Document revX6LGVMp9qrzaXL2Kbpj2xv

396 CHAPTER 16 1955 Guide rating boilers that are not rated in accordance with the SBI or I=B= R Codes. Ratings are expressed oil 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*.. The purpose of this Code is to provide a basis of rating commercial sixes of steel heating boiier-bumer units fired with oil or gas fuel. This code allows a higher rating than is permissible under the -SBl 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 ofThe 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 Ga seous 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 older boilers, 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........................ 1200 sqft of steam radiation Pickup Allowance................................................ 240 sq ft of steam radiation . Maximum or Gross Load.................................... 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 pickup load. Net I = B = R Ratings, SBI Net Ratings, and Net Load Heating Boilers, Furnaces, Space Heaters 397 Ratings of the Heating, 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 mm 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, i0, 11 and 12 of Table 1 (not to be confused with SBl 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 Ngt 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 unnecessaryto 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. , 1,= R = R recommends that allowance for hot water supply load be made only for bathrooms in excess of two, as follows: Instantaneous Coil 12,000 Btu per hour, and ;or Storage Tank installation 120 Btu per (hour) (gallon of tank capacity). For instantaneous coil installations the boiler capacity should not be less than required to heat 2 to 3 gal of water, 100 deg per min. See also Chapter 49. 3. Piping Tax. The estimated heat emission in Btu per hour of the piping con necting the radiation and other apparatus to the boiler. As the heating industry as a whole is not entirely agreed upon piping tax Jo'.vancos for different sizes of installations, it is better to compute the heat emission rom both bare and covered pipe surface in accordance with data in Chapter 28. In verage house heating systems, it is common practice to consider the piping tax to e equal to 25 percent of the Net Load. In determining Net I = B -- R Ratings from i r i = B = R Output, the piping factor allowed varies from 30 percent for small oilers to 12 percent for larger boilers. I ^-Warming-Up or Pick-Up Allowance. The estimated increase in the normal ad in Btu per hour caused by the heating up of the cold system. te 1'*>e warming-up allowance represents the load due to heating the boiler and con ns to operating temperature, and heating up cold radiation and piping. Other items to be considered;in boiler selection are as follows:.