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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale.
He holds a Bachelor of Business Administration and an associate degree in educational administration. He
has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status:
refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg
committee member for the past eight (chairman in 2011 and 2012). Currently he is the treasurer (Johannesburg
council) as well as president (national council).
COMPRESSOR FAILURES
AND MOISTURE IN THE SYSTEM
By Grant Laidlaw
A common problem in the industry is that technicians are not evacuating
systems and this results in the consequence of compressor failures.
Refilwe asks: Hi Grant, we have run into a problem with
compressor failures on the increase. Our suppliers are giving
the reason as incorrect installation procedures – in particular
due to no vacuum having been drawn and moisture in
the system. Why do they say this? What harm can a little
moisture really do?
Hi Refilwe. The purpose of using a vacuum pump on a
refrigeration system is to remove air (non-condensables),
and any moisture from the system. This is very important as we
shall see.
I think firstly let us consider how moisture enters the system,
and this may happen in many ways:
• Moisture can enter the system if charge lines are not purged.
• In a new installation, moisture could remain in a pressure
vessel as part of the certification tests.
• Usage of refrigerant that is contaminated with moisture.
• Moisture could enter the system prior to evacuation, possibly
when the system is open for repair.
• Usage of unsealed refrigerant tubing.
• Oils used in refrigeration systems are hydroscopic and can
easily become contaminated with moisture, therefore
charging a system with contaminated oil will add moisture to
the system.
• On indirect systems: burst tubing in shell and tube heat
exchangers.
• Finally, with air and moisture entering the refrigeration
system, chemical reactions begin to break down the
refrigerant, this can produce more water and acids; a very
undesirable situation.
As a starting point, determine how moisture entered the system?
Take the necessary remedial action.
There are two main areas of concern when moisture enters
the system:
1. It may freeze in expansion devices, causing partial blockage
and system malfunction.
2. It sets up chemical reactions which damage the system.
The first area of concern is moisture freezing in the expansion
device. This is an issue for any system but arguably has a far greater
impact for low temperature and smaller refrigeration systems.
For example: in the case of a system utilising capillary tubing as an
expansion device and running evaporator temperatures below zero
degrees Celsius, moisture moving through the system can freeze in
the capillary tube causing a blockage in the system.
Typically, these systems are controlled by a thermostat.
When the system is no longer cooling, the thermostat will sense
the increasing cabinet temperature and supply power to the
compressor. The compressor now runs against the blockage causing
high side pressures to increase.
As the pressure increases the compressor begins to draw higher
amperage to the point where the overload kicks out and cuts off the
power. The overload then gradually cools down and within a few
minutes, resets and supplies power to the compressor.
The compressor now attempts to start but the blockage still
exists. Whilst struggling to start the compressor again draws
very high amperage causing the overload to activate again. This
cycle will repeat itself, each time causing extreme strain for the
compressor motor and ultimately failure will occur. All this for
one drop of moisture.
www.hvacronline.co.za RACA Journal I September 2020 43