Document 71VLEnMooXk7MobDG6x69Rmp6

'836 CHAPTER 46 1946 Guide theories govern both classes. The type of hood and exhauster, the adap tion of general ventilation, the conveying velocities and duct construction differ substantially. Exhaust systems are also classified by the means employed to collect the material. The dust or refuse may be .collected and controlled by enclosed hoods or open hoods with positive inward air movement or by diluting or exhausting the general air of the room. With some classes of machinery it is not feasible to hood the machines closely and in these cases open hoods over or adjacent to the machines are provided to collect as much as possible of the dust and fumes. This class includes such machines as rubber mills, package filling machinery, sand blast, crushers, , forges, pickling, tanks, melting furnaces, and the unloading points of various types of conveyors. Open hoods should be placed as close to the source of dust or fumes as possible, with due regard to the movements of the operator, and should be placed so that the operator is in no case in the path of the exhausted material. When the hood must be placed at some distance above the machine-it should be large enough to cover a large area as diffusion is usually quite rapid. Some consideration should be given to the natural movement of the fumes. For those that are lighter than air, the hood may be over or above the machine; and where a heavy vapor, or dust-laden air at ordinary temperature is to be removed, horizontal or floor connections are some times preferable. In many cases there are convection currents and other atmospheric disturbances in the work room which should be given con sideration. These disturbances diminish the tendency of dust and fumes to settle from the room air because of their density. In another class of operation the main objective is to prevent the escape of dust into the surrounding atmosphere, and the removal of some dust from the machine or enclosure may be merely incidental. The dustcreating apparatus is enclosed within a housing which is made as tight as practicable, and sufficient suction is applied to the enclosure to maintain an inward air leakage, thus preventing escape of the dust. While the exhaust system is required to handle only the air which leaks in through the crevices and openings in the enclosure, yet in many installations leakages are very high and great care is required to obtain satisfactory results with a system of this kind. The inward-leakage. principle is . utilized for controlling dust in the operating of tumbling barrels, grinding, screening, elevating, and similar processes. Certain dust and fume producing operations are best carried on by isolating the process in a separate compartment or room and then apply ing general ventilation to this space. The compartment or room in which ' the work is performed should be as small as is consistent with convenience . in handling the work. The ventilating system should be designed so that a current of clean air is drawn across the work in such a manner as to carry the.dust or fume away from the operator and out of the work space. Another method of accomplishing the control of this type of installation, . and one usually applied in the case of gases and fumes, is the dilution method; In this case sufficient clean air is introduced generally into the work space tp dilute the contamination ,to a safe level. : . HOOD DESIGN PRINCIPLES2-3 The first step in the design of an exhaust system is to determine the number and size of the hoods and their connections. No general rules, Exhausting and Conveying Systems 837 however, can be given since hood and duct dimensions' are determined by the .characteristics of the operations to which they .are applied. When a tentative decision regarding the set-up has been made, it is then necessary to obtain the suction and air velocities required to effect control. At this point the designer must rely upon the prevailing practice and on such physical data relating to hoods, duct systems and collectors as are avail able. The fan speed must be sufficient to maintain the estimated suction and air velocities in the system.^ In general, the most important require ments4.of an efficient exhaust and collecting systemare: 1. Hoods, ducts, fans, motors and collectors should be of adequate size and type. 2. The air velocities should be sufficient to control and convey the materials collected. 3. The hoods and ducts should be placed so as not to interfere with the operation of a machine or any working part. ' 4. The system should do the required work with a minimum power consumption. 5. When inflammable dusts and fumes are conveyed, the piping should be provided with an automatic damper in passing through a fire-wall. 6. Ducts and all metal parts should be grounded to reduce the danger of dust ex plosions by static electricity. 7. The exhaust system should be readily accessible for inspection and care. ` HOOD SUCTION AND INDRAFT VELOCITIES The removal of dust or waste by means of an exhaust hood requires a movement of air at the point of origin sufficient to carry it into a collecting system. The air velocities necessary to accomplish this depend upon the physical properties of the material to be eliminated and the direction and speed with which it is thrown off. If the dust to be removed isalready, in motion, as is the case with high-speed grinding wheels, the hood must be installed in the path of the particles so that a minimum air volume may be used effectively. It is always desirable to design and locate a hood so that the volume of air necessary to produce results is as small as possible. This will reduce the size of equipment, the power required by the system, and also the heating load requirements in winter. Air Flow from Static Readings The static suction at the throat of a hood is frequently used in practice as a measure of,the effectiveness of control. Where the hood coefficient is known the volume of air flow through any. hood may be determined from the equation: Q = 4005 / A V~ht (1) where Q -- volume of air flow, cubic feet per minute. , A = area of connecting duct, square feet. At = static suction measured 3 diameters from throat of hood, inches of water. = orifice or restriction coefficient which varies from 0,6 to 0.9 depending on the shape of the hood.... An average value of/is 0.71, although for a well-shaped opening a value of 0.8 may be used. The factor f is determined from the equation: ; /= '1 - : (2) where &v is the velocity head in the connecting duct. The static suction is not a good measure of the effectiveness of a/hood unless the area of the opening and the location of the operation with