Document 6b4Qw7wz6ZgG3vV2YG8KnbGyE

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 The maximum rating as defined in the preceding paragraph shall be based upon com bustion of Pennsylvania anthracite, having, the following approximate analysis: Volatile matter 3.5 to 9 per cent; ash content not to exceed 15 per cent; sulphur content Under 1.5 per cent; ash fusing temperature 2750 F, or above (volatile, ash and sulphur content on dry basis in accordance with A .S.T.M. method D271-33); Btu content 12,000 or above; properly sized as follows: A No. 1 buckwheat should pass through a round mesh screen having jKe in. holes and over a similar screen having Y\c in. holes. The undersizing should not exceed 15 per cent and the oversizing should not exceed 10 per cent. A No. 2 buckwheat (rice) should pass through a round mesh screen having holes Ye ,n- in diameter and over a like screen having holes o! Ye in. in diameter. The under sizing, should not exceed 15 per cent and the oversizing should not exceed 10 per cent. Coal Storage It is recommended that the coal bin or closet be constructed so as to bedustproof. Electrical Consumption. The electrical consumption shall not exceed 18 kwhr per 2000 lb of coal burned at any rate of coal feed above 50 per cent of the maximum. Operation Upon Other Sizes of Coal. The foregoing specifications have been drafted for operating with the Nos. 1 and 2 buckwheat sizes of anthracite. In the event that other sizes are recommended, ratings shall be based upon the same efficiency and ash loss requirements. ' Banking. The burner shall be so constructed or controlled as to maintain a fire during an indefinite banking period. Acceleration. When the burner resumes operation after a 12 hour banking period, the time required for the stack temperature to reach a normal maximum shall not exceed 60 min. Stoker-Fired Units Boilers and air conditioners especially designed for stokers are now becoming available. Present designs feature better coordination between the heat absorber and the stoker. Increased setting height and furnace volume and more heating surface than in conversion installations are usually to be found in these stoker units. Class 3 Stokers, Apartment House, Small Commercial This class is used extensively for heating plants in apartments and hotels, and also for small industrial plants such as laundries, bakeries, and creameries. The majority of stokers used in this field are of the underfeed type. The principal exception is an overfeed type having step action grates in a horizontal plane and so arranged that they are alternately moving and stationary, and are designed to advancethe fuel during com bustion to an ash plate at the rear. All of the stokers are provided with a coal hopper outside of the boiler. In the underfeed types, the coal feed from this hopper to the furnace may be accomplished by a continuously revolving screw or by an intermittent plunger. The drive for the coal feed may be an electric motor, or a steam or hydraulic cylinder. With an electric motor, the connection between the driver and the coal feed may be through a variable speed gear train which provides two or more speeds for the coal feed; or it may be through a simple gear train and a variable speed driver for the change in speed of the coal feed; or a simple gear train with a coal feed having an adjustment for varying the travel of the feeding device. With a steam or hydraulic cylinder, the power piston is connected directly to the coal feeding plunger. The stokers in this class vary also in their retort design according to the fuels and load conditions. The retort is placed approximately in the 204 CHAPTER II. AUTOMATIC FUEL BURNING EQUIPMENT middle of the furnace and is provided with tuyere openings at the top on all sides. In the plunger-feed type the retort extends from the inside of the front wall entirely to the rear wall or to within a short distance of the rear wall. This type of retort has tuyeres on the sides and at the rear. These stokers also differ in the grate surface surrounding the retort. In many of the worm-feed stokers this grate is entirely a dead plate on which the fuel rests while combustion is completed. In the dead-plate type, all of the air for combustion is furnished by the tuyeres at the retort. Because of this, combustion is well advanced over the retort so that it may easily be completed by the air which percolates through the fuel bed. With the dead-plate type of grate the ash is removed through the fire doors and it is therefore desirable that the fuel used shall be one in which the ash is readily reduced to a clinker at the furnace temperature, in order that it may be removed with the least disturbance of the fuel bed. In other stokers in this class, the grates outside of the retort are airadmitting and some stokers have shaking grates. These grates permit a large part of the ash to be shaken into the ashpit beneath, while the clinkers are removed through the fire doors. With this type of grate, the main air chamber extends only under the retort while the side grates receive air by natural draft from the ashpit. In still other stokers of this class, the main air chamber extends beyond the retort and is covered with fuel-bearing, air-supplying grates. With this type of grate, the fuel is supplied with air from the main air chamber throughout combustion. Also with this type of grate, dump plates are provided beyond the grates where the ash accumulates and from which it can be dropped periodically into the ashpit beneath. Stokers in this class are compactly built in order that they may fit into standard heating boilers and still leave room for sufficient combustion space above the grates. The height of the grate is approximately the same as that of the ordinary grates of boilers, so that it is usually possible to install such stokers with but minor changes in the existing equipment. In some districts, there are statutory regulations governing such settings. These stokers vary in furnace dimensions from 30 in. square to approxi-. mately 66 in. square. The capacity of the stokers is measured by the amount of coal that can be burned per hour. In general, manufacturers recommend that, for continuous operation, the coal burning rate shall not exceed 25 lb of coal per square foot of grate per hour, while for short peaks this rate may be increased to 30 lb per hour. Although these stokers were designed to burn bituminous coal, types are available for the semi-bituminous coals such as Pocahontas and New River. They can also be used to burn the small sizes of anthracite but at a somewhat lower rate. Class 4 Stokers, Medium Commercial These stokers are usually of the screw feed type without auxiliary plungers or other means of distributing the coal. Rectangular retorts with sectional tuyeres and dead plates without air ports are employed: The unit type of construction is almost universally used, the unit incor porating the hopper, the transmission for driving the feed screw, and the fan for supplying air for combustion. 205