A refrigeration system will be designed to have a maximum duty to balance a calculated maximum load, and for much of its life may work at some lower load. Such variations require capacity reduction devices. Speed control is the most obvious method, but this requires an inverter drive (see next section).
Multi-cylinder machines allow reduction of the working swept volume by taking cylinders out of service with blocked suction or valve-lifting mechanisms. With the blocked suction method, one or more of the cylinder heads incorporate a pressure-actuated valve which closes the supply of suction gas to the cylinder head. In Figure 4.11(a) the normal (full capacity) operating position is with the solenoid valve de-energized. The gas pressures across the plunger are equalized, and the plunger is held in the open position by the spring. When the solenoid valve is energized, the needle valve seats on the upper port, and the unloading plunger chamber is exposed to discharge pressure through the discharge pressure port. The differential between discharge and suction pressure forces the plunger down, sealing the suction port in the valve plate, thus preventing the entrance of suction vapour into the unloaded cylinders.
The valve-lifting method is used with ring plate valves. The ring plate suction valve which is located at the crown of a loose liner can be lifted by various alternative mechanical systems, actuated by pressure of the lubricating oil and controlled by solenoid valves. Typically, an annular piston operates push rods under the valves. Use of these methods allows multi-cylinder machines to have cylinders or banks of cylinders unloaded for capacity reduction. In addition
Valve energized suction port closed reduced capacity
Solenoid coil Needle valve
Port to discharge pressure
Port to suction pressure
Unloading plunger Discharge chamber Suction chamber Spring
Suction port in valve plate
Valve de-energized suction port open full capacity
Figure 4.11 ( a) Blocked suction capacity control mechanism and (b) valve plate showing machined suction port for blocked suction (Emerson Climate Technologies)
Unloaded start can be arranged so that starting current is reduced and build-up of oil pressure occurs before the machine is fully loaded.
Smaller machines may have a valved bypass across the inlet and outlet ports in the cylinder head, or a variable clearance pocket in the head itself. Capacity may be reduced by external bypass piping.
Rotational speed can be reduced by two-speed electric motors or by use of an inverter which provides a variable frequency waveform to vary the motor speed. The lowest speed is usually dictated by the in-built lubrication system.
Posted in Refrigeration and Air Conditioning