ASHRAE Journal - September 2020 - 70

REFRIGERATION APPLICATIONS
Column

Andy Pearson

Deeper into Vacuum
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

In last month's column I wrote about the process of pulling a vacuum on refrigerating
systems and highlighted some good practice points for care of the vacuum pump and
how it is connected to the system for best effect. Here are some more tips for maximizing the beneﬁt and avoiding some horrible pitfalls.

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Don't use a gauge manifold to connect the vac pump
pressure. Strictly speaking this should be understood to
and gauges to the system. Firstly, the hoses are too small
be one millionth of a metre of mercury (in other words
diameter and too long to enable the best effect to be
a "milliTorr") but there always seems to be too much
achieved. Secondly, the gauge needs to be on a different
room for confusion about whether it is a micrometer or
part of the system in order to indicate the true level of
a microinch. The reason I like the unit of Torr is that it is
vacuum that has been achieved.
absolutely unambiguous.
Do make sure to use a sufﬁciently accurate vacuum
To make sense of the whole vacuum process it is essengauge for this purpose. It needs to tell the difference
tial to understand what you are trying to achieve (which
between a system holding a vacuum of 29.7 inches of meris not always the same thing from job to job). As mencury and 29.8 inches of mercury. The gauges on a manitioned last month, removal of air (or pressure test gas) is
fold are just not that accurate. Nowadays it is possible to
one reason but removal of moisture or even free water
buy a simple electronic vacuum gauge for less than $100
is also important. A small amount of water in the system
and a high quality calibrated electronic
will hinder the vacuum process because
Removing water is harder if it turns to ice.
gauge for less than $1,000. The saving in
it will start to boil when the pressure
time spent messing around will pay for
gets down to the saturation point for the
this many times over.
temperature of the water. This is obvious
By the way, do understand the many
if you know what to look for because the
and varied units used to measure
pressure will plateau at a level equivalent
vacuum and be very precise in how you
to the prevailing temperature. However if
use them, especially when specifying
there is a lot of free water inside the sysvacuum procedures to other people. The
tem it could take weeks to boil it dry using
units I used above, inches of mercury,
the vac pump. On the other hand, if the
are particularly devilish because they measure from the
geometry of the system enables the pressure to fall well
other end. Zero inches of mercury (sometimes abbrevibelow the saturation point and it gets down to the triple
ated as "in Hg") is atmospheric pressure and 29.92 inches
point of water (6.1 mBar abs, 4.5 Torr or 29.74 in Hg)
of mercury is absolute vacuum. The higher the number,
then some of the water will freeze and it is much more
the lower the pressure. I prefer to use the unit "Torr"
difﬁcult to remove it through the vac pump. Even worse,
although it is neither Systeme International (SI) nor inch- I was once involved in a project where evaporators were
pound (I-P). Named after the Italian scientist, Torricelli,
being installed inside an existing cold store. The evapoa Torr is equivalent to the pressure required to raise
rators were badly contaminated with
mercury by 1 mm (0.039 in.) in a Fortin barometer. The
water but it froze before the vacuum was
inch of mercury (whichever end of the scale you measure
pulled, so everything seemed ﬁne until
from) has the same origin-nominal atmospheric presthe system was charged and the water
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sure is 760 Torr or 29.92 inches of mercury absolute. I also ﬂooded back to the receiver.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
recommend avoiding the "micron" as a measurement of
70

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ASHRAE Journal - September 2020

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