Document OY7Nrq279D179589Egb6ZV1v

390 . CHAPTER 18 , 1940 Guide Rating Equations for Gravity Warm Air Furnaces9 Until a method of testing and rating gravity warm air furnaces has-been developed, the following empirical rating equations are recommended by the National Warm Air Heating and Air Conditioning Association. Gravity warm-air furnaces of conventional design, having ratios (of heating surface to grate area) of 15 to 1 or greater, and having a ratio of casing area to face area not less than 0.4, are rated by the following equa tions : a. Hand-fired furnaces Converted to Stoker, Gas, or OH Firing. Bonnet Capacity in Btu per hour -- 1785 x S x 1.333 (2) b. Hand-fired furnaces, with ratios of heating surface to grate area greater than IStol and less than SB to 1. Bonnet Capacity in Btu per hour = 1785 x S x 1.333 (3) c. Hand-firedfurnaces with ratios ofheating surface to grate area in excess ofSB tot. where Bonnet Capacity in Btu per hour = 1785 x 25 x G x 1.333 '- (4) 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). The Leader Pipe Rating in square inches, formerly u^ed as a ratiiig unit, may be found by dividing the Register Delivery Rating by 136. Heating Surface of Furnace Prime heating surface is defined9 as surface above the top of the grate having hot gases or live fuel on one side and circulating air over the other, and in all cases is measured on the exterior or air side. The areas of the outer casing, the inner liner, and any radiation shields shall not be considered as heating surface. - in determining the amount of heating surface, extended surfaces are considered to be prime heating surface subject to the following limitations: 1. Extended heating surface may consist of fins, ribs, webs, lugs, or other.projec tions from the prime heating surface. Projections less than i in. thick at the base and extending more than 1 in. from the prim? surface are classified as.fins. 2. Integral fins are continuously welded to, or cast as a part of. the prime heating surface. Both sides are included as heating surface, subject to tne following allow ances: Distance from Prime Surface. Ratio of Effective Area to 1st inch 0.40 2nd inch . 0.30 3rd inch 0.20 . Over 3 in.. None. =- 3. Non-integral fins are spot welded to, or otherwise held in line contact with the prime heating surface.. Both sides toe included as heating surface, subject to the following allowances Distance from Prime Surface. Ratio of Effective Area to 1st inch . 0.30 2nd inch 3rd inch . Over 3 in. . .0 20, : 0.15 None , Heating Boilers, Furnaces, Space Heaters 39l 4. In the case of ribs, webs, or lugs more than 1 in. thick at the base and extending less than 1 in. from the prime surface, the entire surface in contact with circulating air is 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 inch are included as prime heating surface. The portions projecting beyond 1 in. are treated as integral fins. 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 of the firepot. In steel furnaces the nominal grate area is the cross-sectional area inside the firebrick lining. 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 extend from the firepot towards the grate, the areas of any portions of these projections ex tending 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 Healing and Air Conditioning Association recommends empirical equations similar to Equations 2, 3, and 4 for gravity furnaces, except that a constant of 2265 is used in place of the 1785. The following testing and rating codes have been generally accepted in the industry: Commercial Standards CS-109-44 for rating solid fuel-burning, forced-air furnaces having bonnet outputs of 80,000 Btu per hour or less. This provides a method of rat ing small coal-fired forced-air furnaces by test. A Tentative Code for Testing Oil-Fired Furnaces. This code has been adopted by the National Warm Air Heating and Air Conditioning Association for rating oil-fired furnaces by test. The American Gas Association method of rating gas-fired furnaces based upon per formance under tests. This is described in the Approval Requirements for Central Heating Gas Appliances. Commercial Standards 11S-44 is a method of rating oil-burning floor furnaces by test. Commercial Standard CS 104-IS is a method of rating warm air furnaces equipped with pot-type oil burners by test. Various codes covering the construction and performance of appliances as related to fire hazards have been developed by Underwriter Laboratories, Inc. In addition, there are many municipal codes10 which regulate construction and installation of furnace equipment. The yardstick of the National Warm Air Healing and Air Conditioning Association provides criteria for evaluating a furnace design and installation against industry accepted standards. FURNACE EFFICIENCY Rating formulas of the National Warm Air Heating and Air Conditioning Association are based on 55 per cent efficiency for gravity coal furnaces and 65 per cent efficiency for forced air coal furnaces. In the' tentative Oil Testing Code the contemplated minimum efficiency is 70 per cent for X