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#128649 - 09/05/03 10:39 AM Air Conditioning
JohnS Offline
Member

Registered: 07/31/03
Posts: 36
Loc: Cleveland, Ohio USA
I'm curious as to how air conditioners work. I know that a compressor in the air conditioner compresses a gas that in doing so cools the gas and the heat is given off to the outside air. I know the gas is pumped through coils and (inside) air is passed over the coils and the heat in the air is transfered to the coils and the gas, causing the gas to expand. The expanded gas is returned to the compressor to start the cycle over again. Somewhat simplified explanation.

What I don't understand his how all the "work" is calculated.

I saw an advertisement for an air conditioner having a rating of 14 000 BTU. I am assuming they really mean BTU/h. This is suppose to be the rate at which the air conditioner removes "heat energy" from the air. This air conditioner also has an electrical rating of 12 A @ 120 V or 1.44 kW.

14 000 BTU/h x 0.293 071 W.h/BTU = 4.1 kW. From what I see, the air conditioner is able to do 2.85 times the amount of work then what it actually uses. How can this be? Is there not some law of thermodynamics being broken here?

I'm sure the numbers are correct, so can someone explain to me how an air conditioner appears to use more power then it uses?

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#128650 - 09/05/03 11:20 AM Re: Air Conditioning
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
The magic of heat pumps, such as air conditioners or refrigerators, is the property of fluids to take up a lot of energy when boiling.

Huh?

Let's use water for an example. Modern refrigerants have properties making them a bit more difficult to calculate, but principally they work in exactly the same way.

Heating water from 10C to 20C takes up just as much heat as you get back from cooling it from 20C to 10C. Useful for a heat exchanger, but it won't run a refrigerator. But if we start with water to 95C (atmospheric pressure) and want to go to 105C, something interesting happens. From 95 to 100C it takes just as much energy per degree as we expect. But at 100C it boils. Adding more energy doesn't raise the temperature. All the added energy goes to convert water into steam. Once we have converted all the water to steam the temperature begins to rise again.

The stored energy is released if the steam is condensed back to water.

Here comes the trick: Everyone knows that water boils at a lower temperatur if you lower the pressure (e.g. if you are on top of a mountain). In an air conditioner, the pump lowers the pressure so much that the water (refrigerant) boils below room temperature. It doesn't take very much power/energy to lower the pressure and the refrigerant takes up a lot more energy from the air than is used for the pump.

On the other side of the pump, the pressure is high. The boiling point of the refrigerant is no longer below the ambient temperature, but above it. This means it doesn't want to remain like steam. I starts to condense on the walls of the vessel. This condensing releases the energy that the refrigerant picked up when it boiled.

The refrigerant is then returned to the original vessel via a valve that restricts the flow. This restriction keeps up the difference in pressure between the two sides.

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#128651 - 09/05/03 02:55 PM Re: Air Conditioning
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
Your right. Air conditioners can do more work than is put into them...sort of.

A heat pump is probably an easier example. A heat pump is just an airconditioner with the hot coil inside and the cooler coil outside. A heatpump doesn't convert electricit to heat the way a resistance coil does. A heatpump just moves the heat that's already outside into the room. It's more efficient to move heat from one place to another than to convert electricity to heat.

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#128652 - 09/05/03 03:15 PM Re: Air Conditioning
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
And here's my short version of how an a/c works:

The liquid freon is pumped through the "liquid line" to the expansion valve. This is just a tiny orifice. As the liquid sprays through this tiny hole, it evaporates. Try spraying starting fluid on your hand and you'll see how it gets cold when it evaporates.

This evaporation happens inside the indoor coil called the...wait for it...evaporator. Air blowing through this coil gets cold and cools your house. The evaporated freon gets warmed as it absorbs the heat from the air.

This slightly warmed gas is sucked up by the suction side of the compressor where it is compressed to a liquid. When it's compressed, it gets pretty hot.

The discharge side of the compressor is hot. This hot freon is pushed through the outside coil where it is cooled off. From there the freon moves to the the expansion valve and the cycle starts again.

All you're doing is absorbing heat from inside and letting it go outside.

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#128653 - 09/06/03 09:48 AM Re: Air Conditioning
JohnS Offline
Member

Registered: 07/31/03
Posts: 36
Loc: Cleveland, Ohio USA
 Quote:
A heatpump just moves the heat that's already outside into the room.


Thanks for all the answers. I understand now. The air conditioner/heat pump does not convert the heat to another form, such as electricity or mechanical work, it just moves it from the air, through a medium, such as freon back to the air again. Since no conversion is taking place the power the air conditioner actually uses then accounts for the effort to move the heat and some losses of the components; the compressor and the fans. The power (W) to move the air is the product of the volume flow rate (m^/s) and the pressure (Pa or N/m^2), and that would be less then the electrical power supplied, because of losses.

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#128654 - 09/07/03 09:57 PM Re: Air Conditioning
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Thanks to everyone for participating in this discussion!

It's a topic which comes up alot, and the information here will help many people in the future!

On the subject, I would like to compile a database of common package units and split systems - heat pumps and cooling only, which would show FLA per Ton / KBTU / Voltage / Ø, and post it in the Technical reference area.

Any data would be greatly appreciated!

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128655 - 09/07/03 11:34 PM Re: Air Conditioning
Big Jim Offline
Member

Registered: 07/18/03
Posts: 377
Loc: Denver, CO USA
I suspect that database could grow awfully large. y makers x z models x 10 sizes per model. Maybe just a list of approximate values for given tonnage. That might be enough for general design uses. Numbers for the typical 10 SEER unit would work for most everything. If your customer is spending big bucks on a super high efficency unit, it'll draw less and you just overdesigned by a little. A top of the line, 14 SEER, 2 ton will draw about 8 to 9 amps and a generic 10 SEER runs about 12. Hopefully you'll have the actual unit on site to read the nameplate before you actually have to stab a breaker for it.

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#128656 - 09/10/03 08:11 AM Re: Air Conditioning
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
 Quote:

The air conditioner/heat pump does not convert the heat to another form, such as electricity or mechanical work, it just moves it from the air, through a medium, such as freon back to the air again. Since no conversion is taking place the power the air conditioner actually uses then accounts for the effort to move the heat and some losses of the components; the compressor and the fans.


Very good summary! I should add that the compressor work includes not only friction losses and the like, but also the work required to 'lift' the transported heat in temperature.

Any temperature difference can in theory be used to run a motor, i.e. convert the energy to another form. To calculate this efficiency is very simple:

Maximum efficiency = 1-T_low/T_high

where
T_low is the lowest temperature in the cycle
T_high is the highest temperature in the cycle

The temperatures must be in Kelvin or Rankine.

As you can see, the higher the ratio T_low/T_high is, the higher the efficency.

In a heat pump, you effectively invert the formula: The higher the ratio, the less efficient the heat pump becomes. Thus, a freezer is less efficient than a fridge.


[This message has been edited by C-H (edited 09-10-2003).]

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#128657 - 09/10/03 12:20 PM Re: Air Conditioning
JohnS Offline
Member

Registered: 07/31/03
Posts: 36
Loc: Cleveland, Ohio USA
 Quote:
On the subject, I would like to compile a database of common package units and split systems - heat pumps and cooling only, which would show FLA per Ton / KBTU / Voltage / Ø, and post it in the Technical reference area.


I think it is time to dispense with some of these units that only a relative few understand, if they really do. There seems to be a plethora of units used that are really unecessary and somewhat confusing.

Do we really need units like tons and BTUs to describe an air conditioners operation? What is wrong with using the more easily understandable SI units? We can express more understandably the capacity in watts.

I'm sure someone will argue that this is the way it is done. But, an effort can be made to change the situation and make things simpler. Nothing is written in stone.

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#128658 - 09/10/03 12:54 PM Re: Air Conditioning
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I'am game, but what is an SI unit?

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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