Learn To Calculate The Film Temperature Of The Thermic Fluid

When heat is absorbed by thermic fluid in the fired heaters, it is essential to limit the highest temperature attained by the thermic fluid film adjacent to the inside metal wall of the tube through which it flows to prevent undue deterioration of the fluid and the consequent dangers. It is also imperative that there is no localized boiling within the film in order to control fatigue of the tube walls. These requirements in turn make it mandatory that thermic fluid heaters are made such that there is reasonable guarantee that the film temperature al- lowed by the manufacturer of the thermic fluid is not exceeded. The film temperature of the thermic fluid can be calculated if the heat flux through the film is known. The difference between film temperature and the bulk temperature of the fluid would be the product of film resistance and the heat flux. The method of calculation of the film temperature thus boils down to calculating the inside heat transfer coefficient of the thermic fluid and the corresponding heat fluxes at point where high heat fluxes are expected. The maximum heat flux may occur at the point where the visible flame is the closet to the tubes. At this point one can expect the highest radiation from the flame with some additional convection. It is also possible that the maximum heat flux occurs where the flue gases exit from the combustion chamber and enters the convection bank where there may be quite high radiation coupled with high convection. The film temperatures need to be calculated at these points and the highertemperatures need tobe limited to the acceptable value. While calculating the heat flow, the gas side heat transfer coefficient controls the flux since the metal conductivity of the tubes as well as the inside heat transfer coefficient is very large when compared to the gas side coefficient.

In a coil type heater, the radiation heat flux at the point where the flame is closest to the coil is a function of the ratio of flame diameter to the combustion chamber diameter. The equation for radiation can be modified to include effective flame emissivity and the diameter ratio to give the radiant heat flux at this point. A fixed addition of 15 percent to account for additional convection is an accepted practice in industry for normal flames for the calculation of film temperature. For reversing flames the convection coefficient is very much higher and at present need to be back calculated from measured values of flue gas temperature at the exit fromthe combustionchambersince no method of prediction is yet available in published literature.

Provide urjexboilers best Boilers in India,Hot Water Boiler,Thermic Fluid Heater,Boiler Manufacturer and Steam Boiler