In my last article, we have discussed the theory of air compression and the reason for compressing air following an isentropic cycle. Now let’s discuss the actual operation of an air compressor with indicator diagrams.
To understand the operation of an air compressor, let us assume the cycle and indicator diagram for a simple single stage reciprocating air compressor, as shown below. (Click the image to enlarge.)
The simple reciprocating air compressor has a piston which reciprocates inside the cylinder wall and cylinder head. The piston is attached to the crankshaft with the help of a connecting rod and thus the rotation of the crankshaft causes the piston to move up and down inside the cylinder. The crankshaft is mounted on the crank case. The cylinder head contains valve pockets where the suction and delivery valve are fixed.
These suction and delivery valves are of simple pressure differential types. They open and close, due to the pressure difference on either side of the valve plates.
1. When the compressor stops or idles for some time, it is always assumed that there is some residual compressed air left in the cylinder space. This residual air expands when the piston moves down. The pressure drops in the cylinder space at a particular point as the piston moves down, where the pressure inside the cylinder becomes lesser than the atmospheric pressure. Thus this difference in pressure makes the suction or inlet valve open.
2. This opening of inlet valve allows fresh air to be drawn inside the cylinder space as the piston still continues to move in the downward direction. The inlet valve will remain open till there is pressure difference between the atmosphere and inside of the cylinder space. As the pressure difference starts to reduce, the inlet valve starts slowly closing.
The inlet valve closes completely when there is no pressure difference and then the piston reaches bottom dead center (BDC), and it starts to travel in an upward direction. At this position, both the inlet and delivery valve remains closed. Thus as the piston moves up, the pressure starts to build inside the cylinder space.
3. The delivery valve starts to open when there is a pressure difference between the cylinder space and air receiver. Let us assume the air receiver is at a pressure of 7 bar. The delivery valve will not open until the pressure inside the cylinder space is slightly above 7 bar. As the piston moves in upward direction, the pressure increases and at some point the pressure grows beyond 7 bar making the delivery valve open. Thus the compressed air is delivered into the air receiver.
4. As the piston reaches top, the pressure starts to fall and the delivery valve starts to close. The residual compressed air remaining in the space again starts to expand as the piston moves down continuing the next cycle.
Referring to the diagram, the theoretical air compressor P-V diagram can be understood.
The air compressor draws in air from the atmosphere. The atmospheric pressure is P1. The initial volume when the piston is at top is zero (Assuming there is no bumping clearance). Thus as the piston moves from top to bottom, a volume of air V1 is drawn into the compressor. The temperature of air is T1.
As the piston moves up, the air is compressed polytropically (PV^n= C). The pressure of air increases from P1 to P2. The volume decreases from V1 to V2. The temperature increases from T1 to T2.
The compressed air at the pressure P2, volume V2 and temperature T2 is delivered out of the compressor to the air receiver.
Thus we have seen the actual operation of an Air compressor and its theoretical P-V diagram. In my next article, we will discuss the work done by and reasons for using multi-stage air compressors. Also you will come to know some important design reasons that you might have never come across or find in any text books.
Marine Air compressors by Wharton.