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.344 CHAPTER 18 ' 1946 Guide The Institute of Boiler. and. Radiator Manufacturers has adopted a method of rating cast-iron, heating boilers based upon performance obtained under tests. This code6 now applies to sectional boilers having a grate width of 41 in- or less. Eventually the Institute intends to include all sizes of cast-iron boilers in this program of testing and rating. The American Gas Association has adopted a method of rating gas designed boilers based upon performance under tests. This is described in Approval Requirements for Central Heating Gas Appliances. The Heating, Piping and Air Conditioning Contractors National Ajsociation has adopted'a method of rating boilers based on their physical characteristics for those boilers that are not rated in accordance with the SBI or I=B=R Codes. Ratings are expressed on a1 Net Load .basis in square feet-of steam radiation. . BOILER EFFICIENCY The term efficiency as used for guarantees of boiler performance is usually 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. 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 of 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. iI Anthracite, hand fired_..... _1.... ....... .... .......... ............... . . 60 to 75 per cent Bituminous Coal, hand fired............................................... 50 to 65 pa- cent Stoker fired..... .............................. -..................................... 60 to 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.: - | i RATING OF BOILERS : ij In inferring to boiler rating it is necessary to know, the basis on jwhich the rating has been established in order to understand the exacjt meaning of the term. The following example will illustrate the 'meaning, of; three ratings which might be established for the same boiler. i! 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 sq ft of steam radiation 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 bje said to have a net load rating of 1000 sq ft, a design load rating of 1200! sq ft, or a gross load fating of 1440 sq ft, depending on the basis on which the boiler is rated. Heating Boilers and Furnaces 345 On a net load basis the above 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 Ratings of the Heating, Piping and Air Conditioning Con tractors National Association are established on 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 loadx 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 (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 pick-up 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 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 Btq 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 IBR Ratings from Gross I--B--R Output, the piping 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