264. The object of filtration is to arrest dust, smoke, etc., and prevent it from passing into the building. The apparatus employed is of two classes: wet and dry.

A wet filter consists of a coarse netting, which is stretched across the airway and is kept constantly wet or moist. The net may be made of small rope or other rough fiber, and should have a mesh not larger than 1 inch. The water should be allowed to trickle down over it constantly, keeping it wet enough to make the dust adhere wherever it touches. The net gradually becomes loaded with dirt, which requires to be washed off. This may be done automatically by means of an ordinary periodical flushing tank, arranged to empty a liberal supply of water over the screen.

265. A dry screen for a large airway may be constructed as shown in Fig. 87. A set of inclined screens a, a are supported upon transverse bars b and c. These screens are made of wire netting, having a mesh of 2 inches or more; and their purpose is to support other screens made of cheese cloth or light muslin. They are fixed in place, and do not need to be removed for cleaning.

Fig. 87.

The cloth filters are made in the form of V-shaped bags, as shown at d. They are secured in place by fastening the front edges to the bars b; the air entering at the open mouth inflates them so that they lie tightly against the wire screens a. The total area of filtering surface thus exposed should be 8 to 10 times the sectional area of the airway. The filter bags must be removed at intervals and emptied of dust, and they should also be thoroughly washed and dried before they are used again.