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American Society of Heating and Ventilating Engineers Guide, 1936 their normal rating. Natural draft systems have been, and are still being, employed in the operation of large plants during the periods when the boilers are operated only up to their normal rating. When the rate of operation is increased above the 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 as follows: (1) simplicity, (2) reliability, (3) freedom from mechanical ' Fig. 1. General Operating Characteristics of Typical Induced Draft Fan parts, (4) low cost of maintenance, (5) relatively long life, (6) relatively, 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 the pressure difference is the result of a suction, and also the forced draft system in 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, 452 Chapter 26--Chimneys and Draft Calculations Fig. 2. Operating Characteristics of Typical Centrifugal Pump atmospheric pressure, respectively. A mechanical draft system may be used either in conjunction with, or as an adjunct to, a natural draft system. CHARACTERISTICS OF CHIMNEYS In order to analyze the performance of a natural draft chimney, it may be advantageous to compare its general operating characteristics with those of a centrifugal pump and also of a centrifugally-induced draft fan, there being a 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 Fig. 3. Typical Set of Operating Characteristics of a Natural Draft Chimney 453