Design
1 . The more efficient the heat exchanger , the lower the pinch and therefore the higher the thermal power transferred ;
2 . Thus , the more efficient the heat exchanger , the larger the heat transfer surface and therefore the longer the heat exchanger will have to be . Consequently , the pressure losses increase and generate a higher energy consumption ( auxiliaries power consumption );
3 . However , it is clear that the more we want to reduce the pinch , the greater the required effort on the length and therefore over the heat transfer surface .
We point out that the previous remarks 2 and 3 can be subject to some nuances : beyond the heat transfer surface growth due to the length increase , as mentioned in the previous example , there are other possibilities in order to improve the heat exchanger efficiency ( or to decrease the pinch ). Indeed , the heat exchanger pinch can be reduced by :
• Increased heat transfer surface through the use of secondary heat transfer surfaces such as fins ( and therefore without impact on the length of the heat exchanger itself );
• The heat transfer intrinsic performance growth ( quality increasing of the heat transfer ) by means of intensification devices such as , for example and among others , turbulators / inserts , specific surface structuration such as micro-grooved tubes or corrugations , etc .;
• The combined improvement of the heat transfer surface and the heat transfer quality through the implementation of secondary heat transfer surfaces with specific surface structuration , such as segmented fins , offset strip fins ( OSF ), etc .
One of the rational energy management challenges is to use , among other things , technologies that optimise these different criteria .
Appropriate sizing Beyond the heat exchanger ’ s technological development , it is also possible to bring a concrete answer in order to improve the global energy efficiency of the processes using them : this is expressed by their appropriate sizing . The heat exchanger sizing consists in the determination of the heat transfer surface necessary to obtain the desired performances ( power , outlet temperature ). At the same time , it is necessary to determine the pressure losses generated by the fluids circulation ( due to friction against the walls as a result of the material roughness and the fluid viscosity ) in order to verify the conformity with the specifications and the latter ' s objectives . Obviously , a well-adapted , correctly sized , wellmanufactured and correctly used heat exchanger allows a significant improvement of the process efficiency and therefore of the energy consumption . However , the sizing operation remains quite delicate and often not very precise because it relies for a large part on empiricism ( so complex is this science that is called thermal ).
It is thus often common to take sizing precautions and assumptions which lead designers and engineers to design heat exchangers that are often oversized and consequently not fully optimised ( oversizing margin more or less important ). However , this oversizing is wise from the designer ' s point of view . Indeed , during the real heat exchanger operation , this reduces the risk of the heat exchanger not meeting the requirements of the initial specifications : corresponding to an undersized heat exchanger case ( unreachable performances ). This case is undoubtedly much more problematic than a bad optimisation ( important oversizing ).
Ensuring design quality Beyond this energy aspect and its optimisation , it is important to keep in mind the reality on the ground : energy optimisation is often not a priority ( unlike development time and cost , manufacturing / installation / maintenance time and cost , return on investment , etc .) and satisfying a process objective is often the most important . It is therefore necessary to focus on this aspect and sizing a heat exchanger that fits it . The heat exchanger thermal-hydraulic sizing requires a set of preliminary data , which are more or less restrictive , and which will greatly determine the quality of the design . From a project phasing point of view , the following 8 main steps can be established , which are considered essential for the success of such projects : 1 . Drafting of the heat exchanger specifications : definition of the existing process input data ( temperatures , pressures , flow rates ) & the desired output data ( temperatures , pressures / pressure drop ); characterisation of the fluids used and their specificities ( saturation , critical and solidification properties , possible degradation / decomposition risks , ATEX risks , fouling / clogging type and risks , toxicity and corrosion risks ); taking into account the system integration characteristics ( dimensions , available space & associated compactness );
2 . Design point identification : definition of the most disadvantaged operating case from the heat exchanger ' s overall performance ( i . e . the one which requires the largest heat transfer surface );
3 . Technological selection : choice of the heat exchanger type that best fits all the specifications ( for example when a compromise between compactness , risk of clogging , and inspection / maintenance is required );
4 . Thermal , hydraulic and mechanical design : as mentioned above , heat exchanger sizing is a complex exercise and often subject to numerous assumptions . In addition to the previous essential steps , it requires the selection of a suitable heat exchanger modelling method according to the heat transfer type ( single-phase , evaporation , condensation ), the selection of appropriate heat transfer and pressure drop laws for the specified geometries and heat transfer type , and the calculation of the required dimensionless parameters for the correlation validity range . This thermal-hydraulic design must necessarily be combined with a consistent and realistic www . heat-exchanger-world . com Heat Exchanger World April 2023
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