Immersion heaters are a popular application in the oil & gas, petrochemical and manufacturing industries. Their operational principle is simple and is based on the direct heating of a fluid body when the immersed heating element is operating while placed inside the fluid body.
The most common design involves a heating element directly submerged in the target medium. The elements transfer heat to a colder mass via conduction. However, depending on the presence of a flow of the fluid or even the occurrence of a flow due to temperature changes, the heat transfer can also occur via convection.
In most applications, the immersed heater is an electrically powered resistance that reaches efficiencies of 100% in the energy transformation. Since the actual heating resistance is covered, the monitoring of the energy transformation efficiency always presents a short hysteresis which is not to be confused with a loss of efficiency. The electrical energy passing through the resistance is completely transformed into thermal energy which first is used to increase the temperature of the cover and the rest of the mechanical components of the heater, before it can be used to raise the temperature of the actual fluid. Modern controllers include this time delay in the calculations for the ON/ OFF or electrical power decrease/ increase signals to the power source.
Generally, heat transfers via three mechanisms; conduction, convection and radiation.
The first is observed when two solid phases [or non-moving fluids] are in direct contact with each other. The transfer rate, in this case, is proportional to the current [dynamic, in opposition to a steady state] difference of temperatures of the two bodies, the area of contact and a conductivity coefficient for heat transfer, ‘k’. Thus, if one requires to optimize/ modify/ monitor the heat transfer there are three factors to address.
In immersion heaters, the contact area is maximized by the incorporation of high-surface heating elements such as sheets, spirals, coils and similar. In conduction, there are no obstacles when such shapes are used and the heat transfer can be increased with no energy losses, as in the case of convection. Conduction cases are easy to handle since the heated fluid around the heating element gradually allows for the heat transfer to overall content or it can be mixed in a subsequent vessel [like a CSTR] for complete temperature homogeneity. An overall energy balance is adequate for most cases if the subsequent mixing is used: energy input due to electrical power = mass x Cp x DT
Whereas m the mass of the fluid, Cp the heat capacity coefficient, DT the temperature difference produced. If we need to be more accurate for the non-post mixing case, we need to account for the flow induced due to density differences close and away from the heating element. Usually, in conduction, this is not the case.
The second mechanism, convection, occurs when at least one of the bodies involved is a moving fluid, as in the case of air moving around our houses. In convection, the heat transfer depends on the relative velocity of motion of the two materials, the available area of contact, temperature difference and k coefficients that are a function of both bodies. Sophisticated tools address the dynamic temperature distribution and heat transfer.
An example of conduction could be two metal sheets being in contact with another. The temperature profile within the sheets can be calculated in a linear approach [the temperature profiles in the dimension of interest are linear]. When a fluid flows past an immersed heater, various changes take place:
All of the above solutions aim at directing the equilibrium of Calcium Carbonate dissociation towards the ionic/ dissolved phase.
Heat transfer in convection mode for an immersion heater can be quite troublesome to calculate. Practically we follow one of the following paths:
Computational Fluid Dynamics are currently the most popular and accurate tool for the design, control and troubleshooting of heat transfer systems. These packages solve simultaneously the mass balances, energy balances and momentum balances [Navier Stokes equations] for any given system.
The third mechanism of heat transfer is radiation. Radiation is less often encountered in immersion heaters due to the temperature values of the processes. The radiation ratio is very low in simple preheating and heating systems [max 100C]. It’s low in average temperatures found in industrial environments [max 250], and quite high at higher temperatures. Due to the dependence of heat transfer on the fourth power of the temperature, this mechanism becomes dominant in very high temperatures [>700C].
Such applications are industrial combustors and burners. In such very high radiation ratios of the overall heat transfer, specific materials, fluids and infrastructure is used to both maximize adsorption of radiation energy and minimize radiative losses to the environment. The first can be achieved via the choice of highly absorbing fluid media, such as high carbon content fuel in total contrast to natural gas for example. The second is mainly achieved with reflective media around heating vessels that eliminate energy loss to the surroundings. The homogeneous transfer of energy towards each direction makes the prediction of radiation transfer easier than the convection case. Still, modern Computational Fluid Dynamics and multi physics packages [such as COMSOL for example] make the accurate design of such systems even easier.
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