Heat exchanger Bypass flow rate

[Nasser D]

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

 

[LittleInch]

Your sketch has an error as it has 47C in as well as out.

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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Also: If you get a response it's polite to respond to it.

 

[Nasser D]

Thanks LittleInch, sorry for the mistake in the sketch. Inlet temperature is 57C.

 

[Snickster]

I have never seen a bypass around a shell and tube heat exchanger. For one thing the heat transfer efficiency is reduced when the flow velocity over the outside of the tubes on the shell side is reduced if you reduce the flow of the tepid fluid, in addition the heat flow is reduced by the bypass itself so with double the turndown for a single action it will be hard to control. It's been a while since I looked at this but I think normally you would have a conrol valve on the cold water inlet controlled by temperature sensed on the outlet of the tepid liquid which will control the flow of the cold water inlet flow to control the tepid water outlet temperature.

 

[LittleInch]

You say "better control".

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.

 

[Nasser D]

We do have control on the temperature of the cooling tower, but when we are in lower capacity of the plant, we have to also reduce Tepid flow rate to maintain tepid outlet temperature at 47C.

 

[georgeverghese]

Control scheme such as this is done for one or both of the following reasons:
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.

 

[LittleInch]

Or a simple modulating three way valve? This sort of mixing is very common in hot water systems. not sure if people make one big enough, but you could look.

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.

 

[pierreick]

Hi,
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

 

[georgeverghese]

By the way, it should be called tempered water, not tepid water ?

@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.