I think we're ready for this, I haven't processed a pic of a cooling unit yet, or scanned a schematic - but keeping things down to one bite at a time seems like a good idea.
RV fridges can be thought of as a type of "STILL". The cooling units are sealed, and never recharged. When they go bad, you junk them and replace it. For some - the cost of replacement is so close to what it costs to replace the entire unit that it makes no sense not to just get another reefer for the coach. There's just one fly in the ointment:
Will the refrigerator fit through the entry door, or does the door to the coach have to be taken apart?
I am 100% serious about that...
For a double door refrigerator it might even be necessary to remove a slide out to have room enough to replace it.
But the cooling units can be removed from the refrigerator and a new reinstalled right there in the middle of the coach. I've done that before.
This "COOLING UNIT" is located on the back of the refrigerator, and goes all the way from the bottom to the top. It is made of specially formed tubing.
The basis of an RV cooling unit these days is pretty much this, whether it is Dometic, Norcold, or whoever:
A heat source is used to vaporise ammonia (CHANGE IT'S STATE) and cause it to rise to the top of the cooling unit's coils. Because the evaporation, condensing, and boiling point of ammonia is radically different in a HYDROGEN GAS environment, the system has not only Ammonia, but also Hydrogen in it.
One more thing is necessary - something to prevent corrosion. A substance very similar to Zinc Chromate is also inside the cooling unit. If the unit ever leaks - you will see a bright yellow substance in the area of that leak. You can also spot a leaking RV Reefer by a very strong Ammonia smell inside or behind it.
If you look at the back of an RV Reefer, you will see tubing runs (coils) that the coolant travels through. Notice that they are at a very slight angle, and if you feel them they are cooler towards the top than the bottom.
Because we must have flow down through the coolant coils, if the trailer is far enough out of level in relation to a side to side line from one side of these coils to the other, the coolant runs can pool coolant at a bend in the runs, and cause flow blockage.
This may not by itself prevent the coolant from running top to bottom - but it will slow it down a lot! And it may also make the boiler area run dry, and get "COOKED" (not a good thing). An overheated boiler area (READ: BURNT) will result in weakening of the metal at that point, and may change the composition of the corrosion preventive inside. It definitely will not cool well like that.
Most RV Reefers have the back of the fridge mounted next to a side wall, since it is more likely to be level than a wall going from one side to the other.
As a rule, you should maintain a coach level to within about four or five degrees. The closer you get to true level, the better off you are, because the coolant will travel as close as possible to the original design. On the road, you don't have any way to keep it level, but it will change one way and another, so that if coolant is trapped it won't be for long.
It seems very strange to think of causing a cooling effect by adding heat to something, but is it any stranger than using heat of compression in an A/C unit to cause a change of state? (Liquid to gas, and then back).
The main difference here is that when the coolant (ammonia) condenses at the top of the cooling unit, it becomes a liquid again. And it is the liquid state that is cold - unlike an A/C unit, where it cools by entering into a gas state.
BOTH require airflow to draw out heat energy from the coils. This is supremely important! If you have no airflow, you can't cool the coolant in it's different states. And this is why there is a space from the bottom of the reefer compartment to the top of it behind the reefer.
That space is also critical. If it is too wide, the airflow goes around the cooling coils, and much of the cooling effect is lost. One way to correct a situation like that is to place a strip of some material on the outer wall, so that air must rise THROUGH the coils. Electric fans can also be added.
THE ESSENSE OF ANY COOLING PROCESS IS THAT WE ARE TAKING HEAT FROM SOMEPLACE WE DON'T WANT IT, AND PUTTING IT SOMEWHERE ELSE WHERE WE DON'T CARE ABOUT IT!
There is NO SUCH THING as "COLD ENERGY". You need to understand this from the beginning. "COLD" is the absence of "HEAT ENERGY".
That's all it is.
I will get in deeper to the physics of it in the next PASS on this topic. For now this is enough to chew on for a while.
One other note: The reason a corrosion preventive compound was added was because corrosion can cause a blockage above the boiler area. If this happens, the fluids in the boiler never have a way out when they reach vapor temperature (they really are not supposed to boil - I used that phrase because it is the most descriptive).
WHEN THIS HAPPENS: You can tell it has taken place because you will hear a "GURGLING" sound in the pipes in the vicinity of the burner assembly. It's the sound of trapped coolant actually raised to a rolling boil temperature because it has nowhere to go! And no - you can't fix that. It has to be replaced.
1) The coolant is Ammonia, in the presence of a Hydrogen atmosphere - which makes it behave differently.
2) Airflow is needed.
3) Coach levelling is critical to the units functioning.
4) The unit is sealed. If it leaks - we can tell.
ONE MORE THING!
BUGS LOVE burner assemblies, and they really don't care if your ice cream is cold or not....
Check them everytime you get ready to go on the road. A nest in a burner tube is the last thing you want.
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