System control for compressors with capacity control
Control device
The capacity regulators are usually activated depending on pressure, temperature or relative humidity. A multi-stage pressure, temperature or humidity regulator is recommended for high control accuracy. It must be adjusted in such a way that hunting operation is avoided.
A change in demand results in a relatively rapid change in suction pressure. However, due to the buffer effect of the evaporator, this only results in a relatively slow change in temperature.
Control difference for switching the compressor on and off
The control difference for switching the compressor on and off must be greater than that used to activate the capacity regulators. It is advisable to additionally limit the cycling rate of the compressor with a time relay.
Minimise deviations from the set point
CRII capacity regulators can be controlled at a significantly higher cycling rate than CR capacity regulators, allowing for more precise control of the entire system. This is particularly advantageous in systems with a parallel compressors. The permissible deviations from the setpoint (e.g. suction pressure or temperature) can be kept within a narrow range.
Even relatively rapid or steep changes in operating parameters can be corrected by activating the capacity regulators, allowing the capacity to be closely adjusted to the cooling / heating demand of the system. Suction pressure fluctuations are thus kept to a minimum, and the room temperature is maintained at a more stable level.
Systems with only one compressor
Due to the large control range, strong suction pressure fluctuations may occur under certain circumstances. This applies in particular to systems with low refrigerant charge and/or electronic expansion valves. Capacity control below approx. 30% of the residual capacity must be checked very carefully in such systems.
Systems with multiple evaporators
A change in system demand results in a relatively rapid change in suction pressure. However, due to the buffer effect of the evaporator or the goods being cooled, the temperature changes only relatively slowly. The system controller must therefore be set to avoid hunting operation.
Protection againt hunting operation
The capacity demand can be based directly on the cooling demand or suction pressure. The solenoid valves for capacity control should be open for at least 5 s and closed for at least 5 s. Beyond that, no fixed cycle times are required for the capacity regulators. Alternatively, the switching differential of the regulator for a capacity change can be set to a value that guarantees these minimum times.
This concept of variable response times, which deliberately avoids preset cycle times, counteracts long-term hunting operation of the system.