Time to replace defective and potentially dangerous absorption fridge

[davidbrady]

(07-15-2013 13:04)pgchin Wrote:  My 1292 purchased in 2007 came from the factory with the cooling fans......... all this time I have NEVER seen or heard them come on........ But, I am old, blind and deaf!

I see from the online Dometic literature that one or two fans is an option for at least my version of their fridge, the 1292. I think, but I'm not sure, that if the fan was shipped with this option the electrical schematic attached to the back pane of the fridge shows the fan connections.

Generally heat transfer increases with velocity. This is definitely true for the case of natural or forced (with a fan) convection of air on a heat exchanger.

[pgchin]

(07-15-2013 14:51)davidmbrady Wrote:  I see from the online Dometic literature that one or two fans is an option for at least my version of their fridge, the 1292. I think, but I'm not sure, that if the fan was shipped with this option the electrical schematic attached to the back pane of the fridge shows the fan connections.

OK Lil Bro,
So make me get up off my "duff" in my nice AC cooled bus and go look! The nerve!
FWIW... I ordered the unit from PPL and did not ask for any options so my presumption is that whomever ordered it for PPL asked for the "fan option". Mine has 2.The schematic does not show the connections, however, there are 2 red (+)leads with 1 amp inline fuses that attached to position "j3" on the PC board and come from each of the fans. When you look at the schematic on the refer, it shows J3" as "blank", so I think your assumption that fans were options is a good and valid one!

PS The unit from PPL showed up with a factory seal on the crate so PPL did not alter it in any way. I remember at the time they were griping that Dometic would not drop ship directly to me and they had to order it ship to them in Houston from Dometic, then truck it to me...............as the wife wanted stainless steel and residential handles.

[Darrell Snell]

(07-15-2013 15:12)pgchin Wrote:  

(07-15-2013 14:51)davidmbrady Wrote:  I see from the online Dometic literature that one or two fans is an option for at least my version of their fridge, the 1292. I think, but I'm not sure, that if the fan was shipped with this option the electrical schematic attached to the back pane of the fridge shows the fan connections.

OK Lil Bro,
So make me get up off my "duff" in my nice AC cooled bus and go look! The nerve!
FWIW... I ordered the unit from PPL and did not ask for any options so my presumption is that whomever ordered it for PPL asked for the "fan option". Mine has 2.The schematic does not show the connections, however, there are 2 red (+)leads with 1 amp inline fuses that attached to position "j3" on the PC board and come from each of the fans. When you look at the schematic on the refer, it shows J3" as "blank", so I think your assumption that fans were options is a good and valid one!

PS The unit from PPL showed up with a factory seal on the crate so PPL did not alter it in any way. I remember at the time they were griping that Dometic would not drop ship directly to me and they had to order it ship to them in Houston from Dometic, then truck it to me...............as the wife wanted stainless steel and residential handles.

I had two on my 1292.
One on top, wired to a thermo switch attached to the top coil. I hear it occasionally. Gets power from the board.
I removed the lower one when i replaced the coils. It was plugged into a duplex outlet and was probably installed as an attempt to fix the the Norcold recall fix.
Seems to me that it helps direct the heat up thru the vent?

[Arcticdude]

[/quote]Generally heat transfer increases with velocity. This is definitely true for the case of natural or forced (with a fan) convection of air on a heat exchanger.
[/quote]


I believe that's to a point. Beyond that point, you're moving the air too fast to get adequate transfer. I know that is the case with a radiator in a vehicle. Move the water too fast through it and it will overheat. You need at least a restrictor plate in the thermostat housing, in place of the thermostat, to prevent such a condition.


Now, do two 4" fans move too much air? I don't know, but I would at least be suspect, especially when placed right at the inferior edge of the coils.


jm02. Carry on!

[gondolaguy]

Generally heat transfer increases with velocity. This is definitely true for the case of natural or forced (with a fan) convection of air on a heat exchanger.
[/quote]


I believe that's to a point. Beyond that point, you're moving the air too fast to get adequate transfer. I know that is the case with a radiator in a vehicle. Move the water too fast through it and it will overheat. You need at least a restrictor plate in the thermostat housing, in place of the thermostat, to prevent such a condition.


Now, do two 4" fans move too much air? I don't know, but I would at least be suspect, especially when placed right at the inferior edge of the coils.


jm02. Carry on!
[/quote]

Your comparison of air over the hot coils versus water through a radiator seems like apples and oranges. The water pump is pushing the coolant out to the radiating surface whereas in this case the air is passing over the radiating surface and the more the better. It would be like saying that cooling the radiator slowed down at some speed where too much air was passing through it.

[davidbrady]

The higher the Reynolds number the better the thermal conductivity the greater the heat transfer. The Reynolds number is directly proportional to fluid velocity. This is true for the air surrounding cooling fins and tubes of a heat exchanger and for the fluid inside the tubes. If the velocity is great enough then the flow becomes turbulent and the heat transfer is even higher.

Internal combustion engines operate within a very narrow delta of temperatures. For a given water pump flow rate and for a given radiator top tank and bottom tank temperature differential a cooling system needs to be spec-ed that will provide the performance necessary over a likely set of ambient temperatures and eventual radiator fouling. This means that a typical vehicle is engineered with excess cooling capacity. If flow rates are increased by removing tstat bodies then a restrictor may be necessary to restore the designed in cooling system specs.

If the engine's internal materials and tolerances were up to the task we'd run our engines even hotter and we'd increase the fluid flow inside the tubes for better heat transfer and better engine efficiency. Hotter engine, higher efficiency, faster fluid flows, and better thermal conductivity at the radiator.

With today's internal combustion engines, I'm sure we can agree that all things being equal, an increase in the air CFM (that is, air velocity) over the radiator's cooling fins by increasing fan RPMs or improving the fan and air flow properties will allow you to run max HP at higher ambient temps. Why? Because the increased velocity of the cooling air transfers more heat from the radiator tubes and fins. This is true even though the temperature delta, the difference between the radiator input coolant temp and the outside air temperature, is less.

[gondolaguy]

(07-15-2013 22:42)davidmbrady Wrote:  The higher the Reynolds number the better the thermal conductivity the greater the heat transfer. The Reynolds number is directly proportional to fluid velocity. This is true for the air surrounding cooling fins and tubes of a heat exchanger and for the fluid inside the tubes. If the velocity is great enough then the flow becomes turbulent and the heat transfer is even higher.

Internal combustion engines operate within a very narrow delta of temperatures. For a given water pump flow rate and for a given radiator top tank and bottom tank temperature differential a cooling system needs to be spec-ed that will provide the performance necessary over a likely set of ambient temperatures and eventual radiator fouling. This means that a typical vehicle is engineered with excess cooling capacity. If flow rates are increased by removing tstat bodies then a restrictor may be necessary to restore the designed in cooling system specs.

If the engine's internal materials and tolerances were up to the task we'd run our engines even hotter and we'd increase the fluid flow inside the tubes for better heat transfer and better engine efficiency. Hotter engine, higher efficiency, faster fluid flows, and better thermal conductivity at the radiator.

With today's internal combustion engines, I'm sure we can agree that all things being equal, an increase in the air CFM (that is, air velocity) over the radiator's cooling fins by increasing fan RPMs or improving the fan and air flow properties will allow you to run max HP at higher ambient temps. Why? Because the increased velocity of the cooling air transfers more heat from the radiator tubes and fins. This is true even though the temperature delta, the difference between the radiator input coolant temp and the outside air temperature, is less.

I was going to say exactly that, but you beat me to it

[Arcticdude]

unnecessary post

[davidbrady]

Uh-Oh, I think I've been had haven't I??? LOL :-)

I'd like to add that slowing down coolant flow thru a radiator to engineer a top and bottom rad tank temp difference actually decreases heat transfer. The driving force for heat transfer is the temperature difference between the hot coolant and the cooler ambient air. To maximize heat transfer we want this delta-T to be large across the entire face of the radiator. By slowing down the coolant we're decreasing the delta-T across the face of the rad. (Look up log mean temperature difference). If an engine is happy with a 5 deg top/bottom tank delta rather than the more usual 15 deg, then heat transfer would increase. This seems intuitively correct because the difference in temperature between the coolant in the tubes and the ambient air across the full area of the radiator is higher if the top/bottom tank temp diff is less.

Shoot! I did it again didn't I??? Okay, I'll stop now!

[gondolaguy]

(07-16-2013 00:22)davidmbrady Wrote:  Uh-Oh, I think I've been had haven't I??? LOL :-)

I'd like to add that slowing down coolant flow thru a radiator to engineer a top and bottom rad tank temp difference actually decreases heat transfer. The driving force for heat transfer is the temperature difference between the hot coolant and the cooler ambient air. To maximize heat transfer we want this delta-T to be large across the entire face of the radiator. By slowing down the coolant we're decreasing the delta-T across the face of the rad. (Look up log mean temperature difference). If an engine is happy with a 5 deg top/bottom tank delta rather than the more usual 15 deg, then heat transfer would increase. This seems intuitively correct because the difference in temperature between the coolant in the tubes and the ambient air across the full area of the radiator is higher if the top/bottom tank temp diff is less.

Shoot! I did it again didn't I??? Okay, I'll stop now!

Man! I was just about to add that too! Dang you David, you and your Dela T...