A compressor's efficiency is determined by the design of the compressor. The efficiency of a compressor starts with the filling of the cylinder. The following sequence of events take place inside a reciprocating compressor during the pumping action.
A medium-temperature application will be used as an example for the pumping sequence. The refrigerant is R-12 the suction pressure is 20 psig, and the dicharege pressure is 180 psig.
1. Piston At The Top of The Stroke and Starting Down.
When the poston has moved down far enough to create less pressure in the cylinder than is in the suction line, the intake flapper valve will open and the cylinder will start to fill with gas.
A flexible coupling. An extensive procedure is used to obtain the correct shaft aligment. This must be done, or bearing and seal will fail prematurely. Courtesy Lovejoy, Inc.
2. Piston Continues to The Bottom of The Stroke.
At this point the cylinder is nearly as full as it is going to get. There is a very slight time lag at the bottom of the stroke as the crankshaft carries the rod around the bottom of stroke.
An illustration of what happens insisde the reciprocating compressor while it is pumping. When the piston starts down, a low pressure is formed under the suction read valve. When this pressure becomes less than the suction pressure and the valve spring tension, the cylinder will begin to fill. Gas will rush into the cylinder through the suction reed valve.
3. Piston is Starting Up.
The rod throw is past bottom dead-center, and the piston starts up. When the cylinder is a full as it is going to get, the suction flapper valve closes.
When the piston gets near the bottom of stroke, the cylinder is nearly as full as it is going to get. There is a short time lag as the crankshaft circles through bottom dead-center, during which a small amount of gas can still flow into the cylinder.
4. The piston Proceeds to The Top of The Stroke.
When the piston reaches a point that is nearly at the top, the pressure in the cylinder becomes greater than the pressure in the dicharge line. If the discharge pressure is 180 psig, the pressure inside the cylinder may have to reach 190 psig to overcome the discharge valve's weight and spring tension.
When the piston starts back up and gets just off the botttom of the cylinder, the suction valve will have closed, and pressure will begin to build in the cylinder. When the piston gets colose of the top of the cylinder, the pressure will start to approach the pressure in the discharge line. When the pressure inside the cylinder is greater than the perssure on the top side of the discharge reed valve, the valve will open, and the sischarge gas will empty out into the high side of the system.
5. The Piston is At Exactly Top Dead-Center.
This is as close to the top of the head as it can go. There has to be come a certain amount of clearance in the valve assemblies and between the piston and the head, or the would touch. This clearance is known as clearance volume. The piston is going to push as much gas out of the cylinder as time and clearance volume will allow. There will be a small amount of gas left in the clearance volume.
A reciprocating compressor cylinder cannot completely empty because of the clerance volume at the top of the cylinder. Manufactures try to keep this clearance volume to a minimum but cannot completely do away with it.
{source : Refrigeration & air conditioning technology by William C. Whitman,William M. Johnson,John Tomczyk}
