Heat exchangers are often used on equipment to cool the system, but what effect does this have on the oil and on equipment life?
Many heat exchangers are installed at the factory depending on the size
or criticality of the machine. For these types of equipment, the
maintenance staff must keep the heat exchanger operating within a
certain parameter. If these parameters are exceeded, the equipment will
eventually go down, usually because of a safety feature that trips the
equipment due to a high temperature.
If you have had the opportunity to attend one of Noria’s Fundamentals
of Machinery Lubrication classes, you are aware of the importance of
keeping lubricants clean, cool and dry, and equipment balanced, aligned
and well-oiled. These are some of the proactive practices that help
ensure your equipment has the best opportunity to stay as healthy as
possible. Heat exchangers can be utilized as part of one of these
proactive practices — keeping the lubricant cool.
Heat is one of the four pro-oxidants. The others are air, water and
metal catalysts. These pro-oxidants lead to the formation of acids,
varnish, sludge and high viscosity, which normally result from
oxidation. Oxidation is the permanent degradation of a lubricant by
chemical reactions involving oxygen. As oxidation progresses, long-chain
molecules are produced, which promotes the formation of sludge, tar,
varnish and acids.
Individuals who review oil analysis reports typically look at an
increase in viscosity and acid number to determine the health of the
lubricant. While this is important, it is critical to change the
lubricant before the critical limit is exceeded. Don’t forget that the
health of your lubricant is your insurance for maintaining healthy
equipment. Whenever you neglect your lubricants, you are also neglecting
your equipment and allowing sludge, tar and varnish to become
contaminants that can damage bearings, servo valves, etc., and cause
equipment failure. As oxidation increases, so do acids, which results in
corrosion of internal components.
Heat exchangers can play an important role in reducing heat within a
system. Regardless of the type of heat exchanger used, it should be
operated within its limits to ensure that the lubricant is kept cool.
Refer to the manufacturer of the heat exchanger to make sure you are
within the appropriate limits.
You should also keep in mind that for every 10 degrees C (18 degrees F)
over the operating temperature, oil life is cut in half (Arrhenius rate
rule). This means that if you have high temperatures and a heat
exchanger that is not set up properly, it will produce more rapid
oxidation.
In the reliability world, it is understood that oil will need to be
changed more frequently in a machine that is running hot than in a
cooler machine, but you can also apply the Arrhenius rate rule to your
car. For instance, the hotter the oil in your vehicle, the more
frequently you will need to change the oil.
However, do not think that only heat leads to oxidation. You still must
deal with air, water, metal catalysts and all of the other contaminants
that can enter into the lubricant.
So look a little more carefully at the machines in your plant that are
currently running hot. They may be good candidates for a heat exchanger.
In addition, if you can cool the lubricant by 10 degrees C (18 degrees
F) of its operating temperature, you can double the oil’s life.
Remember, a cool lubricant will increase the health of the lubricant
along with the reliability of the equipment.
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