In indirect heating systems, heat from combustion sources such as fuel or electric heating elements is transferred to the hot oil or thermal fluid. This heat is carried to the heat users or exchangers such as reactor jackets, calendar rolls, platens, molds and industrial fryers. The heating system provides the means to control temperatures precisely and to supply heat to the heat users from one primary heating unit or dedicated, multiple units.
All heat transfer oils are petroleum based, be they mineral or synthetic. When these oils are exposed to high temperatures, air or poorly designed circulation piping systems, the oil degrades. The byproducts of degradation are soot, carbon, coke and sludge that circulate through the overall process. These contaminants, which can be as minor as tiny floating particles or as large as thick blobs like road tar, foul the heat exchangers, heater elements or tubes, reducing overall heat transfer efficiency and leading to higher cost of operation, danger of fire and higher cost of maintenance.
One method to improve and increase heat transfer efficiency is by implementing proper filtration of the oil. The filter system can be designed to operate on a side-stream basis (so it does not interfere with normal operation) at or above system operating temperatures. Filter elements clogged with contaminants should not restrict flow as that would further degrade the oil.
Heat transfer fluids have a lower viscosity -- sometimes lower than water -- at high temperatures. Filtering oil at the elevated temperatures helps to remove the contaminants more effectively than when the oil is cooler and more viscous.
Size the filter system to adequately and continuously remove the contaminants from the overall heating loop. Other parameters to consider include:
- System age.
- Type of combustion source.
- Type of heat transfer fluid and its age.
- Nature of the application (batch or continuous).
- Temperature swings.
- Proper analysis of the heat transfer fluid.
By checking a few simple points and sizing the right filter, you can remove the contaminants from the overall heating loop, increasing heat transfer efficiency.
How to ChecklistUse this checklist to find more ways to increase heat transfer efficiency.
- Ensure that the oil velocity in the piping and through the heater is from 7 to 9 ft/sec. Size the piping for the flow rate required by the heat user.
- Implement temperature ramping to heat and cool the oil to and from setpoint operation.
- Ensure that there are minimal leak points across the heating loop to prevent oxidation.
- Implement regular heat transfer fluid analysis and demand proper reporting to understand the source of degradation within the heating system. Install a sample cooler close to the discharge side of the pump, where flow and turbulence provide the best location for drawing the sample directly into a clean container.
- Ensure proper fill and drain procedures, preferably at the lowest point in the heating loop, to evacuate entrained air, preventing cavitation in the pump and premature burn out of the heater element or impingement of the boiler tubes.
- Implement proper piping loops for adequate circulation of the heat transfer fluid around the combustion source. Any stagnation of the thermal fluid will cause it to overheat and lead to sludge from thermal cracking.
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