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Old 07-09-2015, 12:24 PM   #1
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Large Evaporator Line is HOT to the touch...

I have a 2014- 25 foot with the two compartment refrigerator/freezer.

I started the refrigerator this afternoon for cooling it down. Not long afterwards the vent fan to cool the ammonia lines started. The large diameter ammonia line on the right side was too hot to touch. The small lines on the left side were slightly warm.

When I say TOO HOT, you could burn your hand/finger if grabbing it, much like the exhaust manifold on a vehicle... hot.

This is really the first time I considered to check the ammonia lines before. I had trouble with the vent fan snap switch staying on even when the refrigerator was shut down after our last trip and just keeping close attention and cooled to room temperature around 65 degrees.

My question is... is the larger diameter ammonia line being too hot to touch... normal? It was hot enough to seem unusual. The system is on electrical at the moment as we will be leaving in two days and this is the cooling down time and stocking the refrigerator.

Idroba seems to have some insight on the operation of the evaporator sections. The refrigerator is set on #3 at the moment. It is empty and cooling down.

How HOT can I expect this cooling down loading evaporator remain hot, or is that typical for this refrigerator during all operation while in use?
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Old 07-09-2015, 09:57 PM   #2
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If you are talking about the vertical column on the far right, yes, that gets hot when the fridge is working.
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Old 07-09-2015, 10:23 PM   #3
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yup... strange, but it uses heat to cool... do NOT touch these pipes when operating or even a while after shutdown... they do not 'stop' immediately like a 'compressor/freon' based unit..
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Old 07-09-2015, 11:52 PM   #4
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The "large ammonia tube" on the right side will be hot, it is part of the ammonia refrigerator cycle. What it contains is a mixture of ammonia gas and water vapor. That goes to the top fined condenser tube. The water vapor condenses out and returns directly back down the boiler tube where the heat is applied. The ammonia is cooled and condensed in the upper fined section, back to liquid ammonia. That liquid ammonia is then passed through the freezer section where it picks up heat. Then the ammonia passes through the refrigerator section (where the fins are in the upper back of the refrigerator) and picks up more heat from the refrigerator part. The ammonia gas then combines with water, is heated, and starts again.

The above is a gross simplification, and I learned it at the Vintage Airstream Rally presentation at Gunnison last July. I do not claim to be a real expert on these systems.

But two things:

1. Those upper tubes will be hot as they essentially have very hot water/ammonia gas in them.

2. The freezer gets the first cooling, what is left over of the ammonia (that is not evaporated in the freezer) is available for the refrigerator. If you overload the freezer with warm things (even water to make ice on a hot day) you will cause all the cooling power to go into the freezer and none will be left for the refrigerator. Thus you may have a cold freezer but a warm refrigerator section until the freezer load is satisfied. And remember, the total capacity of the ammonia/water RV refrigerator is much more limited than that of compressor (home) refrigerators.

Another thing I learned at the presentation is that if the upper lines are overcooled, not only the water vapor will condense and run back down, but the ammonia can condense in the wrong place and rather than going to the freezer as liquid ammonia to evaporate, it will return with the water and much reduced or no cooling will occur. So we want the upper lines to be hot enough so the ammonia and water vapor condense where they are designed to condense. Too much cooling can cause issues too. Strange huh?

Again, this is stuff I learned from someone else, and I hope I am translating it correctly. I wish the video of his presentation had come out, but it didn't apparently.
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Old 07-10-2015, 03:46 PM   #5
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VENT- keep Closed or prop Open for maximum efficiency?

Idroba, you are a wealth of information. I read some of your other comments on earlier Threads and am getting my Dometic Operation Plan Experience, DOPE for short.

Our Dometic is 10F freezer, 38F refrigerator, set on 3, outside air temperature at 2:30PM is 78F. Fan was OFF and large ammonia line was warm earlier. Currently hot and fan running.

I have been experimenting with the upper vent door being open and use a long piece of trim wood to keep it open with fan running at about 14" up.

I have closed the upper vent to see if I notice anything that tells me that the closed door is better than having it open while the fan is running. Although not a Dometic Engineer research technical sheet... it appears to me that having the Upper Vent CLOSED draws more air through the back of the cooling fins.

By being close to the vent area with the door open, the fan is drawing out a cooler air flow.

By being close to the vent area with the door closed, warmer air is flowing out. It could be possible that by propping open the LOWER Door would assist the air flow on a 100F+ day. This would seem to contradict MY misdirected "common sense" of applying various options to remedy the continuous running of the fan as it "cools" the ammonia lines with upper vent options- closed or propped open. The "cooling fins" really do not seem to be very warm or drawing much heat off of the HOT ammonia line....

Idroba comment about "over cooling" the ammonia system might defeat the system. That is possible, since we are not "compressing" and "expanding" the gases to absorb heat and cooling like in a home's AC and a automobiles AC compressors. During the winter our "garage refrigerator" will not cool as well, since the garage is at 48F minimum with a ceiling furnace running to maintain that setting. I can see the relationship you mentioned earlier.

Well, that larger diameter line in our larger refrigerator is HOT. The smaller unit in our 23 footer never seemed to be that hot. The cooling fan did not run as much, either.

I remember when a relative was cleaning out a deceased relatives cabin it had an ammonia cooling system in their full sized, well insulated, refrigerator. Had the big dome on top. Worked perfectly and was natural gas operated, just a larger system. So Dometic is using a smaller scale 1940's technology that works.

IF anyone has experimented with the Upper Vent CLOSED or propped OPEN, any insight?

Or propped open the LOWER VENT to improve air flow...?
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Old 07-10-2015, 05:36 PM   #6
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Another reason to keep the refrigerator compartment clean and dust free along with the installation of an on-board fire suppression system.
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Old 07-10-2015, 08:30 PM   #7
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The ammonia/water in the system gets boiled! so yes it can be that hot. Everything Idroba says is right on with the theory of the ammonia cooling system. The hot tubing is the condenser and the cooling coils buried in the back of the fridge are the evaporator. As noted when liquid ammonia passes through the fridge, it evaporates and takes heat out of the freezer and fridge. The cooling tray in the freezer has the pipes running through it so it gets good freezing. The fins in the fridge are simply in contact with a small portion of the evaporator, and usually coupled by some conductive putty so they are sized to give about 35 -40 F in the fridge.

The cooling system actually has ammonia, water, and some hydrogen gas. If any of it leaks the system either stops working or works at a very limited capacity. Re-builders can do a refurbished cooling system for about $800. Works just as good as a new fridge for 3 0r 4 times that cost.
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Old 07-11-2015, 08:30 AM   #8
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Here is a ping for ya.....

My head hurts thinking about this... Maybe you can figure it out. Kinda looks like Voodoo to me..... Read it while the link works!

How Absorption Cooling Works
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Old 07-12-2015, 03:39 PM   #9
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The link noted above describes absorption cooling for airconditioning. Ammonia is used for refrigeration. Even so the description is not well stated.

Wikipedia has a much simpler explanation
https://en.wikipedia.org/wiki/Absorption_refrigerator

Basically you evaporate the ammonia dissolved in water by boiling the water. The hot pipe noted above is cooling the hot ammonia gas until it cools enough to liquify. The Liquid Ammonia passes through the coils in the freezer and picks up some temperature from the freezer; even at freezing temperatures, the ammonia wants to get back to a gas. In expanding to a to a gas it takes the heat from the freezer with it and it goes round the loop again. Water and hydrogen gas are present to help move th ammonia from the fridge to the chimney where the flame heats the liquid to start the cycle all over again.

I know ...for you reefer types this is full of holes but it is basic enough to help newbies through the process
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