ASHRAE Journal - February 2013 - 65

refrigeration applications

Coping with Excess Gas
By Andy Pearson, Ph.D., C.Eng., Member ASHRAE
This article is the 12th in a series exploring refrigeration and heat pump concepts
without using jargon. Last month we considered the effect of air or nitrogen in a
condenser. Now, it’s time to consider some effective—and not so effective—strategies
for overcoming this problem.

R

emember that the addition of extra gas in the condenser causes
the compressor to work harder than it ought to. The extra gas is

noncondensable. Within the limits of operation of the refrigeration
system, we cannot make it cold enough to turn it to liquid. This means
it remains stuck in the gassy part of the condenser, unable to drain out
the bottom with the liquid refrigerant. The tactics for dealing with this
pest are simple but require some lateral thinking.

The mixture of gases is allowed to flow
into a separate side-chamber, which is
made very cold, causing the refrigerant
to condense and drain away as liquid.
The low temperature in this side-chamber
should be as cold as possible since the
name of the game is to turn as much of
the refrigerant gas to liquid as possible.
The total pressure in the system should
also be as high as possible during this
process to give the condensable stuff as
much chance as possible to escape. It is
simply a case of making the ratio of pressures in the side-chamber as favorable
as possible, which is done by simultaneously making the pressure high and the
temperature low.
Using last month’s example of R-134a
with some nitrogen in it, the ratios show
that if the total pressure is 212 psi (1462
kPa) absolute then the R-134a will condense at 122oF (55°C) if it is 90% of
the mixture. But, if the temperature is
reduced to 0oF (–18°C) then the R-134a
portion of the total pressure will only
be 10%. We have managed to turn a
February 2013

mixture that was only 10% nitrogen into
one that is 90% nitrogen. However, the
law of diminishing returns looms large:
to increase the nitrogen content to 95%
we would need to cool the mixture to
–27.5oF (–33°C), and to get up to 99%
would require a temperature of –79oF
(–62°C) if the total pressure stays at 212
psia (1462 kPa).
Once the mixture has been lured into
the side-chamber and chilled down to as
low a temperature as possible, the nitrogen can be removed from the system,
usually by blowing it to the atmosphere.
Note that some refrigerant is always lost,
too, and that some nitrogen is always left
behind, so purging should be avoided
completely if possible. It’s always better
to fix the leak than to have to deal with
the consequences.
A purging system that doesn’t pump
the mixture up to high pressure or make
it really cold will lose more refrigerant
than it should, and will need to repeat the
process more often. In a low temperature
cold store or a freezer, a reasonable ratio

can be achieved just using the suction
pressure as the source of cooling, but
in a chill store a small cooling unit running at a lower temperature than the
compressor suction may be necessary to
get to the right ratio to avoid excessive
refrigerant loss.
Some people claim they can do a satisfactory job without a purger unit just
by trapping the gas mixture in the condenser and running the fans to condense
as much refrigerant as possible. This
might just about work on a really cold
winter’s day if they get the pressure up
absolutely as far as possible to start with,
but who feels the need to purge in winter? At the height of summer, even with
an evaporative condenser it is likely that
the refrigerant proportion in the purged
gas will not be lower than 50%. The real
tragedy of this approach is that a decent
purger can be homemade for a cold store
for a few hundred dollars, including fully
automatic controls. But, they are often
not installed as a “cost-saving measure.”
Obviously that’s a decision taken by
someone who doesn’t pay for the replacement refrigerant!
It is fair to say that chill jobs with the
evaporator pressure well above atmospheric are less likely to suffer from air
leaking into the system; on the other hand,
they will still suffer from air left in after
service, or nitrogen that was not removed
after pressure testing. It is even possible
to justify the installation of a purger on
the basis of the time and cost savings
achieved by not having to pull air out of
small components after service, and not
losing refrigerant during the subsequent
“poor ratio” purging; a little joined-up
thinking goes a long way in this case!

Andy Pearson, Ph.D., C.Eng., is group
engineering director at Star Refrigeration in Glasgow, U.K.
ASHRAE Journal

65



ASHRAE Journal - February 2013

Table of Contents for the Digital Edition of ASHRAE Journal - February 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
R-22 Hard Act to Follow: Ammonia Low-Pressure Receiver Systems
Long-Term Commercial GSHP Performance: Part 7: Achieving Quality
Thermally Active Floors: Part 2: Design
Future of DCV for Commercial Kitchens
Standing Columns and Special Sections
Building Sciences
Emerging Technologies
ACREX India 2013 Show Guide
Refrigeration Applications
InfoCenter
Data Centers
IAQ Applications
Special Products
Classified Advertising
Advertisers Index
ASHRAE Journal - February 2013 - Intro
ASHRAE Journal - February 2013 - Cover1
ASHRAE Journal - February 2013 - Cover2
ASHRAE Journal - February 2013 - 1
ASHRAE Journal - February 2013 - 2
ASHRAE Journal - February 2013 - Contents
ASHRAE Journal - February 2013 - Commentary
ASHRAE Journal - February 2013 - 5
ASHRAE Journal - February 2013 - Industry News
ASHRAE Journal - February 2013 - 7
ASHRAE Journal - February 2013 - 8
ASHRAE Journal - February 2013 - 9
ASHRAE Journal - February 2013 - 10
ASHRAE Journal - February 2013 - 11
ASHRAE Journal - February 2013 - Letters
ASHRAE Journal - February 2013 - 13
ASHRAE Journal - February 2013 - Meetings and Shows
ASHRAE Journal - February 2013 - 15
ASHRAE Journal - February 2013 - R-22 Hard Act to Follow: Ammonia Low-Pressure Receiver Systems
ASHRAE Journal - February 2013 - 17
ASHRAE Journal - February 2013 - 18
ASHRAE Journal - February 2013 - 19
ASHRAE Journal - February 2013 - 20
ASHRAE Journal - February 2013 - 21
ASHRAE Journal - February 2013 - 22
ASHRAE Journal - February 2013 - 23
ASHRAE Journal - February 2013 - 24
ASHRAE Journal - February 2013 - 25
ASHRAE Journal - February 2013 - Long-Term Commercial GSHP Performance: Part 7: Achieving Quality
ASHRAE Journal - February 2013 - 27
ASHRAE Journal - February 2013 - 28
ASHRAE Journal - February 2013 - 29
ASHRAE Journal - February 2013 - 30
ASHRAE Journal - February 2013 - 31
ASHRAE Journal - February 2013 - 32
ASHRAE Journal - February 2013 - 33
ASHRAE Journal - February 2013 - 34
ASHRAE Journal - February 2013 - 35
ASHRAE Journal - February 2013 - Thermally Active Floors: Part 2: Design
ASHRAE Journal - February 2013 - 37
ASHRAE Journal - February 2013 - 38
ASHRAE Journal - February 2013 - 39
ASHRAE Journal - February 2013 - 40
ASHRAE Journal - February 2013 - 41
ASHRAE Journal - February 2013 - 42
ASHRAE Journal - February 2013 - 43
ASHRAE Journal - February 2013 - 44
ASHRAE Journal - February 2013 - 45
ASHRAE Journal - February 2013 - 46
ASHRAE Journal - February 2013 - 47
ASHRAE Journal - February 2013 - Future of DCV for Commercial Kitchens
ASHRAE Journal - February 2013 - 49
ASHRAE Journal - February 2013 - 50
ASHRAE Journal - February 2013 - 51
ASHRAE Journal - February 2013 - 52
ASHRAE Journal - February 2013 - 53
ASHRAE Journal - February 2013 - 54
ASHRAE Journal - February 2013 - 55
ASHRAE Journal - February 2013 - Building Sciences
ASHRAE Journal - February 2013 - 57
ASHRAE Journal - February 2013 - 58
ASHRAE Journal - February 2013 - 59
ASHRAE Journal - February 2013 - 60
ASHRAE Journal - February 2013 - 61
ASHRAE Journal - February 2013 - 62
ASHRAE Journal - February 2013 - Emerging Technologies
ASHRAE Journal - February 2013 - 64
ASHRAE Journal - February 2013 - ACREX India 2013 Show Guide
ASHRAE Journal - February 2013 - 64b
ASHRAE Journal - February 2013 - S1
ASHRAE Journal - February 2013 - S2
ASHRAE Journal - February 2013 - S3
ASHRAE Journal - February 2013 - S4
ASHRAE Journal - February 2013 - S5
ASHRAE Journal - February 2013 - S6
ASHRAE Journal - February 2013 - S7
ASHRAE Journal - February 2013 - S8
ASHRAE Journal - February 2013 - S9
ASHRAE Journal - February 2013 - S10
ASHRAE Journal - February 2013 - S11
ASHRAE Journal - February 2013 - S12
ASHRAE Journal - February 2013 - S13
ASHRAE Journal - February 2013 - S14
ASHRAE Journal - February 2013 - S15
ASHRAE Journal - February 2013 - S16
ASHRAE Journal - February 2013 - S17
ASHRAE Journal - February 2013 - S18
ASHRAE Journal - February 2013 - S19
ASHRAE Journal - February 2013 - S20
ASHRAE Journal - February 2013 - S21
ASHRAE Journal - February 2013 - S22
ASHRAE Journal - February 2013 - Refrigeration Applications
ASHRAE Journal - February 2013 - InfoCenter
ASHRAE Journal - February 2013 - 67
ASHRAE Journal - February 2013 - 68
ASHRAE Journal - February 2013 - 69
ASHRAE Journal - February 2013 - 70
ASHRAE Journal - February 2013 - 71
ASHRAE Journal - February 2013 - Data Centers
ASHRAE Journal - February 2013 - 73
ASHRAE Journal - February 2013 - 74
ASHRAE Journal - February 2013 - IAQ Applications
ASHRAE Journal - February 2013 - 76
ASHRAE Journal - February 2013 - 77
ASHRAE Journal - February 2013 - Special Products
ASHRAE Journal - February 2013 - Classified Advertising
ASHRAE Journal - February 2013 - Advertisers Index
ASHRAE Journal - February 2013 - Cover3
ASHRAE Journal - February 2013 - Cover4
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