Document 3eMdMVzZ0QmaQoQjxGJNe0XGy

ff American Society of Heating ami Ventilating Engineers Guide, 1932 rates of combustion. It will be noted that the amount of draft increases as the percentage of volatile matter diminishes, being comparatively low for the lower grades of bituminous coals and highest for the high grades and small sizes of anthracites. Also, when the interstices of the coal are large and the particles are not well broken up, as with bituminous coals, much less draft is required than when the particles are small and are well broken up as with bituminous slack and the small sizes of anthracites. In general, the draft loss through the fuel bed increases as follows: 1. The percentage of volatile matter diminishes. 2. The percentage of fixed carbon increases. 3. .The thickness of the bed increases. 4. The percentage of ash increases. 5. The volume of the interstices diminishes. In making the preliminary assumptions for the draft loss through the fuel bed, due allowances should be made for a possible future change in the grade of fuel to be burned and also in the rate of combustion. A value should be selected for this loss which will represent not only the highest rate of combustion which will be encountered, but also the grade of coal which has the greatest resistance through the fuel bed and which may be burned at a later date. In powdered-fuel and oil-fired installations, there will be no draft loss through the fuel bed since there is rione and, consequently, this factor becomes zero in the general draft equation. All other factors being constant, the height of the chimney in installations of this character will be less than the height in coal-fired installations and in the case of me chanical draft installations the driving units need not be as large since the head against which the fan is to operate is not as great in the former as in the latter. 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. 2. Size of boiler. 3. Rate of operation. 4. Arrangement of tubes. 5. Arrangement of baffles. 6. Type of grate. 7. Design of brickwork setting. 8. Excess air admitted. 9. Location of entrance into breeching. v 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, fit is impossible to group together a set of curves for the draft loss through 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' 244 I f ji I 1t 3 % I !i;A'i g .Ia Chapter 15--Draft and Chimneys proportion of excess air admitted for combustion. The amount of excess air is measured by the C02 content; the less the amount of C02, 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 greater than the loss through one of three or four passes. A 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 to a minimum. 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 Qibt) is given by the general equation: where 0.000194 W*TcfL A*B0WcCbr (15) 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. B0 = atmospheric pressure corresponding to altitude, inches of mercury. 1FC = weight of a cubic foot of breeching gases at 0 deg 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 on 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 (hy) is given by the equation o.oooi94iy*rc hv A1B0WC (161 qnd 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 245 M