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8.2.9 Calculation and designing of the heat exchanger
8.2.9.1 Doublepipe heat exchanger
The following steps may be used to design a doublepipe heat exchanger
The calculations are based on trial and error. If the heat transfer coefficient comes out to be very small or the pressure drop comes out to be very high, this procedure to be redone for different set of diameters in the step1.
8.2.9.2 Shell and tube heat exchanger
The shell and tube heat exchanger also involves trial and error but it is not as simple as in case of double pipe heat exchanger.
The design of shell and tube heat exchanger includes,
a: heat transfer required for the given heat duty
b: tube diameter, length, and number,
c: shell diameter,
d: no of shell and tube passes,
e: tube arrangement on the tube sheet and its layout, and
f: baffle size, number and spacing of the baffles.
The calculation of LMTD can be done if the terminal temperatures are known. However, the design heat transfer coefficient (i.e., heat transfer coefficient including fouling factor) and the area are dependent on each other and thus challenges involve for the estimation. The also depends upon Reynolds number, which depends upon the liquid flow rate, sizes and the number of tubes. Therefore, is a function of diameter and the no of tubes and the parameter provides the area.
Moreover, can also be calculated is based on shell side coefficient but then it requires tube number, diameters and pitch. Thus, the above discussion shows that and A are not fully explicit and requires trial and error method of calculation.
The guideline for shellandtube calculation is shown in below,
Illustration
A heat transfer fluid is leaving a reactor at a rate of 167 kg/s at 85°C. The fluid is to be cooled to 50°C before it can be recycled to the reactor. Water is available at 30°C to cool the fluid in a 12 pass heat exchanger having heat transfer area of 15 m2. The water, which is being used to cool the fluid, must not be heated to above 38°C at the exit of the heat exchanger. The overall heat transfer coefficient of 400 Kcal/hm2°C can be used for the heat exchanger. The water flows through the shell and the oil flows through the tubes. The specific heat of the fluid may be taken as 0.454 kcal/kg°C. Find out whether the heat exchanger would be suitable for the given heat duty?
Solution:
It is given,
f : hot stream (fluid)
c : cold stream (water)
Energy balance across the heat exchanger will be,
Thus the minimum stream will be the hot stream.
Putting the values in the eq. 8.19,
The area 13.2 m^{2} found is less than the available area (15 m^{2}). Therefore, the given heat exchanger will perform the required heat duty.
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