Back to basics
the air – this is normally quite quick, and the second is to flash off
(evaporate) and remove any moisture that is in the system.
When you reach the correct vacuum pressure, the system
pressure must not rise when you switch the vacuum pump off.
If it does with a tight system, then it can only be water that
is evaporating and raising the pressure. It is very difficult if
the system is already cold to get this water out as the driving
force to create the evaporation of any moisture through
temperature difference of components being vacuumed is
greatly reduced.
Other areas where moisture enters the system can be on
plants running in a vacuum, improper oil draining procedures,
a failed heat exchanger where the fluid on the secondary side
of the system is forced into the plant, and general poor service
procedures similar to the pump down procedure mentioned
above. The highest quantity of water is normally found on the
low side of the system, and that’s where we need to look if we
want to remove it. Water also gravitates to oil pots in which,
due to relative densities, oil floats on water and ammonia
floats on oil.
IIAR recommends that at least every three years, all ammonia
systems should have a sample removed and inspected for water
concentration. This might seem excessive to some, but when we
start to look at the performance penalties, we find with water in
the system it makes financial sense.
So, how do we remove it? Well if we have water at 0kPa
which is sea level pressure, it will boil at 100ºC and with
ammonia at 0kPa it boils at – 33.35ºC. This tells us that we
can flash off ammonia vapour from a solution and leave the
water behind. A proficient SAQCC Gas artisan should be able
to do this.
There are automatic air-purgers out there that also de-water
the ammonia on the systems, working on these same principles.
However, it is much more efficient if we do this on the lowpressure
side of the system.
Some of the larger contractors have dewatering machines
that they hire out which makes dewatering really worth doing.
As we know today it is all about efficiency, so let’s get the system
back to pure ammonia. It’s what it is designed for.
I trust that this was interesting. If you really need to find out
more, get yourself a copy of the IIAR bulletins mentioned above –
they are worth every dollar.
Till next time, stay safe out there, and I don’t need to tell
ammonia people – wear your mask. RACA
44
RACA Journal I October 2020
www.hvacronline.co.za