Exploring counterintuitive phenomena that challenge our intuition about heat and fluids.

One of the hardest things to understand about refrigeration is how fluids behave inside a closed system when they turn from liquid to gas and back again. We think we know how this works because every time we make a cup of coffee, we boil some water and our observed knowledge is reinforced. But it’s not like that inside a refrigerator.

The first difference to note is that the gas space above the liquid in a kettle is filled with air, but inside the refrigerator the space is filled with the same substance as the liquid, just in a different form. This means that the pressure in the refrigerator goes up and down with temperature (following a fixed relationship) so when both liquid and gas are present in a space within a refrigerator the liquid is always at its boiling point. Any reduction in pressure will cause more of the liquid to turn to gas; and any increase in pressure turns gas to liquid. This causes several strange things to happen in refrigerators.

The weight of liquid creates a slight pressure gradient with depth, so the liquid at the surface of a pool is ready to boil but deeper in the pool it is slightly subcooled and so boiling is easiest at the top. If a large tank has a float valve controller (like a ballcock) to add more liquid if the level drops then the first thing that happens when the float valve opens is that the level falls even further because the small rise in pressure will condense some of the gas bubbles that are suspended in the liquid. This can cause control instability through a positive feedback mechanism where opening the valve to add liquid reduces the volume of the liquid that’s already there. Conversely when liquid is being drawn out of the bottom of a tank, any pressure drop—for example, at the inlet to a pump—can cause the liquid to boil, creating bubbles that can do great damage when they subsequently collapse again in a phenomenon known as “cavitation.”

The second difference is that boiling water is not a good model of our intuition because the bonds between molecules in liquid water are so high, hence a lot of energy is required to turn the liquid into gas. The high bond strength is because the H₂O molecule is kinked, like a broken stick. The middle of the molecule (the oxygen atom) has a slight negative charge and the ends of the stick (the two hydrogen atoms) are slightly positive. In fridge jargon we say that the latent heat of vaporization of water is high (970 Btu/lb or 2260 kJ/kg). Ammonia comes second to water in terms of latent heat, and for the same reason; the NH₃ molecule is polar so the bonds between molecules are strong, requiring 502 Btu/lb (1167 kJ/kg) to evaporate it at 77°F (25°C). Boiling a non-polar molecule, like R-290 (144 Btu/lb or 335 kJ/kg) or R-134a (76 Btu/lb or 177 kJ/kg) at the same temperature is much easier.

Many of the operational problems in refrigerators, especially large industrial ones, are caused by gas turning to liquid when it shouldn’t or failing to do so when required. Unfortunately this is one area where our intuition based on observation of the world around us can be misleading.