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Shell & Tube
( left to right ) Fixed tubesheet exchanger , U-tube exchanger and Floating-head exchanger with kettle shell
shell-and-tube side heat-transfer coefficients . With the coefficients known , the overall heat-transfer coefficient can be calculated . Knowing this value , it becomes possible to calculate the total heat transfer area required .
A = Q / [ U × LMTD ] where , A = Required heat transfer area ( m 2 ) Q = Heat duty ( W ) U = Overall heat transfer coefficient ( W / m 2 K ) LMTD = Log mean temperature difference (° C ) [ The average logarithmic temperature difference between the shell-side and the tube-side fluids over the heat exchanger length ]
Another important parameter is the pressure drop , which is calculated for the shell-side and tube-side fluids . The pressure drop is a function of the Reynolds number , the type of flow ( turbulent or laminar flow ) and the roughness value of the shell and inner tubes . The calculated required area is compared with the area provided and a check is made to see if the pressure drops are within the design limits . If the calculated required area exceeds the provided area , the geometry of the heat exchanger needs to be redesigned , possibly by increasing the tube length or the tube count . Likewise , if the calculated pressure drop exceeds the maximum defined , a new geometry must be designed to ensure a pressure drop reduction . The trial-anderror process is continued until a satisfactory design with suitable geometry is obtained .
5 ) Mechanical design calculations With the heat exchanger geometry defined , the mechanical design calculations are performed as per the specified code of construction ( such as ASME Sec
VIII Div-1 , 2 ) to determine the optimum thicknesses of each component of the exchanger for the applicable design conditions - pressure , temperature and corrosion allowance . FEA is used to qualify the design for non-standard constructions .
6 ) The manufacturing drawings With all design dimensions of the heat exchanger defined , the manufacturing drawings can be prepared . The accuracy of these drawings is vital to ensure that no design issues – such as fouling – that can delay the project are discovered during manufacturing . Manufacturing drawing generation using 3D-based CAD software identifies and corrects fouling and other issues . An equipment manufacturer must ensure the material , size and quantity of all components specified in the Material Take Offs ( issued for procurement of material ) match the manufacturing drawings . This process is vulnerable to human error , particularly when revisions happen in the design and manufacturing drawings .
Codes focus only on the safety of external pressure boundary The governing codes of construction are very particular about the equipment ’ s pressure boundary not bursting under pressure ( i . e . safety of externals ). However , these codes do not specify any rules for the construction of internals ( being non-pressure parts ) that determine the performance of the equipment . In the case of a shell-and-tube heat exchanger , its thermal and hydraulic performance is mainly dependent on the accuracy of its tube bundle . Apart from the tubes , baffles are vital in facilitating efficient heat transfer between the two fluids . The shell side flow over the tube bundle is guided by the transverse baffles . Inaccuracies in baffle
Tube outlet t 2 Shell inlet T 1
Tube inlet t 1 Shell outlet T 2
Baffles and pass-partition plate guiding the fluid flow
24 Heat Exchanger World February 2023 www . heat-exchanger-world . com