Document VJwJyzZr1xpk08qKDpO809w3q

HEATINC VENTILATING AIR CONDITIONING GUIDE 1940 The draft loss through the boiler and setting (ha) also varies between wide limits and, in general, depends upon the following factors: 1. Type of boiler. 5. Arrangement of baffles. 2. Size of boiler. 3. Rate of operation. 6. Type of grate. 7. Design of brickwork setting. 4. Arrangement of tubes. 8. Excess air admitted. 9. Location of entrance into breeching. Curves showing the draft loss through the boiler are usually based on the load or quantity of gases passing through the boiler, expressed in terms of percentage of normal rate of operation. Owing to the great variety'of boilers of different designs and the various schemes of baffling, it is impossible to group together a set of curves for the draft loss through U 9 1U J3 CU XJ rfv ,, POUNDS OF COAL BURNED PER SQ FT OF GRATE SURFACE PER HOUR Fig. 8. Draft Required at Different Rates of Combustion ' for Various Kinds of Coal the boiler which may even be used generally. It is therefore necessary to secure this information from the manufacturer of the particular type of boiler and baffle arrangement under consideration. When a boiler is installed and in operation, the draft loss depends upon the amount of gases flowing through it- This, in turn, depends upon the proportion of excess air admitted for combustion.. Primarily, the amount of excess air is measured by the COj content; the less the amount of COt, the greater the amount of excess air and hence the greater the draft loss. The loss of draft through the boiler will vary directly as the size of the boiler and the length of the gas passages within. The loss also varies as the-number of tubes high, but not in a direct ratio inasmuch as the loss due to the,reversal of flow at the ends of the.baffles remains constant regardless of the height of the boiler. The arrangement of the tubes, whether the. gases flow parallel to or at right angles to the tubes, has an appreciable effect on the loss. The arrangement of the baffles influences the draft loss greatly, the loss through a boiler with five passes being ' 192 CHARTER 10; CHIMNEYS AND DRAFT CALCULATIONS ' greater than the loss through one of three or four passes: rA poor design and a rough condition of the brickwork will increase'the loss greatly,' whereas a proper design and a smooth condition. will keep the loss at aminimum. The loss through the boiler will be less when the breeching entrance is located at or near the top of the boiler than when it is located at or near the bottom since the gases have a shorter distance to travel in the former instance. The draft loss through the breeching (her) is given by the' general equation: . '., where _ O.OOOl94#rrc/Z, : A'B0WcCbl (16) W = the amount of gases flowing pounds per second. Tc = absolute temperature of breeching gases, degrees Fahrenheit. / = coefficient of friction. L = length of breeching, feet: A -- area of breeching, square feet. "t. Bo = atmospheric pressure corresponding to altitude, inches of mercury. Wc = weight of a cubic foot of breeching gases at 0 F and sea level atmospheric pressure, pounds per cubic foot. Cbr = hydraulic radius of breeching section: ` It has been the general custom to lump off the intensity of the breeching loss at 0.10 in. of water per 100 ft of breeching length regardless of its size or shape or the amount'and temperature of the gases flowing through it. This practice is hazardous and has no more foundation in fact than that of determining the friction head in a water works system without taking into consideration the size of the pipe or the amount of water flowing through it. When the length of the breeching is relatively short, any variation in any one of the factors in the equation will have no appreciable effect on the draft loss. However,' when the breeching is relatively long, the draft loss is affected greatly by the various factors, particularly by the size and shape as well as by the weight of gases flowing. The draft loss due to velocity (ftv) is given by the equation , = 0.000194W'TC A*B0WC (17) and represents the amount of draft required to accelerate the gases from zero velocity to the velocity at which the gases are flowing, or in other words, from a static-gas condition of zero flow to the amount of gases flowing throughout'the installation. This loss corresponds to the velocity head in water works systems. The draft loss due to bends (hod) is equivalent to the loss due to the velocity head for a 90-deg bend. In changing direction of flow, the gas velocity decreases to zero with a loss of velocity head and then increases to its proper value at the expense of a loss in pressure head, the net result being a loss in pressure head equal to the velocity head at the bend. This loss is given by the equation: *Bd 0.000194W*TC A'B0WC 193 (18)