ASHRAE Journal - March 2013 - 79

REFRIGERATION APPLICATIONS

Water on the Inside
By Andy Pearson, Ph.D., C.Eng., Member ASHRAE
This article is the thirteenth in a series exploring refrigeration and heat pump
concepts without using jargon.

I

t is important to remember that when air leaks into a refrigeration system
it brings other stuff with it. Like an onion, the other stuff is mostly water. In

fact, every 9 lbs (4 kg) of air that get into a system bring 1 oz (30 mL) of
water with them. This doesn’t sound like a lot, but when you purge the air
out of the system it leaves the water behind so over weeks and months it
can build up. A system that leaks 1 lb (0.5 kg) of air every day will have
two and a half pints of water (1.2 liter) in it after a year. The effect of that
water depends very much on the type of refrigerant used. I’ve detailed three
very different situations; hopefully, at least one of them is of interest to you.

March 2013

directly to the compressor then the water
will pass through the system and back to
the compressor. All sorts of problems,
from dilution of the oil to rusting of the
compressor internals might occur, but if

©Julien Berthier

Fluorocarbons do not dissolve in water
to any signiﬁcant extent, so the liquid water
passes from the condenser mixed in with
the liquid refrigerant. When it gets to the
expansion valve, if the temperature drops
below the freezing point, the water solidiﬁes
and blocks the valve. Game over. The main
symptom of this situation is low suction
pressure because you can’t get enough refrigerant through the blocked valve to keep
the system going. However, it can be quite a
puzzle because if the system is off for a long
time before it is investigated then the ice
may have melted by the time the “blocked”
valve is opened up for examination.
Ammonia and water are very fond
of each other, and solutions of “aquaammonia” are used in many applications
from window-cleaning to print-making.
They are also, of course, used in absorption refrigerators. This means that water
cannot freeze in the expansion valve and so
it passes, in solution with the liquid ammonia, to the low pressure side of the system.
If the expansion valve outlet connects
directly to the evaporator, which connects

It’s still always better to fix the leak than
to have to deal with the consequences.
the water quantity is small the plant will
probably continue to operate normally.
If the system has a receiver vessel on the
low pressure side, such as a surge drum or
accumulator, then the water will collect in

the vessel and will be pumped around the
evaporator. There are reports of ammonia
systems with up to 25% water content in
the surge drum still functioning, albeit
inefﬁciently. More on that later.
The water builds up in the low pressure
liquid because it is much more soluble in
liquid ammonia than in gas. The evaporated ammonia gas carries almost no water
back to the compressor. The only way it
can get out of the system is to draw off a
sample of ammonia/water liquid and distil
it to boil off the ammonia and leave the
water behind. This process can be easily
automated if necessary, but as I wrote last
month, it is always better to ﬁx the leak
than to have to deal with the consequences.
Carbon dioxide is also soluble in water,
but not to the same extent as ammonia as
any lover of soda pop or champagne can
tell you. There are two additional considerations when carbon dioxide systems are
contaminated by water. Once the solution
is saturated, which requires more than 600
parts of carbon dioxide per million of
water at 32°F (0°C) (but only 120 part per
million at –40°F [–40°C]), any additional
carbon dioxide forms carbonic acid. If
oxygen is also present (and remember
the water probably snuck in with an air
leak) then heavy corrosion can occur.
With very high levels of water, clathrates
may form. These are lattices of bonded
carbon dioxide molecules that form cages,
which trap water molecules. Carbon dioxide clathrates can form solids up to 50°F
(10°C), which is another way of blocking
expansion valves and ﬁlters. They also vanish if the system is idle for too long and so
can be difﬁcult to diagnose.
In all cases, with any refrigerant (except R-718), water spells trouble. If you
can’t keep it out of the system then you
have to get it out, regularly.
Andy Pearson, Ph.D., C.Eng., is group
engineering director at Star Refrigeration in Glasgow, U.K.
ASHRAE Journal

79
ASHRAE Journal - March 2013

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