Document evZ5X12MLKqEwLbjJY3421n6g

392 CHAPTER 16 j 1954 Guide; 2. Liquid and Gaseous Fuels. The combined efficiency of boiler, furnace and burner is the ratio of the heat absorbed by the water and Bteam in the bojler per pound or cubic foot of fuel, to the calorific value of 1 lb or cubic foot of fuel, respectively. The following emciencies 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....................... ft1ors.coal';hand;fired: ....... .......... ; fiO in 7>i :::::::: * OU and gas-fired.................................................................. !!. 70 to 80 eS 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. 1 \ Assume that an installation has the following loads determined iff 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......... ..... ............. .........' 1280 sq ft 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. i 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 anik pickup load. Net I = B = R Ratings, SBI Net Ratings, and Net Load; 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 tow.-l It would be of adequate size for a system in which the sum of the net loci and the piping heal loss did not exceed 1200 sq ft of steam radiation. Tb|-: SBI Ratings shown in columns 1, 2, 3, 10, 11 and 12 of Table 2 (not to Ba; 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 4. 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,.gL ft of steam radiation. Gross 7 = 13= R Output and A .G.A.' Ratings^*6;! established on a gross output basis. ' Kg; In the determination of boiler ratings, the Gross Output is the quantity of heat available at the boiler nozzle, with the. boiler normally insulakp and when dperatiirg under limitations stipulated in the code or methodjMj which the boiler is rated. The boiler may be capable of producing .8!; greater nozzle output, but in doing so would exceed some of these limitation3;;.. 393 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 bathrooms in excess of two, as follows: Instantaneous Coil 12,000 Btu per hour, and for 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 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 percent of the Net Load. In determining Net 7 => B = R Ratings from Gross 1 = B = R Output, the piping factor allowed varies from 30 percent for small boilers to 12 percent for larger Doilers. i. 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 con tents 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. Table 4. Warming-up Allowances for Hand-Fired Low-Pressure Steam and Hot Water Heating Boilers*-b-" PtB0*IWi05;Clp*OTT to Add *oa WabiccwUfo -tum loom? JOO.OOO 2no'^ to 200,000 600mn*tO 60.00 .^DW to 1,800,000 Above 1,800,000 o^Bujij^e k ^ken from the ^ A.S.H.V.E. Up to 420 420 to 840 840 to 2500 2500 to 5000 5000 to 7500 A_b_o_v_e--750w0 Code at Minimum | Requirements for the ~65 60 55 50 Hea444t00i5ng and Ventila ted in t#.TM; Bf' that the second column has been added fox convenience in interpreting the design b o v* 08 equivalent square feet of radiation. nb, Voi?o^i??S Analysis in Starting Heating Apparatus, by Ralph C. Taggart (A.S.H.V.E. Transac- ofeara Heatin* o P- 292); Report of A.S.H.V.E. Continuing Committee on Codes for Testing and Rating T Fuel Boilers (A.S.H.V.E Transactions, VoL 36, 1930, p. 35); Selecting the Right Site 'This *toen "^in Crocker (Heating, Piping and Air Conditioning, March, 1932). *5/* to hand-fired, solid fuel boiler*. A factor of 20 percent over design load is adequate ' *uriT 'C lV~tir'd futU used. Btu PW square foot.