In order to achieve a certain required operating condition a control operation is carried out either on the installation, the compressor drive or on the compressor.
The selection of this control should be done in such a way, that with a minimum of design and energy costs a reliable control of the plant is achieved.
If the control is carried out automatically in such a way that one or several operation parameters are held at a certain level ( in general at a constant level ) in relation to time , this control is then called a regulation .
In the following a few characteristic examples are presented.
Shutting down over a certain time period
of the compressor is often used on small air compressor installations in order to control the delivery pressure. In order to determine the size of the necessary storage volume the pattern of pressure change in the receiver has to be observed. The rise and fall of the pressure in relation to time is proportional to the difference in volume flow between supply and demand and it is in inverse ratio to the size of the receiver. Criteria for the design are the maximum pressure deviations and the allowable frequency of starts of the motor.
Holding the suction valves open
This is a preferred method on multistage compressors fitted with ring or plate valves if speed control is not possible due to the design of the motor drive or if the speed control range is not sufficient for the necessary reduction in volume flow.
The control of the suction valve in its simplest form is carried out via a spring loaded unloader. The force on the suction valve unloader, which acts against the closing force of the valve plate, can be adjusted via the pretension of the spring. The valve closes only at that point where the drag force acting in the direction of valve closure exceeds the spring force. The intake of gas is thereby reduced.
With this kind of regulation the specific work for pumping the gas will be increased. This results from the pressure difference between gas intake and gas delivery when the suction valve is held open.
In compressor installations regulating the suction valve via hydraulically operated and electronically controlled actuators [Steinrück] has been widely accepted. The suction valve unloader will be actuated by a hydraulic cylinder which in turn is controlled via a solenoid valve. The control signal is generated in an electronic unit which receives its information about the actual position of the piston from a top dead center sensor and of the mass flow requirement from the process control system of the plant.
The advantages of this combined control system lies in the fast response and the freely programmable reaction of the movement of the suction valve unloader. This system can coordinate the control of all regulated suction valves , so that also at part load the interstage pressures can be kept at the desired levels. This control can also be used for unloaded start up to reduce the load on the motor.
Increase of clearance volume
by connecting the working chamber with an additional clearance pocket and thereby reducing the volume flow by reducing the volumetric efficiency. This type of control reduces the volume flow whilst reducing the energy input within acceptable limits. The picture below shows such a design [Schutte] .
The additional clearance pocket is fitted on the head end side of a double acting cylinder (fig. above ). The stepless adjustable volume flow is controlled by the movement of a hydraulically positioned conical piston which alters the size of the clearance volume.