Heat exchanger control scheme

[silebi66]

Dear expert :


The waste water is discharged (50 degrees C) through the three plate heat exchanger to be cooled to the anaerobic reactor (35C). To be assuring the microbial activity, the discharge temperatures to be control around 35C. The cooling medium is cooling water is supply at 33C and return at 43 C.

The first control scheme I thought is to throttling the waste water flow inlet, but my boss told me it is quite meaningless because it will block out the waste water normal flow. Therefore, I put the control valve downstream of the cooling water return line, and keep the waste water downstream around 35C.
If the waste water outlet over 35C, the controller will open up the control valve, and vice verse.

The second control option is my boss bring up. He mentions the bypass manipulation (split range control). (Please see the attached graph)

Or we don't have to put any control cause the quantity of cooling water is fixed, and we only to make sure the waste water outlet temperature to be cooling down below 35C.
Please help me out that any control option you encounter in your project is better.

I appreciate your help and any comment if you'd like.

Thanks a lot.

 

[thorium1]

To be frank, I cant make heads or tails what you are drawing. I would suggest something more legible for a start.

 

[MortenA]

Since your cooling water AND your process side flow is fixed (not controllable) then your only chance of success is the "by pass" scheme proposed by your boss. You will most likely need an orifice on the main line in order to create a dP high enough to get a proper range for control. If your exit temperature dosnt really matter then your last suggestion: No control seems like the way forward.

 

[silebi66]

Dear expert :


Please forgive me to rewrite my question the last time I posted.
There is two control scheme to be considered for the three plate heat exchangers
properly functioning, and to maintain the wastewater downstream temperature around 37~35 degree.


1.control valve to be installed downstream of CW return line.
If the wastewater outlet temperature is above 37C(measuring by thermocouple), opening the control valve fully
make higher cooling water rate to maintain the WW outlet temperature within specified range.
(around 35C). If WW outlet temperature drop below 35C, controller can reduce the cooling water
flow by closing the control valve downstream cooling water return line.


2.set up a bypass line between WW inlet line and WW outlet line and control valve to be installed
on the bypass line.

If the WW outlet temperature is above 37C, close the control valve partially make less WW bypass
to WW downstream to maintain the WW outlet temperature within specified range.