Quote:
Originally Posted by ShuttlePilot
Don't forget Latent Heat of Fusion
From a google search refresher.
Removing heat from one pound of water one Btu at a time will produce a linear result until the temperature of the water reaches 32°F (0°C). At the freezing point of water, it will require 144 Btu’s of latent heat to be removed in order to turn all the water into ice without any further drop in temperature, after which the ice will be sub-cooled.
This is the Latent Heat of Fusion of Water, and is 144 Btu’s per pound (or 334 joules (80 calories) per gram).
- Eric
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OK, just a hair over 40 years since I taught physics, so digging deep for some recall here.
This is also known as the enthalpy of fusion. Let's take that one gram of water and start with it as ice at absolute zero (0°K, -273.15°C, -459.67°F). It takes 0.5 calories per gram to elevate the temperature one degree Kelvin/Celsius up to 0°C/+273.5°K, which is the transition point between solid and liquid water at STP (standard temperature and pressure). So, that is approximately 137 calories to elevate the temperature from 0°K to 273.15°K (=0°C=32°F) We now have to pour 80 calories of heat into the ice to transition from solid to liquid, while remaining at a temperature of 0°K. The reverse is true when changing from liquid water to solid ice; the same amount of heat must be removed.
Once the transition of state is achieved, it now takes 1 calorie of heat to raise the temperature for each one degree Kelvin. So, that is 100 calories to elevate the temperature of one gram of liquid water at 0°C to 100°C (32°F to 212°F). One hundred calories to go from liquid water at its freezing point to liquid water at its vaporization point. It now takes an astounding 540 calories just to change that same one gram of water from liquid to steam-steam in which the individual water molecules are at that same 100°C/212°F temperature. This is known as the enthalpy of vaporization.
Simply put, water does not like to change temperature, and it sure as hell does not like to change state. This is why most of costal California never gets extremely hot or extremely cold. Water doesn't like to change temperature and the ocean moderates temperature change both up and down right on the coast.
There is no such thing as a refrigerator/freezer which can keep frozen things frozen, but cannot change water to ice. If, however, the refrigerator does not have the ability to draw heat from a closed system rapidly, it can take a
very long time to freeze something. So the questions becomes, how rapidly can a refrigerator/freezer remove heat from something containing water? It actually matters little whether the item you put in is 90°F or 33°F. It can get the item down to 32°F pretty quickly. Getting the water inside to change from liquid at 32°F to ice at 32°F is what takes a lot of thermal sink, and if the refrigeration system is not very strong, also just takes a
lot of time.
Beatrice
Linear Mode
