A freezer is rated at 1100 watts. 90% of its power is used in the freezing process. How long will it take to make 3kg of ice at -10°C from water at 25°C?

please explain in detail, im having trouble understanding heat.

A freezer is rated at 1100 watts. 90% of its power is used in the freezing process. How long will it take to make 3kg of ice at -10°C from water at 25°C?

3 kg of ice is equivalent to 3 kg of water because mass is conserved.

The freezer takes away heat by doing work.

So heat is taken away from 3 kg of water at 25 degrees to get 3 kg of water at 0 deg.
Then further heat is taken away from 3 kg of water to convert it to 3 kg of ice.
Finally heat is further taken away from 3 kg of ice at zero deg to get 3 kg of ice at -10 deg.

It is easier to understand if we reverse this problem, i.e. supply heat to 3 kg ice at -10 deg to convert it to 3 kg water at 25 deg using a heater.
The heat required for this process will be the same as the heat taken away in the previous case by the freezer.

The heat required will be

Q = 3 x Ci x [0 - (-10)] + (3 x Li ) + 3 x Cw x (25-0) where

Ci = Sp heat of ice, Li = Latent heat of fusion of ice, and
Cw = Sp heat of water. This result will be in calories. Convert to J.

The heater supplies (freezer takes away) Q J of energy at efficiency of 90% . Let E be the energy supplied to to the heater to get Q J of energy. Since the efficiency = 90%, we have Q/E = 0.9 or E = Q/0.9.

Now Power = energy / time So, Energy = Power x time So

E = 1100 x t. Solve for t.

You have not taken into account the coefficient of performance.
The coefficient of performance COP of a heat pump is the ratio between the power of the heating and electric power absorbed from the network. Spread depends on the temperature of cold source and heating source.

So except for the Heat Q on the right, this is what we have:



[LaTeX Error: Can't write to directory]

[LaTeX Error: Can't write to directory]?

The COP of the refrigerator is less or equal to [LaTeX Error: Can't write to directory]

[LaTeX Error: Can't write to directory] is ambient temp.
[LaTeX Error: Can't write to directory] is water temp.

Not sure if it's, just spitting out ideas.

"Now, COP = 0.9 or 90%"-Fals
COP is always supraunitar(2;2,5;3...)
COP" ( coefficient of performance ) si "EER" ( energetic efficiency of refrigeration )
COP=QL/W -corect
.:Kelvin PDC - promovarea si implementarea sistemelor neconventionale de producere a energiei:. (see diagram)