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Old 06-04-2013, 03:00 PM   #127
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I have a Danfoss compressor unit in my Liner and I am very happy with it. Nosier than the absorption type, since there is a compressor to run. It is the 9.5 CF model and takes less amperage than a fantastic fan on high. I have the Tundra by Dometic. I have two 85 watt solar cells to keep the two Optima batteries charges. With solar you must have sun for them to work. They don't work well in the shade.

Bill

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My take on a very workable upgrade to the gas/absorption fridge is a marine unit with a Danfoss compressor, which was designed for the marine industry, can take a pounding of off-shore boat use and work up to 30º off level. These work especially well if you have a decent solar charging system. The Danfoss system is both efficient (read very C-O-L-D and the end of what I call 'RV softserve' ice cream ) and switches automatically between 120VAC and 12VDC power sources. IIRC, the amp draw is around 1 amp on 120VAC and around 3 amps on 12VDC. Also, these units cycle on/off as needed so they are not running on constant duty cycle.

My next trailer will have such a system, as after seeing first hand the damage that a gas/absorption unit fire can cause, I have lost all confidence in such systems. Just my personal opinion.
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Old 06-04-2013, 03:49 PM   #128
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Lew, how much does the Danforth unit cost and does it fit in the space the OEM units take?

Gene
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Old 06-06-2013, 01:05 AM   #129
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Installing the new cooling unit.

First of all, as promised, here is a better picture of the boiler outlet and the discoloration I saw in the metal there. This does not look like a leak to me, just looks like metal that was under some high temperatures.


The baffle as I pulled it out of the gas heat tube. This is something that should be looked at if you ever pull your fridge for any reason. Inspect this! Nothing remarkable here.


So, to pull the unit, after removing all the screws and all infrastructure I placed a pry bar on the coils as shown. A slight tug and it was separated but not free. Note: The piece of wood is there to protect the delicate undersurface.


The cooling unit was loosened but I could not wrestle it out by hand, so i had to apply a little coercive force on th efreezer coils. I stood the unit upright for this step and tapped on the low temp evaporator coil with a rubber mallet for this step. It moved easily and I did not have to be overly aggressive at this point.


I easily pulled the old cooling unit at this point and set it aside to remove the last item - the high-temp coil heat exchanger, AKA the fins in the fridge compartment.


A big surprise for me when I removed the fins off the high temp evaporator - there was NO THERMAL MASTIC at all between the fins and coils. To me there is ineffective heat transfer capability in this situation. The thin layer of adhesive aluminium is not what I would think of as an effective heat transfer layer. I can see this as being part of my problem. Dometic, if you are reading this, I think you can do better than this in my opinion. There is minimal thermal bonding here.


Prior to inserting the new cooling unit I inserted a thermocouple at the functional input to the high temperature evaporator, or what could be thought of as the outlet from the low temperature evaporator. The TC is embedded against the tubing in the foam with some thermal mastic and aluminium tape to hold it all in place. OK, I know this is putting me in the category of a geek and perhaps by some acounts a Loser, but I don't care, I have to know what the temperature of whatever ammonium is reaching this point in case this unit does not work for me. See, if the freezer is COLD, there will be plenty of liquid ammonia reaching the high temperature evaporator to cool the fridge body. The TC is under the strip of aluminum tape above the fins.


I also placed a thermocouple at the inlet to the condenser after the last moisture seperator to see what the temperatures were at that point. Each subsequent TC I placed is hose clamped in place with a piece of fibreglass cloth as a protective barrier between the clamp and the metal.


I also placed a TC at the boiler outlet.


And I have one placed at the lowest point of the condensed ammonia inlet to the fridge. This point the ammonia is (or should be) in liquid form and it is further cooled by the high temperature evaporator as it is paralleled along that circuit on its way to the low temperature evaporator, further cooling it.


I may place another TC at the outlet of the high-temp evaporator prior to going into the absorber coils. I think this will give me a good idea of how this unit is working on the road. I know all of this is a little overboard to place TC's all over this unit like I have, but I don't care. I am totally in learning mode here and I do hope that somehow my data gathering here will allow for us all to learn how these units work in normal or even abnormal situations.
I wish that Dometic had data for what a unit like this runs at so people could better diagnose their units, but they do not. The Dometic "rep" I talked to told me to call my local repair facilities as they did not offer the type of support I was asking for. One minute with my local "certified" individuals told me that they had basic knowledge of these units, but had no in depth knowledge of the basics of the cycle. Faced with a problem like mine and they simply locked up and said "I can't help you".

Anyway, next step is to re-install the fridge and run a new test with all my data logging stuff in service. Watch out!!!!!
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Old 06-06-2013, 06:59 AM   #130
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Very informative photos. Question about the photo of the freezer with the rubber mallet.

I don't remember seeing any tubes sticking into the freezer compartment of my Dometic 3762, its just one big open box. Are the freezer evaporator tubes located on the bottom of the freezer?

Does yours have a cover in the freezer or are those tubes exposed?

Thanks

Kelvin
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Old 06-06-2013, 07:48 AM   #131
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Looks like no obvious leaks in the old unit. I believe leaks show up as yellow stains around the leak area. I am not sure I would throw away the old cooling unit. It might be worth making some bench tests on by simply hooking it to a heater and see what happens.

Perry
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Old 06-06-2013, 09:36 AM   #132
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"OK, I know this is putting me in the category of a geek and perhaps by some acounts a Loser…." I think a better word would be "expert in training."

Gene
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Old 06-06-2013, 11:30 AM   #133
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Quote:
Originally Posted by lewster View Post
My take on a very workable upgrade to the gas/absorption fridge is a marine unit with a Danfoss compressor, which was designed for the marine industry, can take a pounding of off-shore boat use and work up to 30º off level. These work especially well if you have a decent solar charging system. The Danfoss system is both efficient (read very C-O-L-D and the end of what I call 'RV softserve' ice cream ) and switches automatically between 120VAC and 12VDC power sources. IIRC, the amp draw is around 1 amp on 120VAC and around 3 amps on 12VDC. Also, these units cycle on/off as needed so they are not running on constant duty cycle.
Lew, I was interested to read this as I was just recently introduced to such a system this past weekend when sailing with a family member. He had just installed what I believe to be this exact system for his boat, with his deep well in-counter fridge and freezer separates (the RIGHT way to do it for efficency) added with a hydraulic power take off from the boat motor to directly drive the compressor separate from the 1 amp 12V systems. I will be looking at something like this in the future as well. I'm beyond RV soft serve ice cream with my Dometic. I can drink it with a straw.
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Old 06-06-2013, 12:15 PM   #134
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Danfoss is a manufacturer but I think the type of compressor would be called a rotary compressor or scroll pump. This is the type used in most home AC units. The older compressors used pistons.

Perry
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Old 06-06-2013, 12:47 PM   #135
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My Prius has an electric powered AC system. I can be stopped in traffic and the AC still works despite the engine turned off. Eventually the nicad battery drains down and the motor starts up to charge the battery.

It would be cool to have an electric compressor to aid the evaporator system in times of high ambient temperatures or for recovery. I hate having to rush in and out of the fridge to get something out because if the door is left open for a few seconds you loose a bunch of cold air. So if you have your AS on AC the DC powered compressor could run all the time or you could set it to help out the absorbtion unit when you open the door. While on the road if the batteries could be maintained while driving the compressor could run on and off or assist the propane mode. For boondocking hopefully a solar panel could charge the batteries faster than the compressor could pull from it. It probably wouldn't be cost effective to have this combo unit so I'll have to stick with the standard technology.

Kelvin
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Old 06-06-2013, 01:45 PM   #136
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Quote:
Originally Posted by KJRitchie View Post
Very informative photos. Question about the photo of the freezer with the rubber mallet.

I don't remember seeing any tubes sticking into the freezer compartment of my Dometic 3762, its just one big open box. Are the freezer evaporator tubes located on the bottom of the freezer?

Does yours have a cover in the freezer or are those tubes exposed?

Thanks

Kelvin
No, they are against the back wall probably for that freezer. This is not your cooling unit, but it would look something like this -


My fridge has that shelf thing and an aluminium shelf of sorts clamps over the tubs to create the shelf in the freezer. It's primary purpose is to transfer heat, not act as a shelf per se.
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Old 06-06-2013, 01:50 PM   #137
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Perry, no, no obvious leaks, but perhaps the charge was not correct - not enough Hydrogen...who knows. Maybe the bubbler is partially obstructed, mayve the condenser fins are not level. Maybe the lack of thermal mastic for the high temp evaporator forced the fridge to run on high all the time causing things to get so hot that ammonia started to boil & evaporate before it ever got to the low temperature evaporator. Just don't know. It may be the trailer itself hasd some vibration that somehow makes the fridge fail. I have no way of knowing for sure.

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Looks like no obvious leaks in the old unit. I believe leaks show up as yellow stains around the leak area. I am not sure I would throw away the old cooling unit. It might be worth making some bench tests on by simply hooking it to a heater and see what happens.

Perry
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Old 06-06-2013, 02:22 PM   #138
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Did you ever try to burp the thing (turn it upside down)?

Perry
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Old 06-06-2013, 06:24 PM   #139
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Did you ever try to burp the thing (turn it upside down)?
Perry
Oh yes, I pulled the fridge just to do that. Rotated it on each side and top even - every direction. No change - it worked at a standstill and quit as soon as we got on the road.

I would have never bothered to "burp" the unit had I known how the cycle works as I do more now and had I learned what burping really does. There is a thought that it mixes up the solution inside and removes pockets of gas and any trapped water. But, these units are built with a slope in every component such that at a reasonable level state, the liquid state fluids gravitate downward. All burping does is to potentially dislodge some crud that may have gotten inside during the welding of the tubes. A small blockage from welding particles could plug the bubbler pump in the boiler. A "burp" could dislodge that, but only temporarily.

Burping is something to perhaps try if your fridge doesn't work at all and you see no visible signs of leakage.
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Old 06-06-2013, 06:53 PM   #140
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I was quite dismayed to see the total lack of thermal mastic on your high temp. evaporator section! I would send this shot to Dometic and would be very interested in their response. Proper heat transfer can not happen without the right amount of thermal mastic present along the evaporator surfaces for either the freezer or fridge sections.
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