1. A course in turbomachinery……………lecturer: Dr.Naseer Al-Janabi
DEFINITION OF COMPRESSOR
A compressor is a device that transfers energy to a gaseous fluid for the
purpose of raising the pressure of the fluid as in the case where the
compressor is the prime mover of the fluid through the process. The
purpose may also include a desired temperature rise to enhance the
chemical reaction in the process.
Pumps are very similar to compressors but deal primarily with
incompressible hydraulic fluids, whereas compressors generally deal with
compressible gaseous fluids.
TYPES OF COMPRESSORS
The two basic types of compressors are positive displacement and
dynamic.
I POSITIVE DISPLACEMENT COMPRESSOR
The positive displacement compressor functions by means of entrapping a
volume of gas and reducing that volume, as in the common bicycle pump,
and the screw compressor shown in Figure 1. The general characteristics of
the positive displacement compressor are constant flow and variable
pressure ratio (for a given speed).
37 | P a g e

2. A course in turbomachinery……………lecturer: Dr.Naseer Al-Janabi
Positive displacement compressors include:
 piston compressor
 screw compressor
 vane compressor
 lobe compressor
II DYNAMIC COMPRESSOR
The dynamic compressor depends on motion to transfer energy from the
compressor rotor to the process gas. The characteristics of compression
vary depending on the type of dynamic compressor and on the type of gas
being compressed. The flow is continuous. There are no valves and there is
no "containment" of the gas, as in a positive displacement compressor.
Compression depends on the dynamic interaction between the mechanism
and the gas.
Dynamic compressors include:
 ejector
 centrifugal compressor
 axial compressor
Ejector An ejector is a very simple device which uses a high-pressure jet
stream to compress gas. The momentum of the high-pressure jet stream is
transferred to the low pressure process gas. This type of compressor is
commonly used for vacuum applications.
Centrifugal Compressor A centrifugal compressor acts on a gas by means
of blades on a rotating impeller. The rotary motion of the gas results in an
outward velocity due to centrifugal forces. The tangential component of
this outward velocity is then transformed to pressure by means of a
diffuser.
Figure 2 is typical of a single-stage centrifugal compressor. A high-pressure
multi-stage compressor is shown in Figure 3.
38 | P a g e

3. A course in turbomachinery……………lecturer: Dr.Naseer Al-Janabi
Axial Compressor An axial compressor imparts momentum to a gas by
means of a cascade of airfoils. The lift and drag coefficients of the airfoil
shape determine the compressor characteristics. Figure 4 shows a typical
axial compressor. An axial compressor incorporated in a turbocharger is
shown in Figure 5.
39 | P a g e

4. A course in turbomachinery……………lecturer: Dr.Naseer Al-Janabi
40 | P a g e