Ship’s air conditioning and refrigeration system helps in lowering room temperature and in keeping perishable food items and refrigerated cargo at desirable temperature.
As discussed in our article about basics of refrigeration plant – Construction and Working of Ships Refrigeration plant on ships, the reefer and air conditioning systems are installed to handle fluctuating loads i.e. they are responsible for maintaining low temperature of several rooms or cargo holds at the same time.
The efficiency of the refer plant depends on its ability to maintain different rooms or holds at different temperature.
What is Capacity Control?
Capacity control of a refrigeration plant can be defined as a system which controls the output of the plant as per the load in demand. As the load (temperature) of one room is achieved, there will be no more need of the refrigerant for cooling. Hence, the solenoid valve supplying refrigerant to that room will shut. This functionality is called capacity control.
The refrigeration compressor consists of different units working in parallel to cope up with the load. As the load decreases, the capacity control system cut off one or more units (depending upon the load) and maintains the efficiency of the plant by reducing stresses on different parts.
Components and Working of Capacity control system
1. Compressor lube oil pump supply
2. Capacity control valve
3. Capacity control regulating valve
4. Un-loader assembly
The compressor lube oil pump supplies oil to all the bearings and one connection is provided to the capacity control valve.
The capacity control valve is provided with high pressure oil from the compressor lube oil supply pump. This valve has several grooves bored in to its periphery and connected to the un-loader mechanism of different units.
A spring piston is provided which controls the spreading of high pressure oil supply in to the bore chamber. The spring piston is pressed by the oil supplied through an orifice which pushes the piston and aligns the un-loader holes, supplying high pressure oil to the un-loader unit.
The un-loader assembly comprises of a un-loader piston held by a spring. The un-loader piston is connected to a rotating cam ring having lifting pins attached to th suction valve. The lifting pins always act on the suction valve i.e. un-loading the unit at stop condition.
When the bores on control valve aligns with the un loader bores, oil will pass and press the un-loader piston rotating the cam and releasing the un loader pins from the suction valve.
The capacity control regulating valve is responsible to control the pressure (opening and closing of capacity control valve ports with un-loader ports). Its one end is connected to the crankcase and other end to capacity control valve.
As the pressure in the crankcase drops due to reduction in load, oil in capacity control valve is drained into the crank case leading to closing of un-loader ports, lifting of the suction valve, and cutting of the cylinder unit.