Constant air volume systems

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Dear All,
In many of our projects, we have Air handling units with VFD and no VAV system. When simulated with minimum flow ratio as 1 and AHU with VFD, there is no difference in energy consumption. The impact of with and without VFD is negligible. Since there will be a small amount of variation in Airflow in zones due to VFD in AHU, can we reduce the minimum flow ratio from 1 to about 0.8 or 0.7?
Regards,Rathnashree

Rathna Shree's picture
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Hi Rathnashree,
You need to know more about the air handling unit and system (controls, ductwork and zone dampers, if any) to know how to simulate the minimum flow associated with a VFD. An AHU may be designed as a constant volume unit and still use a VFD to balance the system at a fixed flowrate. You don?t automatically get energy savings by using a VFD, and you actually incur a small energy penalty associated with drive losses. If the system does not have any adjustable zone dampers that are automatically controlled, it may be designed to operate at constant volume, and you would use a minimum flow ratio of 1. If the unit only handles a small number of zones, it may be designed to run at constant volume. I suggest you ask the design engineer how the unit will be controlled. You can also look for a controls drawing or talk to a factory rep for that unit.
Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

[Pathfinder-EA-logo-2]T: (585) 698-1956 F: (585) 325-6005
bbishop at pathfinder-ea.com www.pathfinder-ea.com
134 South Fitzhugh Street
Rochester, NY 14608 [cid:image003.png at 01D690BB.2FB08530] Ask me why Carbon Fee & Dividend may be right for you.

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Thank you Mr. Bill for the clarification. This helps.
On Tuesday, 22 September, 2020, 06:07:15 pm IST, Bishop, Bill wrote:

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Hi Rathnashree,

You need to know more about the air handling unit and system (controls, ductwork and zone dampers, if any) to know how to simulate the minimum flow associated with a VFD. An AHU may be designed as a constant volume unit and still use a VFD to balance the system at a fixed flowrate. You don?t automatically get energy savings by using a VFD, and you actually incur a small energy penalty associated with drive losses. If the system does not have any adjustable zone dampers that are automatically controlled, it may be designed to operate at constant volume, and you would use a minimum flow ratio of 1. If the unit only handles a small number of zones, it may be designed to run at constant volume. I suggest you ask the design engineer how the unit will be controlled. You can also look for a controls drawing or talk to a factory rep for that unit.

Regards,

~Bill

?

William Bishop, PE, BEMP, BEAP, CEM, LEED AP

Senior Energy Engineer

?

T: (585) 698-1956????????????? ??????????F: (585) 325-6005

bbishop at pathfinder-ea.com????????www.pathfinder-ea.com

134 South Fitzhugh Street

Rochester, NY 14608?????????????????????Ask me why Carbon Fee & Dividend may be right for you.

?

Rathna Shree's picture
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Hello Rathna,

I would like to add a couple of thoughts. First, we are finding more single zone VAV units that will vary the airflow when the load is reduced, as the fan energy savings can be significant. As you probably know, for a given system, as you reduce the airflow by 25% you can reduce the power required by over 50%.

Second, the energy code is often requiring two speed control at a minimum for single zone systems, which also can greatly improve humidity control as well. With the cost of variable frequency drives so low today, some are finding a VFD more cost effective than a 2-speed motor. Another reason for a VFD on a system without VAV boxes.

As Bill noted, this application should be defined by the design engineer, etc.

Hope this is helpful.

Robert L. (Bob) Towell, P.E., LEED AP, CCP, FASHRAE
[CXE Logo with Text Small]
20A Edwardsville Professional Park
Edwardsville, Illinois 62025-3602
Phone: 618-307-5882
Cell: 314-591-6543

CxE Group is a women (and veteran) owned business.

Bob Towell at CxE Group

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Dear Mr. Bob,
That helps. We have to explore for options here.?
Thank you.
Regards,Rathnashree
On Thursday, 24 September, 2020, 01:59:12 am IST, Bob Towell wrote:

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Hello Rathna,

?

I would like to add a couple of thoughts.? First, we are finding moresingle zone VAV units that will vary the airflow when the load is reduced, as the fan energy savings can be significant.? As you probably know, for a given system, as you reduce the airflow by 25% you can reduce the power required by over 50%.

?

Second, the energy code is often requiring two speed control at a minimum for single zone systems, which also can greatly improve humidity control as well.? With the cost of variable frequency drives so low today, some are finding a VFD more cost effective than a 2-speed motor.? Another reason for a VFD on a system without VAV boxes.

?

As Bill noted, this application should be defined by the design engineer, etc.

?

Hope this is helpful.

?

Robert L. (Bob) Towell, P.E., LEED AP, CCP, FASHRAE

20A Edwardsville Professional Park

Edwardsville, Illinois 62025-3602

Phone:? 618-307-5882

Cell:????? 314-591-6543

?

CxE Group is a women (and veteran) owned business.

?

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