Fridge thermostat capillary tube installation

[Zanshin]

I'm helping a neighboring boat with some issues and came across something I haven't seen before and am not sure about.

The installation make is a Seafrost and the compressor is a standard Danfoss. The thermostat is mounted outside of the fridge, and the capillary tube to measure the temperature is inserted into the fridge but just a couple of centimeters; the rest of it is coiled outside of the fridge and here in the tropics it is in ambient 27-30 degrees Celsius.

So far I've always seen the excess lengths of capillary tube coiled inside the fridge.

I'm not sure if this makes a difference in the thermostat operation. I think that this would only affect the internal pressure of the capillary and thus only change the point on the thermostat where it makes/breaks the connection. But if only 10% of the capillary tube is inside the fridge and 90% in ambient, is that enough area to give accurate inputs to the thermostat?

[Richard Kollmann]

Originally Posted by Zanshin

I'm helping a neighboring boat with some issues and came across something I haven't seen before and am not sure about.

The installation make is a Seafrost and the compressor is a standard Danfoss. The thermostat is mounted outside of the fridge, and the capillary tube to measure the temperature is inserted into the fridge but just a couple of centimeters; the rest of it is coiled outside of the fridge and here in the tropics it is in ambient 27-30 degrees Celsius.

So far I've always seen the excess lengths of capillary tube coiled inside the fridge.

I'm not sure if this makes a difference in the thermostat operation. I think that this would only affect the internal pressure of the capillary and thus only change the point on the thermostat where it makes/breaks the connection. But if only 10% of the capillary tube is inside the fridge and 90% in ambient, is that enough area to give accurate inputs to the thermostat?

The amount of thermostat tube inside or outside box may or may not effect temperature. Where is end of tube located is it touching evaporator?

Is there a problem with controlling box temperature, explain.

[Zanshin]

Hello Richard,

this is an ex-charter boat and the charter company bridged the two Danfoss compressor connections together and removed the thermostat electrically. I am just waiting for the fridge to thaw out enough to look at the capillary tube part inside the fridge. But as I'd always seen most of capillary coiled up inside the fridge and this installation has almost all of it outside the fridge I though I'd ask. I think it won't be a great issue, part of that based on this being a factory Robertson & Caine install, but thought I'd ask.

Tomorrow the thaw should be enough to let me see inside the fridge and reconnect the
thermostat to see if it works as expected.

[OzeLouie]

Hi Zanshin, your observations are interesting, specially as you quote that the capillary sensor "is inserted into the fridge but just a couple of centimeters"

Most mechanical thermostats are derivatives of those used in domestic fridge / freezers where the entire controller and its sensor are located inside the refrigerated area (cabinet) as they are not ambient compensated. Having the controller and most of the sensor outside the cabinet and effected by ambient temperature changes will likely effect it unless it is the rarer ambient compensated type.

With most 'ambient compensated' mechanical thermostats the sensor bulb is typically a larger diameter tube mounted in the material flow being sampled.

Having noted that only a couple of centimeters is located inside the cabinet indicates that some alteration may have occurred since original so I would suggest scrapping that thermostat and installing a digital type and avoid the dramas of the now outdated old gadgets and enjoy the accuracy, reliability and flexibility of the digital. They are not expensive and come with easy to follow instructions.. Suggest an Elitech STC 8000 or similar. (Note: if digital is rated as 110 or 240 VAC, ignore and connect DC as they will work ok off AC or DC input. )

Cheers Louie

[Zanshin]

The installation is factory, they installed the thermostats outside of the units. There is no bulb on the end of these capillary tubes, they are just flat-ended and crimped. As stated before, only about 10% of the length of the capillary is inside the fridge and the rest is coiled behind the thermostats. Electronic thermostats are not an option, first of all I'm in St. Lucia and even Dometic mechanical small units are US$80 and electronic ones are not available locally; buying overseas and paying import duties and a local "agent" makes them financially uninteresting for the boat owner.

The old thermostats seem to be working; it looks like the thermostat wires were removed and the two compressors had their C and T terminals bridged. This means that the units were constantly running - not necessarily a problem for the boat while in charter but something that needs to be corrected for liveaboard off the hook.

Unfortunately the initial question still remains unanswered. I'll rephrase and restate it:

"With an externally mounted thermostat and a straight capillary tube with no bulb, does having 90% of the capillary length outside of the cooled fridge cause inaccurate readings?"

The capillary does not touch the evaporator plate.

[OzeLouie]

Originally Posted by Zanshin

The installation is factory, they installed the thermostats outside of the units. There is no bulb on the end of these capillary tubes, they are just flat-ended and crimped. As stated before, only about 10% of the length of the capillary is inside the fridge and the rest is coiled behind the thermostats. Electronic thermostats are not an option, first of all I'm in St. Lucia and even Dometic mechanical small units are US$80 and electronic ones are not available locally; buying overseas and paying import duties and a local "agent" makes them financially uninteresting for the boat owner.

The old thermostats seem to be working; it looks like the thermostat wires were removed and the two compressors had their C and T terminals bridged. This means that the units were constantly running - not necessarily a problem for the boat while in charter but something that needs to be corrected for liveaboard off the hook.

Unfortunately the initial question still remains unanswered. I'll rephrase and restate it:

"With an externally mounted thermostat and a straight capillary tube with no bulb, does having 90% of the capillary length outside of the cooled fridge cause inaccurate readings?"

The capillary does not touch the evaporator plate.

YES! Get as much inside the cabinet as possible.

[Zanshin]

OzeLouie - thanks, I just came to this page to post that I'd found the following in the documentation/trouble shooting guide:

Remote Thermostats work best when any excess capillary tube is in the cold space

Richard might be right that someone has been playing around with the thermostat and fridge after all.

[moseriw]

Easy BUT you must ensure that the sensor is inside the fride - in total.

[Richard Kollmann]

You have not answered the basic question is the thermostat stopping compressors at your desired temperature? If not change setting to highest temperature setting `on dial and report if compressor stops. Fulling with temperature sense tubing or changing thermostat is not productive at this time.

The end 5 inches of thermostats cap tube must be in contact with evaporator, if now compressor will keep running increase more sense tube contact between the two.

There are two types of refrigeration temperature thermostats zone area and evaporator temperature I assumed yours is evaporator temp control.

[Zanshin]

Richard - what you suggested is what I've been doing the past day - I'm not changing anything until we know that the thermostat is in a position to shut off at the desired tempearture. This installation had the thermostats to both fridge and freezer DISCONNECTED and the C&T terminal bridged until yesterday, so I can't answer all those questions yet. The fridge is cooling down now with the thermostat set to the warmest setting and we'll monitor it to see at which temperature it will turn off. At the default 2000RPM it seems to be running 100% and only bringing the fridge down to 7C, the evaporator temperature is a very warm -6C but unfortunately I have run out of R134a so I can't flush my measurement gear hoses and measure the high and low side pressures, but the running amperage for that evaporator temperature and RPM is a lot lower than the Danfoss manual shows. I put a resistor on the C&T line so that it will run at 2500RPM or thereabouts and tomorrow I'll see what the fridge contents temperature is. There's a 100% lockdown here at the moment otherwise I would have gotten a small refrigerant can. The Seafrost BD system has a constant pressure valve which should output 1-2PSI.

With 2500RPM I think that the system will have enough cooling power to bring down the interior temperature enough to let the thermostat turn off the fridge when at the warmest setting. So the capillary tube question isn't the real issue here, it is a sideline question that has been answered by both the manual and OzeLouie.

The evaporator temperature should be -15c so with the other information, plus being told that someone had "played around" with testing the system and leaking refrigerant, I think that the system has an incorrect amount of refrigerant. But first we want to make sure that the thermostat is working and calibrated for the fridge, then we'll either correct the rest ourselves or get a local refrigeration mechanic to do that.

@moseriw - No, that is not correct, the whole thermostat does not have to be inside the fridge. While this is the most common configuration, the SeaFrost system allows an external thermostat connected by a capillary tube, the documentation states that the thermostat must be within 20 inches of the cooled space.

[Emmalina]

Don't you think you should have thawed it and tested it before posting ? just saying..

[Zanshin]

Emmalina - no, I do not. My question had nothing to do with the state of the fridge, contents, temperature or compressor. It was simply whether the capillary tube should be completely inside the fridge or not.

Everything else in this is ancillary and unrelated to my original question, a good example of thread drift; but I was responding to Richard's questions. The thread actually should have been closed once OzeLouie responded and I found the relevant sentence in the product documentation.

[Emmalina]

Originally Posted by Zanshin

Emmalina - no, I do not. My question had nothing to do with the state of the fridge, contents, temperature or compressor. It was simply whether the capillary tube should be completely inside the fridge or not.

Everything else in this is ancillary and unrelated to my original question, a good example of thread drift; but I was responding to Richard's questions. The thread actually should have been closed once OzeLouie responded and I found the relevant sentence in the product documentation.

OK my capillary tubes are external but I have a bulb on the end in the fridge and its been fine for 12 years. You ask questions and then tell us you will wait to find out the answers ?
So the capillary tube question isn't the real issue here, it is a sideline question that has been answered by both the manual and OzeLouie.

[Zanshin]

Emmalina - I'm confused by your response; I asked a question and it was answered. Richard, being the consummate professional that he is, delved deeper into the problem and asked why I was asking the question which led to a longer thread. There are other issues with the fridge, but I haven't asked for any assistance with those items. If I don't solve the issue with my friend's refrigeration after exhausting my knowledge we'll hire someone to finish the job and I will not be asking for advice here.

But this thread does showcase why several of my cruising friends browse this forum but refuse to post here.

[Richard Kollmann]

Zanshin I am sorry but identifying problem first is a good practice and avoids doing unrecoverable harm like cracking work hardened temperature sense tube when re bending it. I did answered your original question, But other refrigerator related issues are also important if related to a units performance. Louie brought up an interesting point is there a bulb on end of sense tube? No one can drive deeper into a problem without knowing what is the true problem.