Controls in a Clean Room System

[friartuck]

Hi Folks

I need some help on how to control the AHU in a clean room system.

I have constant volume regulators on the terminal HEPA filter boxes which are designed to hold a constant flow rate regardless of the supply system pressure. But heres the BUT...

I would like to control the AHU with a velocity probe and Inverter (VSD) to the fan motor.(to save energy)

My question is, that the controls will in effect be 'open loop.' and there will be a conflict between the inverter control and the CV boxes.

How do I set up the controls? If I set the AHU to perform a constant velocity, what stops the CV regulators from going wild and closing if the airflow from the AHU is a tad too high.

Am i better setting the inverter VSD drive to control from a set supply duct static pressure rather than a set velocity. That way, the CV's will adjust to the main duct pressure.

What I would like to do is save energy by having the fan ramp up gently as the HEPA filters get dirty.By doing a simple set supply duct pressure control, i won't achieve that.

I have considered monitoring the terminal HEPAS and setting the supply duct pressure such that it will adjust in steps according to the highest terminal filter pressure drop. i.e if I monitor 5 terminal hepas, if the fan supply pressure is set initially at say 400Pa, i can adjust its set point in step with the hepa filter incresae. So if the hepa PD goes up by 100pa, I simply increase the control point by another 100Pa to give me 500Pa at the supply fan outlet.

Any suggestions



Friar Tuck of Sherwood

 

[walkes]

Friartuck,

IF the fan is sized for the system pressure drop 200 pa plus a dirty filter pressure drop of 500 pa (700 pa total), 200 l/s @ 700pa. IF the clean filter pressure drop is 100 pa would the cv box not impose the added pressure (400 pa)on start-up when the filters are clean?(Maintaining 200 l/s @ 700pa) Assuming the pressure is within the operating range of the cv box.

 

[ChasBean1]

Friartuck, not a bad idea but it would involve a complex control scenario and would be subject to complications.

Examples:

1. What if the pressure drop across terminal HEPAs rise at different rates due to uneven particulate distribution?

2. Would there be a voting or averaging scheme that would determine fan speed? If it were a voting scheme, which terminals would have precedence? If an averaging scheme, there could be one or two filters that have higher pressure drops and have CV boxes that do not have enough flow.

3. If a room is shut down for any reason (e.g., fogging for decontamination or if a HEPA change is in progress), what does that do to the controls/fan speed?

4. There would be many controlling (analog input) devices that require calibration and that have the potential to fail.

If you think about what operating costs you are trying to save, you should certainly consider installation and maintenance issues with this proposal. I say use the KISS method here. Less controls means less failure. Establish a reasonable discharge static pressure and use some alternate means to alarm if filters are overloaded. Better yet, establish a PM protocol to change the filters before they get to that point.

Did you determine a direction yet? Good luck with this, -CB

 

[SAK9]

I think a control scheme based on HEPA filter pressure drop monitoring may not work.If you run thru the following sequence of events it will hopefully be clear.

1.Assume the system has no controls on it to respond to any changes.Also assume that the fan pressure is just sufficient to overcome clean filter pressure drop(100pa) and the duct losses(say 200 pa)

2.Clean filter allows a flow of 100l/s at a pressure drop of 100pa

3.When the filter gets dirty,the first thing to happen will be a drop in air flow (say down to 70l/s).

4.When the flow rate has gone down,you may not see any increasein DP across the filter

5.The filter supplier will be able to give the pressure drop versus flow curve.So with the reduction in flow you can confirm the new pressure drop which may be less than the initial drop!

Does that make any sense or is it just my imagination?

 

[friartuck]

Dear All

Thanks for all the great interest this posting has generated. I really have appreciated the help and advice.

I haven't particularly come up with a 'right solution' but fortunately I have in place a pretty flexible BMS system that can be adapted to incorporate some or even all of the suggestions.

As a last resort,the system may be simplified to provide a constant pressure which can be adjusted manually to take into account the gradual soiling of the filters. This appears to be the way used at the moment.

I would like to at least try and take advantage of the several remote pressure sensors and the inverter driven fans. It may or may not work.

SAK9, with regards to the PD across the filters and the reduction of flow, I propose that the initial pressure setting is to be set high.(i.e. over pressure by 150Pa or so) The CV boxes would throttle back the airflow and therefore provide the 'right amount'.

As the filters dirty, the CV box would open and let more pressure onto the filter and provide the constant airflow. The increase in pressure would be fed back into the fan speed and gradually increase in line with the system use.

It all sound gobbledegook but If you don't try, you'll never know.

The system is due for completion in a few weeks time. I'll keep you all posted.

Thanks for the interest and advice


Friar Tuck of Sherwood