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L.Ramamurthy
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Join Date: Jul 2009
Posts: 42

Air Balancing in Blowers in Series

11/07/2018 7:55 AM

Hi Team.

I have connected two blowers in series controlled individually by VFDs. First blowers output is connected through a long duct to the second blower. Second blower output is connected to spray drier through a hot air generator. Iam facing challenges in maintaining air flow volume at the same time maintining the air velacity after second blower. request team valuable inputs and some reading materials in this regard.

Thanks
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SolarEagle
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#1

Re: Air Balancing in Blowers in Series

11/07/2018 2:49 PM

Sounds like your hot air generator is causing some back pressure to the airflow...this might be minimized by placing the hot air generator before the second blower or ideally before both blowers...air turbulence can be a velocity killer, therefore the duct layout is very important and might require internal vanes for turbulence reduction...

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Rixter
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#2

Re: Air Balancing in Blowers in Series

11/07/2018 3:37 PM

There will be a pressure drop as air flows through the duct and other parts of the system. The greater the airflow, the greater the pressure drop.

https://apps.engineeringtoolbox.com/air-flow-head-loss-a_16.html

The fans have a response curve (fan curves), pressure difference vs airflow. The lower the airflow, the more the pressure difference. The rest of the system (ducts, etc) have a curve of pressure drop versus flow rate - system curve. The resultant flow rate will be the point where the fan pressure-flow curve crosses the load line. (Total pressure loss equals total pressure supplied by the fans.)

https://www.saylor.org/site/wp-content/uploads/2011/09/Chapter-3.5-Fans-Blowers.pdf

If you have insufficient airflow, you need to either reduce the pressure drop or increase the capacity of the fans so that the operating point is at higher airflow.

https://www.captiveaire.com/MANUALS/AIRSYSTEMDESIGN/DESIGNAIRSYSTEMS.HTM#Fan_Total_Pressure

http://www.sunon.com/uFiles/file/03_products/07-Technology/006.pdf
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L.Ramamurthy
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#3

Re: Air Balancing in Blowers in Series

11/08/2018 1:12 AM

Thanks
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jhhassociates
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#4

Re: Air Balancing in Blowers in Series

11/08/2018 6:18 AM

Use the downstream blower VFD to control the required air flow. Add a pressure sensor in line at the downstream blower intake and control the upstream blower VFD with a PID loop to maintain a constant pressure at the downstream intake. This bi-passes any changes in pressure loss in the long feed pipe owing to different flow rates. Defining the downstream intake pressure as slightly positive, say +2psi gauge, makes setting the PID controller easier as it gives you both positive and negative adjustment options. The upstream blower flow must track the downstream blower flow in order to maintain the set pressure.
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Rixter
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#5

Re: Air Balancing in Blowers in Series

11/09/2018 12:44 PM

I am facing challenges in maintaining air flow volume at the same time maintining the air velacity after second blower.

This may be obvious, but do you have any air filters that need cleaning or replacing?
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#6

Re: Air Balancing in Blowers in Series

11/09/2018 6:19 PM

".... Iam facing challenges in maintaining air flow volume at the same time maintining the air velacity after second blower. ...."

Are these separate issues? Is the cross section variable? Might there be problems with continuity of the ducting?

Unless you have leaks or a nonobvious setup, except for small changes to density volumetric flow rate will be constant down the single duct series blower set up. Air velocity average for similar cross sections at different points in the duct will be simlar at steady state except for leaks and any changes in density.

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PWSlack
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#7

Re: Air Balancing in Blowers in Series

11/12/2018 12:06 PM

Are they positive displacement blowers?

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