You got the prize!
When the compressor stops there is a supply of liquid in the condensor resdy to go through the evaporator but it is stopped by the solenoids in the refrigerated spaces.
When the room temperature rises to the set point the solenoid valve will open, allowing refrigerant through the evaporator, until the room has cooled by the differential setting on the thermostat, when the solenoid will close.
The flow of refrigerant is regulated, usually by a thermostatic expansion valve, which will maintain a constant gas temperature at the outlet to the evaporator.
Sometimes, where several rooms are running at different temperatures but connected in parrallel to the gas system, there will also be a mechanical back pressure valve fitted after the evaporator.
Once the solenoid has opened the suction pressure at the compressor will start to rise until it reaches the set point of the pressure switch, which will then start the compressor.
So long as the solenoid valve is open the compressor should run continously with the suction pressure falling very slightly as the room temperature falls (because the expansion valve will throttle the gas flow slightly to maintain a constant gas temperature).
When the solenoid valve closes the suction pressure will fall rapidly to the set point on the pressure switch and the compressor will stop.
So long as the compressor cylinder head and solenoid valves are gas tight the pressure should stay constant until the solenoid valve opens again.
The compressor should never continue to run when the solenoids are closed because the the gas enters the crankcase and entrains L.O. to lubricate the cylinder heads and piston, the cold suction gas also provides cooling to the compressor, and to the motor where it is integral to the compressor. (Hermetic sealed compressor unit)
So I immediatly suspected faulty cylinder head valves and or solenoid valves.
We did not have any fridge spares on board so I arranged for a Refrigeration Engineer to attend the vessel when we docked.
His investigations showed that the solenoid valve was leaking when it was supposed to be shut ( When the solenoid valve was de-energised and he stood next to it he could actually hear the liquid flowing through it) and also the cylinder head valves of the running compressor were leaking, allowing gas to flow back from the condenser to the compressor suction when the compressor was stopped.
He fitted a new solenoid valve and changed over to the standby compressor, problem solved.
He also ordered new valves for the compressor, to be fitted at the next port call.
I left the ship that Port call so I was unable to check the effect on the running hours, but I would expect fridge compressors to run for 8 hours a day in Northern Europe and 12 hours a day in the Tropics, much more and there has been a serious loss of efficiency!!!
Discussing the problem with the service Engineer and my relief, who had been Chief on the Ship for some time I was able to put together a probable History of events:-
1) The Plant was built with inadequate pipe hygiene.
2) A piece of swarf jammed in the solenoid valve seat preventing it shutting properly.
3)The trickle of liquid through the evaporator when the solenoid was supposed to be closed, left the evaporator with excessive superheat and caused heavy icing on the evaporator.
4) The defrost cycle ceased to be effective because refrigerant was still flowing through the evaporator during the de-frost cycle. These 2 factors led to heavy icing of the evaporator.
5)The ships Engineers recognised the icing problem and solved it by upgrading the defrost heaters and the electrical heating tape on the drains etc, shortening the defrost interval and increasing the length of de-frost.
6)Due to the heavy icing of the evaporator, not all the liquid refrigerant could be evaporated in the evaporator (Restricted air flow through the evaporator and the insulating effect of the ice). Liquid refrigerant returned to the compressor, exploding into gas on contact with the relatively hot cylinder head and producing severe mechanical and thermal stresses in the cylinder head components, this caused the partial failure of the cylinder head valves.
It is always better to ask a stupid question than to do a stupid thing.