Mechanical Details (Reciprocating Air Compressor)

Mechanical Details (Reciprocating Air Compressor) Points : mechanical details (reciprocating air compressor) Shows a simplified section through a single acting air compressor. It will be seen that except for the suction and delivery valves, the construction is similar to any reciprocating engine. Crank is coupled to the output shaft of the prime mover. Flywheel is used to ensure uniform turning moment to be supplied throughout the cycle f operations. Water jackets serve to cool the cylinder and the gas during compression process. The construction of the valves used in reciprocating compressors is explained later in the discussion. In most compressors, the valves are automatic and no valve-operating mechanism is required. Some of the various types of valves used in compressors are reed, plate and thimble, and poppet valves.

(i) Read type valve: This type of valve is shown in Fig. The passage of gas through the valve is shown by arrows. On the compression stroke of the piston, when the pressure developed at A exceeds the pressure in the delivery chamber B the valve plate lifts away from the valve body. These valve plate may be six to eight in number and look like reeds. The gas passes through the opening between the body and the plates uncovered by the plates. To ensure that the valves, plates do not open too far, valve guards are provided. The body also carries an extended portion of a circular cross-section which can be extended in or pulled out slightly. The purpose of this is to vary the clearance volume of the cylinder.

(ii) Plate-type valve: Section through such a valve. The valve plate is kept between the valve seat and the valve guard. The plate is kept is position by a helical coil retaining spring. The side A is communicated to the cylinder and the side B is communicated to the delivery side. When the pressure developed at A due to compressed gas in the cylinder exceeds that at B, the plate is lifted against the spring force and gas flows through the valve as indicated by the arrow in Fig. The valve unit is kept in position by the holdings down screw, which screws into the valve cover plate.

The principle of working of a single acting reciprocating compressor is illustrated.

(a) the piston is at the top dead centre. The suction and discharge valves are closed. The suction valve remains closed due to high pressure of vapour entrapped in the clearance space above the piston and the discharge valve does not open due to its own weight or spring loading. The position of piston is shown by point A on p-v diagram given.

(ii) As the piston moves downward , the pressure of the entrapped vapour decreases, even below than the suction line pressure or evaporator pressure and the suction valve opens. The vapour refrigerant enters the cylinder through suction valve. This stroke of the piston is known as the suction stroke. The reduction of pressure by expansion of entrapped vapour in the clearance space is given by A-B and intake of fresh vapours from the suction line b line BC on p-is diagram. The suction valve opens at point B the piston reaches at C at the end of the suction stroke. Now the suction valve closes usually by spring action. The discharge valve remains closed as before.

(iii) The piston starts moving upward, compressing the vapour refrigerant decreasing its volume, thereby increasing its pressure and temperature. The compression is shown in, by the curve CD. At point D, due to increase in pressure the discharge valve opens and permits the high pressure-high temperature vapours of refrigerant to escape in the discharge line which leads them to condenser. The discharge through the valve is represented by line DA where pressure remains constant. When the piston reaches top dead centre the discharge valve closes due to more pressure in discharge line and less pressure inside the clearance space and also due to the spring loading.