ASHRAE Journal - May 2021 - 62

REFRIGERATION APPLICATIONS
COLUMN

Andy Pearson

Refrigeration
Engineer's Toolkit
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE

It's said that an engineer only needs two things in his toolkit: duct tape and WD-40. If
something moves and it shouldn't-duct tape! If doesn't move but it ought to-WD-40.
If it should move and does, or shouldn't and doesn't, leave it alone. I've long believed
that the refrigeration engineer needs a third vital weapon in his armory: X-ray specs.

©ISTOCK: GCOGHILL

These were commonly seen in the ad pages of the comIn another dangerous scenario a length of a discharge
ics I read as a boy, offering " loads of laughs and fun at
line was isolated on a screw compressor package on a
parties " and backed by a " Money Back Guarantee. " I was
Friday in wintertime. It was verified to be vented and
never persuaded to part with my $1.00 (plus $0.25 shipthen resealed for the weekend. On Monday morning
ping) but it has struck me many times when I've heard
after a very cold weekend, the bolts around the disof accidents on site or equipment that just didn't work
charge check valve were loosened and a high pressure
as expected that these gadgets would be a
jet of liquid ammonia sprayed out. The low
boon to the refrigeration business.
temperature over the weekend caused low
The problem with refrigeration is that we
pressure in the sealed section which sucked
calibrate our understanding on the basis
gas from the system discharge (still at operatof what is familiar. We understand fluid
ing pressure) and condensed it in the sealed
This
is
not
the
weirdest
thing
flow because water is all around us and we
space. I had a similar experience when removabout refrigeration.
interact with it daily (at least I hope we do).
ing the seal cap on a shut-off valve. Fortunately
This calibration serves us well in daily life but doesn't
for me the squirt of liquid shot away from me.
account for the weirdness of fluids in a sealed space-
Once, a non-performing evaporator had horizontal
i.e., a refrigeration system. Here are a few examples of
circuits and was apparently feeding both subcooled
surprising oddities in refrigeration.
liquid and superheated gas to the suction line, which
When pumping out a system before maintenance,
ought to be impossible. The gas velocity wasn't high
if the pressure drops to atmospheric and no more gas
enough to distribute liquid up the walls of the stainless
comes out, it must be empty, right? Wrong! Liquid
steel tubes, and the thermal conductivity of the tube
refrigerant, especially with a high latent heat like
wall was too low to conduct heat into the stream of liqammonia, will get very cold inside the system and lie
uid flowing along the bottom of the tube. Likewise, in
in low lying areas. Carbon dioxide is even more tricky;
the suction of a compressor droplets of liquid can coexit will solidify if the pressure drops below 63 psig
ist with superheated gas because the surface tension of
(4.2 bar g) and then gas off gradually. A design tip is to
droplets inhibits evaporation.
include a service valve at the lowest point of a pipe secSadly, real X-ray specs are not coming any time soon.
tion likely to require to be opened in order to enable
We just need to harness our imaginations and visualize
a temporary line to be connected for the transfer of
what might be happening inside those pipes.
liquid refrigerant under pressure to another part of the
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in Glasgow, UK.
system.
62

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