Regulating Displacement Compressors

The original method for regulating compressors was to use a pressure relief valve to release excess air pressure into the atmosphere. The valve in its simplest design can be spring-loaded, whereby the spring tension determines the final pressure. Frequently a servo-valve controlled by a regulator is used instead. The pressure can then be easily controlled and the valve can also act as an off-loading valve when starting a compressor under pressure. Pressure relief creates a significant energy requirement, as the compressor must work continuously against full counterpressure. A variant, which is used on smaller compressors, is to unload the compressor by fully opening the valve so that the compressor works against atmospheric pressure. Power consumption is significantly lower using this variant method.

Bypass regulation serves the same function as pressure relief, in principle. The difference lies in the fact that the pressure relieved air is cooled and returned to the compressor's inlet. This method is often used on process compressors where the gas is unsuitable or too valuable to be released into the atmosphere.

Throttling is a simple method to reduce flow by increasing the pressure ratio across the compressor, according to the induced under-pressure in the inlet. This method is, however, limited to a small regulation range. Liquid-injected compressors, which can overcome such a high pressure ratio, can be regulated down to 10% of maximum capacity. The throttling method creates a relatively high energy requirement, due to the high pressure ratio.

This is the most common regulation method currently in use. It combines a maximum regulation range (0-100%) with low energy consumption: only 15–30% of full load power with an off-loaded compressor (zero flow). The inlet valve is closed, but with a small opening used at the same time a blow-off valve opens and releases the discharge air from the compressor. The compressor element therefore works with a vacuum in the inlet and low counterpressure. It is important that pressure relief be carried out quickly and that the released air volume is limited, in order to avoid unnecessary losses during the transition from loaded to unloaded. The system demands a system buffer volume (air receiver), the size of which is determined by the desired difference between loading and off-loading pressure limits and by the permitted number of unloading cycles per hour.

Compressors below 5–10 kW are often controlled by completely stopping the electric motor when the pressure reaches an upper limit value and by restarting it when the pressure drops below the lower limit value. This method demands a large system buffer volume or large pressure difference between the upper and lower limits, in order to minimize the heat load on the electric motor. This is an energy-efficient and effective regulation method, provided the number of starts is kept low.

A combustion engine, gas turbine or frequency controlled electric motor controls the compressor's speed and, consequently, the flow rate. It is an efficient method for maintaining a steady outgoing pressure and lower energy consumption. The regulation range varies with the type of compressor and is largest for liquid-injected compressors. Frequently, speed regulation is combined with start-stop at low degrees of loading and pressure relief at standstill.