Example 3: Multiple Pass Exchangers (HTX2TUB)


Ideal counter current exchangers are difficult to construct and even more difficult to dismantle for cleaning. Thus most real exchangers involve flow patterns that are more complicated. Tubes in the exchangers are usually bent at one end so the fluid in them makes two (or more) "passes" before exiting. The following diagram show the flow pattern in a two tube pass exchanger:

unable to show matlab graph

In real exchangers, there are a bundle of tubes, but the flow pattern is the same. The Matlab program htx2tub simulates this model of a heat exchanger. Here is what help tells us about the program. 


>>help htx2tub

  Htx2tub - Two pass counter current heat exchanger
  function 
[Q,ToL,A]=htx2tub(Ti1L,Ti2L,To0,Fcpi,Fcpo,U,dTmin,N)
  Counter Current Heat Exchanger
  Argument List
  Ti1L   Inlet temperature of inner fluid
  Ti2L   Exit temperature of inner fluid
  To0    Inlet Temperature of outer fluid
  Fcpi   Flow*heat capacity of inner fluid
  Fcpo   Flow*heat capacity of outer fluid
  U      Overall Heat Transfer coefficient
  dTmin  Minimum approach temperature
  N      Number of points to plot (omit for no plot)
  xf     fraction of the x dimension to move the text with 
parameters
  yf     fraction of the y dimension to move the text up.
         xf and yf may be omitted and must be if N is not 
given.
  Return List
  Q      Heat Transfer Rate from outer to inner fluid
  ToL    Exit Temperature of outer fluid
  A      Required Heat Transfer Area
  Example:
  >> [Qx,TAout,Area]= htx2tub(25,50,80,5,7,.2,10,20)

Let's try giving the parameters for the exchanger used to heat the reactor feed with the reactor effluent: 

>>Fcpo=1.9249e+05;
>>Fcpi=1.8933e+05;
>>[Qx,TAout,Area]= htx2tub(409.75,894.3,929,Fcpi,Fcpo,1,5,20)
******** Temperature Crossing  *******
dT1 @ x=L    dT2 @ x=L
  -42.6546  441.8954

Qx =
   9.1740e+07

TAout =
  452.4046

Area =
     []

It is not possible to carry out the desired operation in a two tube pass exchanger. The cooling of the effluent with water, however, can be done: 

>>Fcpi=1.9249e+05;
>>Fcpo=5.6785e+06;
>>[Q,To0,A]=htx2tub(929,437.23,305.37,Fcpi,Fcpo,1,5,20)
Q =
  -9.4661e+07
To0 =
  322.0400
A =
   3.0916e+05  <-- compared to 3.0419e+05 for true 
                counter current flow.

The use of two tube passes in this case makes little difference since the temperature difference between the two fluids is so large.

unable to show matlab graph

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