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Cover Story Part 1: Heat Flux And Film Temperature In Fired, Thermal-Fluid Heaters

By R.G. Pelini RGP Engineering |

In a fired thermal fluid heater, a thermal fluid is heated by the metal surfaces of a fired heater or fluegas heat-recovery exchanger. Generally, the heat input rate is controlled to achieve some desired bulk-fluid temperature exiting the heater. However, in being heated to the outlet setpoint, the fluid film in contact with the heating surface is always exposed to a higher temperature than the bulk fluid. In many applications it is the fluid film temperature, not the bulk fluid temperature, that limits the duty of the heater and the usable life of the fluid. (For more on fluid life, see Part 2 of this report) Importance of film temperature Thermal fluids include glycol-water solutions, hydrocarbon oils, silicone oils, molten salts and liquid metals. Most common in the chemical process industries (CPI) are hydrocarbon oils, either petroleum-based or synthetic, which are available for use over a wide range of operating temperatures. Some are optimized for low-temperature operation and others for good thermal stability at temperatures up to 750ºF. For any fluid, there are use temperature limitations. The maximum bulk-fluid-temperature limit is the fluid manufacturer’s recommended maximum use temperature for the flowing bulk-fluid…
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