Explain how the fridge works

[CedarCreekWoody]

I have a Dometic model DM2852 fridge (two actually) and I'm hoping some can explain to me how the cold air moves from the freezer compartment to the refrigerated compartment.
I am currently traveling in 108 degree weather in west Texas. It took several days to get the refrigerated area cold enough even though the freezer was 0 degrees F. I know about the thermistor on the fins and they were in the up position. I'm ok now, headed for cooler Temps, just curious how the heat transfer process.

[TXiceman]

The cold air does not move. This is a gas absorption unit. Ammonia is the refrigerant. The freezer gets the refrigerant first and the residual refrigerant is fed to the refrigerator section.

To aid in cooling you need to make sure the side of the RV with the fridge does not get direct sun on the side. Next, add additional cooling fans to the outside of the fridge to force more air over the condenser. It also helps to put a fan inside the fridge to help circulate the air.

Here is how one works:
https://www.rvrepairclub.com/article...eration-works/

Ken

[CedarCreekWoody]

Thanks for the detailed response. It is an interesting concept. I don't have to explain to you what this texas heat is like this year.
I still don't understand the role of the thermistor, what it controls, and how it's position on the fins regulates temperature.

[bobbecky]

This is the Google description of a thermistor. In the case of these fridges, replace 'ambient air temperature' with the temperature of the fin in the fridge.

A Thermistor is a simple resistance based thermal probe that changes resistance based on the ambient air temperature. The control board it is plugged into will read that resistance and turn on, or off, the heat source as needed to maintain a constant temperature.

[CedarCreekWoody]

Thanks Bob (or Becky?), that helps explain.

[TXiceman]

The thermistor is just a proportional temperature sensor that is connected to the control board to sense the fin temperature. As you move up or down the fin, the fin temperature changes.

I had trouble with one fridge which the position of the fin made little to no difference on the temperature. I bought an aftermarket thermistor and replaced it. I also moved the thermistor one fin to the left and I started getting better cooling from the fridge.

You can pry the thermistor off by slipping a very thin blade between the thermistor and the clip holding it.
Thermister

Ken

[bobbecky]

We've been running our Norcold since 2015 with the ARP device that has the extra fans and the fridge is still working great, even in hot environments. We did have an issue some years ago, and upon doing some checking, found the thermistor and the bracket that holds it on the fin was corroded which kept it from making tight contact with the fin. I ordered a new thermistor and bracket/clip and the fridge was back in good working order. https://www.arprv.com/

[hlh1]

Originally Posted by CedarCreekWoody

I have a Dometic model DM2852 fridge (two actually) and I'm hoping some can explain to me how the cold air moves from the freezer compartment to the refrigerated compartment.
I am currently traveling in 108 degree weather in west Texas. It took several days to get the refrigerated area cold enough even though the freezer was 0 degrees F. I know about the thermistor on the fins and they were in the up position. I'm ok now, headed for cooler Temps, just curious how the heat transfer process.

I don't think your question was answered, and I'm also wondering "how the cold air moves from the freezer compartment to the refrigerated compartment". On my home refrigerator all the cold is created in the freezer part and there's a duct with a fan and door that's connected and thermostat controlled to allow cold air into the refrigerator area. How is this done with these Dometic RV refrigerators?

[chuckster57]

Originally Posted by hlh1

I don't think your question was answered, and I'm also wondering "how the cold air moves from the freezer compartment to the refrigerated compartment". On my home refrigerator all the cold is created in the freezer part and there's a duct with a fan and door that's connected and thermostat controlled to allow cold air into the refrigerator area. How is this done with these Dometic RV refrigerators?

It’s NOT!! As explained, the air doesn’t “move”. In a residential refer there is a compressor like your A/C unit and a fan to move the air through ducting.

In an RV refer the ammonia boils and then starts it journey back down the cooling unit. The freezer is at the top and the plate that the freezer section is screwed into absorbs the greatest amount of heat, cooling the freezer section. As it continues down the unit it pulls more heat through the fins that are screwed into the cooling unit. When it reaches the bottom it has removed as much heat as it can. The LP flame or heating element boils it again and the process starts again.

There is a “thermal mastic” on the cooling unit tubes to aid in heat transfer.

[hlh1]

Originally Posted by chuckster57

It’s NOT!! As explained, the air doesn’t “move”. In a residential refer there is a compressor like your A/C unit and a fan to move the air through ducting.

In an RV refer the ammonia boils and then starts it journey back down the cooling unit. The freezer is at the top and the plate that the freezer section is screwed into absorbs the greatest amount of heat, cooling the freezer section. As it continues down the unit it pulls more heat through the fins that are screwed into the cooling unit. When it reaches the bottom it has removed as much heat as it can. The LP flame or heating element boils it again and the process starts again.

There is a “thermal mastic” on the cooling unit tubes to aid in heat transfer.

Hi Chuck,
I understand the fundamental differences between the two types of "refrigerators", what I don't understand is how the refrigerator part of the ammonia type system is cooled separately from the freezer. Maybe I'm making this too complicated.

[chuckster57]

There is two places that the cooling unit “pulls” heat. If you look in the freezer there are 6 or 8 screws that hold a plate tight against the freezer wall. Since it’s at the top, it has the ability to pull more heat than the bottom section.

The bottom section has fins screwed to the cooling unit to pull what heat it can before getting boiled and starting over.

[hlh1]

Originally Posted by chuckster57

There is two places that the cooling unit “pulls” heat. If you look in the freezer there are 6 or 8 screws that hold a plate tight against the freezer wall. Since it’s at the top, it has the ability to pull more heat than the bottom section.

The bottom section has fins screwed to the cooling unit to pull what heat it can before getting boiled and starting over.

Let me ask this question in a different way; does the refrigerator thermister just change the refrigerator temperature? I'm guessing so, and if so what part of the system is the thermister affecting that's separate from the freezer?

[chuckster57]

The thermistor is telling the control board the fridge temp. If it’s too warm the control board asks for more heat to boil the ammonia. The freezer will get colder as a byproduct of cooling the fridge section.

[JRTJH]

Maybe think of it this way: The cooling unit has two "cooling flasks. One is bonded to the freezer plate. It has a small tube on the exit end that flows to the second flask which is bonded to the refrigerator fins.

When the boiler heats up the ammonia mixture, it percolates into the freezer flask where it starts expanding which causes it to absorb heat from the freezer plate. As the "somewhat cooled and expanded" ammonia flows out of the freezer flask and enters the refrigerator flask, it is not as compressed or as hot, so it is "sort of not as effective, but as it expands in the refrigerator flask, it makes the fins "cold, but not frigid" and absorbs heat from the larger refrigerator box. The ammonia mixture then flows out of the refrigerator flask and makes its way back to the boiler.

The thermistor senses the temperature of the refrigerator fins and if it meets the temperature requirement (calculated by Dometic or Norcold based on each individual refrigerator model requirements to meet specific cooling requirements), and senses that the fins are "at the correct temperature" then the thermistor "opens and the control board shuts down the flame in the boiler. On the other end of the spectrum, if the thermistor does not sense that the fin is cold enough, it remains "closed" and that triggers the control board to continue heating ammonia in the boiler to continue the process....

It is of "no specific consequence" how cold the freezer gets, you just keep making the ice cream harder. The thermistor senses the "end of the cooling flow" and continues cooling until the fins are the correct temperature (which is calculated by Dometic). The "correct temperature on the fin" equates to a temperature range of around 33F to 38F. The fins are "larger than the freezer plate" because the space being cooled is larger than the freezer and the ammonia mixture is "more expanded" which means it is not as capable of absorbing heat as efficiently as it does when expanding in the freezer flask.

Again, the reason the thermistor is in the refrigerator is to measure the cooling capacity at the END of the cooling flow of the ammonia. If that temperature is met, it's assumed that the freezer is colder, and it's of no real consequence how much colder it is, since it will always be "below freezing" if the refrigerator thermistor is "slightly above freezing"....

[hlh1]

Guys, you've been very helpful and I now think I understand. It's interesting that the thermister controls the freezer temps by the refrigerator setting, and that seems kinda backwards to me compared to a household refrigerator. It's also interesting that it has "two cooling flasks" and that's what I was trying to understand, as a household refrigerator only has the one "evaporator" for the freezer and nothing for the refrigerator part other than a temperature controlled vent.



I'm asking these questions because my Dometic refrigerator/freezer runs around 33 degrees in the refrigerator and around 10 degrees in the freezer. I would like for the freezer to get closer to 0 degrees, but any colder in the refrigerator and things will freeze there.

[TXiceman]

Just think of it as magic. You put in heat and out comes cold. I worked in the industrial refrigeration market for my career. The absorption cooling process is a chemical process rather than a mechanical process like a compressor system.

The ammonia is the actual refrigerant. The hydrogen or helium gas is a buffer gas and the water is the transport agent. You have to pick a starting point some where in the process to start the explanation. Water and ammonia have a great affinity for each other. As the ammonia is boiled off from the evaporator(s) the water absorbs Absorber) the ammonia gas (strong solution) and then passes to the boiler where the ammonia is boiled off and the water (weak solution) goes back to the absorber again. The ammonia gas passes to the condenser where it is cooled and condensed to a liquid. The liquid ammonia goes to the freezer coil to absorb the heat and convert some of the liquid to gas. The remaining gas and liquid ammonia goes to the refrigerator and is mostly ammonia gas at this point. The ammonia gas meets the weal solution of ammonia and water in the absorber and picks up the ammonia gas that is not strong solution that goes to the boiler to boil off the ammonia and start all over.

Strong solution is an aqua ammonia mixture that is high on ammonia concentration. The weak solution is an aqua ammonia mixture that is low in ammonia concentration.

This is about as simple a description of the process as I can come up with for a quick answer. I spent 5 years in mechanical engineering college majoring in applied heat transfer and thermodynamics.

So just think of it as magic and let it go.

Ken

[hlh1]

I understand thermodynamics to some extent as I had to pass those classes for my Electrical Engineering degree. But I'm not totally versed in the details of this Dometic system and how it controls the process. I was hoping to figure out a way to cool the freezer more without freezing the refrigerator part. I don't like my ice cream getting soft.


And yes, thermodynamics is kinda like "magic", as is electricity.

[RollaMo]

Originally Posted by hlh1

Guys, you've been very helpful and I now think I understand. It's interesting that the thermister controls the freezer temps by the refrigerator setting, and that seems kinda backwards to me compared to a household refrigerator. It's also interesting that it has "two cooling flasks" and that's what I was trying to understand, as a household refrigerator only has the one "evaporator" for the freezer and nothing for the refrigerator part other than a temperature controlled vent.

I'm asking these questions because my Dometic refrigerator/freezer runs around 33 degrees in the refrigerator and around 10 degrees in the freezer. I would like for the freezer to get closer to 0 degrees, but any colder in the refrigerator and things will freeze there.

Take a look at this image, perhaps it will help explain how these work.
On the image, #5 is the freezer compartment, and #6 is the refrigerator compartment.
This should help explain how the two compartment temps are related, and not controlled separately.

[TXiceman]

Originally Posted by hlh1

I understand thermodynamics to some extent as I had to pass those classes for my Electrical Engineering degree. But I'm not totally versed in the details of this Dometic system and how it controls the process. I was hoping to figure out a way to cool the freezer more without freezing the refrigerator part. I don't like my ice cream getting soft.


And yes, thermodynamics is kinda like "magic", as is electricity.

I am also a ham radio operator, so I understand magic....to some degree, thermodynamics more than electricity. Electricity until you get to the square root of a negative number.

You can't control the temperatures separately in the RV fridge. The thermistor feeds the control board, and the unit is either on or off based on the fin temperature at the location of the thermistor. There is no proportional control.

At one of my 90-minute training sessions for a customer, I presented a Power Point Presentation on Absorption refrigeration comparing Li-Br cooling systems to Aqua Ammonia systems. It has been about 12 years since that presentation. The old knowledge is still there, but slower to find in the old brain cells.

Ken

[hlh1]

Ken, it was building Heat-Kit Ham radios as a teen that got me interested in an Electrical Engineering degree. WB5RQB. I've really done nothing with ham radio for the past 25 years. Now that I'm retired I'm contemplating getting back into HF and CW, but the hobby has sure changed since the 90's.