Control Theory of Heat Exchanger with 3 Control Loops

[D.Stroh]

I am designing a plate and frame heat exchanger system. The process side cools food grade water using the utility side of chilled water. The three things I must control are outlet temperature of the process side, flow rate of the process side, outlet pressure of the process side(must be higher than the utility side so any leaks in the plates will not contaminate the food grade water). The process side is supplied from a food grade water supply header, which then goes through a booster pump, into the heat exchanger, and then is delivered to a storage tank.

My initial design has three control loops:
Control Scheme #1
1) Temperature Control - Process Outlet temp measured by an RTD which controls the utility chilled water flow rate
2) Flow control - Process Outlet flow measured by flow meter which controls a VFD/Centrifugal pump on Process Inlet side
3) Pressure Control - Process Outlet Pressure measured by pressure transmitter which controls a modulating Back pressure control valve

A previous design of this heat exchanger system, switched the flow and pressure control devices. I was not working on this project at the time, and the previous engineer is unavailable to explain their design rationale.
Control Scheme #2
1) Temperature Control - Process Outlet temp measured by an RTD which controls the utility chilled water flow rate
2) Flow control - Process Outlet flow measured by flow meter which controls a modulating flow control valve
3) Pressure Control - Process Outlet Pressure measured by pressure transmitter which controls VFD/Centrifugal pump on Process Inlet side

I'm trying to decide which is the better control theory for this system. Is one control scheme more stable or give better control? To me they are very similar and I'm not seeing much of a difference between the two. I've asked two co-workers and gotten two different opinions. I will need to defend my design choice of which one I pick. I've attached a flow diagram for each option to make this more clear.

A few other notes:
I am not concerned with the temperature control loop, only the flow and pressure.
A positive displacement pump is not an option.
Using a flow bypass loop or re-circulation loop with 3-way valve to control flow is not an option.


Any guidance or thoughts would be appreciated

Thanks!

 

[Compositepro]

All your options can work (or not work). I see many opportunities for control instability due to all the coupling between the control loops and time delays between measurement and controller action. My feeling is that a vfd on a pump is better used for pressure control than flow control since that is more direct and faster response.

 

[georgeverghese]

Agreed, control instability would be likely in either scheme, given that both loops are fast acting in this liquid service. I would leave the pump on fixed speed that can be manually reset from DCS via HIC.

 

[quark]

What causes the variability in clean water flowrate and pressure? If you need to clean water pressure to be more than chilled water, to avoid chilled water leaking in to clean water, then it is better to go with a DP control between the two streams. Secondly, if you don't have any usage point between the HX and tank, water flowrate remains pretty constant.

Simple way to do is to go with fixing the pump speed at required flowrate, as suggested by George, and using a mechanical BPRV to fix the pressure one time. Mechanical BPRVs are standard set up for purified water distribution systems.


If it is necessary to go with entire scheme then your logic is much better than the previous one.

 

[mk3223]

I am wondering what is the purpose of the pressure control by PCV-418 on the Cooled Food Grade Water stream as it goes to the Storage Tank.
What's happened to the pump if the Tank level is too high as the pumping flow is continuing?

 

[D.Stroh]

Thank you all for your input.

@quark the header pressure for the food grade water is connected to a lot of other users. If they are using food grade water, the supply pressure in the header may vary. Also this heat exchanger will be delivering to about 15 different tanks and have drain valves at the end of the line to flush the cooled food grade water lines. Aside from the valves switching during the intial and post delivery flushes, there shouldn't be any downstream pressure fluctuations. I do like the idea of the mechanical BPRV but the plant is not 100% sure on how they are going to use this system yet. The design intent is 100 gpm flow rate of 40°F food grade water. The food grade water and chilled water capacities are limited at the plant. And we are putting in multiple of these heat exchangers to serve different areas of the plant. So during startup or if plant conditions change they may decide to change the flow rates to 50 or 75 gpm. So I am trying to design a system that is flexible and set points can easily be changed without the need mechanically change a BPRV.

@mk3223 the PCV-418 controls the discharge pressure of food grade water from the heat exchanger. This will control the pressure of the food grade water to be higher than the chilled water to prevent contamination in the event of a plate leak. If the tank level is too high than the system would shutdown and stop delivering cooled water to the tank.