Multistage compressors are made up of 1–10 impellers that may be configured in various flow path configurations. The temperature and compression ratio are considered to remain constant during each step. Multistage compressors can be configured in straight-through, compound, or double flow modes. In the straight-through mode, each stage takes in refrigerant from one side of the compressor and emits it into the other side without changing direction. This is useful for low-pressure systems where there is no need for pressure recovery at each stage. Compound-flow compressors have two or more stages connected in a loop such that each stage receives its own outlet pressure before being pumped back into the inlet. Double-flow compressors have two loops with separate inlets and outlets for each stage.
Impellers are attached to the shaft that carries the magnetic clutch that controls power transmission to the compressor. There are two types of impeller arrangements used in multistage compressors: counter-rotating and co-rotating. In the counter-rotating configuration, each impeller spins in the opposite direction as the others. This arrangement is used when the desired effect is reduced speed of rotation at the compressor's output. Co-rotating impellers spin in the same direction. They are designed for higher speeds of operation because they require less energy to drive them than counter-rotating impellers.
Multi-stage compressors are made up of a succession of cylinders, each of which has a different diameter. Between compression stages, the air is cooled by passing through a heat exchanger. Air is driven into an extra chamber in a two-stage compressor, where it is compressed to the desired level. The pressure inside the chamber increases, so the next stage will now have a shorter stroke and thus less contact with the low-pressure side of the system. This means that only half of the cylinders in the next stage will be activated at once.
The third stage uses a smaller cylinder than those used in the first two stages. Only half of the cylinders in this stage are active at any one time. This process continues until the last few strokes of the compressor are done using small cylinders on the outside of the compressor. These final strokes finish off the compression process and send the air back into the main air flow for delivery to users.
The advantage of a multi-stage compressor is that it can deliver high pressures with relatively low mechanical loads on the engine. This is particularly important when trying to improve fuel efficiency.
The disadvantage of a multi-stage compressor is that it is more complex to build and therefore more expensive. It is also possible that some parts might need replacement after a certain period of time. For example, bearings and valves need to be changed every 100 hours or so if you use them often.
Axial Flow Compressor Because the amount of pressure raised by each stage is minimal, the compressor is a multistage machine; a stage consists of a row of revolving blades followed by a row of stator vanes. Each stage removes some of the air from the compression process and increases the pressure.
The seven main stages of an axial flow compressor are: (1) suction, (2) compression, (3) delivery, (4) cooling, (5) regeneration, and (6) exhaust. These stages are shown in figure below:
Suction - The initial or first stage of the compressor. A portion of the incoming air stream is diverted to an area behind the rotor called the "suction zone". This section of the airflow is slowed down by the rotor blades so that it becomes colder and less dense than the remaining portion of the airflow. The resulting difference in density between the two streams causes them to separate, with the denser stream passing through openings in the rotor while the lighter stream continues along the axis of the compressor toward the next stage.
Compression - The second stage of the compressor. In this stage, the majority of the compression work is done as the airflow passes through alternating sets of rotor and stator blades.