Supply and Return Fan VFDs?

[crab master]

What is the best way to control this for a single AHU with both supply and return fans, each with their own VFD? Do you just have the return fan follow the supply fan with a single supply duct static pressure sensor? Or do you get an airflow measurement from both and have the return fan try to match the supply fan's flow? Also tell me the reasoning why you suggest what you do?

A little more info - 2 AHU's, both are basically the same, other than ones about twice the size of the other. Cooling only, chilled water coils with economizer. VAV's with hot water reheat coils. We will have a building static pressure sensor as well, common hallways. *Relief dampers (ducted downstream of the return fan) will modulate with the economizers or as needed due to building pressure. OA and RA dampers hardlinked to each other, relief dampers are about 40 feet away.

Thanks in advance.

[Edited by crab master on 04-20-2006 at 07:43 PM]

[makinmoney]

This is a great question for Umdnj, he can explain this stuff in his sleep.

Where r u umdnj?

[codewriter]

Still need a bit more info

Power exhaust?
or
Relief dampers that modulate with econ dampers?
or
Relief dampers that are part of the return fans housing?
or
etc...

[qtip]

Originally posted by codewriter
Still need a bit more info

Power exhaust?
or
Relief dampers that modulate with econ dampers?
or
Relief dampers that are part of the return fans housing?
or
etc...

Yea that!! Makes a big difference.

[crab master]

Relief dampers that will modulate with the economizers and with building static pressure.

[codewriter]

So the relief dampers are hard linked to the econ dampers?

[crab master]

No. The relief dampers are about 40 feet away from the OA and RA dampers.

[qtip]

Supply an Return track togeather when economizer is not avaliable. As the OA/RA damper start to modulate monitor the duct static between the RA fan and RA dampers, at some point during economization the static will build in this area. At that point I switch the RA speed to ramp to maintain a slight negitve pressure, if you don't the mixing box will become a positive which will prevent OA intake. During this part of operation the building will gain pressure and the EA damper is modulated to maintain building static. As the EA damper opens the static in between the RA fan and EA damper is decreased and the RA fan now ramps to maintain building static.

Don't know if this sound right, kind hard to explain. But with multiple PID loops and a little thought it works quite well.

Now do you want to add a CO2 purge also?

[umd]

Lets not complicate this...Supply fan VFD tracks supply duct static setpoint. Return fan VFD tracks building pressurization (set for slight positive, say .05"wc). Relief dampers are barometric and will do their own thing.

[crab master]

That would be great if relief dampers were barometric but they are not - they have actuators on them.

Yes CO2 purge will be here as well. I think I have that part of it figured out but I am open to suggestions/ideas.

[umd]

"That would be great if relief dampers were barometric but they are not - they have actuators on them."

No big deal...it is quite common to have return air, outdoor air dampers and relief dampers working in unison off the same signal or linkage.

The important thing is that return fan VFD is operating off building pressure. Supply fan will draw outdoor air as required to compensate for a shortfall in return air (which there should be if you are maintaining a positive pressure in the building).

And before you ask, yes it is possible under certain circumstances to draw outdoor air into the mixed air plenum through the relief dampers. This is pretty common.

[qtip]

Originally posted by umd
[B
And before you ask, yes it is possible under certain circumstances to draw outdoor air into the mixed air plenum through the relief dampers. This is pretty common. [/B]

In the sequence I posted earlier that problem is addressed.

[codewriter]

Pretty common?

Does the word efficiency mean anything here?

Why in the world would you modulate the RF VFD to maintain building static in a layout like what has been discribed so far.

An EXACT layout still has not been given, a final answer is simply not possible.

[qtip]

Originally posted by codewriter
Pretty common?

Does the word efficiency mean anything here?

Why in the world would you modulate the RF VFD to maintain building static in a layout like what has been discribed so far.

An EXACT layout still has not been given, a final answer is simply not possible.

Agreed

[umd]

"Pretty common?
Does the word efficiency mean anything here?
Why in the world would you modulate the RF VFD to maintain building static in a layout like what has been discribed so far."


What is the downside of drawing outdoor air into the mixing box via the relief damper? How does it impact "efficiency?"

The priority here is to maintain the building under slight positive pressure under all operating conditions. That has been the accepted standard for the last 50 years.

[codewriter]

Originally posted by umd
What is the downside of drawing outdoor air into the mixing box via the relief damper? How does it impact "efficiency?"

The priority here is to maintain the building under slight positive pressure under all operating conditions. That has been the accepted standard for the last 50 years.

He stated the relief damper is 40 feet away. Now again, since the EXACT layout has still not been stated, I can only assume that the relief damper is not part of the return duct system, but is rather a seperate device all together.

Why would you not modulate the RF based on building static? In simple words: The return fans whole purpose is to maintain the proper pressure (+or- depending on the design) at the inlet of the supply fan. This is needed in systems with an extensive return duct system, etc.

Usually you then have a few options to control building static at this point.
1. Barametric damper(s) throughout the building in designed locations
2. Powered relief damper(s) in the building in designed locations
3. A powered exhaust

theres more than this as well, but these are the basics.


Do we know how many fixed EF's there are in the building? Nope.
Do we know enough about the building to make a good solid judgement?
Nope.


If this was done like you say, and the building static was high, and the RF ramped up, what would the outcome be? I mean the outcome of the whole picture. Think about that.

As far as your method being done like this for the past 50 years, I highly disagree.

There have been units that had relief dampers as part of the RF's housing, when the building static would rise, the relief damper would modulate open, causing the pressure at the SF inlet to drop, thus causing the RF to ramp up to maintain proper inlet press at the SF, and allowing for proper exhausting of the building static. Obviously this all is in tune with one another and modulates accordanly.

[freddy-b]

Code, question for you. Have you ever seen a ahu designed with "volumetric" return control? I have heard the term by engineers, but hae know idea about the principles of operation?
thanks

[codewriter]

Originally posted by freddy-b
Code, question for you. Have you ever seen a ahu designed with "volumetric" return control? I have heard the term by engineers, but hae know idea about the principles of operation?
thanks

Volumetric control simply means that the FAN (does not matter whether its return,supply,exh, whatever) will ramp to maintain the flow (cfm) at a given point in the system. You need an air measurement device, usually a measuring station is custom made, but I have seen some field made peices that seemed to work, even though they were ugly!

Used to see this done alot back when I was involved in Lab stuff, hardly even hear about it anymore.

Is this what your asking?

[freddy-b]

ok, seen that plenty in field, just didnt know that was its name
thanks code

[umd]

Yea, you are right Codewriter, since we can't know every detail of the thread starter's building, we should never discuss possibilities. What is the point of this site? Perhaps your experience is limited to smaller size installations. Perhaps you have never been exposed to large scale building HVAC systems. Have an open mind my friend.

Fact is, fan tracking is commonly used in this industry. Controlling return fan VFD's or vortex dampers is also commonly used in this industry. Sometimes fan tracking is done by actually measuring airflow of the supply fan, subtracting the fixed building exhaust, and attempting to deliver the balance with the return fan. This method can be difficult since there are rarely good locations for reading supply and return airflow totals accurately. If you can accurately measure flows, I have found this method to work well for maintaining building presurization. Sometimes fan tracking is done by fixed percentage (using a reset schedule). This method takes alot of tuning to allow for every posible combination of economizer damper positions and can really be tough when multiple air handlers serve the building. Controlling the return fan from building pressure is a superior method and actually controls what it is intended to control: building pressurization.

Since the original threadstarter stated that the relief dampers are connected to the downstream side of the return fan and that the building has a pressure sensor, we can make some reasonable assumptions, which I have done in my responses.

It is generally accepted by both engineers and architects that buildings should generally be maintained in positive pressure relative to outdoors.

If the building is equipped with return fans and relief dampers it would reasonably follow that the designer did not plan to use exhaust fans and other possible methods.

My original questions still remains unanswered: what is the problem if outside air enters the mixed air chamber via the relief duct? What is the basis for saying this is not efficient?