Hi all,
I have a process with multiple required outcomes, with different devices controlling different process variables, but with each of them affecting the other. Just looking for a sanity check to make sure that (a) it's possible to achieve what I'm trying to achieve, and (b) that I've got it worked out correctly.
Attached is a sketch of the process (please excuse the roughness, I do have proper CAD drawings but they include a lot more ancillary equipment and are a bit convoluted to work through, so in the interests of simplicity I've just drawn in the critical equipment). We have a chamber through which we circulate air using a supply and return air fan. We can draw the supply air from outside and exhaust the return air, or we can recirculate the return air directly to the supply air, or any proportion of the two, using the motorised dampers MD5, 6 and 10. Any exhausted air is drawn through a heat exchanger to warm the supply air coming in. We monitor the fresh air flow using FT1, and the recirculated air flow using FT2. We monitor the pressure in the chamber using PT1.
Dampers MD1 and 4 are not relevant to the application (I don't think so, anyway. I've drawn them in in case someone has an idea for a way they could be used to enhance the process control). They exist because the actual application has duty/standby fans and so we need to be able to open/close as the duty changes to prevent air short circuiting via the standby fan.
All dampers are analog modulating dampers, and all fans are on VSD.
We have three objectives:
1. Maintain a set negative pressure setpoint in the chamber (e.g. -15Pa)
2. Maintain a set airflow through the chamber (e.g. 500L/s)
3. Maintain a set fresh/recirculated ratio of air (e.g. 30%/70%)
To break that down into individually controllable items, I am thinking as follows:
1. Total (actual) airflow through chamber = FT1 + FT2
2. Airflow Setpoint through FT1 = Total airflow setpoint * ratio of fresh air
3. Airflow Setpoint through FT2 = Total airflow setpoint * ratio of recirculated air
And then to control all of these items, I am thinking as follows:
1. Supply Air Fan varies speed to control total air flow (FT1 + FT2)
2. Return Air Fan varies speed to control chamber pressure
3. Supply Air Damper MD10 varies position to control the airflow through FT1
4. Return Air Dampers MD5 and MD6 vary position inversely in tandem to control airflow through FT2
I am envisioning that I will have to have a startup sequence of sorts because each of the process variables depend on the others to an extent. Here's what I'm currently thinking:
1. Open MD10 and MD6 fully, close MD5 (100% fresh air, 0% recirculation)
2. Start return air fan until chamber pressure setpoint reached
3. Start supply air fan until total airflow reached. Return air fan will ramp up to maintain negative pressure setpoint as supply air fan speed increases
4. Start modulating MD4, MD5 and MD10 to achieve required airflows through FT1 and FT2. Supply Air Fan may ramp up or down as airflow is disturbed but as long as I don't have it tuned too aggressively, it should ultimately settle out.
Anyone got any thoughts/expertise/advice to share?
I have a process with multiple required outcomes, with different devices controlling different process variables, but with each of them affecting the other. Just looking for a sanity check to make sure that (a) it's possible to achieve what I'm trying to achieve, and (b) that I've got it worked out correctly.
Attached is a sketch of the process (please excuse the roughness, I do have proper CAD drawings but they include a lot more ancillary equipment and are a bit convoluted to work through, so in the interests of simplicity I've just drawn in the critical equipment). We have a chamber through which we circulate air using a supply and return air fan. We can draw the supply air from outside and exhaust the return air, or we can recirculate the return air directly to the supply air, or any proportion of the two, using the motorised dampers MD5, 6 and 10. Any exhausted air is drawn through a heat exchanger to warm the supply air coming in. We monitor the fresh air flow using FT1, and the recirculated air flow using FT2. We monitor the pressure in the chamber using PT1.
Dampers MD1 and 4 are not relevant to the application (I don't think so, anyway. I've drawn them in in case someone has an idea for a way they could be used to enhance the process control). They exist because the actual application has duty/standby fans and so we need to be able to open/close as the duty changes to prevent air short circuiting via the standby fan.
All dampers are analog modulating dampers, and all fans are on VSD.
We have three objectives:
1. Maintain a set negative pressure setpoint in the chamber (e.g. -15Pa)
2. Maintain a set airflow through the chamber (e.g. 500L/s)
3. Maintain a set fresh/recirculated ratio of air (e.g. 30%/70%)
To break that down into individually controllable items, I am thinking as follows:
1. Total (actual) airflow through chamber = FT1 + FT2
2. Airflow Setpoint through FT1 = Total airflow setpoint * ratio of fresh air
3. Airflow Setpoint through FT2 = Total airflow setpoint * ratio of recirculated air
And then to control all of these items, I am thinking as follows:
1. Supply Air Fan varies speed to control total air flow (FT1 + FT2)
2. Return Air Fan varies speed to control chamber pressure
3. Supply Air Damper MD10 varies position to control the airflow through FT1
4. Return Air Dampers MD5 and MD6 vary position inversely in tandem to control airflow through FT2
I am envisioning that I will have to have a startup sequence of sorts because each of the process variables depend on the others to an extent. Here's what I'm currently thinking:
1. Open MD10 and MD6 fully, close MD5 (100% fresh air, 0% recirculation)
2. Start return air fan until chamber pressure setpoint reached
3. Start supply air fan until total airflow reached. Return air fan will ramp up to maintain negative pressure setpoint as supply air fan speed increases
4. Start modulating MD4, MD5 and MD10 to achieve required airflows through FT1 and FT2. Supply Air Fan may ramp up or down as airflow is disturbed but as long as I don't have it tuned too aggressively, it should ultimately settle out.
Anyone got any thoughts/expertise/advice to share?
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