How does a freezer slow freeze food?

Question

I have been told that the a freezer will freeze food slower if: a) the product load is greater then the capacity b) the capacity is greater then then the load.

In a) I think this means if a freezer can freeze 2kg of food in 12 hours, using more food will cause the freezer to work slower and so freeze slower. Is this the case?

In b) surely if the capacity is greater then the load that means it will freeze fast or at a normal rate? Or do you think it is referring to a much larger capacity i.e. if 1kg of food is placed inside a deep freezer, then it will freeze slower then if 1kg of food was placed inside a smaller freezer. If this is the case, why?

Not sure what is right. Can somebody please explain.

Thanks

Answer 1

Freezers are generally designed to keep the interior at a constant temperature (I think about -20°C). When you put food into the freezer the food heats up the interior of the freezer, and the freezer pumps away the excess heat to get the temperature back down. The heat exchangers in freezers have a finite capacity, so the rate at which they can remove the excess heat is limited.

If you put a small amount of food in the freezer the rate of freezing will be controlled by the intrinsic cooling rate of the food (heat capacity, surface area, conductivity, etc) because the heat flow is well below the limit of the freezer. Adding more food won't change the rate of freezing much.

However at some point the rate of heating of the freezer by the food will become comparable to the rate the freezer can pump the heat away, and at this point the freezing rate will fall and the food will take longer to freeze.