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- Rectangular Duct Sizing Error-Formulae

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2194 Views, 14 Replies

03-03-2010 08:29 AM

Using a ductulator when sizing a 1000 CFM rectangular duct with 0.07 friction , you get a 14" circular, 14x14 square, 25x8 rectangular or equaivalent and so on, with velocity 1200 FPM But when using autocad MEP built in sizing tool it gets the correct circular value as the ductulator but the rectangular values are off and the velocity is off by almost half. Does the sizing tool only works for a circular duct? Is there a solution to make it work for rectangular?

Thank you in advance.

OOPS!!! I don't know how many sizes i messed up.

Thank you in advance.

OOPS!!! I don't know how many sizes i messed up.

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03-04-2010 07:09 AM in reply to:
AddisCAD

Anyone? please

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03-04-2010 01:46 PM in reply to:
AddisCAD

Could it be that your duct roughness and air density settings are way off in your duct system definition?

In a out of the box template here's what I did:

Copied the Standard duct system, named as Standard 0.07in

On the Design Parameters tab, made sure that Friction was selected, and set to 0.07in

Roughness set to 0.0003 ft

Density set to 0.07508 lbm/ft3

Used the DuctAdd command, set the capacity to 1000 cfm, set duct to round, and clicked the calculate button... got 14.5" size (could restrict to 14" if I removed half inch sizes from catalog). 0.0742", 872.0fpm

Set shape to rectangular, set width to 25", clicked the height calculator, got 8", 0.0656", 720.0 fpm

Set the width to 14", click height calculator got 13" (perhaps 14" if I removed odd sizes from catalog), 0.681", 791.2 fpm

Set the height to 8", click width calculator, got 24" , 0.0715", 750.0fpm

The velocity of a 25x8 duct is going to be 720 fpm, not 1200

25in x 8 in = 200 sq in

1000 cfm / (200 sq in / 144 sq in per ft) = 720

The other velocities reported by AutoCAD MEP check out as well.

**Martin Schmid, P.E.**

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

In a out of the box template here's what I did:

Copied the Standard duct system, named as Standard 0.07in

On the Design Parameters tab, made sure that Friction was selected, and set to 0.07in

Roughness set to 0.0003 ft

Density set to 0.07508 lbm/ft3

Used the DuctAdd command, set the capacity to 1000 cfm, set duct to round, and clicked the calculate button... got 14.5" size (could restrict to 14" if I removed half inch sizes from catalog). 0.0742", 872.0fpm

Set shape to rectangular, set width to 25", clicked the height calculator, got 8", 0.0656", 720.0 fpm

Set the width to 14", click height calculator got 13" (perhaps 14" if I removed odd sizes from catalog), 0.681", 791.2 fpm

Set the height to 8", click width calculator, got 24" , 0.0715", 750.0fpm

The velocity of a 25x8 duct is going to be 720 fpm, not 1200

25in x 8 in = 200 sq in

1000 cfm / (200 sq in / 144 sq in per ft) = 720

The other velocities reported by AutoCAD MEP check out as well.

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

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03-15-2010 08:13 AM in reply to:
AddisCAD

Thank you Martin for your response and sorry for belated response.

The ideal cross section air travel shape is circular.Therefore the veloctity is assumed to be 935 FPM of the circular duct. Even in the rectangular duct the provile is assumed to be circular, the corner edges of the rectangular duct is neglegible.

Ashrae fundamentals chaper 35. explains about aspect ration and shows the charts to convert circular duct to rectangular.

The equation a ductultor uses is called the Huebscher formula De = (1.3*(a*b)^0.625)/((a+b)^0.25).

it will be great if we have a way to integrate this equation for my rectangular duct sizing in my autocad MEP.

Thank you,

The ideal cross section air travel shape is circular.Therefore the veloctity is assumed to be 935 FPM of the circular duct. Even in the rectangular duct the provile is assumed to be circular, the corner edges of the rectangular duct is neglegible.

Ashrae fundamentals chaper 35. explains about aspect ration and shows the charts to convert circular duct to rectangular.

The equation a ductultor uses is called the Huebscher formula De = (1.3*(a*b)^0.625)/((a+b)^0.25).

it will be great if we have a way to integrate this equation for my rectangular duct sizing in my autocad MEP.

Thank you,

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03-15-2010 08:36 AM in reply to:
AddisCAD

This does appear to be the same formula used in AutoCAD MEP... back to the same example with the 1000 cfm in a 0.07in/100ft duct resulting in a 14.5" round duct, plugging in the sizes AutoCAD MEP provides for rectangular:

a b De (calculated)

25 7.5 14,34

25 8 14.87 (AutoCAD MEP computed size)

25 8.5 15.39

14 12.5 14.45

14 13 14.75 (AutoCAD MEP computed size)

14 13.5 15.03

Hope this helps,

MS

**Martin Schmid, P.E.**

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

a b De (calculated)

25 7.5 14,34

25 8 14.87 (AutoCAD MEP computed size)

25 8.5 15.39

14 12.5 14.45

14 13 14.75 (AutoCAD MEP computed size)

14 13.5 15.03

Hope this helps,

MS

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

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03-30-2010 08:45 AM in reply to:
AddisCAD

Hay Martin, If you have a ductulator, you would understand what the problem is. MEP works fine with for a circular duct but for a rectangular it needs to do some consideration for the flatness effect.

Thank you,

Thank you,

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03-30-2010 09:44 AM in reply to:
AddisCAD

Hmmm... I do have a couple of ductulators... but addmiddetly it's been a long time since using one. With that I also get 25x8 and appx 14x13 (re attached). Can you clarify what you mean by 'flattness factor', I don't readily see a reference to that in ASHRAE 2009 chapter 21, am I looking the wrong place? If we're missing something somewhere, we'd certailny want to know where.

Thx,

MS

**Martin Schmid, P.E.**

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

Thx,

MS

Product Manager - Revit MEP

Architecture, Engineering, and Construction

Autodesk, Inc.

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03-30-2010 11:42 AM in reply to:
AddisCAD

Seweyew,

We have a Ductulator and believe we understand what is happening. With your ductulator set to .07 WG/100' Friction and 1000 CFM, the Velocity is around 760-800 depending on what ductulator brand you have. On the TABB and Trane Ductulator, at the top in the blue area, line up 1000 cfm with 0.07 friction. Then without moving the wheel, look at the orange area on the right side of the ductulator. Air Volume in CFM is to the Right, and Velocity in FPM is on the inner wheel to the left. When you find 1000 CFM on the right, it matches around 800 FPM.

If you get FPM confused with CFM, and accidentally find 1000 CFM on the left (in the FPM column), it matches 1200 on the right, but that's in CFM. The eye tricks us into thinking think that means 1200 FPM. We can see how getting FPM and CFM swapped may lead you to believe the correct FPM is 1200, when it should be around 760-800.

Hope this helps.

Joshua Benoist, PE

We have a Ductulator and believe we understand what is happening. With your ductulator set to .07 WG/100' Friction and 1000 CFM, the Velocity is around 760-800 depending on what ductulator brand you have. On the TABB and Trane Ductulator, at the top in the blue area, line up 1000 cfm with 0.07 friction. Then without moving the wheel, look at the orange area on the right side of the ductulator. Air Volume in CFM is to the Right, and Velocity in FPM is on the inner wheel to the left. When you find 1000 CFM on the right, it matches around 800 FPM.

If you get FPM confused with CFM, and accidentally find 1000 CFM on the left (in the FPM column), it matches 1200 on the right, but that's in CFM. The eye tricks us into thinking think that means 1200 FPM. We can see how getting FPM and CFM swapped may lead you to believe the correct FPM is 1200, when it should be around 760-800.

Hope this helps.

Joshua Benoist, PE

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03-31-2010 08:00 AM in reply to:
AddisCAD

Joshua and Martin,

Let me try to answer your questions with a question. I am using the same ductulator as the one Martin have. Using ductulator I wanted to size a

2000 CFM duct @ 1000 FPM

I get a .075 WG/100' Friction, with 18.6 circular, and 45x8 rectangular approximately. Correct me if I am wrong.

Now in MEP see attached, that is how I set the HVAC system definition.

I get 42 x 8, with 857.1 FPM,

Let me try to answer your questions with a question. I am using the same ductulator as the one Martin have. Using ductulator I wanted to size a

2000 CFM duct @ 1000 FPM

I get a .075 WG/100' Friction, with 18.6 circular, and 45x8 rectangular approximately. Correct me if I am wrong.

Now in MEP see attached, that is how I set the HVAC system definition.

I get 42 x 8, with 857.1 FPM,

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03-31-2010 09:11 AM in reply to:
AddisCAD

Thank you for the example. We agree with you in the newest example provided. The FPM is not exactly fixed and jumping to 42" has to do with the closest available sizes in the part catalog. See attached Revised Word Document for more info. We will log a Change Request to get this fixed.

Joshua Benoist, PE

Joshua Benoist, PE

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