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402 CHAPTER 16 Rating Equations for Gravity Warm Air Furnaces9 1954 Guideag w ..Sr? . Until a method of testing and rating gravity warm air furnaces has been'J^ developed, the following empirical rating equations are recommended bjr-8 the National Warm Air Heating and Air Conditioning Association. ft Gravity warm-air furnaces of conventional. design, having ratios (off' heating surface to grate area) of 15 to l or greater, and having a ratio ofj! casing area to face area not less than ,0.4, are rated by the following equate tions: cl 1. ffand-fired furnaces converted to Stoker, Gas, or Oil Firing. >.$ Bonnet Capacity in Btu per hour = 1785 X S X 1-333 (2}J 2. Hand-fired furnaces, with ratios of heating surface to grate area greater than 15,-io^ 1 and less than 25 to 1. * Bonnet Capacity in Btu per hour = 1785 X S X 1.333 (3)6. 3. Hand-fired furnaces with ratios of heating surface to grate area in excess of 25 toh. * Ji Bonnet Capacity in Btu per hour = 1785 X 25 X G X 1.333 where S = heating surface, in square feet. G = actual grate area, in square feet. The Register Delivery Rating is equal to 0.75 x (Bonnet Capacity). THef: Leader Pipe Rating in square inches, formerly used as a rating unit, may.l&ij found by dividing the Register Delivery Rating by136. ;||| . ctjlt Heating. Surface of Furnace -'SM Prime healing surface is defined9 as surface above the top of the grate|j having lie! gases or live fuel on one side and circulating air over the other, afiw: in all cases is measured on the exterior or air side. The. areas of the out??! casing, the inner liner, and any radiation shields shall not be considered^. heating surface. ifi In determining the amount ofheatingsurface,extended surfaces,iare;, considered to be prime heating surface subject to the following limitations)! 1. Extended heating surface may consist of fins, ribs, webs, lugs, or other^pro?-' jections from the prime heating surface. Projections less than } in. thick base, and extending more than 1 in. from the prime surface are classified as 2. Integral fins are continuously welded to, or cast as a part of, the prime he&hfi'i surface. Both sides are included as heating surface, subject to the following'allj^l; Distance from Prime Surface Ratio of Effective Area to 1st inch 0.40 2nd inch 3rd inch 0.30 0.20 Over 3 3. Non-integral fins are spot welded to, or otherwise held in line contact f prime heating surface". Both sides are included as heating surface, subject; tg^g" following allowances: d'i'i Distance from Prime Surface . Ratio of Effective Area to Total Area....................... 1st inch 0.30 2nd inch 0.20 3rd inch 0.15 ffes- Nonfg||f Heating Boilers, Furnaces, Space Heaters 403 4. In the case of ribs, webs, or lugs more than J in. thick at the base and extending less than 1 in. from the prime surface, the entire surface in contact with circulating air in included as heating surface. 5. In the case of ribs, webs, or lugs more than 1 in. thick at the base and extending more than 1 in. from the prime heating surface, the areas of both sides of the first ainrcehtrearaeteidncalusdinetdegarsapl rfiinmse. heating surface. The portions projecting beyond 1 in. Grate Area Grate area is defined9 and treated for purpose of rating as follows: 1. The nominal grate area is defined as the total cross-sectional area of the bottom ionfsitdhee tfhireepfioret.bricInk slinteinelgf.urnaces the nominal grate area is the cross-sectional area 2. The actual grate area, used for calculating the ratios of heating surface to grate area, is the nominal grate area minus certain areas that cannot be considered as part of the grate itself.. The following rules govern these deductions: (1) If a solid, con tinuous ledge extends around the grate and inside the firepot, any area of this ledge extending inside of a circle, the diameter of which is 1 in. less than the diameter of the bottom of the firepot, shall be deducted. (2) If separate, solid projections ex tend from the firepot towards the grate, the areas of any portions of these projections extending inside of a circle, the diameter of which is 3 in. less than the diameter of the bottom of the firepot, shall be deducted. (3) In the case of grates which are inclined, or are conical, the projected area is the same as the nominal grate area. The latter should, therefore, be used after making any necessary deductions. Ratings for Forced Air Furnaces For solid fuel burning, forced air furnaces having bonnet capacities between 80,000 and 250,000 Btu per hour, no standard method of test has been accepted, although eventually such codes will be developed. The National Warm Air Heating and Air Conditioning Association recommends the following empirical equations for use in rating solid fuel forced air furnaces: 1. 11and fired furnaces converted to Stoker, Gas, or Oil Firing. Bonnet Capacity in Btu per hour = 2265 x S x 1.177 (5) 2. Hand-firedfurnaces, with ratios of heating surface to grate area greater than lBtol and less than 25 to t. Bonnet Capacity in Btu per hour =. 2265 sSs 1.177 (6) 3. Hand-fired furnaces with ratios of heating surface to grate area in excess of 25 to 1. where Bonnet Capacity in Btu per hour = 2265 x 25 x G x 1.177 (7) S = heating surface, in square feet. C = actual grate area, in square feet. The Register Delivery Rating is equal to 0.85 x (Bonnet Capacity). lowing testing and rating codes have been generally accepted in tne industry: hav:rTi,?frct<d Standard CS-109-44 for rating solid fuel-burning, forced-air furnaces ratino^ ,!lel outputs of 80,000 Btu per hour or less. This provides a method of a ^,,Sma coal-fired forced-air furnaces by test. the Code for Testing Oil-Fired Furnaces. This code has been adopted by fun ^(1Tvl Air Healing and Air Conditioning Association for rating oil-fired fh^A ^ 6St uruler /r'"mcan Gas Association method of rating gas-fired furnaces on performance Gas Ap]' " `s described in the Approval Requirements for Central Heating