Heat exchanger flow rate
Heat exchanger flow rate
(OP)
Hello, could you someone explain to me the effect of increasing the process fluid flow rate, in the tube, in regards to the heat transfer to the cooling water in the shell?
What happens if I increase or decrease the flow rate in the tube by keeping the cooling water flow rate constant?
What happens if the cooling water flow rate in increased or decreased with the process fluid flow rate constant?
I was reading a heat exchanger chart which shows the outlet temperature with respect to the process fluid flow rate..it showed the inlet temp as 500F and with a flow rate of 2GPM, the fluid cooled down to 300F..and with 1GPM it went from 500F to 200F..by increasing the process fluid flow rate, is the heat transfer rate reduced?
What happens if I increase or decrease the flow rate in the tube by keeping the cooling water flow rate constant?
What happens if the cooling water flow rate in increased or decreased with the process fluid flow rate constant?
I was reading a heat exchanger chart which shows the outlet temperature with respect to the process fluid flow rate..it showed the inlet temp as 500F and with a flow rate of 2GPM, the fluid cooled down to 300F..and with 1GPM it went from 500F to 200F..by increasing the process fluid flow rate, is the heat transfer rate reduced?





RE: Heat exchanger flow rate
Then you have two choices. U is constant or U changes with the flow rate.
Sounds very textbookish to me.
Good luck,
Latexman
RE: Heat exchanger flow rate
Really? How did you come to that conclusion? Perhaps you're just being sloppy with terminology. Since you say nothing about your cold side, I'll assume that nothing changed. If nothing changed, then the transfer RATE is constant. What's changed on the hot side is that you decreased the POWER = Joule heat*flow rate. Decreased power results in a larger temperature drop
TTFN

FAQ731-376: Eng-Tips.com Forum Policies
RE: Heat exchanger flow rate
changing the process fluid flow rate will change its exit temperatures.
the overall heat transfer coefficient is dependent on both the cooling water and process water flow rates since it is a function of the convection heat transfer on the shell side and tube side. Any heat transfer textbook will show u a basic understanding on how and why it depends on these flowrates.
when you change the process fluid flow rates, the amount of heat transferred from the cooling water to the process fluid does not change assuming that your inlet temperatures stay constant. What happens is that your U, exit temperatures and hence ΔTLM will adjust to match the "same" heat transferred.
On the other hand, your overall heat transfer area remains a constant.
RE: Heat exchanger flow rate
Let's assume for a moment that the temperature rise of your cooling water is constant regardless of velocity - not true, but hold temperature for a moment.
Your Hx will have a "U" value at a given velociy with units expressed in heat units per unit of tube surface area per time unit - Btu's per hr per square foot per degF in English units. What is happening on each side of the tube will influence the "U" value. For now we are holding the process side constant. Therefore, if you increase the cooling water velocity to infinity, your fluid won't stay in the Hx long enough to absorb any heat. Obviously you wouldn't run it that fast, but that extreme is given to make a point.
On the other hand, as the velocity increases, (to a point) the turbulence increases, so you get better heat transfer at the higher reynolds numbers.
Now add the temperature back in because it is important to the conduction through the tube wall and the ovreall LMTD of the Hx. Slow the fluid down so much that it heats up to its exit temperature too soon and little to no heat transfer occurs in the last few feet/meters of the tube(s).
So it is a balancing act. Picking the velocity that will give you the best heat transfer for the surface area available.
rmw
RE: Heat exchanger flow rate
RE: Heat exchanger flow rate
Good luck,
Latexman
RE: Heat exchanger flow rate
Academic related posts are discouraged. If you post topics related to undergraduate course-work or projects, your posts will be red flagged and marked for deletion. Students generally have vast resources available to them (libraries, professors, etc), and need to learn how to use those sources instead of relying on the Eng-Tips.com fora (it is similar to having someone else do your homework for you; not a good idea.)
RE: Heat exchanger flow rate
By what magic do you get this??
RE: Heat exchanger flow rate
On the process fluid side:
Before: Q1 = w1Cp1ΔT1
After: Q2 = w2Cp2ΔT2
Now replacing the symbols with numbers one gets the following:
Q1 = (2×200) Cp1
Q2 = (1×300) Cp2
Assuming Cp1 ≈ Cp2 → Q1 = (400/300) Q2
Then, to understand what happened, see Latexman's answer, for the overall equation.
By keeping A constant and changing the others, since Q dimished, it means U×LMTD decreased.