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lewster · 06-04-2013, 01:11 PM

Quote:

Originally Posted by Buttercup

It would be interesting to know if these fail from a rust situation or a cracked welds more often.

Most of the failures I have seen (and I have been involved in a law suit on a Norcold 1200 series recall that led to a coach fire) have been at the welds and not where the tubing has corroded thru. You would think that both Norcold and Dometic would realize this (actually, they probably do and don't are to address the situation, just like the class action suit now progressing on the Norcold 1200 series recall where Norcold is liable for numerous coach fires, yet does nothing to re-design the welding flaws in their cooling units) but they find in easier to simply ignore the obvious!

Gene · 06-04-2013, 01:16 PM

I don't know what a compressor that would withstand the insults of the road would cost. It would only be for cooling with 120 v. hookups. The ones in refrigerated trucks can hold up, so it can be done. The absorption unit would only be used for boondocking and driving. To make it competitive, the manufacturer would have to accept less profit and/or find automated ways to make the thing.

Until a company wishes to challenge the big guys (Norcold and Dometic), this isn't going to be even considered. Though the present fridges can be fixed, we still have to buy the parts from the same manufacturers so they can make even more money.

Inventors, get to work.

Gene

lewster · 06-04-2013, 01:18 PM

Quote:

Originally Posted by Gene

Lew, what's the alternative? Perhaps one that operates a standard compressor on 120 v. and switches to gas/absorption on the road and while boondocking. Gauging by how cheap window air conditioners are, I would think a compressor would be pretty cheap and get the box cold so the gas/absorption unit would only have to maintain it between shore power opportunities. They could also insulate the box better.

Gene

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.

lewster · 06-04-2013, 01:22 PM

And thanks, Gene, for forcing me into my 4000th post! I don't know if I should weep or celebrate!!!

KJRitchie · 06-04-2013, 01:24 PM

Only two fridge manufacturers left, Dometic and Norcold, although I hear Atwood is got a helium fridge. I guess they can say our fridges won't Hindenburg. Not much competition to innovate or make changes.

Kelvin

lewster · 06-04-2013, 01:38 PM

From my recent brush with being involved in a lawsuit over a Norcold-fueled fridge fire and subsequent total loss of the RV to said fire, it is not the hydrogen that causes the ignition in RV fires, but super heated ammonia blowing out of a small rupture in the cooling unit under high pressure and hitting either an open LP flame or hot (600ºF) electric heating element. The hydrogen in the cooling unit never makes it below the upper half of the cooling unit, and the majority of RV fridge fires occur right behind the boiler section where the LP burner and heating element(s) are located.

Many of the recall devices are designed to shut the 12VDC power to said control boards and with no DC power being applied to the LP gas valve, the solenoid in this valve will shut the gas flow, as it is designed to be normally closed; requiring 12VDC to stay open and keep the flame lit.

This scenario does NOT APPLY when the fridge is being operated on 120VAC electric heating elements, as they get really hot (600º +/-), are encased in steel tubes which are also at that temperature, and then wrapped in heavy fiberglass insulation. This helps keep the heat in when operating normally, but in the event of a rupture, also retains the heat and provides an ignition source for the super heated ammonia which WILL and DOES cause ignition!

How long does it take for you to put your hand on an electric range after it has been operating on HI after you turn off the power?

wkerfoot · 06-04-2013, 02:00 PM

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

Quote:

Originally Posted by lewster

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.

Gene · 06-04-2013, 02:49 PM

Lew, how much does the Danforth unit cost and does it fit in the space the OEM units take?

Gene

Buttercup · 06-06-2013, 12:05 AM

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!!!!!

KJRitchie · 06-06-2013, 05:59 AM

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

perryg114 · 06-06-2013, 06:48 AM

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

Gene · 06-06-2013, 08:36 AM

"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

Westfalia · 06-06-2013, 10:30 AM

Quote:

Originally Posted by lewster

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.

perryg114 · 06-06-2013, 11:15 AM

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

KJRitchie · 06-06-2013, 11:47 AM

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

Buttercup · 06-06-2013, 12:45 PM

Quote:

Originally Posted by KJRitchie

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.

Buttercup · 06-06-2013, 12:50 PM

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.

Quote:

Originally Posted by perryg114

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

perryg114 · 06-06-2013, 01:22 PM

Did you ever try to burp the thing (turn it upside down)?

Perry

Buttercup · 06-06-2013, 05:24 PM

Quote:

Originally Posted by perryg114

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.

lewster · 06-06-2013, 05:53 PM

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.