Document Dv2YzD4a5dkjwzLBRz772kkma

Heating Ventilating Air Conditioning Guide 1938 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 N 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, as the system used in-its production. creates a pressure difference below, or above, atmospheric - 180 Chapter 10. Chimneys and Draft Calculations Fig. 2. Operating Characteristics of Typical Centrifugal Pump pressure, respectively'. A mechanical draft system may be used either in conjunction with, or as an adjunct to, a natural draft system. 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 181