Refridgerators: Are two fans better than one?

[xf021209]

Originally Posted by dunnnc

The can at the bottom is the accumulator tank and just stores the liquid ammonia. If the fan is doing it's job, the can should be cool. The ammonia is pulled from the tank and boiled in the stack.

My burner is directly under the can, must be a different design than the larger units. The refrigerator is in a pop up so it's one of the very small units. The top is where the line leads down and seems to be the return. Moving air down there would not be good for the flame and I doubt the ammonia would get hot enough to move up to the top of the tubes to condense and pull the heat out of the refrigerator. It's most likely just designed this way for the compact size.

[OldCoot]

Originally Posted by xf021209

My burner is directly under the can, must be a different design than the larger units. The refrigerator is in a pop up so it's one of the very small units. The top is where the line leads down and seems to be the return. Moving air down there would not be good for the flame and I doubt the ammonia would get hot enough to move up to the top of the tubes to condense and pull the heat out of the refrigerator. It's most likely just designed this way for the compact size.

How about some pictures? I take it is is not like the diagram or pictures I posted.

[xf021209]

Originally Posted by dunnnc

How about some pictures? I take it is is not like the diagram or pictures I posted.

No it looks much more compact, but has to use a similar principle, and I know it's an ammonia unit. I'd be happy to post pictures except my cousin purchased the pop up from me, and he lives in Ohio. I'm in Texas :-) I've seen the bigger replacement units that look much more like the diagram. After rebuilding the pop up, using it for a few years, and finally selling it, I'm in the process of rebuilding a FR Salem (FEMA Trailer) now. I'm still trying to decide on using a conventional small office style refrigerator (inexpensive, reliable, will need ice blocks during travel time) or going with an RV unit (very expensive, not as reliable, no need for ice blocks!). Since I stay in parks most of the time, I'm leaning towards conventional. I already have a nice black office fridge that would work well. I could pickup a small black freezer for around $200.00. Freeze a couple of milk jugs and cool the fridge before travel. Once at the camp ground, put the milk jugs in the freezer in case there's food left after camping (rarely) for the drive home. I could do similar to what the original question here was - since the condenser is on the back of the fridge, just like with an RC fridge, a small muffin exhaust fan may not be a bad idea to keep the heat from collecting inside the trailer and to cool the fridge faster. One thing that really surprises me is how such a small low cost device improves the efficiency of RV fridges...and why isn't the fan included?

[M109Rrider]

Originally Posted by dunnnc

The can at the bottom is the accumulator tank and just stores the liquid ammonia. If the fan is doing it's job, the can should be cool. The ammonia is pulled from the tank and boiled in the stack.

When i first jury rigged my fan it was pulling air directly over that tank and was barely warm to the touch...my fridge warmed up overnight and spoiled everything in it. Removed the fan and with in a few hours it cooled back down. I don't understand the thermodynamics of how heating ammonia can cause a cooling effect but I do understand your thoughts on how the fridge works by looking at the drawing. I say you need to be careful with exactly what your cooling back there only from personal experience.

[OldCoot]

Originally Posted by M109Rrider

When i first jury rigged my fan it was pulling air directly over that tank and was barely warm to the touch...my fridge warmed up overnight and spoiled everything in it. Removed the fan and with in a few hours it cooled back down. I don't understand the thermodynamics of how heating ammonia can cause a cooling effect but I do understand your thoughts on how the fridge works by looking at the drawing. I say you need to be careful with exactly what your cooling back there only from personal experience.

All I can say is where my fan is has worked great and the accumulator is always cool and never hot whereas before it was warm/hot to touch before installing the fan. The burner portion is to the right of the accumulator tank on my unit and is completely enclosed.

[M109Rrider]

OK. Is the one in this pic (with bar code sticker) a different tank? This isn't my picture but looks similar to mine. I had my fan wedged right above it in the coils when I had problem

P.s. my wife was really pi$$ed when I did that..lol!!

[dnaraG_1M]

Originally Posted by M109Rrider

When i first jury rigged my fan it was pulling air directly over that tank and was barely warm to the touch...my fridge warmed up overnight and spoiled everything in it. Removed the fan and with in a few hours it cooled back down. I don't understand the thermodynamics of how heating ammonia can cause a cooling effect but I do understand your thoughts on how the fridge works by looking at the drawing. I say you need to be careful with exactly what your cooling back there only from personal experience.

Yep. You right.

But:
We don't really need to understand the detailed thermodynamics of the
ammonia cycle though. It's pretty clear that the ammonia gas - boiled
from the storage pot - is what produces the cooling effect. Knowing
that, it's easy to understand anything that reduces the amount of
ammonia gas available - like a leak, less heat in the boiler, etc - will
reduce the amount of cooling. 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.

And the wifey do get POed, finding spoiled food........


cheers,
johnd

God, I HATED my Thermo class.....still do, even after 40 years.....

[OldCoot]

Originally Posted by dnaraG_1M

...We don't really need to understand the detailed thermodynamics of the
ammonia cycle though...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...

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!

[TURBS]

Originally Posted by dunnnc

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!

Good question

[Herk7769]

It works on the principle of "Magic"

See link more complicated reason.

Heat of Vaporization - ChemWiki

[M109Rrider]

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)

[xf021209]

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?

[OldCoot]

Originally Posted by xf021209

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.

[dnaraG_1M]

Originally Posted by dunnnc

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,
johnd

[OldCoot]

Originally Posted by dnaraG_1M

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..."

[xf021209]

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.

[xf021209]

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.

[dnaraG_1M]

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..."

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

[xf021209]

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.

[Mark S]

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