Page 68 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das
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64     Chapter 3 Double pipe heat exchanger




                    IF (abs((f o,new e f o )/f o,new ) < 0.02) and (abs((f o,new e f o )/f o,new ) < 0.02) THEN
                       GO TO Step 14
                    ELSE
                       GO TO Step 12
                    END
                ELSE
                   Calculate T w from Eq.3.8 and proceed to calculate h i from Eq. 3.6.
                   GO TO Step 14
                END
             14. Calculate U D (Eq. 3.3). Calculate LMTD using F T from Eq. 3.22 if series-parallel configuration
                 is chosen, else LMTD to be calculated directly from {T h,in ,T h,out ,T c,in ,T h,out ,}.
             15. Calculate A o (Eq. 3.1). L total ¼ A o /(pD i,o ), N HP ¼ L total /(2L std ); Round off L total to next higher
                 value of L std so that there are integral number of hairpins.
             16. Calculate f i corresponding to Re i (Eq. 3.16). Calculate D e ’(Eq. 3.13c). Calculate Re o
                 (Eq. 3.15b). Calculate f o corresponding to Re o (Eq. 3.16 def).
             17. Calculate DH f,o (Eq. 3.18a), DH f,o,bend (Eq. 3.19) and DH n (Eq. 3.20)
                Calculate DP o (Eq. 3.21a).
                Calculate DH f,i (Eq. 3.18b). Calculate DP i (Eq. 3.21b).
             18. IF DP i > DP i,max THEN
                    Switch fluids and check for pressure drop.
                       IF even after switching fluids, the pressure drop limits are exceeded THEN
                       connect annuli in parallel and tubes in series. Recalculate F T using Eq. 3.22.
                       Go to step 9.
                    END
                ELSE
                   Print Design output and fill up the rest of the form shown in Table 3.1.
                END

             3.4.2 Design example

             Problem: Design a double-pipe heat exchanger to cool 2000 kg/hr of 5% w/w caustic solution from 80 C

             to 40 using cooling water available at 33 C. The maximum return temperature for the cooling water


                                                                                       2
             stream is 45 C. The dirt factor for caustic and cooling water may be taken as 0.00035 m K/W and

                      2
             0.00018 m K/W. The maximum pressure for the cooling water and the caustic pump header are 5 and
                                                                              2
                    2
             4kg/cm (g), respectively, and the maximum allowable pressure drop is 0.7 kg/cm for both the fluids.
                          Viscosity variation of cooling water and 5% w/w caustic lye with temperature

               T( C)          30      40       50      60      70       80      90      100
               Water m w (Pa.s)  0.8  0.65     0.55    0.47    0.40     0.35    0.31    0.28
                                10  3   10  3    10  3    10  3    10  3    10  3    10  3    10  3
               5% w/w Caustic  1.03   0.83     0.69    0.58    0.50     0.43    0.38    0.33
               m c (Pa.s)       10  3    10   3    10  3    10  3    10  3    10  3    10  3    10  3
   63   64   65   66   67   68   69   70   71   72   73