One of the starting points for heat transfer design calculations is the assumption that flow through the tube bundle will be uniform. In the vast majority of cases this is a reasonable assumption but in cases where frictional pressure drops are low the distribution on both tubeside and shellside should be assessed. Failure to properly distribute the fluid flow will always lead to reduced thermal performance, and heat exchanger modelling software is very limited in its ability to either identify the problem or predict its effects.
No one deliberately creates a design with unevenly distributed flow. However, in some cases the design constraints lead to conditions where maldistribution is a real possibility. For the tubeside, single pass designs are particularly problematic, especially where the tube length is constrained. Single pass designs may be necessary for a variety reasons, including large volume flows, close temperature approaches and vertical heat exchangers handling two-phase flows. Once the tube length constraint is reached the only way to increase surface area is to increase the bundle diameter, leading to designs with low velocities, poor heat transfer coefficients and low frictional pressure drop.
CFD studies carried out by CALGAVIN confirm that, where the bundle frictional pressure drop is low relative to momentum losses in the headers, maldistribution is likely occur. It has also been shown that low pressure drop exchangers are also more susceptible to other effects such as the influence of asymmetrical piping arrangements to/from the unit.
Where tube-side maldistribution is suspected to be a problem hiTRAN thermal systems have proved to be beneficial, both in new designs and in retrofit. hiTRAN systems are ideally suited to situations where a single pass bundle is required, offering improved heat transfer rates while using the allowable pressure drop to force proper distribution of the fluid through the bundle. In new designs this will typically lead to dramatic improvements in the efficiency of the heat exchanger.
In retrofits, the increased flow resistance of the hiTRAN Thermal System can be used to correct an existing maldistribution problem. The low pressure drop of the installed heat exchanger usually allows a hiTRAN turbulator to be installed within process constraints and without mechanical modifications. In addition to correcting the maldistribution the retrofit will also increase the heat transfer rate leading to significantly improved performance at very modest cost.