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.
EFFECTS OF WATER IN AN
AMMONIA SYSTEM…
By Andrew Perks
CONTINUED
I can’t imagine my whiskey without some ice, but it has some pretty negative effects
when we add it to a refrigerant.
As mentioned in my previous article, water is very important to
us as is evidenced in the Western Cape from the summer of
2018/19 with the droughts and ‘Day Zero’.
That given, ammonia can cope with water much better, but don’t
fool yourself because there is a penalty to be paid. The International
Institute of Ammonia Refrigeration (IIAR) bulletins No 108. ANS/
IIAR 6-2019 and ANSI/IIAR 2 refer to the comments we are discussing
further in this article on the subject.
Water contamination in the system occurs over a period of
time. Due to this gradual process the effects slowly build up and
go unnoticed and are just seen as a gradual decrease in efficiency,
usually felt to be related to general wear and tear in the plant.
I can remember being called out to a plant at a winery where the
client complained of rust in his compressor – it was the white wine
cellar and that section of his cooling system only operated 2 months
maximum a year. The client was doing a top end inspection and
noticed the rust.
Being a first time for me, we stripped the compressor and
found evidence of corrosion throughout the machine, even on the
crankshaft which cleaned up nicely with some very fine emery paper.
The winery’s ammonia was super saturated with water which on a
normal 24/7 plant would never have caused this corrosion, but since
the plant stood for 10 months of the year, the corrosive effect of the
resultant Ammonium Hydroxide was noticeable. We purged the
ammonia and drew a very deep vacuum (at least -95 kPa) which we
broke 3 times with dry Nitrogen.
There are some graphs in the IIAR bulletins mentioned which
highlight the effect of water dilution on a system running at –18 ºC
room temperature and a nominal suction pressure of 106.9 kPa. With
10% of water dilution, due to the impact on the ammonia saturation
conditions, the system now needs to run at 89.63 kPa to hold the
same conditions of –18 ºC in the cold room which means the system
now needs to run in a vacuum.
The effect of water dilution is much more pronounced in low
pressure systems, resulting in overall compressor capacity loss,
and therefore the requirement for more compressors to run. Which
of course results in more power being used to achieve the same
results. A lose-lose situation.
In the United States of America (US) they do not enforce stress
relieving of ammonia vessels after construction like we do in South
Africa. During construction of pressure vessels there is a lot of stress
set up in the vessel which needs to be relieved.
Ammonia is known to create serious issues where there is
localised stress but it has been found that a small concentration of
0.2% (2000 parts per million) of water can reduce the stress corrosion
cracking effect as it scourges the loose oxygen particles which
contribute to the problem.
So, a little bit of water has an advantage where there is the
possibility of stress corrosion cracking taking place in the system.
Water content above 2% or higher results in energy penalties.
Sometimes you can get too much of a good thing.
So, where does this water come from? Usually bad service
procedures such as poor initial vacuum being drawn on the plant at
construction stage (should be at least -95 kPa.) to remove any air
and moisture.
It can also happen when systems are being blown down during
maintenance into water drums to absorb the smell of the ammonia
being released. What happens is that as the last of the vapour is
vented from the system the remaining vapour condenses due to the
reduced pressure/temperature relationship in the plant, drawing
whatever water is in the hose back into the plant which is now under
a vacuum. This can easily result in 200l being drawn into the system
if the technician is not vigilant.
Before we pull a full vacuum (-95kPa) the plant needs to be
properly pressure tested to ensure it is tight. As mentioned, there
are two reasons why we pull a vacuum on a plant: firstly, to remove
www.hvacronline.co.za RACA Journal I October 2020 43