How Double Stage Reciprocating Air Compressor Works
Double stage or two-stage reciprocating air compressors consists of two cylinders. One is called a low-pressure cylinder and another is called a high-pressure cylinder.
When the piston in a low-pressure cylinder is at its outer dead center (ODC) the weight of air inside a cylinder is zero (neglecting clearance volume), as the piston moves towards the inner dead center (IDC) pressure falls below atmospheric pressure & suction valves open due to a pressure difference.
The fresh air is drawn into the low-pressure cylinder through an air suction filter. This air is further compressed by the piston and pressure inside & outside the cylinder is equal, at this point suction valves are closed.
As the piston moves towards ODC compression of air took place and when the pressure of air is in a range of 1.5 kg/cm² to 2.5 kg/cm² delivery valves open & this compressed air is then entered into a high-pressure cylinder through the intercooler. This is called low-pressure compression.
If suction & discharge stroke took place on both sides of the piston then it is called Double Acting Low-pressure compression.
Suction valves of a high-pressure cylinder open when the air pressure in a high-pressure side is below the receiver pressure & air from the low-pressure cylinder is drawn into the high-pressure cylinder.
As the piston moves towards the ODC, first stage air is further compressed. When air pressure from the low-pressure cylinder and inside the high-pressure cylinder is equal, suction valves are closed.
Now the air is further compressed by piston until the pressure in the High-Pressure Cylinder exceeds that of the receiver & discharge valves open. This desired high-pressure air is then delivered to a receiver.
The same procedure is repeated in every cycle of operation. If suction & discharge stroke took place on both sides of the piston then it is called double-acting high-pressure compression.
In the double stage, reciprocating air compressor air pressure can be developed in a range of 5.5 kg/cm² to 35 kg/cm².
Normally where we required air pressure above 7.0 kg/cm² & delivery of air above 100 cubic feet/min. this double stage reciprocating air compressor is used.
This is the most common model used in various engineering plants. If we required air pressure above 35 kg/cm², this double stage reciprocating air compressor is not useful.
There you have to use three-stage air compressors.