To prevent the piston from striking the valve plate, all reciprocating compressors are designed with a small amount of clearance between the top of the piston and the valve plate when the piston is at the top of its stroke. The volume of this clearance space is called the clearance volume and is the volume of the cylinder when the piston is at top dead center.
Not all the high-pressure vapor will pass out through the discharge valves at the end of the compression stroke. A certain amount will remain in the cylinder in the clearance space between the piston and the valve plate. The vapor which remains in the clearance space at the end of each discharge stroke is called the clearance vapor.
Reference to Figs. 7.2 and 7.3 will help to clarify the operation of the compressor. Figure 7.2 is a time-pressure diagram in which cylinder pressure is plotted against crank position. Figure 7.3 is a theoretical pressure-volume diagram of a typical compression cycle. The lettered points on the TP and PV diagrams correspond to the piston positions as shown in Fig. 7.1.
At point A, the piston is at the top of its stroke, which is known as top dead center. When the piston is at this position, both the suction and discharge valves are closed. The high pressure of the vapor trapped in the clearance space acts upward on the suction valves and holds them closed against the pressure of the suction vapor in the suction line. Because the pressure of the vapor in the head of the compressor is approximately the same as that of the vapor in the clearance volume, the discharge valves are held closed either by their own weight or by light spring loading.
As the piston moves downward on the suction stroke, the high-pressure vapor trapped in the clearence space is allowed to expand.
Fig. 7.2. Theoretical time-pressure diagram Fig 7.3. Pressure-volume diagram of
of compression cycle in which cylinder pressure typical compression cycle.
is plotted against crank position.
The expansion takes place along line А—В so that the pressure in the cylinder decreases as the volume of the clearance vapor increases. When the piston reaches point B, the pressure of the re-expanded clearance vapor in the cylinder becomes slightly less than the pressure of the vapor in the suction line; whereupon the suction valves are forced open by the higher pressure in the suction line and vapor from the suction line flows into the cylinder. The flow of suction vapor into the cylinder begins when the suction valves open at point В and continues until the piston reaches the bottom of its stroke at point C. During the time that the piston is moving from В to C, the cylinder is filled with suction vapor and the pressure in the cylinder remains constant a: the suction pressure. At point C, the suction valves close, usually by spring action, and the compression stroke begins.
The pressure of the vapor in the cylinder increases along line C-D as the piston moves upward on the compression stroke. By the time the piston reaches point D, the pressure of the vapor in the cylinder has been increased until it is higher than the pressure of the vapor in the head of the compressor and the discharge valves are forced open; whereupon the high-pressure vapor passes from the cylinder into the hot gas line through the discharge valves. The flowof the vapor through the discharge valves continues as the piston moves from D to A whilethe pressure in the cylinder remains constant at the discharge pressure. When the piston returns to point A, the compression cycle is completed and the crankshaft of the compressor has rotated one complete revolution.