Hi all,
I posted recently about a HVAC-style process control application I'm working on, and have a related question that probably warrants its own thread.
I have a chamber with a supply air fan and a return air fan. In the scenario I'm currently trying to control, there is no recirculation of air; we just draw air directly from outside with the supply air fan, and exhaust it directly back outside with the return air fan.
In this scenario, Priority One is to maintain a negative pressure in the chamber, say -20Pa. This is achieved by varying the speed of the return air fan.
Priority Two is to maintain a set amount of air flow through the chamber. This is achieved by varying the speed of the supply air fan.
Now, if the system has been well designed by the HVAC designers, it should be the case that no matter how fast the supply air fans are running, and how much air they're pushing into the room, the return air fans will always have enough capacity to maintain the negative pressure in the room. I'd like to think that this will be the case. But let's just entertain the ludicrous idea that maybe this isn't the case, and that if I ran the supply air fan flat out, the return air fan wouldn't be able to keep up with demand and I'd lose my negative pressure setpoint. This would be undesirable; maintaining that negative pressure is Priority One.
So, if that were the case, I would need some way for the supply air fans PID loop to look at the return air fans PID loop, note that the return air loop is at (or close to) 100% and not maintaining setpoint, and to limit its output (or back off slightly).
Is there a PID feature available for this? Or does anyone have any ideas on how to achieve it neatly? I feel like there should be a more elegant solution than just brute force or hardcoded limits.
I posted recently about a HVAC-style process control application I'm working on, and have a related question that probably warrants its own thread.
I have a chamber with a supply air fan and a return air fan. In the scenario I'm currently trying to control, there is no recirculation of air; we just draw air directly from outside with the supply air fan, and exhaust it directly back outside with the return air fan.
In this scenario, Priority One is to maintain a negative pressure in the chamber, say -20Pa. This is achieved by varying the speed of the return air fan.
Priority Two is to maintain a set amount of air flow through the chamber. This is achieved by varying the speed of the supply air fan.
Now, if the system has been well designed by the HVAC designers, it should be the case that no matter how fast the supply air fans are running, and how much air they're pushing into the room, the return air fans will always have enough capacity to maintain the negative pressure in the room. I'd like to think that this will be the case. But let's just entertain the ludicrous idea that maybe this isn't the case, and that if I ran the supply air fan flat out, the return air fan wouldn't be able to keep up with demand and I'd lose my negative pressure setpoint. This would be undesirable; maintaining that negative pressure is Priority One.
So, if that were the case, I would need some way for the supply air fans PID loop to look at the return air fans PID loop, note that the return air loop is at (or close to) 100% and not maintaining setpoint, and to limit its output (or back off slightly).
Is there a PID feature available for this? Or does anyone have any ideas on how to achieve it neatly? I feel like there should be a more elegant solution than just brute force or hardcoded limits.