Document zQdVN2zr6b8EwkBdGZ4zgzL0z

350 CHAPTER 18 1948 Guide 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. The combined 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 per cent Bituminous Coal, hand fired............. 1__________________ 50 to 65 per cent Stoker .fired.'._____~___ ....:_________ _____ _____________ , 60 Ip 75 per cent ... Oil and Gas fired....: 70 to 80 per cent 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-------------Piping Tax.-----,--.............................. Design Load---------------------------------------------Pickup Allowance.--:------------------------ Maximum or Gross Load__________________ 1000 sq'ft of steam radiation 200 sqft of steam radiation 1200 sq ft of steam radiation 240 sqft 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 food 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 100.0 sq ft of steam radia tion 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 Ratings of the Heating, Piping and Air Conditioning Con tractors National Association are establishedon this basis. On a design load basis the boiler would be rated 1200: sq ft of steam radiation 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 radi ation. The SBI Ratings shown in columns 1, 2, 3, 10, 11 and 12 of Table 2 (hot 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 Heating Boilers and Furnaces 351 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 Factors Involved in Boiler Selection : 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 6, 8, and 14. The sum of the calculated heat losses for all the rooms represents the total required heat emission of the connected radia tion 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 equivalent square feet of equivalent radiation. 2. Hot Water Supply Load. The estimated maximum heat in Btu per hour required to heat water for domestic use. . When the hot water supply is heated by the building heating boiler, this load must be taken into consideration in sizing the boiler. A common practice is to add 240 Btu per hour to the radiation load for each gallon of storage tank capacity. For more specific information see Chapter 50. 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 allowances for different sizes of installations it is better to compute the heat emission from both bare and covered pipe surface in accordance with data in Chapter 28. In average house heating systems,.it is common practice to consider the piping tax. to be equal to 25 per cent of the Net'Load. In determining Net. f = B = R Ratings from Gross I~B=R Output, the pipirig'factor allowed varies from 30 per cent for small boilers to 12 per cent for larger boilers. .4. Warming-Up or Pick-Up Allowance. The estimated increase in the normal load in Btu per hour caused by the heating up of the cold system. ' The warming-up allowance represents the load due to heating the boiler and contents to operating temperature, and heating up cold radiation and piping. The factors to be used for determining the. allowance to be made should'be selected from Table 4. ' ...Other items ..to be considered in boiler selection are: . (a) Efficiency with hard or.soft coal, gas, or oil firing, as the case may be. (h) Grate area with hand.fired coal, or fuel burning rate with stokers, oil, or gas. . (c) Combustion space in the furnace. (d) Type of heat liberation, whether continuous or intermittent, or a combination of both. (e) Convenience in firing and cleaning. ` (/) Adaptability to changes in fuel and kind of attention. (g) Height of water line. . ...(A) Miscellaneous items such as draft available, possibility of future extension, pos sibility of.break-down, and head room in the boiler room. . (*) The most economicalsize of boiler is usually one that is just the right size for the load. Either larger or smaller boilers may be less economical. Selection of Cast-Iron Boilers Net load ratings of cast-iron boilers are usually available from manu facturers' catalogs. They may also be obtained, conveniently from