In my exam, I was given 2 options to design a process. Assuming a saturated liquid coming out from the separator, A) should the stream pass through a valve to reduce pressure, then to heat exchanger to heat it up, or B) should the stream be heat up first, then passes the valve to reduce pressure? Assuming the final temp and pressure in both cases are the same.

What's the justification to that selected option?

  • $\begingroup$ What have you been able to figure out yourself so far? $\endgroup$ Oct 18 '17 at 3:54
  • $\begingroup$ I was thinking option A would be the best choice because the valve placed after separator can be used as a level control valve. And for option B, placing valve after HEx might causes instability in the temp parameter. However, I'm just not convinced with my statement and would like to hear your thoughts $\endgroup$
    – Semme
    Oct 18 '17 at 16:33
  • $\begingroup$ Reduce the pressure first : should mean the heat exchanger is cheaper as lower design pressure, but that may be relative... $\endgroup$
    – Solar Mike
    Oct 21 '17 at 15:38

I think the key information is that the liquid is saturated. This means any reduction in pressure will lead to partial vaporization and two phase flow.

For heat tranfer best overall heat transfer coefficient is always direct to liquid rather than vapor.

You still will have vaporization once the saturated liquid once it enters the heater, but the effect will be less.

For same heat duty the required size in heat transfer area of a vapor heater will be much larger than a liquid heater.

I would put the pressure reducing valve after the heater.


Lower pressure heat exchanger is going to be easier to build, but will also need a larger surface area because the heat transfer coefficient is higher for compressed fluids. I suggest trying to ballpark your heat transfer coefficient for both pressures (and velocities) and seeing how that affects your other parameters.



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