Filtering Devices

Gas Compressors

These machines are employed for bringing gases to a higher pressure level and for transporting them. The number of types of compressors is large: rotary compressors, piston compressors, diaphragm compressors and others.

Piston compressors have in principle the same construction as piston pumps. But the piston speeds and the number of strokes per unit of time are much higher. These compressors can be constructed for any capacity and delivery pressure.

The most usual type is that with lubricated cylinders. The presence of solvents in the gas does not present so much difficulties in this case as in the rapidly rotating vane compressor. But there are cases when no lubricating oil is admitted into the cylinders, e.g. in compressing oxygen. Some other lubricant must then be used or the cylinders must not be lubricated at all. In the latter case the cylinder liner or the piston ring must be made of graphite. The maximum speed is about 200 ft/min. and the maximum pressure is about 300 lb/min.

Diaphragm compressors is of almost the same construction, as the diaphragm pipe for liquids. The diaphragm can be flat or shaped as a tube or bellows and may be constructed of steel or rubber. The absence of the leaky stuffing box makes these compressors suitable for handling valuable or toxic gases. But it is expensive and vulnerable.

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The separation of comparatively small amounts of solids from larger amounts of liquids is done by settling, followed by decantation of the supernatant clear liquid, or by the use of a filtering device. The separation of very small amounts of liquids from large amounts of solids is done by draining, or by centrifuging.

Settling out the solid is the cheapest method, and is frequently used. If the operation is extensive, a large capacity in settling tanks is required: these act also as storage tanks. If the liquor is hot, the tank may be insulated by strips of wood. As a rule, this process is too slow; also the mud collected in the bottom of the tank still contains too much water, and is usually sent through a filter, a combination of settling and filtering.

Filtering devices include gravity filters, pressure filters, more commonly called filter-pressers, and suction filters. Several points must be considered when studying a filtration problem: whether the cake or the filtrate is the valuable part, whether or not washing is required, and whether the operation should be continuous or intermittent. These considerations will have weight in the choice of the device.

The suspension to be filtered is called a slurry, if its content in solids is not so high that it does not flow and cannot be pumped; slurries contain from less than 1 per cent to 40 per cent of solids are more properly called sludges. The solid separated in the filter device is called the cake, the clear liquor running off is the filtrate. filtering devices such as filter-presses and suction filters are really frames for the support of the filtering medium, the filter cloth, which may be cotton duck of various thicknesses, muslin, paper, wool flannel, or metal wire woven into cloth, such as iron wire, nickel wire, and monel wire.

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Дата добавления: 2014-10-17