Refrigerator: Newsletter Challenge (01/31/06)

Here we go again. Back to the 3 laws of thermodynamics:

You can't win

You can't break even

You can't quit the game

Refrigerators (as with any device) are always less than 100% efficient. That is, more energy is pumped into the refrigerator than the refrigerator can remove in the form of heat. All (well essentially all) of the "wasted" energy that is not directly converted for cooling by the refrigerator is given off as heat! This means a net rise in the room's temperature over time.

It is true that opening the door will momentarily chill the room as the cold air is released and heat from the room is transferred into the refrigerator, the energy required to drive the refrigerator compressor (and don't forget the refrigerator light!) will soon overwhelm the cooling effect and the room gets hotter. Remember the 3 laws!

You can't win

You can't break even

You can't quit the game

Better to go jump in the pool!

We really need more challenging questions for the Challenge!

Couldn't have said it better myself . . . However, if you put the kids to a steady routine of soaking large bath towels in the pool and pasting them on the condenser coils (assuming, of course they're the convecting type mounted on the back of the fridge), wringing out the hot water and re-wetting the towels with cool pool water, etc., etc., it may be possible to slightly cool the room with the fridge. 'Course, the kids will be hotter than normal, so maybe not.

You take the Spray /rinse hose from the sink.spray cold water over the external condensor coils. Let the heated water out of the room through a drain in the floor. Hence, you now have a heat pump. Get oput that sweater!

Do a simple energy balance... Since there is energy coming into the refrigerator, courtesy of the power company and your timeshare "fees" it must be a) stored or b) converted. Since the refrigerator does not store energy, it's converting to heat, a little noise, and light when you open the door. Since the vast majority is heat, the room will eventually warm up. Sorry kids.

They overworked the compressor and blew out the fridge.

From my simpleton's perspective, the refridgerator has to be removing somekind of heat from the air. Since the heat cannot be destroyed only created the fridge couldn't keep up with the amount of heat that it is generating. CJ

It could work if the heat from the condenser were vented to the outside. Otherwise, if the system were 100% efficient, there would be no net change in room temperature since the heat removed from the evaporator would be exactly canceled out by that coming off of the condenser.

But because of losses mentioned by previous posters, the room will heat up. Some of the losses come from resistive losses in the electronic circuitry, friction of the fluid flow in the heat pumping system, hysteric and eddy losses in the rotors and stators inside the motors, as well as bearing friction losses.

The question was will it work. We aren't given enuff info to say for sure, but we can take guesses. The standard obvious answer is no. BUT! Since its a time share is in Florida, we could gess that its very cheaply built, so they probably didn't bother insulating it. So if you closed the doors and didnt cause too much turbulance, the hot air rising to the ceiling would leak and radiate thru the ceiling and upper walls. Stopping the cold air from leaking away thru gaps at the bottom of the doors woud also help. Reducing the space woud be a big help, like Homer Simpson did by setting up a tent attached to the frij.

That's the best idea I've heard so far, change the reference frame.
First off, the simple answer is no, it will heat the room in the long run because as a closed system, the only thing going in is electricity that will generate heat through the imperfect system of refridgeration.
Secondly, the fridge is cold inside and has notably more 'cold energy' than the coils have on the back, so by opening up the door to the fridge and unplugging it from the wall, air will circulate through the fridge, cooling the floor around it at least for a few minutes.
finally, for my best solution yet, position the fridge with the heating coils outside an outer door of the dwelling and the open end of the refridgerator towards the inside. Insulate the gaps with some blankets or something and enjoy a nice game of cards on the floor gathered around the A/Cerator.
cheers.

I actually did something like this by placing the fridge next to the door and constructing a barrier which the door would line up with if it were opened a certain distance. The fridge was in a small room and would otherwise cause thermal runaway as the losses in the system would raise the room temp and make it work harder. Eventually, the room temp would become too high for the refrigerator to get cold enough inside to turn off. Thus, I made the barrier to get rid of the heat output. Then, the leakage from cold compartment actually helped to lower the room temperature--like free A/C.

By door, I mean house door to the outside.

If you didn't learn from the first response to the challenge, nothing is free, just free to you. I hope you weren't the one paying the electric bill and I feel sorry for whoever was.

Sure, it is not free in the absolute sense. But the A/C effect was. I was paying for refrigeration and getting the A/C at no extra charge--free.

Homer is one funny guy.

Funny AND an engineering super genius. He built a time machine out of a toaster once!

The bit about 100% efficient isn't quite right. Maximum theoretical efficiency, that of an ideal (Carnot) heat engine operating in reverse is less than 100% (2nd law, the real one, not Anon. Cowards, though it's a good joke) so even in this case heat put into room = heat removed from fridge contents + mechanical energy input from the motor. So the room gets warmer (whether the fridge door is open or not). For a real fridge the situation is worse.

He said if .

Precisely. Why consider a situation that is not possible in priciple, let alone in practice?

One reason may be to make it known that it is not possible. In theory, there are losses processes like super conduction.

It is known that there are no known systems that are perfect and all this individual was doing was establishing the fact that in the realm of reality, including the least amount of inefficiency causes an unsastisfactory result.
I'll have to give you credit for not forgetting about carnot effeciency, but in this case, it's more like preaching to the converted.

This would help if you would move the heat exchanger coils on the refriferator outside. Unfortunately, they are usually on the back of the refrigerator and sitting in your kitchen. Heat removed is expelled to the air again plus some extra heat due to losses.

Yep it works. You can cool the room with the refigerator. Take the top off the GE monitor top refer. Stick it in the window. Close the window so the coils are outside and the ice box is inside the room. Plug it in. The old monitors would run even when out of position. The heat will be rejected outside the room. Ran for ever too! One of the things they taught you in the Edison program is not to design them like that.

This was a question from my "Intro to Classical Thermodynamics" class in 1974! As a previous poster noted in the"Thermo for Dummies" version of the 3 laws of thermodynamics, 1. You can't win 2. You can't break even. Ried

The refrigerator releases the heat that it extracts from its cold box to the environment in which the refrigerator is located. Whatever heat the refrigerator extracted from the room would be transferred right back to the room with the additional heat caused by a compressor running almost constantly. In effect, the open refrigerator door would in a short period of time cause a hotter room.

Opening the refrigerator door causes the opposite of the cooler desired temperature. The refrigerators compressor only has the ability to cool the cubic meter area of the refrigerator plus about 20% more, provided the room temperature is not more than 50oC. With the refrigerator door open, the cool air will only last a short time, because the refrigerator is now an inefficient heater. How? Normally when the door is closed, you have two separate environments. The one inside the refrigerator and the one outside the refrigerator. Now... with the door open, the inside and the outside are in the same environment. A regrigerator basically has two coils of piping. One of the coils is pressurized with a compressor pumping freon refrigerant causing the coil to become very cold. The other unpressurized coil is fed by the first coil through an orface. As the freon is released into the lower pressure tubing its temperature increases causing the coil to become very hot. An opposite and equal reaction. Cool air inside and hot air outside. There is one thing in life that I have learned, it is... NOTHING IS FREE The compressor generates heat because it is inefficient. Over a period of time the room would increase in temperature. Kev

You are right, but I think you got the part about the cooling/heating relative to pressure reversed. ;-)

If memory serves, a gas, in this case freon, that goes from a high pressure to a low pressure state where the volume is constant should drop in temperature.

PV=nrT

The volume in the low pressure tube remains essentially constant, but the pressure of the freon drops, so the temperature should as well. Pressurizing the gas would increase its temperature, or did I miss something?

Thanks MD (685), you are correct.

Rare when that happens! :-)

It's slightly more subtle than that. The refrigerant is a liquid at the compressor output pressure of a few bar and room temp, but a gas at the lower pressure and the fridge temp. The cold gas is compressed approx adiabatically by the compressor and comes out as hot gas (about 80°C, judging by touch). As the gas cools in the coil it condenses to liquid. As the liquid goes thru the throttling valve it evaporates and cools because of latent heat, not just the PV = nRT bit. The trick is to find a refrigerant that has suitable properties. Early fridges used ammonia, but it's nasty stuff. Freons are ideal, but then we had the scare about CFCs and atmospheric ozone, so got banned. Nowadays I believe various hydrocarbons are used, but somebody might correct me on that.

"Shut that fridge, my beer has got warm. Get your swimming things on, slap on a load of waterproof sun block and go sit outside in the breeze or have a swim"

While you're waiting for the beer to cool, go have a really hot shower, because when you get out of it, perversely, you'll feel cooler (see the CQ on sweat evaporation).

Drink the now cool beer and wonder about fitting a lock to the fridge, or selling the kids.