Document 4vr6Dmow4OQnQYd5OzbxLegKx

520 CHAPTER 35 1960 Guide 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 2. Hot Water Supply Load. The estimated maximum heat in Btu per hour required to heat water for domestic use. I = B = R recommends that allowance for hot water supply load be mode only for bathrooms in excess of two, as follows: Instantaneous Coil 12,000 Btu per hour, and fur Storage Tank installation 120 Btu per (hour) (gallon of tapir 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 mia. See also Chapter 56. understand the exact mining of the term. The following 3. Piping Tax. The estimated heat emission in Btu per hour of pTftmpIp will illustrate the meaning of three ratings which the piping connecting the radiation and other apparatus to the might be established for the same boiler. boiler. Assume that an installation has the following loads deter mined in accordance with the section Selection of Boilers: As the beating 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 cov Net Load................................... 1000 sqft Piping Tax................................. 200 sqft Design Load............................... 1200 sqft Pickup Allowance.................... 240 sqft of of of of sssstttteeeeaaaammmmrrrraaaaddddiiiiaaaaaIOesttttniiiviiroooodueennndnetdrpaerugptteehti,premhethopineusipuisnierpngfaigphcNieetnaeagxttininIftaoga=ccbstoceyBorsret=edaqmalulnoRsacw,leReittodawti2siivnt0hagcrpsoideemfasrrcomtaefmrononitnmGoprfCor3atsh0hcsaetIpipcNete=erecrtetBo3nL2t=oc.oafoInRdnr. Maximum or Gross Load___ 1440 sq ft of steam radiation small boilers to 12 percent for larger boilers. 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 4. Warming-Up or Pickup Allowance. The estimated increase in the normal load in Btu per hour caused by the heating up of the cold system. rating of 1200 sq ft, or a gross load rating of 1440 sq ft, de pending on the basis on which the boiler is rated. On a net load basis the boiler would be rated 1000 sq ft The warming-up allowance represents the load due to heating the boiler and contents to operating temperature, and heating up cold radiators and piping. 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 Mechanical Contractors Association of America are estab lished on t-hia bass. 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 Other items to be considered in boiler selection are as fol lows: a. Efficiency with hard or soft coal, gas, or oil firing, as the case may be. b. 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 intermit tent, or a combination of both. loss did not exceed 1200 sq ft of steam radiation. The SBI e. Convenience in firing and cleaning. Ratings shown in columns 1, 2, 3,11, 12, and 13 of Table l j. Adaptability to changes in fuel and kind of attention. (not to be confused with SBI Net Rating) are established on g. Height of water line. 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. h. Miscellaneous items such as draft available, possibility of future extension, possibility of breakdown, and head room in the boiler room. i. The most economical size of boiler is usually one that is just the right size for the load. Either larger or smaller boilers may be less economical. SBI Gross Output, Gross I = B = R Output, A.GA. Ratings and ABA2 Ratings are established on a gross output basis. Cast-Iron Boilers 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 limita tions 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 limita tions. Net load ratings of cast-iron boilers are usually available from manufacturers' catalogs. They may also be obtained conveniently from published tables of I = B = R ratings,* or from recommendations of the Mechanical Contractors Asso ciation of America,* and can be used in selection of boilers, unless the heating system contains an unusual amount of bare pipe, or the nature of the connected load is such that the SELECTION OF BOILERS General Factors normal allowances for pipe loss and pickup do not apply. In such a case, the selection must be based on the gross output. . Steel Heating Boilers 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 con vection coils) to be installed. SBI catalog ratings, in accordance with the previously mentioned Steel Boiler Institute code, are intended to corre spond with the estimated design load. When the heat emission of the piping is not known, the net load to be considered for the boiler may be determined from Tables l and 2. The differ ence between design load and net load represents an amount 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 calculated heat losses for all the rooms represents the total required heat emission of the con nected 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. which is considered normal for piping loss of the ordinary heating system. Boilers with 294 sq ft or less of heating surface and hav ing SBI net ratings (steam) of not more thqp 5620 sq ft if mechanically-fired and 4120 sq ft if hand-fired, are classified as Table Steel Boilers, formerly Residential boilers. An in- Heating Boilers, Furnaces, Space Heaters 521 pnUteH Table boiler for oil, gas, or stoker firing may carry a raaviTnum net load, expressed in square feet of steam radia tion, of 19.1 times the square feet of heating surface in the boiler, provided the boiler has been tested in accordance with the SBI Code for testing oil-fired steel boilers at output rates of 85,100 and 115 percent of the SBI gross output. The SBI net rating for a water or steam boiler, automatically-fired, ex pressed in Btu per hour, is 4590 times the boiler heating sur face. SBI Net Ratings for water and steam boilers are based on a piping and pickup factor of 33Vs percent. The SBI net rating (sq ft steam) for band-fired Table boilers is not greater than fourteen times the heating surface. If the heat loss from the piping system exceeds 20 percent of the installed radiation, the excess is to be considered as part of the net load. Table 6.... Practical Combustion Roles for Cool-Fired Heating Boilers Operating at Maximum Load on Natural Draft of From H In. to H In- Water* Kind of Coat Sq Ft Grata lb of Coal per (Sq Ft Grate) (Hr) No. 1 Buckwheat Anthracite Anthracite Pea Anthracite Nut and Larger Bituminous Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 Up to 9 10 to 19 20 to 25 Up to 4 5 to 9 10 to 14 15 to 19 20 to 25 Up to 4 5 to 14 15 and above 3 4 5 5 5M 6 8 9 10 11 13 ____^ 9.5 12 15.5 * Steel boilers usaeU? hers MgW combasttoo rates (or Crete srese lSeqft thso those indie*ted is this table. Hiwf Gross Output ratings for Table l and Table , as well as for Table S Steel Boilers are shown in the SBI Rating Code (1958). Packaged Firetube Boiler ratings as published by the Amer ican Boiler and Affiliated Industries are Gross Output ratings. It is recommended that boilers of this type be selected with ratings equal to the Gross Load imposed on the boiler, which is the sum of the Radiation Load, Hot Water Supply Load, Piping Tax, and Pickup Allowance. If the Piping Tax and Pickup Allowance cannot be deter mined, a maximum allowance of 25 percent is generally con sidered to be a safe maximum for the sum of both allowances to. be added to the Radiation Load and Hot Water Supply Load. Heating Surface and Grate Area Basis Where neither the net load nor gross output ratings based upon performance tests are available, a good general rule for conventionally designed boilers is to provide 1 sq ft of boiler heating surface for each 14 sq ft of equivalent radiation (240 Btu per sq ft) (hr) represented by the design load. This is equivalent to allowing 10 sq ft of boiler beating surface per boiler horsepower. In this case it is assumed that the maxi mum load including the warming-up allowance will be pro vided for by operating the boiler in excess of the design load, that is, in excess of the 100 percent rating on a-boiler-horse power basis. SBI ratings for hand firing are based on 10 sq ft of heating surface per boiler horsepower. Due to the wide variation which may be encountered in manufacturers' ratings for boilers of approximately the same capacity, it is advisable to check the grate area required for heating boilers burning solid fuel by means of the following formula: a G CXFXE (1) where G = grate area, square feet. H ** required gross output of the boiler, Btu per hour (see Selection of Boilers). C -- desirable combustion rate for fuel selected, pouods of dry coal per (square foot of grate) (hour). (See Table . 6.) P ** calorific value o! fuel, Btu per pound. R -- efficiency of boiler, usually taken as 0.60. Example 1: Determine the grate area for a required gross output of the boiler of 500,000 Btu per hour, a combustion rate of 6 lb per hour, a calorific value of 13,000 Btu per pound, and an efficiency of 60 percent. 500,000 10.7 sq ft 6 X 13,000 X 0.60 The boiler selected should have a grate area not less than that determined by Equation 1. With small boilers, where it is desired to provide sufficient coal capacity for approximately an eight-hour firing period plus a 20 percent reserve for ig niting a new charge, more grate area may be required depend ing upon the depth of the fuel pot. Gas-Fired Boilers After determining the net load for the installation, gasdesigned boilers can usually be selected from manufacturers' tables of net load ratings which are based on piping and pickup allowances varying from 56 percent for small steam boilers and 33.3 percent for small hot water boilers to 28'B percent for very large boilers. II the piping and pickup load or other factors create an unusual load, a boiler should be selected which has an A.GA. output rating equal to the maxi mum output required. Detailed recommendations for selec tion of gas designed boilers are given in the A.GA. publica tion, Comfort Heating* SPACE LIMITATIONS Boiler rooms should, if posable, be situated at a central point with respect to the building, and should be designed for a maximum of natural light. The space in front of the boilers should be sufficient for firing, stoking, ash removal, and cleaning or renewal of flue tubes, and should be at least 3 ft greater than the length of the tubes. A space of at least 3 ft should be allowed on at least one side of every boiler for convenience of erection and for ac cessibility to the various dampers, cleanouts, and trimmings. The space at the rear of the boiler should be ample for the chimney connection and for cleanouts. With large boilers the rear clearance should be at least 3 ft in width.