Document 5kky4QB2dYY1YmyqpQxOOXK20

American Society of Heating and Ventilating Engineers Guide, 1937 The draft loss through the boiler and setting (hs) 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. 9. Location of 5. Arrangement of baffles. 6. Type of grate. 7. Design of brickwork setting. 8. Excess, air admitted, 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 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. 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 (h&T) is given by the general equation: = 0.000194. . kBl------- A'B0WcChr <16) where 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. PFC = 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. 476 Chapter 26--Chimneys and Draft Calculations 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 (Ay) is given by the equation hv 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 (hsa) 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: 0.000194 "Bd - A'BoWc (18) The friction at a right-angle bend is sometimes expressed as the equivalent of a straight length of flue of a certain length for a certain diameter, similar to the procedure used in estimating the loss due to bends in piping systems conducting water. Most flues, however, par ticularly breechings, are built square or rectangular in section and no general equation based on the shape of the flue can be'conveniently expressed. The draft loss due to sudden contraction of an area (he) is given by the equation: 0.000194JKeJF`Tc he = A\BaWc (19) where Kc = coefficient of sudden contraction based on -r^f the ratio of the areas of the A\ smaller to the larger section = 0.5 ^ 1 -- ^ . -f xL.______tl___ When the flue or passage through which the gases flow is suddenly contracted, a considerable portion of the static head in the larger section 477