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(d) Louvers and Grilles <br /> In calculating free area of all louvers and grilles, <br /> consideration shall be given to the blocking effect <br /> of louvers, grilles or screens protecting openings. <br /> Screens used shall not be smaller than 1/4 inch mesh. <br /> If the free area through a design of louver or grille <br /> is known, it should be used in calculating the size <br /> opening required to provide the free area specified. <br /> If the design and free area is not known, it may be <br /> assumed that wood louvers will have twenty to twenty- <br /> five (20-25) percent free area and metal louvers and <br /> grilles will have sixty to seventy-five (60-75) percent <br /> free area. <br /> (e) Alternate Methods of Supplying Combustion Air <br /> In lieu of requirements of Table 13-1, combustion <br /> air supply may be designed in accordance with recog- <br /> nized engineering principles when first approved by <br /> the Administrative Authority. <br /> -6- <br />