Sanity CHeck - Process Control

One example of process impact by equipment design for this application is the extent of air leakage into the chamber. If it is tightly sealed, the pressure response and instrument performance will be much different than if the chamber has openings to allow product or people to move through. With high leakage, the pressure loop may be naturally slow to respond to exhaust fan speed, and it turns out that total flow can be more aggressively tuned even though it is not highest priority.
That makes sense. The chamber will be very well sealed - there are access doors for loading product in and out, but they are all monitored, and when any door is open I'll basically put all the loops in software manual and have them hold current state until the doors are closed.

In this application you have the flexibility (or curse) of more control elements --fans and dampers -- than measurements. It is difficult to specify the optimal arrangement of the loops without knowing the extent of interactions, which can be effectively evaluated by equipment testing during start-up.

My experience base is not heavily in HVAC-type control systems, though my instinct was to start with basically the same arrangement originally proposed. It is advisable to reserve time in the schedule for testing, tuning, and possible reconfiguration of the overall strategy. And/or bring in a consultant you trust that "does these things all the time" if the additional cost is justified.
Yes, I've allowed plenty of time for tweaking and testing during the startup phase. Functionally there's very little to the system - I'll have I/O checks and communications etc sorted inside of the first day or two, and then I've got another week or thereabouts just to play with tuning and try different things. And an Ignition SCADA system with Historian sitting over the top, so I'll have plenty of data and trending to help!

I was suggesting that the same PID output value used to set the speed of the supply air fan be added to (bias) the output of the pressure controller; this sum would then set the speed of the return air fan. (Basically, as more flow is called for, both fans will increase their respective outputs by the same amount, assuming the pressure setpoint is satisfied). This would require more programming overhead.

Regardless of how you go about it, you'll have to experiment a bit to determine how best to control this process.

Good luck and have fun with it.
Gotcha, that makes sense. So rather than waiting for the increased supply air flow to cause a drop in pressure, the bias immediately increases the speed of the return air fans to hopefully prevent the pressure from dropping at all. I've never really looked much into what the bias functions in a PID are used for, I'll do some reading and see how it might be applied to my situation here.
 
...The chamber will be very well sealed - there are access doors for loading product in and out, but they are all monitored, and when any door is open I'll basically put all the loops in software manual and have them hold current state until the doors are closed.

This is some speculation...though it seems like the well-sealed chamber design will increase the interaction between individual controllers in the multi-loop design, leading to challenges in getting them all to work together. What I see as a potential concern is recirculating from the discharge of the return fan to the suction side of the supply fan. It could be tricky to get the two fans working together, especially at high recirculation levels. You might casually ask the HVAC system designers if they considered ducting the recirculation flow directly into the chamber (like a controlled leak), or maybe downstream of the supply discharge. They may have a good reason for going to the supply side suction, such as filters or other flow restrictions that would limit recirculation flow. If not, this might be a plan B if it seems impossible to reliably decouple the chamber pressure and total flow loops by just controller design or tuning.
 
That unfortunately is not an option - under certain process conditions we have to operate on full outside air with no recirculation, to avoid a build up of gas in the chamber. So we need to be able to easily switch from no recirculation, to some recirculation, to all recirculation.
 
OK, but to be clear: what I'm suggesting is to keep MD5 to regulate recirculation flow, but to (attempt to) physically decouple it from supply-side flow control by ducting directly to the chamber or the exhaust side of the supply fan. Consulting HVAC designers is/was recommended since there could be unintended consequences from this process change (e.g., the inability to maintain negative pressure with 100% recirculation). Definitely will be an interesting process control application.
 
Oh right, I'm following you now. I'll definitely keep that one up my sleeve as an option if things aren't behaving!
 
Time for an update! The system is up and running and behaving beautifully!

I ended up with three PID loops:
1: Room pressure, controlled by return air fan speed
2: Total air flow, controlled by supply air fan speed
3: Recirculated air flow, controlled by MD5
It wasn't necessary to control MD6 or MD10 at all; they are left at 100% all the time. The heat exchanger provides a non-insignificant restriction to the air flow, so as soon as I open MD5, air will bypass the heat exchanger even with MD6 fully open, because it's the path of much less resistance. So as long as my supply air fans are controlling the total air flow, and MD5 is controlling the amount of recirculation, then the amount of outside air flow must by simple math also be correct. And again, even with MD10 fully open, the supply air fans will draw air from the recirculation line if at all possible because it's easier than drawing it through the heat exchanger.

I set up the system so that each of the three loops would hold current output in manual mode until the previous loop was stable. Then I had each loop bias the one before it proportionally. So, the return air fans would start and establish room pressure. Then when the supply air fans start, a proportion of their PID's CO gets added to the bias of the return air fans PID, so that the return air fans ramp up in anticipation of the additional supply air load. Likewise, when MD5 starts to open, a proportion of its position gets added as a negative bias to the supply air fans PID, because the more you open that damper, the less hard the supply air fans have to work to achieve the same air flow.

Thanks for all the tips, between the 10-20 of us we got the job done. Good work team! Drinks are on me!* 🍻

*come visit me down under to claim
 

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