Document Oqk2pZ4Jj5qraxkyyd24DDQp

414 CHAPTER 16 1957 Guide sociation has adopted a method, based on their physical characteristics for rating boilers that are not rated in accordance with the SBI or I=B=R Codes. Ratings are expressed on a Net Load basis in square feet of steam ' radiation. The HP&ACCNA, has also adopted a Testing and Rating Code for Boiler-Burner Units.6 The purpose of this Code is to provide a basis of rating commercial sizes of steel heating boiler-burner units fired , with oil -or gas fuel. This code allows a higher rating than is permissible under the SBI Code. A gross output is" established with certain limiting factors applying to flue gas temperature, carbon dioxide, efficiency and quality of steam. This output is divided by 1.5 to determine the net rating.- BOILER EFFICIENCY The term efficiency, as used for guarantee of boiler performance, is usu ally construed as follows: 1. Solid Fuels. The efficiency of the boiler alone is the ratio of the heat absorbed by the water and steam in the boiler per pound of combustible burned on the grate, to the calorific value of 1 lb of combustible as fired. The combined efficiency of boiler, furnace and grate is the ratio of the heat absorbed by the water and steam in the boiler per pound of fuel as fired, to the calorific value of 1 lb of fuel as fired. 2. Liquid and Gaseous Fuels. Thecombined efficiency of boiler, furnace and burner is the ratio of the heat- absorbed by the water and steam in the boiler per pound or cubic foot of fuel, to the calorific value of 1 lb or cubic foot of fuel, respectively. The following efficiencies apply to current designs of boilers operated, under favorable conditions at their gross output ratings. Some olderboilers, designed primarily for hand firing, may have lower efficiencies: when automatically fired. Anthracite, hand-fired.......................................... ................... 60 to 75 percent Bituminous coal, hand-fired...>........................ 50 to 65 percent Stoker-fired. .................................. ........................................... 60 to 75 percent Oil and gas-fired....................................................................... 70 to 80 percent -- Higher efficiencies, for hand-fired bituminous coal may be obtained by,, careful firing of either a regular or a smokeless boiler. - RATING OF BOILERS In referring to boiler rating, it is necessary to know the basis on which the rating has been established in order to understand the exact meaning of the term. The following example will illustrate the meaning of three ratings which might be established for the same boiler. Assume that an installation has the following loads determined in accordance with the section Selection of Boilers: : Net Load.............................................................. 1000 sq ft of steam radiation Piping Tax.. , -200 sq ft of steam;radiation Design Load........ Pickup Allowance Maximum or Gross Load 1200 sq ft of steam radiation 240 sq ft of steam radiation . 1440.sq ft of steam radiation A boiler that is just large enough to carry this system might be said'to have a net load rating of 1000 sq ft, a design load rating of 1200 sq ft, or a gross load rating of 1440 sq ft, depending on the basis on which the boiler is rated. On a net load basis the boiler would be rated 1000 sq ft of steam radiation and would have sufficient excess capacity to supply the normal piping and Heating Boilers, Furnaces, Space Heaters 415 pickup load. Net I = B = R Ratings, SBI Net Ratings, and Net Load Ratings of the Healing, Piping and Air Conditioning Contractors National Association are established on this basis. On a design load basis the boiler would be rated 1200 sq ft of steam radia tion and would have sufficient excess capacity to supply the pickup load. It would be of adequate size for a system in which the sum of the net load and the piping heat loss did not exceed 1200 sq ft of steam radiation. The SBI Ratings shown in columns 1, .2, 3, 10, 11 and 12 of Table 1 (not to be confused with SBI Net Rating) are established on a design load basis. On a gross output basis of rating, the boiler would be rated 1440 sq ft of steam radiation and would be of adequate size for a system in which the sum of the net load, piping load, and pickup load did not exceed 1440 sq ft of steam radiation. Gross I-- 'B = R Output and A.G.A. Ratings are established on a gross output basis. In the determination of boiler ratings, the Gross Output is the quantity of heat available at the boiler nozzle, with the boiler normally insulated and when operating under limitations stipulated in the code or method by which the boiler is rated. The boiler may be capable of producing a greater nozzle output, but in doing so would exceed some of these limitations. SELECTION OF BOILERS General Factors The Maximum Load or Gross Load on the boiler is the sum of the four following items. The Design Load is the sum of items .1, 2, and.3. :' The Net Load is the sum of items 1 and 2. 1. Radiation Load. The estimated heat emission in Btu per hour of the connected radiation (direct, indirect, or forced convection coils) to be installed. The connected radiation is determined by calculating the heat losses for each room in accordance with data given in Chapters 9,11 and 12. The sum of the calcu lated heat losses for all the rooms represents the total required heat emission of the connected radiation, expressed in Btu per hour. As practically all boilers are now rated on a Btu basis, it is unnecessary to convert the radiation load to square feet of equivalent direct radiation. 2. Hot Water Supply Load. The estimated maximum heat in Btu per hour re quired to heat water for domestic use. . B -- R recommends that allowance for hot water supply load be made only for oattirooms in excess of two, as follows: Instantaneous Coil 12,000 Btu per hour, and ir storage Tank installation 120 Btu per (hour) (gallon of tank capacity). For nstantaneous coil installations the boiler capacity should not be less than required 0 neat 2 to 3 gal of water, 100 deg per min. See also Chapter 49. Piping Tax. The estimated heat emission in Btu per hour of the piping conting the radiation and other apparatus to the boiler. all^8 *^e bating industry as a whole is not entirely agreed upon piping tax from?KnC^Bu0r <^eren*' sizes of installations, it is better to compute the heat emission ever i "are and, covered pipe surface in accordance with data in Chapter 27. In be 6i "ouse heating systems, it is common practice to consider the piping tax to Grossf ^ percent of the Net Load. In determining Net I = B -- R Ratings from i,,.= B = R Output, the piping factor allowed varies from 30 percent for small 001 ers to 12 percent tor larger boilers. load or R^k-Up Allowance. The estimated increase in the normal n Btu per hour caused by the heating up of the cold system. tenTse,,Warmi^:uP allwance represents the load due to heating the boiler and con10 operating temperature, and heating up cold radiation and: piping.