Bag Filters

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.

Text 26

Coolers

In chemical engineering, water cooling is widely used and gas washing, quenching and atomizing are equally important accompaniments of process apparatus.

Except in comparatively few cases where an ample supply of pure cool water is available, it is necessary to cool artificially the water used for industrial purposes, such as for continuous process work in chemical plants. Methods generally available for cooling water include: a natural pond of large area, a cooling tower, or a spray cooling plant arranged over a pond. Where water supply is abundant, or where there is sufficient space, the first method may be adopted; but for ordinary industrial establishments this is generally impossible because of the large space required for the pond. Moreover, this is a very inefficient method of dissipating heat contained in water because a still pond limits the surface exposed to the air for cooling; unless the air temperature is considerably lower than that of the water, cooling is slow. In most places a natural cooling pond system cannot be used in hot weather. A cooling tower occupies less space than a natural pond and is more suitable.

With a cooling tower, the water is introduced into the top of the tower and flows either over a series of baffles, or over packing, commonly in the form of wooden slats and the interfacial area is increased. The air current is produced either by natural convection caused by the lower density of the gas within the tower, or by means of a forced draught produced by a fan of blower. In most cooling towers, the air current is in an upward direction and counter-current flow of the air and water is obtained. Sometimes, however the air current is produced by the wind blowing horizontally into the tower. Counter-current flow is not then obtained but fresh air is introduced throughout the length of the tower. Higher gas velocities are obtained in forced draught systems and therefore the transfer coefficients are greater.

Text 27