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Shell

& Tube

Being the only equipment that deals with two fluids with different properties ( one on the shell side and the other on the tube side ) - rubbing shoulders and yet not being in contact - shell-and-tube heat exchangers are the most difficult exchangers to operate .

By Haresh Sippy , MD & Chief Founder of Tema India Ltd .

Although the expertise lies in the hands of the licensor of the processing plant , and the process engineering , thermal and mechanical engineering on the EPC , the actual design concept involves probing in the dark and thrives on substantial safety factors and software . It is so because , in reality , there are several unaccounted variables in the manufacturing of customised equipment . More so for non-standard and proprietary products .

The design of a shell-and-tube heat exchanger 1 ) Properties of fluids To work on the thermal design of the heat exchanger , the following properties of the two fluids under heat transfer are required : density , specific heat , thermal conductivity and viscosity . The right way is to get the values of these four parameters at different temperatures in the form of a heating or cooling curve for the application . The more we understand the physical properties of the fluids in question , the more precise the design of the heat exchanger will be .

2 ) Selection of exchanger construction and allocation of fluids The very first and most important thing to determine is the allocation of the two fluids to the shell side and the tube side . Generally , the hotter , high-pressure , corrosive fluid is assigned to the tube side of the exchanger . Other factors such as the phase of fluid , allowable pressure drop , fluid velocity and heat transfer coefficient also influence this decision . Thereafter , the designer selects the type ( fixed , floating or U-tube construction ) of the heat exchanger by considering factors such as thermal expansion stresses and the need for bundle cleaning from the shell side . Later , the design temperature , pressure and maximum permissible pressure drop are established for the exchanger .

3 ) The energy balance Heat lost = Heat gained

Q _{shell side}

= Q _{tube side}

M _s

× c _s × ( T1-T2 ) = M _t

× c _t

× ( t2-t1 ) where , Q = Heat load ( W ) M = Mass flow rate ( kg / h ) c = Specific heat capacity T1 , T2 = Inlet and outlet temperatures of shell side t1 , t2 = Inlet and outlet temperatures of tube side

Once the physical properties have been correctly defined , it is time to define the flow rate , and in and out temperatures . With values of two of the above parameters known , the third parameter is computed . The designer then chooses the geometry of the heat exchanger .

Tube OD × Thk × Length × Qty × Pitch and Pattern

Once the tube layout is finalised , the outer tube layout diameter and the shell diameter are determined . Based on the tube geometry selected , the heat transfer area can be calculated .

4 ) Thermal calculation At this stage , the design engineer performs a thermal calculation . The objective is to obtain the

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