Heat exchanger Bypass flow rate
Heat exchanger Bypass flow rate
(OP)
HI every buddy, I have a vertical two pass heat exchanger with below sketch. It is cooling tepid water with cooling water with below temperatures. Tube side: Cooling in 25 C. Cooling out 32 C. Shell side: Tepid in: 57C, 15 psi, 2900 gpm. Tepid out: 47 C, 5 psi, 2900 gpm. I am going to install a bypass on the Tepid side for better controlling of Tepid output temperature. (See attached sketch)
1. Do we have any standard about percentage of the fluid that can be bypassed?
2. How can I calculate the flowrate of the bypass, knowing size of the bypass, inlet and outlet pressure and the delta H. I know I need to use Bernoulli, but I could not get my answer.
Thanks all for the help
1. Do we have any standard about percentage of the fluid that can be bypassed?
2. How can I calculate the flowrate of the bypass, knowing size of the bypass, inlet and outlet pressure and the delta H. I know I need to use Bernoulli, but I could not get my answer.
Thanks all for the help
RE: Heat exchanger Bypass flow rate
What are you trying to achieve exactly?
If the inlet flow remains the same then surely your non bypass fluid will cool down more as you have greater residence time?
Wouldn't controlling the cooling flow work better?
But flowrate of the bypass is a simple pipe friction thing - however again the less flow through the HX, the lower the DP between inlet and outlet currently at 10 psi so the lower the flow through the bypass. Sod all to do with Bernoulli.
So eventually the flow will get to a steady state where the two DPs equalise.
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RE: Heat exchanger Bypass flow rate
RE: Heat exchanger Bypass flow rate
RE: Heat exchanger Bypass flow rate
So is there any control at the moment?
The bypass can only raise temperature at the outlet though this would be a slow change until a new steady state is reached.
Control of the cooling flow is the standard approach.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Heat exchanger Bypass flow rate
RE: Heat exchanger Bypass flow rate
a) Get tighter control of tepid water exit temp, especially when there are fast cooling load swings - thermal inertia related response lag of the tube bundle metal mass is avoided.
b) Keeping CW rate constant at a high velocity will help to minimise biological growth related corrosion on tubeside, which will otherwise occur with low velocity.
You may most likely need 2 control valves for this - one on the tepid water feed ( or tepid water exit) side, and the other on the tepid water bypass side in order to get the required turndown. Obviously, for 100% turndown on cooling load, the tepid water feed control valve to the HX would be fully closed, while the bypass line control valve would be handling the flow relevant at 100% turndown. We dont know the details of the all tepid water users and the various operating scenarios.
RE: Heat exchanger Bypass flow rate
The issue with the 47C water is possible legionnaires - it will need to be considered.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Heat exchanger Bypass flow rate
My experience with this type of set up is 2 valves in split range (TCV) on the mainstream and Bypass with a TIC at the discharge (mixed solution).
I imagine the cooling water set at a fixed flow rate to cope with the availability of CW.
Pierre
RE: Heat exchanger Bypass flow rate
@LI,
Agreed, legacy control loops for these were with 3 way control valves. In modern installations, we use 2 control valves in split range as @pierre says, for better reliability.