Compressors are mechanical devices. We use them to increase the pressure of gases by decreasing their volume. Compressors are almost similar to the pumps in operation. Since both serve the same purpose of increasing the pressure on the fluid and transporting it through the pipes. However, in the case of pumps, most of the liquids are incompressible so their volume doesn’t decrease while pumping. The volume does change in compressing the gases.
The compressor generally acts as a part of the system that provides compressed gases for a variety of applications. For an instance, in industry, we use it for the processing of the instrumentation air. Compressors can be divided into the following two categories.
Positive Displacement Compressors
A positive displacement compressor is one that traps a certain amount of gas and then displaces it positively. The most common example of a positive displacement compressor is a reciprocating compressor.
In the reciprocating compressor, the gas enters the cylinder from the suction valve and leaves through the discharge valve. The pressure of the gas increases as the piston forces the gas into a smaller volume (cylinder).
The simplest form of a displacement compressor is the bicycle pump. We draw air into the cylinder and then compress it out with the help of a piston. A piston compressor works on the same principle. However, we accomplish its forward and backward motion by attaching it to a connecting rod and a rotating crankshaft.
Another example of positive displacement compressors is the screw compressor.
In a dynamic compressor, the pressure increases at the same time while the gas flows through it. The rotating blades of the impellers increase the velocity of the gas. After which it passes through the diffuser section where it slows down under expansion. This results in increased pressure at the outlet. The dynamic compressors have the following two types.
The centrifugal compressor draws the gas into the centre of a rotating impeller with radial blades. The impeller then throws it towards its periphery (by centrifugal force). Before letting in the gas towards the centre of the next impeller (stage) it passes through a diffuser. The air temperature at the inlet of each stage (impeller) has a very important effect on the compressor’s power requirements. Therefore, cooling between the stages is carried out. Centrifugal compressors like P.D compressors can have multiple stages.
In an axial compressor, the gas flows along the axis of the shaft. The axial compressors have a set of rotor and stator blades. The rotor blades force the gas to flow. The stator blades act as diffusers which decrease the velocity of the gas thus increasing its pressure.
The blades are generally smaller towards the end. The reason is that the stator blades partially convert the K.E into pressure. And the rest of the conversion takes place at the exit due to the smaller sized blades. As the gas moves through the lesser space (volume) its pressure increases.
Cooling in the compressors is generally carried out for air. We need very dry air when we use it for the instrumentation. Cooling serves two purposes. If we use it between the stages, the volume of air decreases thus decreasing the compression work for the next stage ultimately increasing the efficiency of the compressor. Since cooling occurs at a constant pressure it decreases the volume of the air according to Boyle’s law. The other purpose of cooling is the precipitation of water. This helps in the partial removal of the moisture.
However certain dryers (desiccant dryers, refrigerant dryers) are also used to achieve maximum removal of the moisture. The aftercoolers are used at the end of the compression to remove water.
The compression ratio is one of the most important parameters in the design of the compressors. It is actually the ratio of absolute (stage) discharge pressure to absolute (stage) suction pressure.
The compression ratio serves two purposes:
1. The calculation of the final outlet temperature since it is always critical to keep it under limits. A high discharge temperature may lead to the damaging or sometimes failure of internal components. Once we know the compression ratio (r) that is:
r = Pd/Ps
We can determine the theoretical discharge temperature by using the following equation:
Td = Ts x r(1-k/k)
where Td = Temperature at the discharge
Ts = Temperature at the suction
r = compression ratio
k = ratio of the specific heat, i.e. Cp/Cv
2. The other important function is to determine the required power. Since a high compression ratio means high power is required. To read more about the compression ratio click here.
What is Surging in Compressors?
Surging is the momentary reversal of the flow in the compressor from the system. If the compressor is part of a very high capacity system, it also has the demand of a very high gas flow through it. Due to any reason, if the demand for the gas decreases downstream of the compressor, the pressure in the system increases. Consequently, the head needed to maintain the flow also increases. If the situation continues and the head needed to maintain the flow increases above the maximum head of the compressor, the flow stops.
As a result pressure in the compressor becomes less when compared to the pressure in the system. This causes the flow to reverse back to the compressor. For a maximum head of the compressor, there is always a minimum value of the flowrate beyond which surging starts. In the characteristic curve of the compressor, we call it the surge point. This can seriously damage the compressor equipment.
How to avoid surging?
In order to avoid surging, we generally provide it with a bypass line. Therefore, when the demand for the gas by the system decreases, the bypass line opens and it throws the flow back to the suction inlet. Anti-surge systems are always installed at a flow rate that is 10% higher than the surge point flow.