Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/756816
ML www.machinerylubrication.com | November - December 2016 | 47 heater. The temperature should be set to allow the oil to remain fluid but not too high so as to cause premature oxidation and hot-wall thermal degradation. Most of these heaters must be fully submerged. Otherwise, they can become damaged due to overheating. The risk of thermal/oxidative degradation from an immersion heater is influenced by four factors: the temperature of the adjacent fluid, the thermal/oxidative stability of the fluid (additive and base oil type factors), the fluid's viscosity (low oil viscosity is less risky), and the skin temperature of the heating element (heater watt density). When inspecting the heater, any evidence of sludgy, carbonaceous deposits on element surfaces indicates that the skin temperature is too high. A good rule of thumb is 15 watts per square inch (2.4 watts per square centimeter) maximum for lower viscosity and/or briskly circulating fluids. For non-circulating and/or higher viscosity fluids, a watt density maximum of 10 watts per square inch (1.6 watts per square centimeter) should be maintained. In the automotive world, many heaters are actually external to the oil sump. They work by heating the surrounding metal, with the heat then transferring to the oil to keep it fluid. These heaters also need to be thermostatically controlled but have a lower risk of causing localized thermal damage to the oil, as they utilize more of a radiant heating process. The downside to these heaters is that they usually require more energy and take longer to become effective. In my experience working on refrigeration compressors, I found it was common to use this type of heating apparatus to keep oil moving inside the compressor when it became cold. In the heating, ventilation and air condi- tioning (HVAC) industry, these are referred to as "crankcase" heaters. They have a small heating element that wraps around the outside of the compressor. The heaters can then be tied into a thermostatically controlled relay to kick on and off based on the ambient and running conditions. When Oil Heaters Make Sense While heaters make sense in areas where the equipment will be operating in extremely cold conditions, this doesn't mean that they are suitable for all applications. Don't use an oil heater if the in-service lubricant doesn't require it. If the lubricant remains fluid enough to move and adequately lubricate at all in-service temperatures, a heater will only put more stress on the lubricant and shorten its life. In addition, don't utilize heaters if the lubricant has a chronic problem with fuel dilution, as some heaters can get very hot. Fix the source of the fluid ingression and then continue using the heater. Also, exer- cise caution with heaters in applications that employ a lower refined oil or one that is at the end of its oxidative life. A heater may just be the catalyst needed to push the oil over the edge and kick-start the oxidative failure process. By selecting the right oil heater and lubri- cants, you can help ensure your equipment will work well at all expected temperatures. Just remember that heaters are not a one-size- fits-all solution to problems involving high viscosity brought on by lubricant cooling. However, if you do your homework and routinely inspect your heaters and oils, they can provide tremendous benefits to your lubrication program. About the Author Wes Cash is the director of technical ser vices for Noria Corporation. He ser ves as a senior technical consultant for Lubrication Program Development projects and as a senior instructor for Noria's Oil Analysis I and Machinery Lubrica- tion I and II training courses. Wes holds a Machine Lubrication Technician (MLT) Level II certification and a Machine Lubricant Analyst (ML A) Level III certification through the International Council for Machinery Lubrication (ICML). Contact Wes at wcash@noria.com.