Design a heat exchanger manufacturer specialised in copper alembics to design a number of new condensers to double the condensate flow . The design was carried out by the said company and the condensers were manufactured and installed . Unfortunately , the first performance tests on site did not give satisfaction since the condensate flow rate obtained was 30 % lower than expected , thus not fulfilling the customer ' s objectives . An expert evaluation took place and one of the first points analysed was the design of the condensers . Technical justification notes were requested from the supplier ; however , the latter was in fact unable to provide a detailed and substantiated calculation note . In fact , after investigation , the supplier ' s idea was to consider that the condensate production capacity could be doubled by extending the heat exchange surface compared to the initial condenser ( shell-and-tube technology ) which was satisfactory . The supplier initially wanted to double the tube length , but an integration constraint did not allow this adjustment , so the heat exchanger was provided with a doubled heat transfer surface by means of the tube bundle duplication . Nevertheless , no analysis of the internal and external flows was carried out . This led to this malfunction ( 30 % lower condensate ) because the internal heat transfer phenomena , and in particular the single-phase glycol water circulation , although easy to determine , were not subjected to the appropriate calculation analyses . Indeed , as the fluid makes only one pass inside the tubes , the bundle duplication ( i . e . a multiplication by 2 of the tubes number ) has led to an increase by 2 of the available cross section for this fluid and thus divided by 2 its intra-tubular speed ; or simply illustrated as :
m � m � m ⋅ � v1 v = and v = , if S = 2 S so v = and v = .
2 2
1 2 2 1 2 2
�⋅S1
�⋅S2 �⋅ ⋅S1
This decrease in liquid velocity led to a change in the flow regime from an initially transient flow ( close to the turbulent flow ) to a laminar flow . This change , although easily identified , was not anticipated by the manufacturer and therefore resulted in a drastic decrease of the heat transfer coefficients , which is the main cause of the underperformance observed during the commissioning of the heat exchangers . In this context , intensification techniques such as inserts could have been used to compensate for this performance reduction , but the process constraints did not allow for such devices . Thus , only a complete redesign and a new manufacturing of the device could ensure that this problem was overcome . In parallel to this analysis , the complete study carried out by GRETh has allowed us to identify operating margins on the specifications and to optimise the design of the heat exchanger . It should be noted that the critical point was not at all related to the complex thermal phenomenon to be evaluated , which is the extra-tubular multicomponent mixtures condensation of alcoholic vapors with non-condensable gases , but only on the singlephase part which was the condenser thermal brake ( higher thermal resistance ).
Conclusion This short case study shows the essential value of the expert ' s knowledge , but it also reminds us of the imperative need to master , through training and feedback from associates , customers and previous generations , the knowledge of sizing and best practices necessary to achieve an optimised design that correctly fulfills the needs . It can be said that there is no engineering field which is not faced with thermal issues , which makes the thermal knowledge necessary and if not essential , then strategic . Thus , it is advisable today , in our opinion , to also maximise efforts towards learning and raising awareness of current and future actors to new technologies and good design practices that will lead to appropriate technological developments and choices , and consequently to well sized heat exchangers : in a word , efficient heat exchangers for an efficient industry .
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