Document qvLR55GnqbgZ2LOYO8z26d9G

American Society of Heating and Ventilating Engineers Guide, 1932 controlled by enclosing hoods, open hoods, inward air leakage or by exhausting the general air of the room. With some classes of machinery it is not feasible to closely hood the machines and in these cases open hoods over or adjacent to the machines are provided to collect as mufch 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. The 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. When the hood has to be placed at some distance above the machine it should be large enough to encompass an area of considerable extent as diffusion is usually quite rapid. Consideration must also be given to the natural movement of the fumes. For those that are lighter than air the hood should 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 required. If it is attempted to remove heavy dust such as lead oxides by an over head hood the conditions may be worse than if no exhaust were used at all, owing to the rising air current carrying the dust up through the breathing zones. The principle to keep in mind in all cases is to take advantage of the natural tendency of the material to move upward or downward. In another class of operation the main object is to prevent the escape of dust into the surrounding atmosphere, the removal of some dust from the machine or enclosure. being merely incidental. The dust-creating apparatus is enclosed within a housing which is made as tight as prac ticable, 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 only required to handle 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 strong current of clean air is drawn across the operator, and away from him toward the work, where the dust is picked up and carried from the room. REQUIREMENTS OF EFFICIENT SYSTEM The most important requirements of an efficient exhaust and collecting system are as follows: 1. Fans, collectors, hoods, and ducts should be of adequate size. 2. Air volume and velocities should be adequate for the work to be accomplished, ; 346 Chapter 25--Ventilation of Industrial Buildings; Exhaust Systems \ 3. The exhaust hoods should not interfere with the operation of the machine or with access to its working parts. , 4. The system should not increase the fire hazard. -- 5. The system should not increase the dust explosion hazard. 6. Where power is expensive, it should do the required work with a minimum power consumption. 7. In cold climates, it should not remove any more air than necessary from the building. 8. Where power is comparatively cheap, first cost should be low, even if the power required to operate is slightly higher. DESIGN OF EXHAUST SYSTEMS The first step in designing systems employing hoods to trap the material is to determine the number and size of the connections for each individual machine. At this point the designer's past experience is of great value, as, while it is possible to set certain general standards, yet in actual practice the sizes are considerably affected by the local conditions which the lay out man finds in the field, and he bases the pipe sizes, and hoods on his judgment, being guided by his experience and the general practice. The size of hoods and connections are determined by the size and type--, of machines or apparatus to be handled by the exhaust system, by the kind of material worked, by the duty of the machines and by other local conditions. It is impracticable to lay down any general rules for deter mining size connections for the various types of machines and Tables 1 to 3, giving sizes as used in some of the common industries are only intended to serve as a general guide. Under certain favorable conditions smaller connections may be supplied. Open bottom exhaust hoods of the canopy type, where it is impractical to enclose completely the point of origin of the dust or fumes, should extend over the machine or operation at least 12 in. in every direction if the hood is not elevated more than 2 ft. For each additional 2 ft of elevation, the size of the hood should be increased at least 12 in. in all directions. It is desirable to make the area of the connecting pipe not less than one-fifteenth of the total hood area. In systems employing inward air leakages the area of connections must be proportional to total leakage area in the enclosing housing. Tumbling barrels have connections ranging from 4 to 8 in., bucket con veyors 6 to 12 in., and screening machines 6 to 10 in. In general room exhaust, large connections should be provided so that the air may be handled at low velocity and with a minimum power consumption. Air Velocities Required After having determined on the proportions of the exhaust system as regards hoods and connections it is then necessary to choose' the air velocity or suction at the hood connections; suction at the hood con nections being a measure of the air velocity.at that point. The air velocity required is dependent upon the specific gravity of the material, the fineness of the particles, and their physical characteristics. 347