Fridge Problem

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Big Pete
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Fridge Problem

Postby Big Pete » Thu Apr 08, 2010 12:33 pm

I have recently done a short relieving trip on another ship.

I discovered that the domestic refrigerating compressor was running normally to bring down the Meat & Veg room temperatures, but once they were down to temperature the compressor was short cycling. It would run for about 5 or 6 seconds, then stop for a couple of minutes before running again for a few seconds.
Occassionally I also noticed ice on the gas suction pipes.
I also noticed that the compressor was running for 12 or 13 hours a day.
There was plenty of gas in the condenser sight glass and no gas bubbles in the liquid line sight glass.
The compressor was cutting in and out at the correct pressures according to the makers manual.
There was no excessive frost on the evaporators.
The door seals were in good condition and the Cooks were not keeping the doors open too long.

Any ideas what was wrong???

BP
It is always better to ask a stupid question than to do a stupid thing.

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JK
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Re: Fridge Problem

Postby JK » Thu Apr 08, 2010 1:05 pm

Yeck, a fridge question :cry:

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The Dieselduck
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Re: Fridge Problem

Postby The Dieselduck » Thu Apr 08, 2010 1:22 pm

I've seen in the past, this could be caused by the thermostatic valve icing up. You could melt it, then properly apply insulation. Like like many, refrigeration is not a common issue, so I don't often get a chance to get my hands dirty and troubleshoot with any accuracy this system.
Martin Leduc
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JK
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Re: Fridge Problem

Postby JK » Thu Apr 08, 2010 1:25 pm

I find my tagline is getting truer every day, but I have a thought which I won't post yet.
Maybe some other more brilliant engineer reading this will get over their stage fright and post their experiences.

Matthias
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Re: Fridge Problem

Postby Matthias » Fri Apr 09, 2010 10:45 am

Is there a crankcase pressure regulating valve fitted before the compressor? On our setup we have these installed with their main purpose being to prevent overloading of the compressor by limiting the crankcase pressure during defrost cycle or after a normal shutdown period.

Our compressor was behaving similar to yours and it ended up being that this valve was sticking, resulting in a restriction on the gas side which would cause the compressor to start & stop in short intervals.

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Big Pete
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Re: Fridge Problem

Postby Big Pete » Fri Apr 09, 2010 11:21 am

Hi Mathias,

No crankcase pressure regulating valve, and the compressor was running normally when the liquid solenoid valves were energised.

BP.
It is always better to ask a stupid question than to do a stupid thing.

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D Winsor
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Re: Fridge Problem

Postby D Winsor » Sat Apr 10, 2010 9:07 am

There are 2 things you should check
1 Check the LP pressure switch to insure that the differential pressure setting and the pressure range is correct for the gas being used.
2 Usually there is an electric solenoid valve for each room controlled by the room thermostat in the high pressure liquid line. If the room thermal expansion or TX valve is freezing it is a good indication that the solenoid valve is leaking and causing the compressor to short cycle
Troubleshooting 101 "Don't over think it - K.I.S.S. it"

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Big Pete
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Re: Fridge Problem

Postby Big Pete » Sun Apr 11, 2010 12:38 am

Congratulations Winsor,
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.

BP
It is always better to ask a stupid question than to do a stupid thing.

Pengze
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Re: Fridge Problem

Postby Pengze » Mon Nov 12, 2012 8:27 pm

Dear Sir,
What is this mechanical back pressure valve fitted after the evaporator ?
Any photo of the valve will help.
Thanks.

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Big Pete
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Re: Fridge Problem

Postby Big Pete » Tue Nov 13, 2012 1:09 am

Hi again Pengze,

You are very busy posting questions!
I do not have a photograph of one of these valves. I recently bought a Fridge Book from Amazon to update my Refrigeration knowledge, it is called "Refrigeration Equipment, a servicing and installation hanbook" written by A. C. Bryant and I would recommend it.
I have attached a scan of the relevant section.
In my experience the valves consist of a vertically mounted copper cylinder about 20 mm diameter maybe 150 mm tall with the inlet on one side and the outlet on the other. There is a knurled cap on the top. When this is unscrewed the adjusting screw is underneath.

Regulating Valves.jpg


I hope this helps.

BP
It is always better to ask a stupid question than to do a stupid thing.


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