Document N2YMbM8ebwrr0YQqN4j8k1DQ

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 25 per cent for steam systems and 35 per cent for hot water systems is preferable to ignoring entirely the load due to heat loss from the supply and return lines, but better practice, especially when there is much bare pipe, is to compute the emission from both bare and covered pipe surface in accordance with data in Chapter 43. A chart is shown in Fig. 1 indicat ing percentage allowances for piping and warming-up which are appli cable to automatically-fired heating plants using steam radiation. With direct radiation served by bare supply and return piping the percentages may be higher than those stated, while in the case of unit heaters where the output is concentrated in a few locations, the piping tax may be 10 per cent or less. Warming-Up Allowance The warming-up allowance represents the load due to heating the boiler and contents to operating temperature and heating up cold radiation and piping. (See Item 4.) The factors to be used for determining the I Fig. 1. Percentage Allowance for Piping and-Warming-up allowance to be made should be selected from Table 3 and should be applied to the estimated design load as determined by Items 1, 2 and 3 While in every case the estimated maximum load will exceed the design load if adequate heating response is to be achieved, there is, however, no object in over-estimating the allowances, as the only effect would be to reduce the time of warming-up by a few minutes. Otherwise, it might result in firing the boiler unduly and increasing the cost of operation. Performance Curves for Boiler Selection In the selection of a boiler to meet the estimated load, the A.S.H.V.E. Standard Code for Rating Steam Heating Solid Fuel Hand-Fired Boilers recommends the use of performance curves based on actual tests con ducted in accordance with the A.S.H.V.E. Performance Test Code for Steam Heating Solid Fuel Boilers (Code No. 3), similar to the typical curves shown in Fig. 2. It should be understood that performance data apply to test conditions and that a reasonable allowance should- be made for decreased output resulting from soot deposit, poor fuel or inefficient attention. 244 t$1 \ CHAPTER 12. HEATINC BOILERS Selection Based on Heating Surface and Crate Area Where performance curves are not 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 square foot) represented by the design load consisting of connected radiation, piping tax and domestic water heating load. As stated in the section on Boiler Output, this is equivalent to allowing 10 sq ft of boiler heating surface per 1000 000 600 700 600 500 400 300 WEIGHT. LB. FUEL CAPACITY. L0. FUEL AVAILABLE, LB. FUEL OEPTH,IH 0160 651 41140 FIXED CARBON ASH SULPHUR MOISTURE BTU PER LB. 550-.464? 2-66 3.00 13.655 Fig. 2. Typical Performance Curves for a 36-in. Cast-Iron Sectional Steam Heating Boiler, Based on the A.S.H.V.E. Code for Rating Steam Heating Solid Fuel Hand-Fired Boilers boiler horsepower. In this case it is assumed that the maximum load including the warming-up allowance will be provided for by operating the boiler in excess of the design load, that is, in excess of the 100 per cent rating on a boiler-horsepower basis. Due to the wide variation 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: r_____ H u 7 C X FXE (1) 245