13/06/2023
Maximize fluid life, part 1: Film temperature https://bit.ly/3KcROsw
-----------The film temperature is defined as the temperature of the heated surface inside a heater.
Figure 1: Heat transfer fluid. Source: Paratherm
For fired heaters, the film temperature is measured inside the tube at the wall. For electric immersion heaters, it is measured at the surface of the element.
Because heat flows from high temperature to lower temperature, the film temperature will always be higher than the temperature of the surrounding fluid. If the velocity of the fluid drops below the turbulent flow rate or the heat flux (measured as BTU/hr-ft2 or watts/in2) is too high, the film temperature can be 200° F to 300° F higher than the fluid flowing a short distance away.
If this film temperature exceeds the fluid’s maximum recommended film temperature, the fluid will become “overheated.” This causes thermal cracking, which produces the low boilers that cause pump cavitation, loss of heating capacity and high system pressure.
To prevent overheating, make sure to maintain the right fluid flowrate under all operating conditions. When building an immersion heating system, check with fluid suppliers about their recommended watt density. Finally, if a user has a fired heater, the burner alignment and components should be checked periodically to avoid flame impingement.
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