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More turbulence inside tubes

Hitran wire matrix inserts improve heat transfer
More turbulence inside tubes

Matrix elements like the Hitran thermal system have progressively been used as an effective and economical way to improve the thermal performance of tubular heat exchangers and reactors. The characteristics of the fluid dynamics in tubes equipped with Hitran wire matrix inserts are fundamentally different to those in a plain tube design. In the past, these systems have been limited to single phase applications. Significant improvements can now also be achieved in two-phase applications.

Dr. Peter Drögemüller

The conditions provided by fluid flow dynamics though plain tubes are not ideal for either heating or cooling. A thermally inefficient boundary layer, with minimal radial movement to or from the tube wall, is created due to frictional drag at the wall and viscous shear forces within the fluid. This laminar layer can significantly reduce the tube-side heat transfer coefficient and hence significantly affect the overall heat transfer.
Experiments conducted using dye stream injection at the tube wall provide a good illustration of the mechanisms involved in heat transfer improvement. Figure 1 shows that dye streams injected by the tube wall in the laminar flow regime remain fixed to the wall (A), but on meeting a wire matrix insert they are immediately mixed in with the bulk flow (B). Using PIV (Particle Image Velocimetry) measurements, the impact of Hitran on the tube-side flow can be visualized in more detail. Small aluminium tracer particles are fed into the flow stream and the movement can be recorded and visualized with a laser light sheet. In figure 1, the left side shows the recorded flow pattern at Re = 500 in a plain tube. As indicated in the colour scale, high fluid velocities are shown as red arrows, whereas blue vectors refer to low flow velocities. As anticipated in a plain tube, the maximum flow velocity is observed in the centreline of the flow. The lowest velocities are measured towards the tube wall. When measured just behind the Hitran wire matrix element, a reversed flow pattern can be observed, as seen in the right graphic of figure 1. The minimum flow velocity is measured in the centreline of the flow, which can be explained by the presence of the matrix element’s core wire. The maximum flow velocity (red arrows) is present near to the tube wall. It can also be observed that the flow direction is partly pointing towards the flow centreline. Besides considerable increased heat transfer coefficients, the following benefits can be anticipated:
  • higher wall shear forces, and
  • shorter residence time of liquid at the wall.
Both observations indicate the beneficial effect in terms of improved heat transfer and reduced fouling. As a consequence, the tube-side heat transfer coefficient is considerably higher than in a plain tube.
Improved heat transfer
Figure 2 shows the so-called heat transfer factor (j-factor) as a function of Re. For the plain tube case, the Sieder-Tate correlation in the laminar region and the Dittus-Boelter correlation for turbulent flow are shown as solid lines. The area outlined by the dotted line represents the Hitran design range. The low border of the range is characterised by inserts with low packing density, whereas the highest heat transfer improvement is achieved with high-density wire matrix inserts. The packing densities of the wire matrix insert between these two extremes can be varied continuously, allowing adaptation to the pressure drop require-ments of the exchanger.
In the Reynolds range from 100 to 2300 the improvement effect is most pronounced compared to the plain tube. Here, tube-side heat transfer can be improved up to 20 times for a given Reynolds number. At a constant Reynolds number, an improvement in heat transfer yields an increased pressure drop when fitting the tubes with Hitran turbulators. By reducing the number of passes in new exchanger designs or in revamp scenarios, it is possible to reduce the total exchanger tube-side pressure drop below the plain tube design. This is demonstrated in the example of an oil pre-heater in Figure 2. The eight-pass plain tube exchanger operates at an Re number of 1600, causing a pressure drop of about 1 bar. Under this operating condition, a heat transfer factor of 3.8 is achieved [1].
The run of the pressure loss curve for the pre-heater fitted with Hitran turbulators (blue squares) shows an equal pressure drop for a two-pass arrangement. As a consequence, the Reynolds number drops to 400 and the heat transfer factor increases 5-fold to about 20 [2]. Even the reduction to a one-pass arrangement [2*] yields a 350 % higher tube-side heat transfer coefficient than the plain tube case. In this case it can be noted that the tube-side exchanger pressure drop is only one fifth of the plain tube design pressure drop.
Application example
The real, practical application benefits of Hitran can be seen through a case study of a lube oil cooler. Lube oil is cooled from 71 to 55 °C and the maximum allowable tube-side pressure drop is 1.4 bar. Table 1 compares the plain tube design with an enhanced Hitran design. Compared to the plain tube, the size of the unit for the same heat duty is reduced considerably. As a result, only one third of the fan power calculated in the plain tube design is necessary for the Hitran enhanced design. By reducing the number of passes from 8 to 2, the tube-side pressure drop for the unit remains constant. The length of the flow path and also the fluid residence time at the wall are reduced considerably. In conjunction with a wall temperature approaching the bulk temperature, beneficial effects with respect to the tendency to foul/wax can be assumed.
Two-phase applications
This article has considered the use of Hitran turbulators in single-phase applications, but significant improvements can also be achieved in two-phase applications. If Hitran wire matrix inserts are installed in vertical condenser tubes where the condensation takes place on the tube side, a considerable increase in tube-side heat transfer can be observed. This is especially the case for multi component mixtures and in applications where inert gases are present – the heat and mass transfer is improved. The required heat exchanger area can often be reduced to less than half of the plain tube area.
In thermosiphon and forced reboilers, with the liquid entering under sub-cooled conditions, the sub-cooled region can be shortened considerably by installing Hitran wire matrix elements. As a result of this, a greater area can be utilised for evaporation.
When film boiling with poor heat transfer is present, the installation of Hitran elements breaks up the gas layer between the tube wall and the liquid, yielding an improvement in heat transfer. In applications where the vapour needs to be superheated, wire matrix elements act in the same way as described for single-phase applications.
Software tool to calculate improved heat exchangers
On request, Cal Gavin Ltd can send out free licenses for their software tool, hitran.SP. This program runs under Windows and calculates the heat transfer and pressure drop for single-phase heat exchangers equipped with Hitran wire matrix elements. A screenshot is shown in Figure 3. It is possible to import files from the HTRI and HTFS/Aspentech (TASC & ACOL) software programs that are commonly used in industrial heat exchanger design.
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