ASHRAE Journal - May 2024 - 80

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Reverse Cycle
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
A couple of months ago, I mentioned that defrost methods are like baseball teams:
everyone has a favorite and won't hear a bad word said about it. In that sense, what I
am about to say makes me feel like a Mets fan in a Yankees bar.
My preferred defrost method, when it is possible,
is to swap the evaporator and condenser of the
refrigeration plant and run it as a heat pump for a
short time. There are lots of reasons why I like this-I
will try to explain some of them here-but first here is
an overview of what happens.
The plant is fitted with a four-port ball valve, located
in the discharge line between the oil separator and the
condenser with the wet return line from the evaporator
to the low side receiver passing through
it. By rotating the ball in this valve
through 90°, the discharge gas can be
directed in the reverse direction up the
wet return line from the compressor to
the evaporator.
The gas gives up its latent heat in the
evaporator, turning to liquid which
flows in reverse from the evaporator
to the condenser, passing through an
expansion device on the way and then
evaporating in the condenser at low
pressure. The evaporated gas passes back through the
ball valve and into the low side receiver, and then on to
the compressor suction.
I said that I prefer this " when it is possible. " It is
It turns out that reverse cycle is easier
than it looks.
through a small vent line, typically settling at about 5°F
(about -15°C). This might take five minutes and then the
ball valve is turned through 90°. Once the valve position
is confirmed, the compressors start in sequence and
the refrigeration capacity is gradually increased. The
discharge pressure rises quite rapidly until frost on
the coil starts melting, which slows down the rate of
pressure rise. When the last of the frost has gone, the
rate of pressure rise increases again and the defrost
is terminated when the discharge
pressure reaches a set level, usually
about 70°F (about 20°C). At this point
the compressors stop and the pressures
equalize again, then the valve gets
turned back through 90° and the system
is ready to start refrigerating again.
The whole process, including pressure
equalization and drain down, can be
completed in less than 30 minutes. The
compressors typically only run for a
fraction of this time.
best suited to a system with one evaporator and one
condenser, although it is possible to serve multiple
evaporators provided they all defrost at once. It is
best with an evaporative condenser, although we
have had very good success in recent years with aircooled
condensers. It can be done with water-cooled
condensers, but this is a much trickier proposition and I
wouldn't recommend it.
The sequence of a good reverse cycle defrost is as
follows. The compressors stop, and pressure is equalized
between the high and low pressure sides of the system
80
ASHRAE JOURNAL ashrae.org MAY 2024
It is necessary to restrict the flow of liquid from
the evaporator during defrost, but as the operating
conditions are always more or less the same, this
doesn't require any sophisticated control and is usually
done with a fixed orifice. It is very easy to set the
defrost up to perform reliably, and it eliminates the
two biggest problems with traditional hot gas defrost:
pipe fractures caused by liquid hammer and leakage
from the high to low pressure sides of the system
causing gross inefficiency.
When all's said and done, provided attention is paid
to the small details, reverse cycle defrosting is easier
than it looks.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
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ASHRAE Journal - May 2024

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