Back to basics
ANDREW PERKS
Andrew Perks is a subject expert in ammonia refrigeration. Since undertaking his apprenticeship in Glasgow in the
1960s he has held positions of contracts engineer, project engineer, refrigeration design engineer, company director
for a refrigeration contracting company and eventually owning his own contracting company and low temperature
cold store. He is now involved in adding skills to the ammonia industry, is merSETA accredited and has written a
variety of unit standards for SAQA that define the levels to be achieved in training in our industry.
WE DON’T WANT WATER
IN OUR REFRIGERATION SYSTEMS
By Andrew Perks
As I mentioned in my last article, I was so looking forward to my overseas visit
to my first IIAR conference in Orlando US. I have been to many exhibitions and
conferences in the past but the thought of a real live Ammonia one really captured
my imagination.
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Well we all know what happened – the whole world went crazy
and we are still a long way from the end of it. I was chatting
to someone about the last pandemic, the Spanish Flu of 1918. The
scary bit is that it lasted for 2 years and the second wave was more
deadly than the first as by then most people were just fed up with the
restrictions placed on their lives. What’s that old adage? “If we don’t
learn from the past, we are destined to repeat it”. So, everyone out
there just stay safe and take care.
So, what is this about water? We all know H₂O is pretty good stuff,
71% of the earth’s surface is covered with it. Our adult bodies are
between 57 to 60% of it, and if you read the world statistics, our total
consumption as a country is 791l per person. I just can’t imagine my
whiskey without its ice and the good news is water has no calories. All
in all, it’s pretty good stuff in the right place.
However, (there is always a however isn’t there?), it’s not good
stuff when we get it into our refrigeration system. I attended a
recent webinar from IIAR on “Water Contamination with Ammonia
refrigeration systems” and found it quite enlightening.
Anhydrous Ammonia as we know is a robust industrial refrigerant
that due to its affinity with water can, and does, absorb water
easily, resulting in it becoming what is known as an Aqueous (wet)
Ammonia. Unlike with the Hydrocarbons, water mixes readily with
Ammonia and does not separate thus freezing up in the system.
Whilst with Ammonia this may be seen as a good thing that doesn’t
totally wreck the refrigeration process, it’s not supposed to be there
and does have an effect on the system’s operation and efficiency.
IIAR have published various standard publications on the effect of
water in the system initially with Bulletin 108 which has been updated
with ANSI/IIAR 6-2019 and the later ANSI/IIAR 2 update revision and I
am going to refer to these as we go.
When water and Ammonia mix, Ammonium Hydroxide is
formed which can be extremely corrosive as concentrations rise
unnoticed over a period of time. There are also other detrimental
RACA Journal I September 2020
effects on the system with high levels of water in the Ammonia.
Whilst it is still a functioning refrigerant its saturation pressures
need to be depressed to achieve the correct evaporation
temperature required in the system.
Some of the symptoms of water in the system are as follows:
• Pressure/temperature relationship is affected
• Chemical changes occur in the compressor oil
• Pressure drops increase in piping
• There is an overall decrease in the plant’s performance
• To maintain cold room temperatures compressors must
operate more
Commercial grade Ammonia is not really dry but is classed as
anhydrous Ammonia with a purity of 99.995% Ammonia. Our
systems are permitted to have some water content in the region of
between 50 to 5 000 ppm, whilst the limit for oil is 50 ppm, without
affecting overall plant performance.
The net effect of water in the system results in us having to
push down suction pressures to achieve the correct temperature
of refrigerant at the evaporator. As the water content increases
this effect increases resulting in lower and lower suction pressures
in the system. It should be remembered that as the suction
pressure decreases so does the gas density, meaning that there is
less refrigerant per cubic meter compressed by the compressor.
Less refrigerant flowing around the system means less of a
cooling-effect. The system tries to balance this out by increasing
compressor operating with an overall impact on the wear and tear on
compressors and energy efficiency, resulting in a lose-lose situation.
As a yard stick, for every 1º C the system pressure or saturation
temperature drops the compressor capacity drops by approximately
4.5% due to the change in the gas density.
This is quite a lengthy subject that needs some serious attention
and I will be continuing on this topic in the next issue. RACA
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