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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.