> >> manual once
> >> stated that the fridge would work more efficiently if you kept it 
> >> full of stuff.  That got me thinking.
> 
> 
> The thermal content of air is somewhat over 1000 times lower 
> per volume than for water. For very rough figuring you can 
> consider a cc of water and a litre of air equivalent as long 
> as you don't involve any phase changes. If the air is phase 
> changing in your application then you probably need other advice :-).
> 
> So the air in a 200 litre 'fridge is about 200cc of water 
> equivalent. ie every time you open the door it's equivalent 
> to adding about a cup of water at ambient temperature. This 
> assumes that all the cold air is lost and that the convection 
> flow does not scrub more coolth off surfaces when you leave 
> it open longer. This shows why it's bearable to open vertical 
> fridges if you don't do it too often but a good idea to do it 
> as little as practical.


It takes 1 calorie (not food calorie) to heat 1 mL of water (1cc) by 1
degree celcius.  We need to drop 20 degrees, so 200 x 20 = 4000 calories of
coolth needed.  Say 25% efficiency, make it 16000.

That's not much, in food terms (K calorie) that's a jellybean or two.

Aparently a calorie is about 0.00000116222222kWH, more commonly known as
'not much'.

My power bill says a kilowatt hour is worth $0.12AU, so opening the fridge
door costs me $0.0022, or less than a quarter of a cent.

It appears fridge compressors are fairly crap these days, so limiting run
time wouldn't be a bad thing.

Tony

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist