Air Conditioner Question 1

[Locheim]

Hello everyone. I'm not an engineer and this is probably a stupid question to you guys but I have $20 riding on it.

My buddy is insisting that a standard window air conditioner produces more hot air than cold, is this true?

 

[MintJulep]

Maybe.

More hot comes out the back than cold comes out the front. But that does not necessarilly mean that there is more hot air than cold air.

 

[Locheim]

Ok, now I am confused, could you elaborate while keeping it simple enough for us non-engineers to understand?

 

[djack77494]

Air conditioners and heat pumps do not create heat or "cold", they simply move it.

If we neglect inefficiencies in the system, then the heat lost by the room air in an A/C unit will equal the heat gained by the outside air. You could have more, less, or equal amounts of hot air and cold air. If you have less hot air than cold air, then the hot air will have a larger increase in its temperature than the decrease in the cold air's temperature.

This is not that difficult a subject, but I'm foundering a bit here. It's hard to put this into non-engineer-ese. Post back if you're not satisfied with this explanation.

Doug

 

[BigInch]

I think the bet has everything to do with including efficiencies, and therefore what Bud says is true. More hot air is produced than cold. This is true not only for window units. It is true for all air conditioners and refrigerators.

Otherwise Bud, being a wise guy, probably would not have made the bet, because if you neglect inefficiencies as djack says... heat is just moved from inside the room to outside of the room and .... nobody would win the bet.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BronYrAur]

You may have to cough up $20 on this one.

Cold air is produced when the air conditioner removes heat from it. This heat must then be rejected to the outdoors, which is why half of the unit hangs outside. It is a one-to-one enthalpy relationship; however, it takes mechanical energy to transfer this heat. This energy creates more heat which is also rejected to the outdoors.

Think of a dehumidifier. It is basically a window air conditioner that throws the rejected heat back into the room. it also throws the mechanical heat into the room which is why a dehumidifier ultimately adds a net heat gain to the room.

Tell your buddy he owes me half of his winnings!

 

[Locheim]

Doh! Thanks for the info guys! That's actually very intuitive and I should have thought of it before (I do understand what heat is and whatnot). I blame the Guinness!

Another friend of mine (he is a neuroscientist so what does he know, right?) told me an a/c is like and refrigerator with an extra fan. He then explained that common sense dictates that if your fridge does not make your house hotter then it makes your food cold then an a/c expels more cold than hot air.

Anyway, thanks for losing my $20

 

[Locheim]

Sorry for the double post, I don't see an edit button.

This is actually how my buddy explained it.

the EER is basically the lay man version of te physical unit of effiiciency, which is essentially a ratio of the energy put out divided by the energy used. It's not so much an issue of the heat generated--its how efficient the AC is in turning the energy you put in (in, electricity) to energy out (cold air)
Efficiency is uaully a number from 0 to 1 (obviously, noting is 100% effiicient, some energy is lost. The EER takes taht decimal and makes it a rating, I think so an effiiciency of 0.5 (which means only half the energy is gets put out) is a rating of 5. So rating of 8-10 would mean 80%-100% efficiency (though I am pretty sure that no AC gets a 10, 9.9 maybe or 9.99999).

The total heat produced (not just passed through, i.e. hot air expelled from within to out) is generated by the engine. This heat is directly 3.41 BTU's of heat per watt used by the engine. So...a 10000 Btu air conditioner would need to have close to a 2932 watt engine to be taking a loss on a standard AC (which has an EER of about 16 these days, I am now finding). And this is assuming a high rate of efficiency.
Most air conditioners are under 100 watts. Which means, that in an inefficient air conditioner (100 watts and 10,000 BTUs), it is still generating a net of 9971 (appr) BTUs of cold. I'd say at a ratio of 341-1, that shitty air conditioner is definitely still generating far more cold than hot. There is still some lost due to unprocessed heat energy being leaked by the system, but that is generally little to none.

At this time another friend told me of a time when they forgot to stick the tube on one of those portable a/c's out the window and the room heated up quite a bit. I relayed that, to which he responded:

Alrighty, portable air conditioning units with the hose are not the same as older portable units, or window mounted/giant compressor-type air conditioners. The hose ones use a reservoir of water is used to cool and humidify the air through evaporation. Not sure on the details, but in the end it seperates the warm water (humidity) out of the air and kicks it out the hose, and then releases the cooler less humid air back into the room. Should you leave the hose in the room you get no heat removed, but a little more added from the machine.
Bigger types (like window units) are refrigerators, except smaller and with one added fan. This one sucks air out of the room, cycles it through the parts that need cooling, and then kicks it out the back at a hotter temperature. The temperature increase is not, however, significant enough to counteract the amount of cooling given.

 

[BigInch]

I think both your friends split the 20.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[TurbineInvention]

Yes the heat of compression needs to be rejected outside. This is usually 17,000 BTU's per ton of cooling (12,000 BTU's). However scroll compressors are more efficient then recips so this number may go down. Also the condensers are now oversized (the outside part) this allows for greater heat transfer and lower head pressures and less required heat of compression. In case you did not know 1 ton of cooling is the amount of energy required to melt 1 ton of ice in 24 hours. 2000 pounds x 144BTU's Per pound (latent heat of solidification of water)288000 BTU's/24=12000 BTU's of cooling. Its a through back from before mechnical cooling.

 

[BigInch]

That's a cool fact that I've never thought about; why 2000 lbs should equal 12000 Btu. Thanks!

I remember once I saw ice loaded into the top of the Amtrak cars. It looked like a ton, but I think it melted in less than 24 hours. They were loading it again in the afternoon.

But, that was in Texas.



Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[MintJulep]

A refrigerator does make your house hotter.

This has nothing to do with efficiency. It is the Second Law of Thermodynamics. Heat cannot be moved from a cold thing to a hot thing without expending energy to do it. The theoretical amount of energy needed is defined by the temperature difference.

There are so many "efficiencies" with air conditioning that without defining exactly which one you are talking about the term is meaningless.

 

[Ashereng]

I look at it a bit more simply.

Put an air conditioner inside your room room. Close all the doors and windows. Turn it on. The room's temperature will start rising.

Why? Because the electrical cord is bringing power in, the a/c is doing work, generating heat.

So I guess your friend wins the bet. The a/c will creat more heat.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[BigInch]

Will that work in my igloo?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

Only if you have a place to plug the a/c in?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[BigInch]

In the summer... its solar 24-7. That's why I need the AC too.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[sreid]

Although you may have lost on the Hot Air-Cold Air question, you might be able to win it back on a question of Heat In vs. Heat Transfered. The heat in (AC Powr to run the compressor) is usually two to three times less that the amount of heat transfered (Coefficient of Performance).

 

[MintJulep]

The other way around.

COP of 3 means three units of heat transfered for one unit of power in.

 

[willard3]

The answer invloves the laws of thermodynamics:

First Law of Thermodynamics....the best you can do is break even

Second Law of Thermodynamics....you can't break even