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Old 08-25-2012, 04:34 PM   #71
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Originally Posted by herk7769
It works on the principle of "Magic"

See link more complicated reason.

Heat of Vaporization - ChemWiki
I agree.. P.F.M... even with the afternoon sun baking the fridge side of the trailer mine is magically staying a steady 36 degrees with a single 12v computer fan neatly zip tied to the top cover (fridge in slide, top cover mounted vertical)
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Old 08-25-2012, 06:03 PM   #72
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Originally Posted by M109Rrider

I agree.. P.F.M... even with the afternoon sun baking the fridge side of the trailer mine is magically staying a steady 36 degrees with a single 12v computer fan neatly zip tied to the top cover (fridge in slide, top cover mounted vertical)
I wonder if during cooler weather it takes more heat to vaporize the ammonia in large enough quantity to maintain a lower fridge temp? I never tried to turn the heat up to medium or high during cool evenings when I noticed the fridge temp going up, and the problem didn't always happen. I also wonder what the effect of humidity is on the cooling. Higher humidity I believe would aid cooling of the evaporator heat, but by how much?
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Old 08-25-2012, 06:21 PM   #73
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Originally Posted by xf021209 View Post
I wonder if during cooler weather it takes more heat to vaporize the ammonia in large enough quantity to maintain a lower fridge temp? I never tried to turn the heat up to medium or high during cool evenings when I noticed the fridge temp going up, and the problem didn't always happen. I also wonder what the effect of humidity is on the cooling. Higher humidity I believe would aid cooling of the evaporator heat, but by how much?
How do you turn the heat up on a frig? The cooling (ammonia) system is a closed system and humidity would have no effect on the system itself.
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Old 08-25-2012, 06:33 PM   #74
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Why do the frigs work so much better in cold weather (say 45°)then? The whole outside of the unit is colder and inside the camper is about 72°?

Just asking!
Could it be because the DeltaT between the evaporator and condenser
is much less than in summer???
Maybe that is why fans on the condenser coils help in
summer, eh?

For a test, maybe keep the condenser coils iced down in
summer and see if you don't get the same efficiency as
in winter...........betcha do.......

(The interior temp of the TT- which may be the same winter or
summer - should not enter the picture; the evap is in the
fridge(cold) and the condenser is in the outside air(cold).....)

cheers,
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Old 08-25-2012, 06:41 PM   #75
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Could it be because the DeltaT between the evaporator and condenser
is much less than in summer???
Maybe that is why fans on the condenser coils help in summer, eh?...
I agree whole heartedly, but what about this?
"...Now think about boiling a cold liquid vs boiling a hot liquid. Clearly more heat input is required for the former. With a fixed amount of heat available - from propane or AC - less ammonia will be boiled if the liquid is cool, compared to if the liquid was hot. Less ammonia gas; less cooling effect..."
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Old 08-25-2012, 06:48 PM   #76
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Originally Posted by dunnnc

How do you turn the heat up on a frig? The cooling (ammonia) system is a closed system and humidity would have no effect on the system itself.
My fridge had three settings on the burner, low, medium, and high. The fridge did not have a thermostat, and yes humidity does effect cooling systems. The more moisture content in the air allows for better absorption of heat from the evaporator fins. Do you know why VW Beatles (the original air cooled) would overheat up in Alaska? Lack of humidity did not allow the heat to dissipate from the fins on the engines, the fins were too small for the very low winter humidity up there. Though the temps were sub freezing, the heat transfer was not good enough and the engine would overheat or burn up. This is also why AC units in homes sometimes freeze up. High humidity can be problematic for AC systems. Evaporators release the heat through transfer to the air, much in the same way sweat does off the skin. Higher humidity allows more heat to dissipate from the evaporator fins to the air. That could cause the unit to cool quicker or possibly remove the heat too quickly making the unit work harder to keep the fridge cool. If humidity had no effect, neither would fans.
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Old 08-25-2012, 06:50 PM   #77
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Quote:
Originally Posted by dunnnc

I agree whole heartedly, but what about this?
"...Now think about boiling a cold liquid vs boiling a hot liquid. Clearly more heat input is required for the former. With a fixed amount of heat available - from propane or AC - less ammonia will be boiled if the liquid is cool, compared to if the liquid was hot. Less ammonia gas; less cooling effect..."
Ah, that's it, the assumption that the heat is fixed. On my pop up camper the heat was not fixed. I could set it to low, medium, or high.
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Old 08-25-2012, 07:06 PM   #78
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Originally Posted by dunnnc View Post
I agree whole heartedly, but what about this?
"...Now think about boiling a cold liquid vs boiling a hot liquid. Clearly more heat input is required for the former. With a fixed amount of heat available - from propane or AC - less ammonia will be boiled if the liquid is cool, compared to if the liquid was hot. Less ammonia gas; less cooling effect..."
Yes, if the amount of heat input is fixed (mine is), there should
be less ammonia available for the cooling cycle in winter than
in summer. However, my guess - and it is a guess - is that this
is a second order effect. Clearly, the DeltaT between the evap and
condenser coils is a first order effect. We may see that is true when
a large improvement occurs with fans on the condenser coils.

Not saying that a second order effect can be ignored, just that it
may exist, but be swamped out by the first order effect. IE improved
thermal efficiency when the DeltaT is small. In the summer, when
thermal efficiency is poorer, the second order effect may play a
greater role............

And then, of course, there is the consideration that even with less
ammonia available in winter due to heat loss, you may NEED less
ammonia to pump the heat out of the fridge, due to improved
thermal efficiency when the DeltaT is smaller.....

These things DO get involved, don't they?

cheers,
johnd
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Old 08-25-2012, 07:15 PM   #79
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Quote:
Originally Posted by dnaraG_1M

Yes, if the amount of heat input is fixed (mine is), there should
be less ammonia available for the cooling cycle in winter than
in summer. However, my guess - and it is a guess - is that this
is a second order effect. Clearly, the DeltaT between the evap and
condenser coils is a first order effect. We may see that is true when
a large improvement occurs with fans on the condenser coils.

Not saying that a second order effect can be ignored, just that it
may exist, but be swamped out by the first order effect. IE improved
thermal efficiency when the DeltaT is small. In the summer, when
thermal efficiency is poorer, the second order effect may play a
greater role............

And then, of course, there is the consideration that even with less
ammonia available in winter due to heat loss, you may NEED less
ammonia to pump the heat out of the fridge, due to improved
thermal efficiency when the DeltaT is smaller.....

These things DO get involved, don't they?

cheers,
johnd
Yes, very. Isn't dissipation of heat, still dissipation of heat no matter if the system is closed or open? The question still remains why on most days/nights my little pop up camper fridge would work well, then with no setting changes, it would start to warm up. Then the following day in most cases it would start to cool down. This was fairly random. The only thing that was not consistent was the weather, so I'm still unsure why ammonia fridges vary so much in cooling.
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Old 08-25-2012, 07:24 PM   #80
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So after 8 pages of discussion, numerous scientific theory, scores of photographs and even a little bit of debate, the answer to my original question that started this thread, "are two fans better than one" the answer is a definate maybe. I am glad we got that resolved

I added the second fan just above the intake and will know for sure when I camp again at the end of September how it works. I may try moving it up to the horizontal exhaust vent if it's not as good as I hope . Thanks for all the input.

Ps. Man, there are some very intelligent folks on this forum. What a great resourse and source of entertainment.

Mark
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