How to adjust a volumetric compressor

A compressor, as already said, has as its main feature the presence of several compression chambers for air or gas, sealed on the inlet/suction side. When gas or air are sucked in, the volume inside these compression chambers decreases until the required value is reached; after that, it is possible to discharge the compressed elements through the outlet system. 

A REMINDER: Dynamic Compression and Volumetric Compression

Once we understand how volumetric compression works, we can ask ourselves a new question: Is it possible to adjust a volumetric compressor? If so, how? 

How to adjust a volumetric compressor

Compressors can of course be adjusted, and this operation can be carried out in six different modes: pressure adjustment, bypass, inlet throttling, pressure discharge, start / stop, regulation.

Pressure discharge

Originally, a safety valve was used to regulate the pressure of a volumetric compressor and it had the exact task of discharging the excessive air. Today, however, in most cases a solenoid valve controlled by a regulator is used. This element allows us to have greater control over the pressure but, above all, it overtakes a limitation of these valves and that is to be able to guarantee unload in the event that the compressor is switched on under pressure.  For small compressors, it is also possible to unload the machine simply by opening the entire valve, thus leaving the compressor working under atmospheric pressure (allowing savings and energy efficiency). 

Bypass

The compressor regulation with Bypass adopts the same principle as the pressure discharge, with the difference that the discharged air is cooled and then returned to the compressor. It is a regulation tool used mainly in process compressors. 

Intake narrowing

This is perhaps the least effective method of all, it is in fact used to a limited extent. It consists in reducing the air or gas flow by increasing the compression ratio according to the diminished pressure induced at the compressor suction. The method of regulating the throttle valve at the intake leads to high energy requirements, being able to “regulate” only 10% of the maximum capacity of a machine. 

Discharge with throttled suction

Pressure discharge is undoubtedly the most popular method of regulating a volumetric compressor. Thanks to its efficiency, it is in fact widely used to guarantee the maximum adjustment range with low energy consumption: with a compressor in unload mode, the energy consumption is equal to 15-30% of the power at full load condition. This efficiency is due to an inlet valve which is closed, and to a discharge valve on which a small opening is applied that allows gas and air to run out of the compressor. With this configuration, we talk of inlet depression (closed valve) and low back pressure (small opening on the discharge valve). In order not to have any leakage during the compression-decompression, it is essential that air and gas be discharged quickly, being careful to leave a very limited volume of air. 

Start / Stop

Another regulation method, perhaps much more drastic but widely used for compressors with power below 5-20 kW, is that of stopping the electric motor when the pressure reaches its limit value. The engine will then be restarted as soon as it drops below this value. We can consider this method very efficient in energy terms as long as the start-ups are not many. 

Speed regulation 

The last method to regulate the pressure of a volumetric compressor is to act directly on the speed, that is on its flow rate.  The capacity of a compressor is given by the engine which can be combustion, electric or gas, controlled in frequency. By acting on the flow rate, it is possible to keep the outlet pressure constant, thus reducing energy consumption. 

Riem Italy: compressor analysis and energy optimization 

The Riem Italy team is very sensitive to the issue of energy saving and the efficiency of its machines. For this reason, it adopts effective countermeasures based on each type of compressor. So, in spite of the necessary standardized processes, at Riem Italy there is always a careful analysis on each component of the systems to find perfect and ad hoc volumetric regulation solutions. 

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