Document ZBKpkMaEGkwMe1YLZE2MvKLN7

506 CHAPTER 35 1959 Guide 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, de pending 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 Ratings of the Mechanical Contractors Association of America are estab lished on this hasis. On a design load baas 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 radiation. The SBI Ratings shown in columns l, 2, 3,10, 11, and 12 of Table 1 (not to be confused with SBI Net Rating) are established on a design load basis. On a gross output baas 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.GA. 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 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 dung so would exceed some of these limita tions. saEcnoN of boilers General Factors The Maximum Load or Gross Load an 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. 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 loses 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. 2. Hot Water Supply Load. The estimated matininm heat in Btu per hour required to beat water for domestic use. I = B -- R recommends that allowance for hot water supply load be made only for bathrooms in excess of two, as follows: Instantaneous Cod 12,000 Btu per hour, and for Storage Task installation 120 Btu per (hour) (gallon of capacity). For instantaneous coi] installations the boiler capacity should not be less than required to heat 2 to 3 gal of water, 100 deg per min Bee also Chapter 50. 3. Piping Tax. The estimated heat emission in Btu per hour of the piping connecting the radiation and other apparatus to the boiler. As the heating industry as a whole is not entirely agreed upon Eiping tax allowances for different sizes of ingtalUtipnn it is etter to compute the heat emission from both bare ana cov ered pipe surface in accordance with data in Chapter 32. in average house heating systems, it is common practice to con sider the piping tax to be equal to 25 percent of the Net Load. In determining Net I = B = R Ratings from Gross I = B = R Output, the piping factor allowed varies from 30 percent for small boilers to 12 percent for larger boilers. 4. Warming-Up or Pickup Allowance. The estimated increase in the normal load in Btu per hour caused by the hpAting 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 radiators and piping. Other items to be considered in boiler selection are as fol lows: 0. Efficiency with hard or soft coal, g*, or oil firing, as the case may be. b. Grate area with hand-fired coal, or fuel-burning rate with stokers, oil, or gas. r c. Combustion space in the furnace. d. Type of heat liberation, whether continuous or intermit tent, or a combination of both. e. Convenience in firing and /. Adaptability to changes in fuel and kind of attention. g. Height of water line. h. Miscellaneous items such as draft available, possibility of future extension, possibility of breakdown, and head room in the boiler room. 1. Hie 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. Cast-Iron Boilers 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 ean 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 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 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 1 and 2. The differ ence between design load and net load represents an amount which is considered normal for piping loss of the ordinary heating system. Boilers with less than 294 sq ft of heating surface and hav ing SBI net ratings (steam) of not more than 5000 6q ft if mechanically-fired and 4120 sq ft if hand-fired, are classified as Table Steel Boilers, formerly Residential boilers. An in sulated Table boiler for oil, gas, or stoker firing may carry a net load expressed in square feet of steam radiation of not more than seventeen times the square feet of heating surface in the boiler, provided the boiler ba been tested in accordance with the SBI Code for testing oil-fired steel boilers at output rates of 125,150, and 175 percent of the steam SBI net rating. The SBI net rating for a water boiler, automatically-fired, expressed in Btu per hour, is 270 times the steam SBI net rating in square feet. The net water rating is 12VV percent greater than the steam rating because a lower piping and pickup factor is generally required in a water system. The SBI net rating (sq ft steam) for hand-fired Table boilers is not greater than fourteen times the beating 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. Gross Output ratings for Table 1 and Table , as well as for Heating ;Bbi!ers, Furnaces, Space Heaters ' 507 Table 3 (a new'designation) Steel Boilers are shown in the seventh edition of the SBI Rating Code. 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 heating 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- Tabfe 6.... Practical Combustion Rates for Coal-Fired Heating Boilers Operating, at Maximum Load on Natural Draft of From > In. to H bt Water*. Kind of Cool Sq ft Greta lb of Cod 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 3M 4 4M 5 5 5H 6 8 9 10 11 13 '9.5'" 12 15.5 * beaten umaUj1 tevv v--cotobiistaoo ntez tor into into exceeding IS tq ft then those indicated in this table. 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: H G CXF X B (1) tohere G * grate area, square feet. H ~ required grosz output of the boiler, Btu per hour (see Selection of Boilers). C = desirable combustion rate for fuel selected, pounds of dry coal per (square foot of grate) (hour). (See Table 6.) F = calorific value of fuel, Btu per pound. B - efficiency of boiler, usually taken as 0.60. Example 1: Determine the grate area for a required grots output of the boiler of 500)300 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)300' 10.7 sq ft 6 X 13)100 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 333 percent for -small hot water boilers to 28.8 percent for very large boilers. If 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 possible, 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. The boiler room height should be sufficient for the location of boiler accessories, and for proper installation of piping. In general, the ceiling height for small steam boilers should be at least 3 ft. above the normal boiler water line. With vapor heating, especially, the height above the boiler water line is of vital importance. CONNECTIONS AND FITTINGS Steam outlet connections should be the full size of the manufacturers' tappings, in order to keep the velocity of flow through the outlet reasonably low, and to avoid fluctuation of the water line and undue entrainment of moisture, and should extend vertically to the maximum height available above the boiler. A steam velocity in boiler outlets not ex ceeding 25 to 30 fps at maximum load is recommended, un less data are available to show that a higher velocity is satis factory. See further data on pipe connections to boilers in Chapters 26 and 28 and in the ASMS Boiler Construction Code for Low Pressure Heating BoUers. Where a return header is used on a cast-iron sectional boiler to distribute the condensate to both rear tappings, it is advisable to provide full size plugged tees instead of el bows where the branch connections enter the return tap pings. This aids in cleaning of sludge from the bottom of the boiler sections through the large plugged openings. An equiva-