Document rBqByJ1Vode6gNn8rr6mo84kv

American Society of Heating and Ventilating Engineers Guide, 1934 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. Ba -- 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 ldngth 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 (hv) is given by the equation hv 0.000194 W*TC A*B0WC (16) 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 (hub) 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: fiBd 0.000194W*TC A*B0WC (17) 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: 360 Chapter 26--Chimneys 0.000194XctFir,c hc = AlB0Wc (18) where Kc = coefficient of sudden contraction based on smaller to the larger section. As -- area of the smaller section. the ratio of the areas of the When the flue or passage through which the gases flow is suddenly contracted, a considerable portion of the static head in the larger section is converted into velocity head and a draft loss of some consequence, par ticularly in a short breeching, takes place. A sudden contraction should always be avoided where possible. At times, however, due to obstruc tions or limited head-room, it is necessary to alter the size of the breeching, but a sudden contraction may be avoided by gradually decreasing the area over a length of several feet. The draft loss due to a sudden enlargement of an area (ho) is given by the equation: 0.000194.go fF*rc ho AlB0Wc (19) where K0 = coefficient of sudden enlargement based on -r~, the ratio of the areas of the At smaller to the larger section. When the flue or passage through which the gases flow is suddenly enlarged, a portion of the velocity head is converted into static head in the larger section and, like the loss due to sudden contraction, a loss of some consequence, particularly in'short breechings, takes place. A sudden enlargement in a breeching may be avoided by gradually increasing the area over a length of several feet. In large masonry chimneys, the area of the flue at the region of the breeching entrance is considerably larger than the area of the breeching at the chimney, and a sudden enlargement exists. The draft loss through the economizer (h&) should be obtained from the manufacturer but for general purposes it may be computed from the following general equation: te 6.6 WnNTQ 10s (20) where IFn = pounds of gases flowing per hour per linear foot of pipe in each economizer section. N = number of economizer sections. An economizer in a steam plant affects the draft in two ways, (1) it offers a resistance to the flow of gases, and (2) it lowers the average chimney gas temperature, thereby decreasing the available intensity. In the case of a natural`draft installation, both of these factors result in a relative increase in the height of the chimney and, in the case of a large 361