I'm working on a heat exchanger design project and I'm stuck at one of the first steps - energy and rate balance. In the problem, we have a hydrocarbon stream getting cooled by a water stream. We are given the following values for the Hydrocarbon stream: Vdot=200GPM, T1=135 F, T2=100F, P1=321 psia, with minimum pressure drop of 285 psig, X1=0.684 C3H6, X2=0.316 C3H8. And for the water stream we are given: T1=85 F, P1= 60psig, with an allowable pressure drop of 50psi. I found the values for mCp(T2-T1) for the hydrocarbon stream, but I'm only given values for the inlet temperature and pressure of the cooling water stream. I found the specific heat of the water stream, but I'm still left without the mass flow rate and exit temperature. Can anyone provide some advice on how to obtain one of these values so I can balance both of the streams? Any help would be greatly appreciated. Thank you!
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In heat exchanger design, the outlet temperature of the water is usually one of the factors that has to be decided upon.
It has an upper bound limit of the lowest temperature of your HC stream so 100°F (as you want it to cool the HC stream), but practically you want it lower than this so there is a temperature gradient between the two fluids at all points. In this case you might want to look at a water outlet temperature of 90°F or 95°F.
Which of these is better will depend on the economics of your system as the first requires more water, but a smaller HEX (as you have a higher driving force) whereas the second needs less water but a larger HEX.
