help- my bar fridge is wacko?!

I've inherited my parents' bar fridge that they inherited from the previous occupants of their house. It's old - an Ariston - and the ID plate on the back is in Italian, so it's travelled a long way to Australia and I'm reluctant to "put it down". They told me it has a tendency to ice up fairly quickly, so I've fitted a new gasket - the most obvious deficiency (the old one was seriously leaky). This is another $50 reason for not getting rid if it and trying to fix it (the problem is coming...)

I also noticed that the tube running from the expansion valve through the back of the fridge into the evaporator actually runs up between the back of the fridge panel and the heat exchanger, and at one point was actually pressing against the heat exchanger. So I've put a block of polystyrene between them to at least stop them from touching.

THE PROBLEM:
I've had it running for a couple of days now and it's certainly cold but already it's icing up all along the aforementioned tube and on the evaporator plate as well. The compressor runs continuously, even when I turn the "temp" knob right down to "stop". When I switch the power off and on, there's a lag of a few minutes before the compressor kicks back in.

Inside the fridge there's a long wire about the thickness of a coathanger sheathed in a clear plastic sleeve that's coming from the plastic "control box" up the top. This was floating around loose so I've fixed it to the back inside face of the fridge. I assume this to be a temperature probe, but I thought if it was part of the thermostat there would be a "reservoir" at the end of it? Should this wire be closer to the evaporator/freezer box, which is suspended from the inside roof of the fridge? ie would having this in the wrong spot be making the fridge "think" that it wasn't cold enough?

I've had a look at a few websites and some engineering textbooks and I understand how the cycle works but not how it's controlled. Is the thermostat a "hydraulic relay" where the "sensing fluid" acts to open and close a valve in the cooling line? or does it connect to an electrical relay that switches the compressor on and off? or does the compressor have a pressure sensor on the expansion valve side that detects when the working fluid is moving/stopped? what does turning the control knob on the fridge actually do? is it changing the threshold/switching temp of the thermostat? if so, then why doesn't the compressor cut out when I turn it down? Is it OK to insulate the tube running from expansion valve to evaporator to improve efficiency? can I buy some sort of tape to do this?

I'd appreciate any enlightenment from readers, and as a last resort I'll pay someone to come and look at it, but I'd really prefer to try and sort it out myself, and hopefully learn something in the process.

Thanks in advance,
Cheers, Ben
Univ. of Queensland, St Lucia, Australia

Quote:

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Originally posted by Fridgetech
I've never found out why little domestic fridges have the thermostat mounted to the evaporator plate.

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I think it is because they use 1 type of thermostat which can be used for fridges or freezers. I know some we use as replacements are actually freezer 'stats but utilise the alarm contacts which are at about 8° (I think) above the normal contacts.

Back to Ben's problem, I would suspect that the thermostat itself is faulty - they don't last forever:D

Also Ben the reason the compressor didn't restart straight away after you switched the power off and on again was because you overloaded it on the restart.

When you trun off a small running compressor such as in a fridge you need to allow the pressures within the system to equalise between the high and low pressure sides. The compressor is NOT designed to start against a large pressure difference and will switch itself off on it's internal, or external, motor overload. Not a good habit to get into as it tends to kill compressors, so if you turn it off give it about 3 to 5 minutes before you switch it onn again.

Nice to know the old beastie is still running though;)

I've never found out why little domestic fridges have the thermostat mounted to the evaporator plate.
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It's because a domestic fridge thermostat works at a much lower temperature like -27 -35 for example and so it has to be attatched to the evap plate for it to function correctly .A normal fridge stat say set to 4'c would not regulate the temperature properly in such a confined space as like a domestic fridge.I typically replace fridge stats with Ranco VL9's, VL5 for freezer and VB7 for bottle coolers(which by the way is an air probe for +1'c-+8'c)and VL1 for iceboxs etc. etc.

Hoppa:D

On domestic refrigerators, the control senses the evap coil because it is the freezer section that is actually being controlled most often. The refrigerator section is simply controlled by a vent regulating how much cold air or draft in some cases is allowed to enter the refrigerator section from the freezer. Usually... not always though.

That is a bit of a twist, since most commercial refrigerators sense the evap coil temperature and commercial freezers sense air temp.