Bulk Temperature And Film Temperature In Thermic Fluid Heating
Thermic fluid heaters and its working temperature is always a perplexing term and it is also mostly misunderstood expressions. Generally, all thermic fluid manufacturers recommend certain economic operating temperature or optimum fluid bulk temperature based on its useful life of the product. Hence, the manufacturer of Thermic Fluid Heaters recommends a continuous operating temperature of 300° degrees Centigrade for such a heater in their expectations or estimation of temperature profiles.
But it is important to understand various aspects of temperature when thermic fluid flows through tubes of heater. When fluid flowing in the coils or tubes of the heater, a very thin layer of the fluid all along the metal periphery vehemently adheres to the solid metal very strongly. This layer never moves and form a film or layer which is a stationary oil film. This film creates very high resistance to heat flow from outer heated metal surface to circulating thermic fluid. This resistance depends on the velocity and thickness of the film formed. Thus, the more heat is passed through this film and gain heat in the bulk oil and the resultant temperature of the film is always higher than bulk temperature of the oil and bulk temperature depends on the velocity of the thermic fluid flow in the heater coil.
Therefore, lower film thickness permits higher bulk temperature through higher thermic fluid flow velocity of fluid in tube. Difference between bulk temperature and film temperature with higher flow velocity can be estimated as low as 20°-25° degrees Centigrade. On other side, with low velocity of the fluid in tube can make film temperature difference to the tune of 70° -75° degrees Centigrade. So, film temperature is very crucial for the life of tube and thermic fluid. Therefore, thermic fluid flow velocity through helical tube coil is very critical and it is one of the determinants of film temperature. When heater outlet bulk temperature measured is 280° degrees Centigrade and its film temperature with a good design for optimum velocity can limit film temperature above 20° to 25° degrees Centigrade to bulk temperature and film temperature will be in the tune of 300° to 305° degrees Centigrade. In case of low fluid flow velocity due to inadequate design, the film temperature can go to 70° to 75° degrees Centigrade above operating bulk temperature which results in fluid acquiring film temperature of 350° to 355° degrees Centigrade and it is near to the dangerous level and boiling point of even synthetic thermic fluid and mineral oils is always disadvantageously have lower boiling point.
The film temperature goes above boiling point, vapour is formed along the metal periphery and this is much less viscous and thinner than liquid oil film and adhere far more rapidly and it has high resistance which is hundred times higher than liquid film resistance. This vapour film in the periphery of the tubes interfere and will not allow to transfer the heat to thermic fluid and tube surface get exposed to the hot combustion gases and heat at the outside of the tubes. This kind of overheating on the metal surface is also called “hot spot” heating and it will cause cracks on tubes whereby explosive low flash point hydrocarbon gases are formed. The vapour film above the desired temperature is capable enough to crack the fluid to lighter hydrocarbon gases or low boilers thereby reducing flash point of fluid drastically.
So, it is vital to understand the significance of bulk temperature and film temperature for the thermic fluid heater manufacturers and their proper and appropriate usage.
(Author is presently working as a Principal Consultant – Nirmal Encon Services, Director Technical – Heatran Engineers Pvt Ltd, and Group Executive Chairman & Managing Director – Nirmal Energy Limited & its associated companies, Ahmedabad and having more than three decades of experience in the similar fields.)