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The weight of quantity of solids which air or gas can carry varies directly as the velocity; the higher velocity, the greater the carrying capacity. The velocity of a dust-laden gas travelling in a pipe may be lowered by enlarging the pipe; at such enlarged places, the gas will drop part or all of its load. The enlargement need not be very great to produce a considerable reduction, for example, if a 6 inches pipe is widened to 24 inches a velocity of 160 feet per second becomes 10 feet per second. The dust collector is such an enlargement. It is made up of two truncated cones; the upper cone flares out downward; the lower cone meets the upper one and flares in downward, so that the box is widest half-way up (посередине). The pipe bringing the gas is so placed that the air is delivered along the wall and acquires a circular motion as it travels spirally around and down the box. The dust drops out and collects at the bottom, while the comparatively clean gas leaves by an internal pipe set halfway down the collector. The size of the dust collector must fit the volume of air or gas handled, so that the required reduction of velocity is reached.
The bag filter is a dust-arresting device consisting of bags made of cloth, with a number of them within a housing so set up that the dirty air enters the inner part of the bag, depositing its solids there, while the clean air passes through, or so that the dirty air surrounds the bags and deposits its load on the outer side, while clean air travels through to the inner side, to be pulled away by a fan. Alternatively, the bag filter may be fed by a blower, so that the dirty air is under pressure, and the clean air also. In any scheme, the dust is arrested by the cloth, and the coating is removed periodically by rapping or shaking the bag; the housing is mounted on hoppers in which the deposits accumulate. As example of the bag filters, the multi-bag filter will be described. Its individual bags are tall cylinders 6 inches across and 9 feet 6 inches high, made of cotton sateen cloth; both napped and unnapped cloth (ворсистая и неворсистая ткань) are used. Each cylinder bottom fits over a collar in the floor, and is held in place by a quick detachable bottom band, while its top is fastened to the bag-shaking shaft. Three individual bags are sewn together as one element is referred to as a single filter bag; it measures 17 inches across. The dust-laden air travels over the hoppers to the inside of the bag, and is pulled through its meshes which retain the dust in the form of a coating. Periodically, a motor-operated shaker mechanism gives the bags an upward and downward motion, loosening the coat of dust, which drops to the hoppers below. The bags are mounted in a housing, which is on the suction side of fan, so that any repair necessary may be made in dust-free air.
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