Boosting tube-side heat transfer
Tube insert technologies can improve heat exchanger performance by enhancing tube-side heat transfer coefficients in two-phase applications.
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For single-phase applications, it is generally understood that tube inserts will yield the greatest benefits in viscous or otherwise slow-moving fluids, due to the low heat transfer coefficients arising in laminar and transitional flow. In such cases, it can be possible to significantly optimise the design or performance of the heat exchanger.
The criteria which determine whether tube inserts could be effective in two-phase flow conditions are more complex than for single phase. The presence of a second phase necessarily introduces concerns about phase change equilibria, flow separation, and mass transport. The design engineer first needs to consider how these factors contribute to the overall thermal performance, to determine whether the enhancement mechanism of a particular tube insert will usefully improve or change the flow conditions.
This article will identify some general examples of two-phase applications where tube-side enhancement with Calgavin’s hiTRAN Thermal Systems can be particularly effective. We will discuss the underlying flow regimes in these cases and how they are augmented. This will allow us to define some basic criteria to help the design engineer decide if it would be worthwhile to consider using tube-side enhancement.
Various forms of tube-side heat transfer enhancement technologies have been used widely in industry to help improve the performance of tubular heat exchangers. The adoption of these technologies is driven by the need to reduce the overall cost of new equipment and to increase the efficiency of existing processes. Using tube-side enhancement can be an attractive proposition if the tube-side heat transfer is a limiting factor, and provides an additional degree of freedom in the design.
hiTRAN is one example of a tube-side enhancement technology. It is a type of removable tube insert which consists of a matrix of wire loops supported with a central core wire (see Figure 1). The wire matrix generates turbulence by interrupting the normal fluid flow, which increases the rate of convective heat transfer and virtually eliminates the transition between laminar and turbulent flow (see Figure 2). The packing density of the wire matrix can be finely adjusted to suit a prescribed pressure drop, and the geometry of the loops is varied to achieve optimum performance.
For applications with single-phase fluids, the criteria to decide the suitability of tube inserts are straightforward. Laminar and transitional flow conditions typically yield poor tube-side heat transfer coefficients, which subsequently limit the overall performance of the heat exchanger. In these conditions, tube inserts can often provide significant increases in the heat transfer coefficient. Calgavin provides a freely downloadable software tool, hiTRAN.SP, which can assist with the specification of hiTRAN elements in single-phase applications.
While multi-phase streams are preferentially allocated to the shell-side of a heat exchanger – owing to the greater volume and design adjustability – there are nevertheless many exceptions where allocation to the tube-side is necessary. For example, in air-cooled heat exchangers, operating with a high pressure fluid, when using expensive corrosion-resistant materials, or where such allocation would yield a more optimal thermal design.
In two-phase applications, there are additional factors to consider due to the added complexities of mass transport, phase separation, and vapour-liquid equilibrium. A tube insert will not only generate turbulence but will also disrupt the interface between the two phases, provide additional contact surface for the liquid phase, and alter the phase change behaviour due to the increased pressure drop. Therefore, it is important to consider which phenomenon controls the performance of a given heat exchanger, and how this may be affected by the enhancement mechanism of a tube insert.
Through studying cases where hiTRAN has been used in two-phase fluids, it has been possible to determine some criteria for cases where it is likely to provide measurable and significant improvements in performance.
Although there is no rigorous definition of a vaporiser, the term is typically applied to heat exchangers where a liquid stream is converted completely into a vapour. Some vaporisers also require sensible heating of the sub-cooled liquid, and super-heating of the vapour.
In many cases, the process fluid will arrive at sub-zero temperatures, with the heating medium significantly hotter. The local temperature differences can exceed 100°K, which often leads to the occurrence of film boiling – a condition where the liquid at the heat transfer surface rapidly vaporises and forms an insulating vapour layer. In regions of film boiling, the heat transfer coefficient is very low and can significantly limit the performance of the heat exchanger.
Additionally, at near-complete levels of vaporisation, high velocities can cause liquid droplets to become suspended within the vapour and form a mist. The liquid droplets do not easily evaporate, because the rate of heat transfer through the vapour phase and into the droplets is relatively poor. Consequently, the presence of mist flow can lead to liquid carry-over in the outlet of the heat exchanger. In applications where liquid cannot be tolerated in subsequent parts of the process, mist flow must be avoided.
Film boiling and mist flow can not only reduce the effectiveness of a vaporiser, but also create significant uncertainties in the prediction of the performance. hiTRAN offers a potential solution for an affected vaporiser. The increased convective heat transfer cools the internal surface of the tubes, and the wire matrix introduces a physical disruption within the vapour film. The combination of these effects suppresses the onset of film boiling, increasing the local heat flux. The technology can also disperse mist flow by increasing the rate of heat transfer through the vapour into the droplets and breaking them up through collisions with the wire matrix.
A tube-side ethylene vaporiser, designed as a BEU-type shell-and-tube heat exchanger (702 tubes, 4m straight length), was found to perform below the required duty when in service. Analysis of the process conditions and thermal design indicated significant areas of film boiling were present. The exchanger was retrofitted with hiTRAN, and the subsequent increase in heat transfer performance indicated the suppression of film boiling. After the retrofit, the vaporiser was able to meet its required duty (see Table 1).
Vertical thermosyphon reboilers
A thermosyphon reboiler (calandria) is a common case of a distillation column reboiler, whereby the force driving the circulation is due to the density change of the boiling fluid. In a vertical thermosyphon, the process fluid is assigned to the tube-side and partially boils along the tube length.
In lower pressure applications, a disadvantage of the vertical thermosyphon is the high static pressure head which effectively sub-cools the liquid at the inlet. Therefore, a certain portion of the tube length is needed to first heat the liquid to its boiling point. This reduces the total amount of surface area available for carrying out the intended process of boiling the liquid.
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