Adiabatic compression is that compression where no heat is transferred to or from the gas during the compression process.
A thermodynamic process in which there is no heat transfer into or out of a system is called an adiabatic process. To perform an ideal adiabatic process, a system would have to be surrounded by a perfect heat insulator. If a compression or expansion of a gas takes place in a short time, it can be considered a nearly adiabatic, such as the compression stroke of a gasoline or a diesel engine.
Adiabatic compression is isentropic (i.e., the entropy remains constant). If temperature–entropy diagrams are available for the gas involved, the theoretical discharge temperature can be read directly.
In an adiabatic process, change is occurring within a system because of transfer of energy to or from the system in the form of work only; i.e., no heat is transferred.
The adiabatic process obeys the relation:
We can put this into words by saying that pressure (p) times the volume (V) raised to the power of k, which is the ratio of specific heats is equal to a constant, C. (Ratio of specific heats is defined as the specific heat at constant pressure divided by the specific heat at constant volume, which is sometimes referred to as the adiabatic index or the heat capacity ratio.)
To get a clearer idea what adiabatic compression is, let’s consider a simple example: Consider a piston cylinder in a reciprocating compressor. As we push the piston from point B to C, the volume of the cylinder decreases, causing the pressure to increase.
Pressure versus stroke diagram for a reciprocating compressor.
This is illustrated in pressure vs. volume graph aside. Since the piston is moving at a high rate of speed, there is likely very little heat added or removed from the compressor cylinder during the compression process. In this example, we can conclude this situation approximates an adiabatic process. For similar reasons, we usually assume positive displacing compressors, such as reciprocating and screw compressors, can be reasonably described as an adiabatic process.
One useful calculation for predicting the discharge temperature of a reciprocating compressor is the theoretical discharge temperature equation for adiabatic processes. The equation is written as shown here:
