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Transcript:
Greenheck. Building Value in Air.
Welcome to Greenheck’s Virtual Lab where we have a working Variable Air Volume (VAV) Exhaust System. I have three chemical fume hoods, I have a snorkel, a storage cabinet and all of this exhaust is being tied into one common manifold. Over on the right I have air intake, so there are three control loops that were monitoring the system. The first is overall pressurization. Typically what we find in our lab space is that we have negative pressure. In other words there are more red arrows leaving the space than blue arrows coming into the space.
The next control area is how much flow is actually going out of our chemical fume hoods. And notice that I can open and close these sashes. But there's a little valve above each of these chemical fume hoods that's providing a consistent flow based on the available area. So, when I open the sash that valve opens. As I close the sash that valve closes.
So that the most simplistic control is a sash sensor. When my sash is open it tells my damper to open, when that sash closes that damper closes.
Now, the final control has to do with overall pressurization. Note this little green light which is monitoring duct pressure. When my sash closes I have a pre-engineered exhaust fan trying to pull air in from that system. But as that valve closes there's less available air. So notice when I close a sash I go to red for a few seconds. That means there's too much negative pressure in the duct system. So what we introduce is a bypass damper to bring in outside air to make up for what's not coming in through our chemical fume hood sashes. As I open the sash, now I need more air from the system, this damper starts to close down. I’m going to open up this final sash, and this damper is going to close.
So all day long, as these sashes are opening and closing, that bypass damper is modulating to maintain consistent duct pressure. And the value of this system is for nights and weekends, and other times when there's low usage you reduce the amount of flow exiting the building, which helps us reduce the amount of makeup air that comes into the space. So it's a safe and very effective way to reduce the cost of operation of that system.
