Document JNK9b1Q8B32jQDmYOVmp52nee

American Society of Heating and Ventilating Engineers Guide, 1936 through the fuel bed since there is none 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 (he) also varies between wide limits and, in general, depends upon the following factors: 1. Type of boiler. .2 Size of boiler. 5. Arrangement of baffles. 6. Type of grate. 3. Rate of operation. 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 i Pounds of Coal Burned per Sq.Ft of Grate Surface'per Hr? Fig. 7. Draft Required at Different Rat'es';of Combustion \ for Various Kinds <5f Coal 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 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 COi content; the less the amount of. C02, the greater the amount of excess air and hence the greater the draft loss. 462 ;* TV Chapter 26--Chimneys and Draft Calculations 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 at 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 (hsr) is given by the general ! equation: .where 0.000194 W*TcfL A*B0WcCbr (14) 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. 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. 1 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 (hy) is given by the equation Av 0.000194 W*TC ' A'B0Wq (15) 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 463 i