Before you can learn about the different compressors and compression methods, we first have to introduce you to the two basic principles for the compression of gas. After that, we will compare the two and look into the different compressors in these categories.
There are two generic principles for the compression of air (or gas): positive displacement compression and dynamic compression. These principles are grounded in the theory of how air is compressed and discharged.
In positive displacement compression, the air is drawn into one or more compression chambers, which are then closed from the inlet. Gradually, the volume of each chamber decreases, and the air is compressed internally. When the pressure has reached the designed built-in pressure ratio, a port or valve is opened. The air is then discharged into the outlet system due to the continued reduction of the compression chamber's volume.
In dynamic compression, air is drawn between the blades on a rapidly rotating compression impeller and accelerated to a high velocity. The gas is then discharged through a diffuser, where the kinetic energy is transformed into static pressure.
Positive displacement compressors provide consistent airflow, regardless of system pressure. They compress air by trapping a fixed volume and mechanically compressing it, such as with pistons or rotary screws.
These compressors deliver higher pressure ratios even at lower speeds and are ideal for smaller, stable applications like manufacturing and automotive industries. Their simple design ensures reliability and easy maintenance.
Dynamic compressors use high-speed rotating blades to compress large volumes of air.
Their flow rate and pressure vary with operating speed, making them suitable for high-volume applications like power generation and HVAC. Their complex design is optimized for variable flow rates and efficient high-speed operations.
A bicycle pump is the simplest example of a positive displacement compression. Air is drawn into a cylinder and is compressed by a moving piston. Piston compressors have the same operating principle. They use a piston whose forward and backward movement is accomplished by a connecting rod and a rotating crankshaft.
If only one side of the piston is used for compression, this is called a single-acting compressor. If both the piston's top and undersides are used, the compressor is double acting. The pressure ratio is the relationship between absolute pressure on the inlet and outlet sides.
Accordingly, a machine that draws in air at atmospheric pressure (1 bar(a)) and compresses it to 7 bar overpressure, works at a pressure ratio of (7 + 1)/1 = 8.
In the two graphs below, you'll find the pressure-volume relationship for a theoretical compressor and a realistic diagram for a piston compressor illustrated (respectively).
The stroke volume is the cylinder volume that the piston travels during the suction stage. The clearance volume is the volume underneath the inlet and outlet valves and above the piston. It must remain at the piston's top turning point for mechanical reasons.
Differences between the stroke volume and suction volume are due to the expansion of air remaining in the clearance volume before the suction starts. The practical design of a compressor, e.g. a piston compressor, results in a difference between the theoretical p/V diagram and the actual diagram.
The valves are never completely sealed and there is always a degree of leakage between the piston skirt and the cylinder wall. In addition, the valves can not fully open and close without a minimal delay. This results in a pressure drop when gas flows through the channels. The gas is also heated when flowing into the cylinder as a consequence of this design.
In a dynamic compressor, the pressure increase takes place while the gas flows. The flowing gas accelerates to a high velocity by means of the rotating blades on an impeller. The velocity of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser.
Depending on the main direction of the gas flow used, these compressors are called radial or axial compressors. Compared to displacement compressors, a small change in the working pressure of dynamic compressors results in a large change in the flow rate.
Each impeller speed has an upper and lower flow rate limit. The upper limit means that the gas flow velocity reaches sonic velocity. The lower limit means that the counter pressure becomes greater than the compressor's pressure build-up, which means return flow inside the compressor. This, in turn, results in pulsation, noise, and the risk of mechanical damage.
To understand more about how dynamic compressors are regulated and how performance is optimized, read this guide on dynamic compressor regulation.
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