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Old 09-11-2011, 02:10 AM   #1
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Just for fun.....real situation....


Verify size a standard residential grille.

A/C 2,5 tonnage
Grille 30x14
Gross Area 29x11

Please, consider that we don't have AK factor.

CFM per tonnage _____? Why?
FPM Used ___________? Why?

A) Grille is correctly configured for the AC?
B) Grille is oversized for the AC by ________(sq ft)?
C) Grille is undersized for the AC by _______ (sq ft)?


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Old 09-11-2011, 06:33 AM   #2
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Just for fun.....real situation....


Quote:
Originally Posted by JJboy View Post
Verify size a standard residential grille.

A/C 2,5 tonnage
Grille 30x14
Gross Area 29x11

Please, consider that we don't have AK factor.

CFM per tonnag 400__? Why? Nominal CFM rating of A/C
FPM Used _300_______? Why? Noise and filtration ability if its also a filter grille

A) Grille is correctly configured for the AC?
B) Grille is oversized for the AC by ________(sq ft)?
C) Grille is undersized for the AC by __1.66__ (sq ft)?

A return only grille can have a higher velocity through it and not make noise, then a return filter grille can. So which is it?

A 30X14 return grille with 1000 CFM going through it would make an air velocity of 500 FPM and a pressure drop of .029". While allowable, it can create noise.

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Old 09-11-2011, 04:45 PM   #3
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Just for fun.....real situation....


Return only grille

I used gross area is the area that is associated with the inside dimension of the frame. 29x11 = 2.22 sq ft

1000CFM/ 300FPM = 3.33 sq ft - (200FPM if filter grille)

3.33 sq ft - 2.22 sq ft = 1.11 sq ft

Grille is undersized by 1.11. sq ft

beenthere, could you explain me how to get a pressure drop of .29" and why you used 30x14 instead 29x11 ??? also I couldn't find 1.66 sq ft

I'm studying Manual T ISBN: 1-892765-06-3


Thanks again beenthere
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Old 09-11-2011, 09:00 PM   #4
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Just for fun.....real situation....


I use grille listed size, not the measurement of the where the vanes start. So I used the 29X11 as the listed grille size.
The PD is from hart&Cooleys engineering specs. The PD is .029", not .29".

You can down load all of Hart&cooly's specs/engineering data from their website. Same for most other manufactures.


Lots of reading in those ACCA manuals, huh.
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Old 09-12-2011, 01:09 PM   #5
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Just for fun.....real situation....


Is this rule of thumb authentic?

Return Grilles

Return grilles themselves are another critical factor for air flow and again many in the industry use in an incorrect "rule of thumb" for sizing the return grilles of 1 square foot per ton or 100 square inches per ton. When in fact a grille will generally flow air through a grille at about 2 cubic feet per minute per square inch.

As an example lets look at that same house with 3 tons of cooling and assume it has a 20 inch x 25 inch return grille which has a total of 500 square inches of gross area. We then multiply the 500 square inches by the standard 2 cubic feet per square inch of air flow and we get 1000 cubic feet per minute (CFM). That is 200 cubic feet per minute (CFM) short of what is actually needed for a 3 ton system which is a minimum of 1200 CFM.


As a result many systems return ducts and return grilles are undersized by as much as 30% which means that you give up 30% of your air conditioning systems capacity.

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Old 09-12-2011, 07:08 PM   #6
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Just for fun.....real situation....


That would be a bad rule of thumb.

While a 20X25 return grille will move 1200 CFM through it, its velocity would be at 500 FPM. At 960CFM velocity is 740FPM, and at 720CFM its 300FPM.

If you want to use a rule of thumb for return grill sizing for cooling. Use W x H X 1.6

20X25=500X1.6=800. So you would have a CFM of 800 at a velocity of roughly 340FPM(I extrapolated that velocity).

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