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Sizing a heat exchanger 1

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tomb7us

Chemical
Feb 21, 2008
9
I'm an up in coming engineer yet to have taken heat transfer, and im curious how you would size a heat exchanger. I am going to start working on a project in which im going to have to size an exchanger, with help of course.

Water will be entering the exchanger at approx 120*F then will need to be heated to 150*F by using hot condensate from a process else where in the plant. I dont even know where to start looking or what to consider. The flow rates into the exchanger will be 20 gpm and the water out will just be going to fill a tank.

Where do i start?
 
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You can start with any of the references below.

1. Process Heat Transfer by DQ Kern
2. Fundamentals of Heat Transfer by JP Holman
3. Heat Transfer by Incropera and Dewitt

Good luck,

 
First you must understand what you're expecting the heat exchanger to accomplish. Look at both sides (fluids) of the exchanger. For the fluid being cooled, it will lose a certain amount of heat given by m*Cp*(T2-T1) where m = mass flowrate, Cp = heat capacity, and T2 & T1 are the final and initial temperatures, respectively. [Note that this is for sensible heat changes. If there is a phase change, a different equation is used.]

The heat lost by the fluid being cooled is equal to the heat gained by the coolant or the fluid being heated. That quantity of heat is termed the exchanger's duty.

The equation governing the performance of an exchanger is:
Q = F * U * A *(DT)lm
where Q=duty
F=Factor to deal with non-ideal exchanger configuration
U=Overall heat transfer coefficient
A=Exchanger surface area
(DT)lm=Log mean temperature difference

F is obtained from nomographs published by TEMA, the Tubular Exchangers Manufacturers Association. U depends on your fluids, their properties, and the velocities on tube and shell sides. Many companies have their own tables of U values for various services. A is what the principle parameter you are designing. It is the closest thing to a single variable that could about fit the definition of "sizing" the exchanger. (DT)lm is obtained from the inlet and outlet temperatures of each side of the exchanger. The end result of a successful calculation will enable you to develop the process datasheet for the exchanger. This is then turned over to a specialist who develops the mechanical design of the exchanger.
 
If you're at school level, heed to Quarks recommendation and design it from first principles. Not pretty scenario, but probably what your teacher is looking for at this stage, specially if you haven't taken any heat transfer courses

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
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