Capacity Control or Regulation for Refrigeration Compressor on Ships
Capacity control or regulation in refrigeration compressors enables the refrigeration plant to run at part loads according to the demand of the refrigerant in the system.
Capacity control is obtained when the solenoid valve, fitted in the top cover, closes the access to the two cylinders, positioned under the same top cover. This makes the inlet pressure to the cylinder drop to zero bar. At the same time the compressor capacity is reduced to 50%. However, a little gas will be flowing through the closed solenoid valve, hereby ensuring the necessary cooling and lubrication of the cylinders. This capacity control control permits a certain reduction in power consumption.
(A) Controlled operation:
With the solenoid valve energized, the suction port in the corresponding cylinder head is shut off by means of a servo valve; the pistons of this cylinder row run idle without gas pressure.
(B) Normal operation:
With the solenoid valve de-energized, the gas ports in the valve plate and cylinder head are open.
Note:
Stop the compressor and check that the solenoid valve works correctly. At a current impulse to the solenoid valve the characteristic valve stroke must be heard!
Refrigeration Compressor Starting Unloader
At a star-delta start of electric motors it is often considered necessary to limit the compression work of the machine at the starting moment in order to reduce the starting torque of the electric motor. Usually, a solenoid valve is used in a by-pass arrangement which in the starting up phase short circuits the discharge side to the suction side of the compressor. At the same time, a non-return valve must be fitted in the discharge line to the condenser preventing the return flow of discharge gas to the compressor.
When the electric motor has reached its max. number of revolutions per minute, a switch takes place from star to delta start. The solenoid valve is closed and the compressor now works under normal conditions.
Fit the non-return valve in the discharge line after the compressor. Check that the solenoid valve is fitted with the flow arrow pointing from the high pressure to the low pressure side. A wrongly fitted valve or a leaky valve will lead to overheating
and breakdown of the compressor.
In a few cases a delaying relay must be used, keeping the solenoid valve open until the motor has reached its max. number of revolutions. The delaying relay is set at a delay of 1-2 sec. after a switch from star to delta has taken place.
Heating Rods for Refrigeration Compressor Oil Heating
- The refrigeration compressors are delivered in a standard execution with built-in heating coils or rod in the crankcase.
- The purpose of the heating coil or rod is to keep the oil in the crankcase warm even during standstill of the compressor. This ensures a low content of refrigerant in the oil.
- Too much refrigerant in the oil makes it loose its lubricating properties. This may lead to damage of the movable parts in the compressor.
- Further, the danger exists that the oil, during start-up of the compressor, foams so vigorously that the lubricating pressure will disappear.
- Before start-up the heating rod should be switched on for at least 8 hours.
- The heating coil or rod must not be switched on if the oil level in the vessel is below minimum in the sight glass. While the compressor is operating, it is usually switched off.
- Further, remember to switch off the heating rod if the compressor crankcase is opened for inspection.
REFERENCE: Instruction Manual for SABROE Reciprocating Compressors SBO 21, 22, 41, 42, 43
Related Links:
Pressure Testing Refrigeration Plant on Ships
Pumping Down of the Refrigeration Plant on Ships
Maintenance of Reciprocating Refrigeration Compressors on Ships
Troubleshooting of the Reciprocating Refrigeration Compressor Plant on Ships
Capacity Control or Regulation for Refrigeration Compressor on Ships