Heating of ultraviolet oils

The best technology for heating ultra-hazardous fuel oils

We know that to obtain a good combustion of any type of fuel oil it is essential that the oil is nebulized, or better, atomized to the maximum possible degree. However, the fundamental parameter that determines the greater or lesser difficulty in atomizing the oil is its viscosity. This in turn is directly linked to the temperature. The higher the oil burning temperature, the lower its viscosity and therefore its atomization.

Although the heating of ultra viscous oils is problematic in resistance heaters, gas or fuel oil heaters, due to the great possibility of cracking the oil, with the introduction of the Inductive Heater the task of heating ultra viscous oil has become a simple and safe operation, even taking into account the high viscosities of these oils at higher temperatures. For example, the kinematic viscosity of a 9A oil at 275ºC, that is, the recommended temperature for nebulization and firing, has a viscosity of 20.12 centistokes, approximately 20 times greater than water at room temperature.

These high viscosities introduced in empirical correlations of the most used (for example, formulas by Hausen, Sieder-Tate and others) for the calculation of the Nusselt number and therefore the average coefficient of heat exchange by convection, lead to temperatures of tube wall of the Inductive Heaters that are excessively high, totally incompatible with the practical results obtained in several measurements made in the field. In practice, for ultra viscous oils, the temperature difference measured between the tube wall and the fluid in several Inductive Heaters in operation is between 8 and 25ºC.

Although none of the best known empirical correlations for calculating the Nusselt number proved to be appropriate to explain the low temperature differential between the wall and the ultra viscous oil flowing in the stainless steel tubes of the Inductive Heater, more careful studies Brascoelma led to conclusions that confirm the excellent results measured.

As already mentioned, due to the non-negligible viscosity presented by ultra viscous oils even at the atomization temperature, it is practically impossible to obtain a flow with a high Reynolds number. For example, in the case of 4A oil, entering this at 120 ºC in the heater, and leaving at 180 ºC, the viscosity will be 156.56 and 22.84 centistokes, respectively, with 51.54 centistokes at an average temperature of 150 ºC .

In a concrete example of a 750 KW heater project for 4A oil heating, if we adopt a maximum head loss of 2Kgf / cm², the power density of 0.9 W / cm² and the flow rate of 21 m³ / h, whatever the diameter of the tubes used in the heater, it is impossible to achieve an oil flow with a Reynolds number above 4000, minimum condition to obtain a transition flow from laminar to turbulent with ultra viscous oils. (With the data specified above, a Reynolds number equal to 835 was obtained with a given tube configuration.)

It is known that a high convection heat exchange coefficient is related to turbulent flow, with which the thickness of the boundary layer is reduced, which favors the transfer of heat from the tube wall to the fluid.

How then to explain the low difference between the wall or film temperature and the temperature of the oil flowing in the tubes, in view of the low number of Reynolds involved in the flow of ultra viscous oils?

Brascoelma, using an intuitive concept, which we can apply to long tubes, a concept that simplifies the analytical treatment of the subject, managed to justify the reduced difference of wall-fluid temperature in the heating of ultra viscous oil, emphasizing that only the Inductive Heater has the privileged conditions, inherent to its design, which allow such simplified treatment.

The Inductive Heater, due to the fact that the currents are induced in the tube walls, is an equipment that guarantees an extremely uniform heat flow throughout the internal surface of the oil flow, without any physical possibility of the existence of more localized points hot.

This, together with an adequate design of the heat exchange surface, the length and diameter of the tubes, justifies the low wall-fluid temperature difference. With a perfectly uniform heat flow, as in the case of the Inductive Heater, the wall temperature and the fluid temperature increase linearly with a slight convergence in the direction of flow, convergence that for sufficiently long tubes, brings the oil temperature closer to the outlet of the heater with the temperature of the inner wall of the tube, which explains in practice the low differences in oil - wall temperature at the outlet of the Inductive Heater.

Regarding the resistances, which also use electrical energy to heat ultra viscous oils, the Inductive Heater has fundamental advantages: energy savings (between 25 and 30%), superior heat exchange and much lower wall temperature. The greatest energy saving of the Inductive Heater in relation to the resistances, comes from the fact that these, being shielded, have the heating resistor isolated by magnesium oxide from the tube that constitutes the shield. Therefore, the heat developed in the resistance must overcome the thermal barrier made up of magnesium oxide and also the metal barrier made up of the resistance protection tube.

This means that when calculating the global heat transfer coefficient for an armored resistance immersed in oil, 3 partial heat exchange coefficients must be considered, until it reaches the fluid being heated.

In the Inductive Heater, there is only one heat transfer coefficient (by convection) that coincides with the overall heat exchange coefficient of the Inductive Heater.

In order to maintain a certain heat flow in the resistors, the temperature of the heating element must work at a much higher temperature than the external surface of the resistor, which means a greater expenditure of electrical energy.

In the Inductive Heater, the heat does not have any thermal barrier, not even the tube wall intervenes as an obstacle to the transfer of heat, since it does not propagate from the outer surface of the tube to the inner surface, but is generated inside the tube wall.

The other advantage of the Inductive Heater in relation to the resistances lies in the fact that they are submerged inside a flanged cylinder, usually of a large diameter, through which the oil flows.

Due to the always high cross-sectional area of this cylinder, the oil practically has an extremely slow flow, which means heat transfer from the resistance surfaces to the oil almost exclusively by conduction, and the heat exchange by convection is very low. This situation also forces the resistances to maintain a very high surface temperature, which means an additional consumption of electrical energy and a real danger of cracking the oil on the resistance surface.

Practice has shown that in most cases the Inductive Heater replaces resistance heaters with an installed power savings of 25 to 30%.

Although the unit cost of electricity is higher than that of fuel oil or natural gas, in many cases a final advantage was also observed in the replacement of oil or gas heating systems with Inductive Heater.

The multitubular and multilayer construction and the great constructive versatility of the Inductive Heater provide flow characteristics that favor high efficiency in heat exchange. It is not without reason that the overall performance of the Inductive Heater is 98% as already confirmed by companies abroad.

The uniform heat flow throughout the wall of the tubes guarantees heating of the oil without the possibility of hotter zones than others, thus eliminating any possibility of an oil cracking start, a condition that does not fully exist in heaters with resistances and even less on heaters that use fossil fuels.

The Inductive Heater has resistance to gas and fuel oil over the heaters, another very important advantage when it comes to heating fluids: very low thermal inertia, and therefore a very fast response time to temperature variations. , not offered by any other existing or known fluid heating system. In the heating of ultra-viscous fuel oils, this guarantees a perfectly stable nebulization temperature and viscosity, which results in a more efficient burning of the fuel oil with better yield. The other advantage of the Inductive Heater, unlike the other systems, is that it allows to raise the nebulization temperature above those recommended by the suppliers of ultra viscous oil, without danger of cracking, which represents an effective factor in increasing the burning efficiency, whereas an increase in temperature is always associated with a decrease in viscosity.

We did not mention here, for the sake of brevity, the electromagnetic design necessary for the development of the Inductive Heater, which by a happy coincidence found in the use of austenitic stainless steel tubes (non-magnetic) an extremely favorable design element, which allowed to carry out the most modern and efficient machine for converting electricity to heat.