Most manufacturers have a selection software program that can help solve this problem.
The Titus selection software program TEAMS is available as a free download from the Titus website. TEAMS contains many grille sizes not published in the catalog, and allows the user to input air volume (CFM) in increments other than those published.
If software like TEAMS is not available, the following method should be used.
1. Determine the nominal duct area in square inches by multiplying the nominal length and width of the desired grille size in inches.
E.g. 64 x 10 = 640 square inches
2. Convert the duct area from square inches to square feet by dividing the total square inches by 144 and rounding to a 2-place decimal.
E.g. 640 square inches / 144 = 4.44 square feet
3. Locate a published grille size in the catalog for which the nominal duct size is approximately the same. The nominal duct area in square feet is the column directly to the right of the listed nominal duct sizes.
E.g. 4.44 square feet approximates to the 4.50 square feet shown for a 36 x 18 supply grille, and is less than is less than a difference of 2%. Duct leakage often accounts for more of a discrepancy than 2% in terms of performance.
4. Locate the nearest approximate desired volume listed for the approximated grille size.
E.g. A desired volume of 2300 CFM falls approximately midway between the listed volumes of 2110 and 2532, therefore the median throw and NC values between the values for 2110 and 2532 should be used.
The median value of NC 23 and 28 is 25 after rounding to an integer value.
The median values of the 0 degree deflection values 39-59-84 and 47-65-93, are 43-62-88.
Mark Costello - GRD Product Manager