Document RarrdJ8r6dwjNB2bKvJRe7q8k

American Society of Heating and Ventilating Engineers Guide, 1932 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 (/tBd) 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: , 0.000194rrc ted = AVoWT" (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: 0.00Q194gcH,,rc A\B0Wc (18) where Kc = coefficient of sudden contraction based on A1 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: o.oooi94jc:ow*rc ho = AlBoWc (19) where Ko = coefficient of sudden enlargement based on the ratio ot the areas ol the 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 246 Chapter 15--Draft and Chimneys 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 (hE) should be obtained from the manufacturer but for general purposes may be computed from the following general equation: Ae 6.6iy'wrc (20) where Wa - 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) Offers a resistance to the flow of gases, and (2) 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 plant, may add as much as 20 to 30 ft to the height. The decrease in the temperature of the gases after they have passed through the economizer has an extremely important effect on the performance of a natural draft chimney and also a fan. The draft loss through recuperators and regenerators (hR) is generally relatively small and may be disregarded. However, when it is evident that these losses are of some consequence, the friction loss through the regenerator may be calculated from Equation 15 and the losses due to sudden contraction and expansion from Equations 18 and 19, respectively. CONSTRUCTION OF CHIMNEYS For general data on the construction of chimneys reference should be made to the Standard Ordinance for Chimney Construction of the National Board of Fire Underwriters. Briefly summarized, these provisions are as follows for heating boilers and furnaces: The construction, location, height and area of the chimney to which a heating boiler or warm-air furnace is connected affect the operation of the entire heating system. Most residence chimneys are built of brick and may be either lined or unlined, but in either case the walls must be air-tight and there should be only one smoke opening into the chimney. Cleanout, if provided, must be absolutely air-tight when closed. . The.walls of brick chimneys shall be not less than 3% in. thick (width of a standard size brick) and shall be lined with fire-clay flue lining. Fire-clay flue linings shall be manufactured from suitable refractory clay, either natural or compounded, and shall be adapted to withstand high temperatures and the action of flue gases. They shall be of standard commercial thickness, but not less than % in. AH fire-clay flue linings shall meet the standard specification of the Eastern Clay Products Association. The flue sections shall be set in special mortar, and shall have' the joints struck smooth on the inside. The masonry shall be built around each section of lining as it is placed, and all spaces between masonry and linings shall be completely filled with mortar. No broken Hue lining shall be used. Flue lining shall start at least 4 in. below the bottom of smoke- 247