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Refrigeration

08/26/2010 7:40 AM

what happened when a refrigerator door is opened for a long time in a room....will it heat the room or cool? If so...why?

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#1

Re: refrigeration

08/26/2010 7:49 AM

A refrigerator is simply a box with a heat pump. It literally moves heat from inside the box to outside the box. In theory the room would be neither heated nor cooled, the heat would just be recirculated. In practice you would get some heat generated due to inefficiencies in the motor, the compressor and the refrigerator light, but I doubt this would have a noticable effect on room temperature.

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#2

Re: Refrigeration

08/26/2010 11:50 AM

This is a simple black box problem.

You have a smaller box (fridge) within the bigger box (room).

Assume that the bigger box does not transfer heat to its surroundings.

You add energy to the smaller box, but the small box cannot contain that energy so it is released to the larger box.

Depending on the thermal mass within the bigger box, your temperature will rise accordingly as energy is put into the smaller box.

I contend that this will not be insignificant.

If your fridge runs at 8 amps, the amount of energy relased into the room will be:

120V x 6 A = 960 watts

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#3
In reply to #2

Re: Refrigeration

08/26/2010 12:29 PM

How much of that 960 watts is generating heat?

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#4
In reply to #3

Re: Refrigeration

08/26/2010 1:59 PM

All of it.

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#5
In reply to #4

Re: Refrigeration

08/26/2010 2:39 PM

I think not.

How does it generate 960watts of heat?

Where does the energy come from to drive the compressor?

My refrigerator is rated at 80 watts. Probably 15 watts for the light and 65 watts for the compressor. Assuming about 12 watts heat from the light, and assuming the compressor motor is an inefficient shaded-pole motor (I don't know, I've never looked) let's say about 40 watts of heat from this, so 52 watts in total. Less than one of them there old-fashioned 60w incandescent bulbs. (Much less than the halogen kitchen lights my wife insisted on!). Not much.

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#6
In reply to #5

Re: Refrigeration

08/26/2010 3:05 PM

The conservation of energy principle applies here. Electricity is energy. The electrical energy that I introduce into the black box eventually manifests itself as physical energy or heat.

Note my assumptions too.

Your refridgerator only uses 80 watts. Bully for you. If your refridgerator was much larger and it consumed about 8 amps at 120V, then you would be putting about 960 watts of heat into your room.

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#7
In reply to #6

Re: Refrigeration

08/26/2010 3:29 PM

You said it yourself: "physical energy or heat", except that you've missed light and sound. So I'll ask again, if your 960 watts of electrical input produces 960 watts of heat, what drives the compressor?

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#8
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Re: Refrigeration

08/26/2010 3:45 PM

What eventually happens to the light energy in the black box?

What eventually happens to the sound energy?

What eventually happens to the mechanical energy that is added to the compressor?

Where do they go?

If you can answer these, then I can answer your question.

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#9
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Re: Refrigeration

08/26/2010 5:33 PM

It is really not that complicated. There are only two things in a fridge that draw power, the lightbulb, usually 15-25watts, and the motor driving the compressor. Assuming your figures are correct, then your motor must be at least 935 watts. That's a pretty damn big motor for a domestic refrigerator, and I have yet to see or hear of a 935watt motor that gives out 935watts of heat. Furthermore, with the fridge door closed and in normal operation, and assuming a COP of say 2.5 your condensor coil would be chucking out about 2.5kw in heat.

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#10
In reply to #9

Re: Refrigeration

08/26/2010 7:05 PM

I agree. Energy in equals energy out.

I dont care what your compressor uses or your lightbulb uses. If you measure the amp draw of your refridgerator, the amount of heat that the room will gain with the door open is the voltage mulitplied by the amperage. All of the elctrical energy will eventually manifest itself as heat.

It doesnt matter what size motor, light bulb, compressor, etc. Whatever energy you consume is the energy that will become heat.

I am done discussing. I hope this makes sense to the original poster of this question.

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#11

Re: Refrigeration

08/26/2010 10:48 PM

Heat from the room air transfers into the evaporator inside the fridge. To this is added the motor energy (and the light too). This total energy transfers from the condenser outside the fridge back into the room air. Depending on the COP (coefficient of performance), wild guess 2.0, the original room heat would be 1920 watts, plus 960 from the motor/light; total heat rejection 2880 watts. The net effect is the same as a 960-watt heater. It should be noticeable.

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#12

Re: Refrigeration

08/27/2010 4:35 AM

1) Yes, energy in equals energy out. The refrigerator compressor requires energy to drive it in the form of work (kinetic energy), and this is supplied by the electric motor. So if we have, say, a 950watt compressor motor it might be supplying, say, 750watts of energy to drive the compressor. If it is also supplying 950watts of heat, then the total output is 1700watts for 950watts input. Quite a clever motor. I'd like to have some of these.

2) Almost 3kw of heat from the back of your fridge? That's an awful lot, think of how much heat you get from a 2kw electric fire. It sounds quite dangerous from the back of a domestic fridge. Don't be fooled into thinking that, because your fridge is rated for 8 amps it draws 8 amps continuously. The starting current for the motor is considerably higher than the full load current, so if your fridge is rated at 8 amps it might normally draw only 3 amps under continuous operation. This will equate with a motor of about 350watts, which is a lot more reasonable (I'm informed that domestic fridges and fridges/freezers are usually between 100-300watts).

I might be very wrong about this (it has been known, just ask my wife!). What I would like to understand is, if all the input electrical energy manifests itself as heat, where does the mechanical energy come from to drive the compressor? The principle of the heat pump is really not much different to a water pump, it uses mechanical energy to move something from one place to another. The heat pump compressor requires a certain mechanical power just as the pump requires a certain hydraulic power. In the case of a heat pump it moves heat, in the case of a water pump it moves water. If your 1.1kw pool pump or water pump has a hydraulic power of 800watts, the motor supplies this amount of power to pump the water and the rest is lost in heat, etc. It doesn't supply 800watts in mechanical energy and 1.1kw in heat.

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#13
In reply to #12

Re: Refrigeration

08/27/2010 5:39 AM

The exact wattage of the fridge is completely irrelevant; it's the principle that counts. Motor ratings are mostly based on full-load amps, not starting or overload amps. The motor in this example is not supplying 750 watts of mechanical energy plus 950 watts of heat; it is consuming 960 watts, all of which winds up as heat input to the room.

The 750 watts of mechanical energy is reasonable in this example, plus about 210 watts of motor and light bulb heat. However, the 750 mechanical watts go into the heat of compression of the refrigerant, so that collectively the entire 960-watt motor input becomes heat in the room.

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#14
In reply to #13

Re: Refrigeration

08/27/2010 6:29 AM

Thanks. Now I understand the principle of energy conservation. Just a couple of points:

I agree that motor ratings are based on FLA, not starting or overload current. But appliance ratings are not, they are based on the maximum current draw. Amps in normal operation will be considerably less than rated Amps. Thus a fridge rated at 8amps might consume only 300watts, so this is the power level that should be considered.

I agree that the exact wattage is irrelevant in terms of the principle of energy conservation, but maybe not in terms of the OPs question. MrGeneRall's assumptions in post#2 are quite valid when considering the principle, but in a real-life situation of a refrigerator in a room the room-heat will be dissipated. You might need a refrigerator as large as the room in order to draw 960watts, so this would almost certainly have a noticeable effect. With a normal-sized fridge in a normal-sized room, I still maintain that the 100-300watts heat generated would not be noticeable.

I notice that someone has rated all my comments as OT, whilst all MrGeneRall's have been rated GA. I don't care if this is petty-mindedness, crass stupidity, just plain bullying, or what; whether it seems fair or not, it is not becoming of CR4 users. It might make guests wonder what sort of people CR4ites really are. A good, clear, concise explanation to a question, leading to greater knowledge and understanding, is enormously more valuable than smug answers and spurious ratings.

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#15
In reply to #14

Re: Refrigeration

08/27/2010 2:21 PM

The OP didn't ask how noticeable the added heat would be (0.1°, 1°, 10°, ?). This would depend on how large the fridge is and how small and well insulated the room is. Whatever the wattage consumed by the fridge, it will be added as heat to the room. (Unless someone does something weird like cut a big hole in the wall and put the condenser outside.)

If I remember, I can take my ammeter home and see what a typical fridge draws....

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