Will a refrigerator cool down a room?

[PolymerTim]

Originally posted by: gururu2
the best way to answer such questions is to K.I.S.S (keep it simple stupid).

the motion in air is not destroyed by a refrigerator. it is transferred through an efficient conduit (refrigerants). a crazy amount of air has to cycle over a refrigerant (which traps heat like noones business) to strip its heat off. all that air is released into the surroundings.

a refrigerator has enough refrigerant surface area to cool a certain volume of air. if you open the door, the refrigerant will begin to attempt to cool much more air than it was designed to. the refrigerant will absorb its maximum heat capacity and will stay maxed out even though the fan/air system is working at maximum capacity. at some time, the refrigerator will simply stop cooling.

The temperature change in the room will be related to the volume and heat capacity of the refrigerant. Obviously the molecules of refrigerant can be expected to hold more energy than air molecules, thus the overall energy of the room will decrease by that much, which may or may not result in a detectable temperature change.

Hehe, that's great! I totally agree with your first sentence, but I think the rest of your post violates your first sentence.

I have a few problems with your logic. First, you completely ignore the motor/pump so I guess you can at least claim 100% efficiency to avoid that (although un-realistic).
Next the surface area of the refrigerant coils will primarily determine the rate of heat transfer, not the amount. This would only be important when trying to determine the steady-state equilibrium temperature of the room. I don't believe the refrigerator will ever stop working. It will continue to cool the inside of the refrigerator while (for real refrigerators) putting heat out the back. Leaving the door open will simply allow air from these two thermal zones to mix and counteract each other. The temperature inside the refrigerator, even with the door open, will probably continue to be cooler than near the rear coils. The difference in temperature of the two zones will depend largely on the rate of convective heat transfer through the air.

Your final statement would only apply if you turned off the refrigerator and opened the door allowing the system to come to steady state without applying any more work to the system.

 

[BrownTown]

OMFG, the fact that this thread went on more than maybe 5 posts really makes me question the reasoning capabilities of people here. The "correct" answer is that it will make the room warmer, thats the answer any type of physics teacher wants you to think of. However like DrPizza pointed out if you want to get super technical and draw a graph the initial opening will cool the room caused by a subsequent warming due to the energy being put out by the motors. Now if you want to throw a monkey in and say the fridge vents outside the room then you can go and do that and make more stupid arguments, but the fact is that the correct answer is "warmer". I think people need to learn the difference between "technically correct" and actually correct. In the real world people don't think your witty or insightfull for comming up with some side case to make the wrong answer seem right, they think your being silly/annoying. For ANY question you can make up some strange case to justify the wrong answer, but part of being able to answer a question is to figure out what the person asking it really wants and in this case they want you to say "hotter", saying it is cooler is WRONG, no matter how many cases you can point out where "cooler" is somehow technically correct it is still WRONG.

And just FWIW talk of refrigerent heat capacity or motor efficiency is not related to this discussion. Maybe a magical 110% efficient motor would make you right, or a magical refirgerent with a heat capacity 1000 times anything know to man would also cool the room, but unfortuantely these thing do not exist in real life and should not be considered here.

 

[CycloWizard]

Originally posted by: BrownTown
OMFG, the fact that this thread went on more than maybe 5 posts really makes me question the reasoning capabilities of people here. The "correct" answer is that it will make the room warmer, thats the answer any type of physics teacher wants you to think of. However like DrPizza pointed out if you want to get super technical and draw a graph the initial opening will cool the room caused by a subsequent warming due to the energy being put out by the motors. Now if you want to throw a monkey in and say the fridge vents outside the room then you can go and do that and make more stupid arguments, but the fact is that the correct answer is "cooler". I think people need to learn the difference between "technically correct" and actually correct. In the real world people don't think your witty or insightfull for comming up with some side case to make the wrong answer seem right, they think your being silly/annoying. For ANY question you can make up some strange case to justify the wrong answer, but part of being able to answer a question is to figure out what the person asking it really wants and in this case they want you to say "hotter", saying it is cooler is WRONG, no matter how many cases you can point out where "cooler" is somehow technically correct it is still WRONG.

And just FWIW talk of refrigerent heat capacity or motor efficiency is not related to this discussion. Maybe a magical 110% efficient motor would make you right, or a magical refirgerent with a heat capacity 1000 times anything know to man would also cool the room, but unfortuantely these thing do not exist in real life and should not be considered here.

What you haven't yet realized is that not everyone who posts in this forum is an engineer. Some of us like to go way overboard and explain things in detail for the benefit of everyone here, including the engineers and non-engineers alike. I, for one, am glad that people do this because it often gives me a new perspective on things that I thought I had considered very thoroughly. Bottom line: this is a forum for people to post their ideas. If we want to create a rambling thread about every possible permutation of how a refrigerator and a room (or a plane and a treadmill) interact, then we are free to do so, just as you are free to not read any of it because, like you said, most of it is irrelevant to reality. Thought exercises are still an important part of the learning process, even if what we're thinking about could never be practically manifested in any physical system.

In other words, chill out, drink some :beer::beer:, and enjoy the forum instead of trying to police it.

 

[BrownTown]

Originally posted by: CycloWizard
What you haven't yet realized is that not everyone who posts in this forum is an engineer. Some of us like to go way overboard and explain things in detail for the benefit of everyone here, including the engineers and non-engineers alike. I, for one, am glad that people do this because it often gives me a new perspective on things that I thought I had considered very thoroughly. Bottom line: this is a forum for people to post their ideas. If we want to create a rambling thread about every possible permutation of how a refrigerator and a room (or a plane and a treadmill) interact, then we are free to do so, just as you are free to not read any of it because, like you said, most of it is irrelevant to reality. Thought exercises are still an important part of the learning process, even if what we're thinking about could never be practically manifested in any physical system.

In other words, chill out, drink some :beer::beer:, and enjoy the forum instead of trying to police it.

I'm just saying it gets hard to figure out the "right" answer when you have 10 different people talking about 10 different situations. While thought excersizes may be good for some it can also be very confusing with so many different people proposing different situations and solving their own situation and not nescecarrily the problem in the OP.

From a techical point of view this is simply because you cannot create or destroy energy, you are just moving it around, so when you coll down a fridge you equally warm up the room. However you can never even do this perfectly and in real life you always produce more heat than you do "cool", so if the fridge is all in one room then it will slowly warm the room.

 

[CycloWizard]

Originally posted by: BrownTown
I'm just saying it gets hard to figure out the "right" answer when you have 10 different people talking about 10 different situations. While thought excersizes may be good for some it can also be very confusing with so many different people proposing different situations and solving their own situation and not nescecarrily the problem in the OP.

From a techical point of view this is simply because you cannot create or destroy energy, you are just moving it around, so when you coll down a fridge you equally warm up the room. However you can never even do this perfectly and in real life you always produce more heat than you do "cool", so if the fridge is all in one room then it will slowly warm the room.

We all arrived at the same conclusion for the main question. If we hadn't, that would be depressing. However, the OP postulated an alternative scenario, which I answered (albeit in very simple fashion) in the first response in this thread. After that, it's a free for all.

 

[DrPizza]

Okay, suppose you have one of those little refrigerators operating inside a larger refrigerator. How much cooler can it get inside?

 

[BrownTown]

Originally posted by: DrPizza
Okay, suppose you have one of those little refrigerators operating inside a larger refrigerator. How much cooler can it get inside?

Well shit now I am going to have to pose multiple situations due to your vaugeness . First off if you just put the fridge on in there its temperature sensors will realise that it is already at the temperature it wants to be and not even run and therefore the inside will be the same as the inside of the large fridge. However if you mean to ask what is the maximum cold it could be if we ran both fridges at full blast then thats another question. A quick Google search I can find that at least one (apparently there are several possible variations) of the refrigerents used has a boiling temperature of -30 degrees C, so since your fridge is still well above that temperature the is still the possibility to cool down much further. Also of note is that the freezer section in a normal fridge might me 25 degrees F cooling from as high as 80 (delta of 55), cooling down another 55 degrees would be -30 degrees F which is just below -30 C, howerver we know this is impossible because the fridge cannot generate that temperature since its refridgerent will not boil at that point. So we know it is possible to cool down all the way to -30 degrees C, however I *believe* you can only asymtotically approach this value and not reach it, so for a double cascaded refridgerator using standard Freon I would say ~-25 degrees C is achievable (but hell one fridge running 100% of the time might make this too, I certianlly don't know how much of the time a normal fridge runs?).

 

[gururu2]

Originally posted by: PolymerTim

Hehe, that's great! I totally agree with your first sentence, but I think the rest of your post violates your first sentence.

I have a few problems with your logic. First, you completely ignore the motor/pump so I guess you can at least claim 100% efficiency to avoid that (although un-realistic).
Next the surface area of the refrigerant coils will primarily determine the rate of heat transfer, not the amount. This would only be important when trying to determine the steady-state equilibrium temperature of the room. I don't believe the refrigerator will ever stop working. It will continue to cool the inside of the refrigerator while (for real refrigerators) putting heat out the back. Leaving the door open will simply allow air from these two thermal zones to mix and counteract each other. The temperature inside the refrigerator, even with the door open, will probably continue to be cooler than near the rear coils. The difference in temperature of the two zones will depend largely on the rate of convective heat transfer through the air.

Your final statement would only apply if you turned off the refrigerator and opened the door allowing the system to come to steady state without applying any more work to the system.

yes my point was to make it clear that the small capacity of the refrigerator would very quickly become overburdened, most definitely because of lack of refrigerant and a system to cool it. larger refrigerators require larger volumes of refrigerant and stronger fan sytems. one thing you may have experienced is that when you accidentally leave your refrigerator open, all of the food gets warm. this is one outcome of the inside of the refrigerator reaching equilibrium with the outside. if you were to touch the refrigerant coils after having left the door open several hours, you will notice that they will be much warmer than if you open the door randomly.
the motor contributes work and heat but i think this makes the problem less interesting because that alone doesn't encourage further thought on the intended purpose of the appliance. any old motor is gonna make a room warmer.

 

[spikespiegal]

Next we'll be arguing if you walk out of a house falling of cliff a millisecond after it hits the ground you won't get hurt.

.....At least it worked for Bugs Bunny.

The 'fridge' scenario here is a simple example of thermodynamics. If the room (system) is closed, and there's an electrical cord powering the fridge dumping energy into the system, then the entire state of the system is to gain energy, be it thermal, kinetic or other - doesn't matter.

The movie 'Musquito Cost' with Harrison Ford had some interesting plays on the classic fridge.

 

[Rubycon]

Originally posted by: spikespiegal
The movie 'Musquito Cost' with Harrison Ford had some interesting plays on the classic fridge.

Yes it did. Since Ammonia was mentioned and a fire was used to initiate the refrigeration cycle one could presume that the system was an R717 based absorption system.

 

[Born2bwire]

Ok, ok. But can the fridge still keep my beer cold is the important question.

 

[DrPizza]

Originally posted by: BrownTown

Originally posted by: DrPizza
Okay, suppose you have one of those little refrigerators operating inside a larger refrigerator. How much cooler can it get inside?

Well shit now I am going to have to pose multiple situations due to your vaugeness . First off if you just put the fridge on in there its temperature sensors will realise that it is already at the temperature it wants to be and not even run and therefore the inside will be the same as the inside of the large fridge. However if you mean to ask what is the maximum cold it could be if we ran both fridges at full blast then thats another question. A quick Google search I can find that at least one (apparently there are several possible variations) of the refrigerents used has a boiling temperature of -30 degrees C, so since your fridge is still well above that temperature the is still the possibility to cool down much further. Also of note is that the freezer section in a normal fridge might me 25 degrees F cooling from as high as 80 (delta of 55), cooling down another 55 degrees would be -30 degrees F which is just below -30 C, howerver we know this is impossible because the fridge cannot generate that temperature since its refridgerent will not boil at that point. So we know it is possible to cool down all the way to -30 degrees C, however I *believe* you can only asymtotically approach this value and not reach it, so for a double cascaded refridgerator using standard Freon I would say ~-25 degrees C is achievable (but hell one fridge running 100% of the time might make this too, I certianlly don't know how much of the time a normal fridge runs?).

LOL, I didn't expect anyone to even half seriously answer that question.

 

[jagec]

Originally posted by: BrownTown

Originally posted by: DrPizza
Okay, suppose you have one of those little refrigerators operating inside a larger refrigerator. How much cooler can it get inside?

Well shit now I am going to have to pose multiple situations due to your vaugeness . First off if you just put the fridge on in there its temperature sensors will realise that it is already at the temperature it wants to be and not even run and therefore the inside will be the same as the inside of the large fridge. However if you mean to ask what is the maximum cold it could be if we ran both fridges at full blast then thats another question. A quick Google search I can find that at least one (apparently there are several possible variations) of the refrigerents used has a boiling temperature of -30 degrees C, so since your fridge is still well above that temperature the is still the possibility to cool down much further. Also of note is that the freezer section in a normal fridge might me 25 degrees F cooling from as high as 80 (delta of 55), cooling down another 55 degrees would be -30 degrees F which is just below -30 C, howerver we know this is impossible because the fridge cannot generate that temperature since its refridgerent will not boil at that point. So we know it is possible to cool down all the way to -30 degrees C, however I *believe* you can only asymtotically approach this value and not reach it, so for a double cascaded refridgerator using standard Freon I would say ~-25 degrees C is achievable (but hell one fridge running 100% of the time might make this too, I certianlly don't know how much of the time a normal fridge runs?).

A normal fridge doesn't run very often, actually. The better the insulation, the less often it runs. I would venture to say that as long as your insulation is decent, a normal fridge can cool down pretty much all the way to the boiling point of the refrigerant at the evaporator pressure if you run it 24/7. Cascade refrigeration systems used for extreme cooling are necessary only due to effective temperature ranges of different refrigerants. If a refrigerant evaporates at a very low temperature, it won't want to condense at any reasonable pressure above room temperature, so heat must be rejected to another refrigeration loop with a more moderate temperature refrigerant. That, or use a multistage compression cycle to force it above room temperature, but that's appallingly bad engineering, and much more expensive and dangerous to boot.

In this case it doesn't make much sense to cascade two cooling systems which are running the same refrigerant at the same pressures. It's true that if there was a heat source (or poor insulation) in the inside refrigerator, cascading them might yield a slight decrease in the temperature, but it would be a horribly inefficient design and you'd be much better off with a larger-capacity single loop.