Thermal design

• flow rates of both streams.• inlet and outlet temperatures of• both streams.• operating pressure • allowable pressure drop• fouling resistance• physical properties• heat duty• type of heat exchanger.• line sizes• preferred tube size• maximum shell diameter• materials of construction• special considerations

Important parameters for thermal design

Process (thermal) design procedure

Shell and tube heat exchanger is designed by trial and error calculations.

Kern method

• Step #1 Obtain the required thermophysical properties of hot and cold fluids thermal conductivity heat capacity, thermal expansion viscosity • Step #2 Perform energy balance and find out the heat duty (Q)

• Step #3 Assume a reasonable value of overall heat transfer coefficient (Uo,assm)

• Step #4 Decide tentative number of shell and tube passes. Determine the LMTD and the correction factor FT

Continued

Determine LMTD• Assumptions1) U is constant over entire length

of path2) Cp and flow rates are also

constant.3) No phase changes in the system4) Heat losses are negligible

continued

• Determination of correction factor

• Step #5 Calculate heat transfer area (A) required

• Step #6 Calculate the number of tubes Calculate tube side fluid velocity

• If u <1 m/s, fix np so that,

Step#7 Determine the equivalent diameter of shell.

Step 8 Decide type of shell and tube exchanger (fixed tubesheet, U-tube etc.).• A triangular (or rotated triangular) pattern

will accommodate more tubes than a square (or rotated square) pattern

• For services that require mechanical cleaning on the shell side, square patterns must be used

Baffle spacing and baffle cut

• Step #9 Assign fluid to shell side or tube side

• Step #10 Determine the tube side and shell side film heat transfer coefficient and calculate the overall heat transfer coefficient

Shell side heat transfer co-efficient (ho):

Overall heat transfer coefficient U

• Step #11

go the next step # 12. Otherwise go to step #5,calculate heat transfer area (A) required using calculated U and repeat the calculations starting from step #5.

• Step #12 Calculate % overdesign

Typical value of 10% or less is acceptable.

Step 13

• Step #13 Calculate the tube-side pressure drop

continued

• Shell side pressure drop

Performance analysis

Ratio of quantity of heat removed form a fluid to the maximum which might have been removed