Friday, Feb 10, 2012 at 22:53
You're welcome Barry,
yes, try it and see what the results look like.
And this is to
test Allan B's nerves :)
....Any circulation of air, either cold or warm within the fridge has an effect on the temperature distribution within the fridge only. It has nothing to do with the removal of heat and thus power use. Nor does it affect heat conduction through the insulation.....
Allan, maybe you can re-think your statements above.
Let me explain one more time why you must consider convection on both sides of the fridge walls.
The rate of heat conduction through the insulation is determined by the temperature gradient between the outer and inner surface of the walls.
If you allow this gradient to be steeper, the rate of conduction will increase (the colder the wall surface gets on the inside, and the warmer the outer surface, the harder the compressor is made to work by the thermostatic control unit).
I think that's agreeable?
Because the heat source (condenser), and the heat sink (evaporator) are on the opposite sides of this wall the direction of the heat flow (as long as there's means of facilitating this flow) is from the condenser to the evaporator (duhh...).
So what facilitates heat flow between condenser and evaporator?
Because still air conducts heat very poorly, but nevertheless expands and contracts with temperature, it readily starts moving vertically if it's in contact with a wall surface of unequal temperature.
So what you get is a rotating mass of air on the outside and on the inside (air moving down along the relatively cool outer surface, and up on the relatively warm inner surface. Of course, on the surfaces of the evaporator/condenser, the direction of airflow is reversed, which completes the inner/outer circles of air movement.
And bingo, now we've got convection on both sides of the wall.
Convection increases the temperature gradient between the two surfaces, warm air is fed from the condenser (and ambient) to the outer surface and cold air from the evaporator to the inner surface.
If you artificially stop or at least restrict the air movement on either one, or both sides of the wall, the temperature gradient inside the wall decreases, and there's less heat flowing through this wall.
And I betcha, a towel is far more efficient stopping convection, than conducting heat from the relatively warm inner surface towards the cold surface of the evaporator.
The end result is a slightly warmer inner wall surface, thus a reduced temp. gradient inside it, and lower rate of heat conduction through that wall.
And a lower rate of heat conduction means less work to be done by the compressor, because now the temp. gradient between condenser and evaporator can be kept up more effortlessly.
To prove this, all you have to do is, stuff towels between the content and all wall surfaces (no towels between evaporator and content of course), and see how much more efficient your fridge will suddenly work.
....In point of fact, it is undesirable to restrict air movement within the chamber unless you are trying to achieve temperature gradients within the contents. Many modern domestic refrigerators use fan circulation to obtain more even temperature throughout the fridge....
Not restricting it between evaporator and content, but restricting it along the walls will definitely help.
Convection forming between the walls and content has to be restricted as much as possible.
Sure, fan assisted cooling is a slightly different kettle of fish (not sure if this pertains to this case).
I still remember how inefficient a fridge works if the evaporator is all iced up, and things don't get really cold inside although the compressor is running all the time.
I guess the main purpose of the fan inside is to reduce the temp. gradient between evaporator and content.
The associated losses due to the increased heat transfer rate through the walls are more than compensated for by the efficiency gains of a fan assisted refrigeration cycle (stuff is getting colder more quickly).
Of course it would be ideal to retain the low wall-temp-gradient, while the fan moves air between content and evaporator, but you can't have it both ways unfortunately, and some of this airflow will always increase the wall-temp-gradient to some extent (unless there's a towel shielding the walls from the cold draft).
cheers, Peter
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