ASHRAE Journal - October 2020 - 58

REFRIGERATION APPLICATIONS
COLUMN

Andy Pearson

Testing, Testing
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

This is the third of three columns focused on the process of evacuating a refrigeration
system and focuses on the test element of that process, including a look at how the
vacuum ﬁts within the rest of the test regime.

NOON - REST FROM WORK (AFTER MILLET), BY VINCENT VAN GOGH

In last month's column I noted that the objective of
Others see it as their mission to ﬁnd a leak. Their mental
pulling a vacuum is not the same from job to job. In
attitude is that the leak exists and the challenge is to nail
a new build ﬁeld-erected system a strength integrity
it. It is far easier to be motivated to ﬁnd a needle in a haytest will be required for all the site-installed pipework,
stack if you really believe in the needle. The second type
and often this will extend to some of the factory built
of person will aim to run the tightness test at as high a
equipment, too, if it cannot reasonably be isolated
pressure as possible. That makes needle ﬁnding easier.
from the test. The site strength test will typically use a
Once the tightness test has been completed and the
gas as the test medium because of the
nitrogen has been safely vented to atmodifﬁculty of drying out the system after
sphere, it is necessary to get the vacuum
a hydraulic test. This is a dangerous
pump connected and persuade the rest
procedure-probably one of the most
of the test gas to leave the scene. For the
hazardous things that we do-because the
dedicated needle hunters this is a second
newly installed pipework is being raised
bite at the cherry, if I may mix my metato a pressure it has never seen before.
phors. Some systems will be tight under
Vincent wasn't really that
Usually oxygen-free nitrogen is used as
pressure but will leak under vacuum. For
committed to tightness testing.
the strength test medium. Oxygen under
example, if there is an inclusion in a weld
pressure would be highly dangerous and air is nearly as
that blocks the gap when pressurized but is pushed
bad.
in by atmospheric pressure when the system is under
Once the strength test has been passed the pressure
vacuum, leaving a pinhole which then leaks refrigerant
can be slightly reduced and a tightness test can be conwhen the system is charged.
ducted. This involves getting close to the unit and huntIn other circumstances the vacuum process is trickier.
ing down any possible leaks that would result in refrigWhen the refrigerant is being changed, for example a
erant loss if the plant was charged. The strength test
conversion from R-22 to an HFC blend like R-422D, the
does not need to be maintained for long, but a valuable
O-ring seals in the plant may have been leak-free before
part of the tightness test is leaving the pressure high for
the process of conversion started, but once they have been
several hours and seeing whether it varies. Variation, of
pulled out of shape by the vacuum then pushed back again
course, might be due to temperature changes as well as
by the new refrigerant, they will leak badly. The leak is not
leaks, but as a colleague said to me recently, "It's a funny caused by the new refrigerant, but by the journey to get
thing but nobody ever reports the tightness test presfrom old to new. It is quicker and easier to recover the old
sure going up due to temperature change."
refrigerant, open up the system while it's empty, change
These test procedures highlight the differences in
all the seals, box it up, pull a vacuum and
mental attitude to the task in hand. Some people view
then charge the new gas than it is to mess
the exercise as a box to be ticked on the way to project
about trying to change some of the seals
completion. They will run the tightness test at as low a
once the new refrigerant is already in the https://bit.ly/3bajige
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pressure as they can get away with and their heart will not system and is already leaking.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
really be in it. Then they will tick the box and move on.
58

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ASHRAE Journal - October 2020

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