Can software replace the expert engineer?
„Do what I mean“: , this demand on calculation software sounds tempting. It means minimising the default values by standardised default values; but be careful! Is this not the lack of expertise on the part of the user?
Unconditional trust in a calculation left to “others” should not lead to uncritical acceptance of the results, as this would circumvent the important “four eyes principle“. Every engineer must know that HE is the interface to the programme and must evaluate the result with his expertise, but can also influence it in any direction.
A variety of software is available for the calculation of tube bundle HEs. The hydraulic and thermal correlations are known and published e.g. in the VDI Heat Atlas. The heat transfer equations for the pipe and shell side based on measured data describe the heat transfer quite accurately. The accuracy of the hydraulic and thermal calculation of a complete pipe bundle exchanger is given as +- 15%, which also applies to the LV programme WTS. As the input values are checked by the programme, and messages indicate limiting values, the user always receives a fully functional exchanger. But is it at the optimum? However, warnings such as “window too small” should definitely be respected in the first step. Only experience allows the interpretation of hints and the optimisation of results.
There are things for which the user is responsible alone
Fouling Factors
Heat exchangers where fouling occurs are designed with a fouling factor. This factor is selected in such a way that the exchanger provides the necessary performance at the end of the minimum service life. Fouling factors have a great influence on heat transfer and can cause an increase in surface area of up to 50%. This far exceeds the accuracy of the thermal calculation. In many cases, the operator has determined these factors for his applications himself from operational data and does not leave it to the supplier.
Physical Properties
The viscosity has the greatest influence, especially when cooling higher viscous media. If a shell-side heavy fuel oil is cooled with relatively cold cooling water, an area can form in the bundle where the viscosity is so high that the medium is displaced from the bundle and flows across the gap between the bundle and the shell and no longer participates in the heat exchange. Just know that.
Bypass flows in the shell-and-tube exchanger
As calculated – so it must be built
If the calculated dimensions are not adopted in the final design, the exchanger may not perform as desired in the worst case. This applies in particular to the dimensions of the shell side, such as the design of the tubesheet and the baffles. The shell side often determines the exchanger size due to the greater resistance.
Vibration
Associated with the hydraulic design is the tendency for vibrations to occur at the bundle of the exchanger, leading to damage. Here, the engineer’s decision is doubly challenged, because the avoidance of vibrations interferes with the geometry of the shell side and thus influences the heat transfer. The LV programme WTS verifies a pending need for action by rigorously considering all bundle areas; the engineer must incorporate the required changes into the design.
LV software and an experienced engineer form a winning team
Lauterbach Verfahrenstechnik develops software in the field of process engineering and pressure vessel design. The basic idea of also offering special solutions at low cost is made possible by the concept of the ATLAS modular system created in 1978, so that, in addition to standard software, unique products such as customer programmes can also be implemented quickly, incorporating the special company knowledge.
The calculation of all exchanger types offered by LV can be designed both thermally and hydraulically and simultaneously, via an interface to the strength software mechanicallyaccording to AD / ASME / Euronorm. Thus, the uniform programme operation and the appearance in the documentation are maintained. An essential European requirement, the traceability of the results through the presentation of the equations and graphics used (traceability) is guaranteed for the customer.
Our aim is to support the user with suitable software tools. The correct handling of the freedoms offered by versatile software, which can optimally influence the result, requires experience; this experience can be gained through many projects or through practical seminars.
Every user can therefore achieve optimal calculation results.
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