Document gaB9QmNEQg0ByG82DNB64Gvq9

368 CHAPTER 17 1949 Guide Table 1. Capacity op Gas Piping Nominal Diameter of Pipe in Inches Length of Pipe in Feet 1 * * i) 2 Capacity--Cu Ft Per Hr with a 0.6 Sp Gr Gas and Pressure Drop of 0.3' Water Column 15 .172 345 750 30 120 241 535 850 45 99 199 435 700 60 86 173 380 610 75 77 155 345 545 90 70 141 310 490 105 65 131 285 450 920 120 120 270 420 860 150 109 242 380 780 180 100' 225 350 .720 determined from the approved input and an average efficiency stated in the Approval Requirements and is the heat available at the outlet. Sizing Gas-Fired Heating Plants Although gas-burning equipment usually is completely automatic,, maintaining the temperature of rooms at a predetermined figure, there are some manually controlled installations. In order to overcome effec tively the starting load and losses in piping, a manually-controlled gas boiler should have an output as much as 100 per cent greater than the equivalent standard.radiation which it is expected to serve. Boilers under thermostatic control, however, are not subject to such severe pick-up loads and consequently, it is possible to use a lower selec tion factor. For a gas-fired boiler or furnace under thermostatic control a factor of 20 to 25 per cent is usually sufficient for pick-up allowance. In those installations, in mild climates where 100 per cent outside air is used, furnaces should be of larger size in order to provide adequate capacity and quick pick-up under intermittent heating conditions. The factor to be allowed for loss of heat from piping will vary somewhat, the proportionate amount of piping installed being greater for small installations than for large ones. For. selection factors to be added to installed radiation under thermostatic control see Chapter 18. Table 2. Multipliebs Fob Vabious Specific Gravities For Use With Table 1 Specific Gravity Multiplier Specific Gravity Multiplier . .35 1.31 1.00 ' .775 . .40 1.23 1.10 .740 .45 1.16 1.20 .707 .50 1.10 1.30 .680 .55 1.04 1.40 .655 .60 1.00 1.50 .633 .65 .962 1.60 ' .612 .70 .926 1.70 .594 .75 .895 1.80 .577 .80 .867 1.90 .565 .85 .841 2.00 .547 .90 .817 2.10 .535/ Automatic Fuel Burning Equipment- - 369 Appliances used for heating with gas should bear the approval seal of the A.O.A. Testing Laboratories on the manufacturer's nameplate, together with the official input and output ratings. It is not permissible to operate a gas heating unit above its stated rating. It may be necessary to operate below this rating at elevations above 2000 ft unless the appliance has been tested and approved for operation at altitudes up to 5200 ft as shown on name plate. Installations should be made in accordance with recommendations shown in the publications of the American Gas Association. Controls Temperature controls for gas burners are described in Chapter 34. Some central heating plants are equipped with push-button or other manual control. The main gas valve may be of either the snap action or throttling type. Automatic electric ignition is available. SIZING OF GAS PIPING Piping for gas appliances should be of adequate size and so.installed as to provide a supply of gas sufficient to meet the maximum demand without undue loss of pressure between the point of supply (the meter) and the burner. The size of gas pipe necessary to install depends upon the following factors: 1. Maximum gas consumption to be provided. 2. Length of pipe and number of fittings. 3. Allowable loss in pressure from the outlet of the meter to the burner. 4. Specific gravity of the gas. To obtain the cubic feet per hour of gas required by the burner divide the Btu input at which the burner will be adjusted by the average Btu heat ing value per cubic foot of the gas. Capacities of different sizes and lengths of pipe, in cubic feet per hour, with a pressure drop of 0.3 in. of water column for a gas of 0.60 sp. gr. are shown in Table 1. In adopting a 0.3 in. pressure drop, due allowance for an ordinary number of fittings was made. To convert the-figures given in Table 1 to capacities for another gas of different specific gravity, multiply the tabular values by the multipliers shown in Table 2. FUEL BURNING RATES The burning rate for automatic fuel burning devices is determined by the gross heat output required of the boiler, or furnace, to carry the net heating load plus allowances for system losses, and pick-up. General values for these allowances previously have been noted. Detailed infor mation for piping and pick-up allowances for steam, and hot water systems is given in Chapter 18 and for warm air systems in Chapters 21 and 22. When the gross output, operating efficiency, and heat value of the fuel are known, the required rate of burning can be determined by means of Figs. 21, 22 and 23 for the several fuels. As the rate of fuel burning is directly proportional to . the load for a given efficiency, these charts can be extended by moving- the decimal points the same number of digits in both vertical and horizontal scales.