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American Society of Heating and Ventilating Engineers Guide, 1935 Chapter 26--Chimneys and Draft Calculations *|d `OM3I3UJ3 YVSlNVtOii* operation is increased above their normal rating, some form of mechanical draft is employed as an auxiliary to overcome the increased resistances or draft losses. Natural draft systems are used almost exclusively in the smaller size plants where the amount of gases generated is relatively small and it would be expensive to install and operate a mechanical draft system. The principal advantages of natural draft systems may be summarized i as follows: (1) simplicity, (2)'reliability, (3) freedom'from mechanical parts, (4) low cost of maintenance, (5) relatively long life, (6) relatively j low depreciation, and (7)-no power required to:operate. --The principal- disadvantages are": (1)"lack of flexibility, (2) irregularity, (3) affected by surroundings, and (4) affected by temperature changes. Mechanical Draft . Artificial draft, or mechanical draft, as it is more commonly called; is a difference in pressure produced either directly or indirectly by, a forced draft fan, an induced draft fan, or a Venturi chimney as the pressure transformer. The intensity.of mechanical draft is dependent for the most part upon the size of the fan and the speed at which it.is operated. The element of temperature does not enter into the creation of mechanical draft and therefore its intensity, unlike natural draft, is independent of the temperature of the gases and the atmosphere.. Mechanical draft includes the, induced .'and Venturi types, of draft systems in . which thepressure difference is the result of a suction and also, the forced draft system in 1 which the pressure difference is the result of a blowing. Mechanical draft systems tend to produce a vacuum, or.a plenum, according as the system used in its production creates a pressure difference below, or above, atmospheric pressure, respectively. A mechanical, draft system may be used either in conjunction with, or as an -adjunct to', a natural' draft I system. CHARACTERISTICS OF CHIMNEYS 1*04 intOM SMV * * ' " xmJ3 3d xaWttxO S 3 ' t ` 8 8 9 _ <o-w,*'r o**y**is 424 HWUU<. < *- UQ go . 5Ha<f6xs] tUad o <H CWS >cu' Zw H o! ; In: order to analyze the performance'of a natural draft'chininey-,: it is advantageous to compare its general operating characteristics with thos4 of-a-centrifugal pump and also a cehtrifugally-induced draft fan', there being a close similarity among the three. Figs. 1, 2 and 3 show the general operating characteristics of a typical centrifugally-induced draft fan, a typical centrifugal pump, and a typical natural draft chimney, respectively. The draft-capacity curve of the:Chimney corresponds to the head-capacity curve of the pump and also to the dynamic-head capacity of the fan; the efficiency curve of the chimney to the efficiency curves of the pump and fan; and the gas horsepower curve of the chimney to the brake" horsepower curves of the pump and fan. When the gases in the chimney are stationary, the draft created is termed the theoretical draft. When the gases are flowing, the theoretical intensity is diminished by the draft loss due to friction, the difference between the two being termed the available draft. The general equation for the available draft'intensity of a natural draft chimney with a circular section is as follows: M12&W'TcfL D`BaWc (1) 425