stevenal wrote:
When placing my order, though, I saw something interesting. The same page shows the 210W 12V DC element. So the original problem was that Dometic put a 354W element in a tube designed for 325W, and now folks are advocating putting a 325 Watter in the 210W rated DC tube. Am I the only one seeing the problem with this idea?
Steve
What lead you to believe the tube to hold the 12 volt tube was rated for any type of wattage? What mechanically makes a 210 watt or 325 watt tube welded to a boiler tube rated?
Where did we get to that Dometic actually designed a tube for any size wattage?
The problem here is believed to be the "rates" of thermal expansion of different thickness parts working against each other creating high stresses and the weaker part breaks first. In this case the main boiler tube verses the actual weld attaching it to the electric element tube. The boiler tube shrinks at a different rate then the actual weld. Cold fluid is inside the boiler tube being heating actually cools the boiler tube yet the weld stays hotter longer. The rates at which the weld and boiler tube heat/shrink are different. This fits to why the boiler tube tear was at the end of the weld. It is the point getting the most stress where all this heating/cooling expansion is going on.
If the 12 volt DC tube had a much thicker cross section of weld than the 120 VAC tube, Or the actual element tube was thicker, then yes I could agree using a higher wattage element on the 12 volt pocket may make the matters worse.
But we do not have indication of the welds being larger or the 12 volt tube or the tube being thicker. Moving the element to a different zone that has not been stressed has merit behind it for 2 reasons, even if that element tube is identical to the 120 VAC tube.
If someone's unit has the thermal expansion problem, to prolong life until failure, they might try:
1. Reduce the heat to reduce the stresses. = A proper rated element.
2. Stop stressing the affected area. Move the element from the stresses pocket. = move from 120 VAC to 12 VDC.
Thermal cycle fatigue once you have it, is time cycle driven. It is predictable that it will fail. It will just take a number of thermal cycles in order for it to happen.
After thinking more on this,
It would fit the pattern of thermal cycle fatigue that Cooling units with heating tubes that have very thin welds would have less issues of fatigue than welds of thick cross section.
After seeing what has been presented so far, my take on this issue is the actual welding concept and or the size of the weld bead is the design/manufacturing problem that is agravating this. If the element tube was held against the main boiler tube with a thermal expansion design concept in mind, it would not matter much if we had a 354, a 325 or a 210 watt element.
It would not be surprising that fridges with very light welds with 354 watt elements would have fewer failures then 354 watt units on thick welded tubes. Providing the actual run time on electric was the same.
Now to the DC system,
I would think 12 volt systems are normally lower wattage to keep the DC amps from going real high and creating large current draw on a battery system. The 12 volt option is usually mentioned to be used only as a maintaining cooling mode, not to bring a hot fridge down cold.
Since Dometic missed the entire thermal expansion problem all together the first time, or lack of trying to control it, why would they put a whole lot of effort into making the 12 volt tube actually rated to only handle the lower wattage?
