PUMPING LATENT HEAT

But not always. If a block of ice is heated, its temperature rises only until it reaches the melting point; then the mixture of ice and water keeps absorbing heat without increase in temperature until the last bit of ice has melted. Only thereafter does the temperature resume its rise as the water is heated.

Quite similarly, the temperature interrupts its rise when the water reaches the boiling point, and resumes it (if the heat is still on) only after all of the water has been evaporated as shown in the filgure.

There is a certain amount of heat required to melt ice, and an even greater amount to evaporate water (or other liquids); to evaporate water, for example, takes 540 calories per gram (and about as many BTU's per pound) an unusually high value among liquids.

Now we know that energy, such as heat, can neither be created or destroyed; where then, did the heat go that went into the ice while it was melting or into the water while it was evaporating without increase in temperature?

Into the molecular structure of the substance; the energy is "hidden' in it, but this latent heat (as it is called) is retrieved when a vapor condenses or a liquid freezes. When a glass of water is put out in the frost, its temperature will drop until eventually it has the same temperature as its environment, by which time it is all ice. But the decline in temperature was temporarily interrupted at the freezing point (see figure above), where the temperature remained constant until all of the water had turned to ice. The heat required to keep the ice warm at the freezing point (at least for a time) when the air was much colder came from the water, which released its latent heat of fusion.

Quite similarly, a liquid requires the latent heat of vaporization to be turned into a vapor, and it will return it when it condenses. When a liquid evaporates, it will draw this heat of vaporiation out of its immediate surroundings as anyone knows who has stood wet and naked in a draft.

Suppose now that we condense a vapor at point B. transport the liquid to point A, evaporate it there, pipe it back to point B,, and 1et the substance circulate as shown above. Then at the point of evaporation A, the fluid will draw the latent heat from the environment, transport it to B, where it will release it as the vapor condenses. Point A will get colder, and point B warmer.

The arrangement is called a heat pump, though that name is not usually given to its most common application - the refridgerator. The working fluid is forced to evaporate and to condense mechanically (in a compression refrigerator) or thermally (in an absorption refrigerator) by heating a mixture of ammonia and water to evaporate the ammonia, which then condenses at higher pressure sabsorbed by the water.

In both cases the evaporation takes place at low pressure and condensation at high pressure, which is really no more than raising the boiling point of a liquid with increased pressure (a principle also used in a pressure cooker).

A refrigerator, like any other heat pump, moves the heat from one point to another, specifically, from the cooling coils inside the refrigerator to the condenser coils outside it - and it therefore always warms the kitchen.