ASHRAE Journal - February 2024 - 72

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Weird Science
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
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
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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
Gary and Wyatt couldn't figure
out what was going on in their
refrigerator.
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.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
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ASHRAE Journal - February 2024

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