Document 0gRj8XLR98R7npn3bqDaaJYrJ

5S899KPS-*' Heating Ventilating Air Conditioning Guide 1939 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 0f* operation is increased above the normal rating, some form of mechanics 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 Chapter 10. Chimneys and Draft Calculations essure, respectively. A mechanical draft system may be used either in conjunction with, or as an adjunct to, a natural draft system. Fig. 1. General Operating Characteristics of Typical Induced Draft Fan i 1 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 % l,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 J 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 draftj systems tend to produce a vacuum or a plenum, as the system used in itsj | production creates a pressure difference below, or above, atmospherics 186 I Draft Control To obtain the maximum efficiency of combustion, a definite minimum supply of air to the combustion chamber must be maintained. To pro vide this condition, it is necessary to have some mechanical means of draft control or adjustment, because of variable wind velocities, fluctua tions in atmospheric temperatures and barometric pressures, and their effect upon draft. For this purpose there are various mechanical devices which auto matically control the volume of air admitted to the combustion chamber. Mechanical draft regulators designed to control or adjust draft, should Fig. 3. Typical Set of Operating Characteristics of a Natural Draft Chimney 187