Efficient use of waste heat through optimization of structural tubes

Dr Alexandra Harless (MSc)

VT supervisor:  Prof. Eberhard Franz

Versuchsstand Rohrbündelwärmeübertrager mit Strukturrohren

This project involves the development of a method for optimizing the energy and cost efficiency of waste gas heat exchangers. The aim of the optimization is to maximize the heat transfer and minimize the manufacturing costs. These objectives contradict each other, since changes in geometry which result in an increase in heat transfer usually lead to higher production costs. Pareto optimality has proven to be an adequate assessment criterion for such multi-criteria problems. The optimization of the heat transfer leads not only to energy efficiency but also to cost and raw material efficiency. A higher heat transfer rate allows a reduction of the heat transfer surface and thus leads to material and cost savings. This can be achieved by using structural tubes. It is clear that energy-efficient heat exchangers will only be used on a larger scale in practice if the cost efficiency provides an economic incentive.

Structural tubes are tubes with a deformed surface on the inside and/or outside. These include internally finned tubes, swirl or indented tubes. The deformations cause disturbances of the flow (detachments, turbulences), which have a positive effect on the heat transfer. Depending on the combination of structural geometry, flow velocities, and material properties, different forms of flow occur in the tube. These not only influence the heat transfer, but also produce a self-cleaning effect in the structural tubes and thus reduce fouling behaviour. The fluid can spiral along the structure on the inside of the tube. This leads to a higher flow velocity. Flow separation and recirculation also occur. This repeatedly disturbs the thermal boundary layer that hinders the heat transfer. Both result in a higher heat transfer, but at the same time cause a greater pressure loss.

Certain process conditions must be observed when designing heat exchangers. These include a maximum pressure drop across the tube bundle or a specified heat flow. In addition, manufacturing specifications such as standardized tubesheet layouts or tube lengths must be observed.

These constraints have a significant influence on the design of the heat exchanger and thus on the production costs.

The production costs of a shell-and-tube heat exchanger can be divided into manufacturing and material costs. Heat exchangers with structured tubes generally result in lower material costs than comparable smooth-bore tube heat exchangers. Due to the higher resistance coefficients of structured tubes, however, the tube or bundle diameter must be increased, which leads to higher manufacturing costs. Furthermore, there are costs associated with the structuring. An optimal compromise has to be found between reducing material costs and increasing labour costs or structuring costs. Furthermore, the material costs depend on the alloy surcharge, which is determined monthly and is subject to significant fluctuations. The cost-optimal design of structured tube heat exchangers is therefore dependent not only on the process and manufacturing constraints, but also on when the tubes are procured.

An essential part of the work to be performed is the experimental determination of the thermohydraulic behaviour of structural tubes. The data required as a basis for this is determined using a test rig designed for this purpose.

Furthermore, a method for the design and calculation of waste gas heat exchangers is developed, which independently determines the most cost-effective design that meets all constraints. These findings will also be used to estimate the suitability of certain structural geometries for cost-optimal use in shell-and-tube heat exchangers on the basis of the dimensionless parameters of Nusselt number and friction coefficient. This can be used for future developments of the structural geometries.

The use of structural tubes in shell-and-tube heat exchangers can save up to 11% of the production costs compared to a cost-optimized design with smooth-bore tubes. The volume of the apparatus can be reduced by about 33%. This subsequently leads to further savings in thermal insulation. The compact design made possible by structural tubes also makes structural tube heat exchangers interesting for retrofitting and special applications where space is limited.