Absorption Refrigerator Quality of Heat

[Fridge Defend]

We were contacted after posting the term ‘quality of heat’, the iRV2 reader was confused by this. They ask some questions that promoted a thoughtful exchange of knowledge, values and experience among RV enthusiasts using absorption refrigerators.

Quality of Heat Absorption Fridge Boiler Curve

Above is a boiler curve for a Norcold fridge. We feel that this is the best way to teach what 'quality of heat' means. This fridge came to us with the complaint:
"your product keeps turning off my fridge for no reason".

Upon investigation, there was a failed electric heater on this fridge, and there is no way to know if one of the two heaters fails. The Norcold recall will not detect this situation. Here is an explanation of the curve, we fixed the heater, then preformed this test:

A) The refrigerator goes through a normal cooling period with both heaters connected and working properly. The refrigerator thermostat turns on the cooling unit; the temperature rises, and then holds at about 187°C.

B) One heater is disconnected, and then the refrigerator is allowed to go through 3 more cycles.

C) Note that the maximum temperature is now 192°C on startup. First clue that there is an issue.

D) The temperature drops to 179°C rather than the 187°C with both heaters. Second clue.

CONSEQUENCES: If the fridge is allowed to keep running, the boiler eventually overheats. One may ask these questions:

• Why the boilers curve changes with just one heater operating?
  
• Why would the boiler overheat when there is not enough heat?

The answer is simple, and it is reflected in the fact that the fridge is not cooling properly, and this is a 'quality of heat' issue.

A1) One will find in both Dometic and Norcold service manuals that “the proper amount needs to be applied to the boiler”.

A2) When not enough heat is applied to the boiler, the pump in the boiler is either less efficient, this results in poor cooling on either LP Gas (propane), or Shore Power (120VAC heaters) for a multitude of reasons. In this case, one heater failed, and the boiler eventually stopped working.

This creates a dangerous situation that the end user never knows about, the recalls do not address this situation. Only if one monitors boiler temperature does one know that this problem is occurring. A lot of folks find this out by their fridge failing. When they put in a new cooling unit, they replace the heaters, this solves the electric heater issues. If it is a gas problem, then they end up having to solve the gas issue.

We hope this answers this particular question, and we can keep this thread going as we are more than happy to answer any questions.

[TandW]

One can measure the quantity of heat. But heat has no quality. Neither low nor high. Heat has energy that may be classified as potential or kinetic. The energy that produces heat may be classified by quality.

[TXiceman]

I am a retired mechanical engineer that spent my career in industrial refrigeration, thermodynamics and heat transfer. I am totally confused with your term
"quality of heat".

You are posting info that is way over 99% of the folks on this forum. I have designed large industrial ammonia absorption cooling units and the concept is totally foreign to most folks. They can understand mechanical refrigeration, but not the absorption process which is really a chemical process. Putting in heat to get out cold just does not compute.

The biggest issue has come over the years with both Norcold and Dometic, cheapening the unit to the point of poor performance. Comparing older units which were more tolerant of misuse than the new units, the new units weigh much less. They have reduced the coil size and coil thickness. Now they have to run the boiler to higher temperatures which is less tolerant of misuse. It also stresses the coil more which leads to premature failure both chemically and mechanically.

The recalls from the manufacturers were nothing more than a patch to shut the unit off before it reached the point of damage. As far as I am concerned, both Dometic and Norcold are junk. I applaud you for coming up with your add on system to help improve the operation and safety of the ammonia absorption units. If I ever own another ammonia absorption unit, it will definitely have your system added to it.

Ken

[Fridge Defend]

Originally Posted by TXiceman

I am a retired mechanical engineer that spent my career in industrial refrigeration, thermodynamics and heat transfer. I am totally confused with your term
"quality of heat".

You are posting info that is way over 99% of the folks on this forum. I have designed large industrial ammonia absorption cooling units and the concept is totally foreign to most folks. They can understand mechanical refrigeration, but not the absorption process which is really a chemical process. Putting in heat to get out cold just does not compute.

The biggest issue has come over the years with both Norcold and Dometic, cheapening the unit to the point of poor performance. Comparing older units which were more tolerant of misuse than the new units, the new units weigh much less. They have reduced the coil size and coil thickness. Now they have to run the boiler to higher temperatures which is less tolerant of misuse. It also stresses the coil more which leads to premature failure both chemically and mechanically.

The recalls from the manufacturers were nothing more than a patch to shut the unit off before it reached the point of damage. As far as I am concerned, both Dometic and Norcold are junk. I applaud you for coming up with your add on system to help improve the operation and safety of the ammonia absorption units. If I ever own another ammonia absorption unit, it will definitely have your system added to it.

Ken

Hi Ken, one engineer to another.
As you said "recalls from the manufacturers were nothing more than a patch to shut the unit off before it reached the point of damage."

This is just not correct because the boiler can still be damaged and fail due to the fact that the recalls do not monitor the boiler temperature and turn off the fridge before damage can be done.

I posted the following video so you can better understand the failure mode:

https://www.youtube.com/watch?v=FjuW3kN2xQs&t=27s

Posting information is important, this is because it is the engineers job to keep the public safe.

[Fridge Defend]

Originally Posted by TXiceman

I am a retired mechanical engineer that spent my career in industrial refrigeration, thermodynamics and heat transfer. I am totally confused with your term
"quality of heat".

Ken

Hi Ken, please let me explain the thermodynamics definition (lettered):

a) During a vaporization process, a substance exists as part liquid and part vapor.

b) This describes the absorption refrigeration boiler found in Dometic and Norcold cooling units. That is, there is a mixture of saturated liquid and saturated vapor.

c) To analyze the aforesaid mixture, properly, a term needs to be defined for analysis of the proportions of the liquid and vapor in the mixture. See YouTube video in last post.

d) To put it all together, a property called quality is defined which is a ratio of the mass of vapor to the total mass of the mixture for thermodynamic analysis.

When one is on the cutting edge of technology, such as our patented product, terms need to be defined that describe in scientific terminology what is happening. Thus, we choose 'quality of heat' because if an improper amount of heat is applied, then the quality is wrong for the boiler to work. The quality of the mixture is affected by heat, thus we coined the term quality of heat.

Thanks for asking, I hole the video helps

[TandW]

There is saturation temperature at specific pressure and then there is superheat, otherwise known as dry steam.

[bigb56]

Is the recommendation not to use the 12V mode on the highway for units with dual heaters only?

[Fridge Defend]

Originally Posted by TandW

There is saturation temperature at specific pressure and then there is superheat, otherwise known as dry steam.

Thanks for including this, the Fridge Defend measures the phase change temperature of the water and ammonia mixture in the Dometic and Norcold boilers.

If you read our patent, you will see that we in effect our patented a flow monitor measuring the phase change temperature of the fluids within the boiler of the Dometic and Norcold refrigerators. Dometic and Norcold liked it so much, they tried to re-patent our product, but the US Patent Office rejected their applications because our patent is prior art (term of art).

So the saturation temperature at 350psi in the cooling unit is around 180degC. If the temperatures rise, then the quality has changed, and then when the water boils, your fridge fails by slow death

[TandW]

Perhaps you should copyright your term and elaborate more on the weak and strong ammonia cycle.

[Fridge Defend]

Originally Posted by bigb56

Is the recommendation not to use the 12V mode on the highway for units with dual heaters only?

Thanks for asking this question, one poster stated that the fridge can be run when diving, it says so in the manuals. But, lets quote Dometic training document:

"Only use DC heating element when driving from location to location or to maintain temperature"

We used DC heaters until we found out why our fridge never cooled well on DC.
Note that Dometic states maintain temperature

This is a key statement, we have about 10,000 miles of data collection from Dometic and Norcold boilers while driving down the road. One critical issue arose using the 12VDC heaters, just as Dometic stated, the DC heaters would maintain temperature if switched from say shore power or LP gas to the 12VDC heater. But, if the fridge turns off because it is cool enough, the DC heater did not have enough heat to restart the process. This is where we coined the term quality of heat (among other things like wind stagnation while driving).

[Fridge Defend]

Originally Posted by TandW

Perhaps you should copyright your term and elaborate more on the weak and strong ammonia cycle.

I think we also coined the term 'SPAR', single pressure absorption refrigerator. The entire Dometic and Norcold cooling unit is one container (pressure vessel). I did this in our patent because I was using single pressure absorption refrigerator over and over.

Please see this video, it does a good job of defining the strong and weak solution:

https://youtu.be/3JB0QyPvEAo

It also shows why the boiler stops cooling in the situations described above.

Thanks gentlemen

[TandW]

Thank you!

[TXiceman]

When I was having problems with a Never-Cold, a few years back I finally got to talk to a fellow at Norcold about the issues. All he could do was to refer me to a local technician....ha ha ha. He said that they could not provide that type information due to pending litigation. They knew they had problems and were just hiding from the issue. I pointed out to him that his recall which had just come out did not solve the problems, just tried to stop the unit before there was damage and only addressed part of the problem.

I managed to get it cooling better by adding two additional fans outside for better cooling (it already had two running on a thermostat). I wired the 12 VDC fans in series to operate at a slower speed on 6 volts. I added a 3" fan inside to get better air circulation, replaced the thermistor and moved it one fine to the left.

Without getting into the thermodynamics and chemistry, the unit now cooled much better. I was looking at getting your Defender, but we sold the RV and bought a unit with a residential fridge, 6 VDC battery bank and a 2800-Watt inverter. I was a much happier camper. It kept the Blue Bell ice cream frozen and the beer and steaks cold as well.

We have had older RVs with ammonia absorption units that cooled well and lasted 30 years. In the past, they could design a reliable unit that worked, but the all-mighty accountant rules and the engineer gets kicked to the curb. Just design things to last through the warranty and no more.

I am happy to be retired and not having to fight those battles any longer.

Ken

[bigb56]

Originally Posted by Fridge Defend

Thanks for asking this question, one poster stated that the fridge can be run when diving, it says so in the manuals. But, lets quote Dometic training document:

"Only use DC heating element when driving from location to location or to maintain temperature"

We used DC heaters until we found out why our fridge never cooled well on DC.
Note that Dometic states maintain temperature

This is a key statement, we have about 10,000 miles of data collection from Dometic and Norcold boilers while driving down the road. One critical issue arose using the 12VDC heaters, just as Dometic stated, the DC heaters would maintain temperature if switched from say shore power or LP gas to the 12VDC heater. But, if the fridge turns off because it is cool enough, the DC heater did not have enough heat to restart the process. This is where we coined the term quality of heat (among other things like wind stagnation while driving).

Thanks for explaining, I didn't know about the 12 volt heater not being able to re-start the cooling process, maybe that's why it bypasses thermostat control? The 12 volt on my Dometic 8555 did not maintain the temp at all, after I discovered it was only seeing 11.5 volts with the engine running I rewired it straight from the battery bank (fused of course) with #8 wire and now get 13 volts while driving (gained 32 watts) but it still loses ground while driving longer distances just not as bad. This year I may try driving with it on gas, have not tried that with this rig. I have done numerous mods to my 8555 including fans & Fridge Defend, corrected the bad venting, insulation and venting flue to the outside instead of letting it dump out under the condenser like Winnebago left it