Book - School of Science and Technology

496 Fans and air treatment equipmentEliminator platesSpray headerSpraynozzlesPerforatedinlet plateEliminatorwash-downVisionpanelAir flowPumpWater tankOverflowStrainer tankFigure 17.24 Air washer, single bankspray typeThe casing has an access door and internal illumination, and may be of galvanised steel,or constructed of brickor concrete, asphalted inside.As the mist-laden air is drawn through the chamber it requires to have the free moistureremoved, and for this purpose eliminator plates of zigzag formation are provided to arrestthe water droplets. Some makes precede these with scrubber plates, down which water iscaused to run by a spray pipe at the top in order to flush down dirt which has collectedfrom the air. Galvanised eliminator plates are liable to rapid deterioration; better materialsare copper and stainless steel. Glass plates with serrated ribs have been used successfully,and they are permanent.A single bankair washer will not saturate the air more than that corresponding toabout 70% of the wet bulb depression. A double bankwasher may reach 90%. Therecirculated water may be cooled or heated for full air-conditioning and dehumidification.Where it is desired to saturate at the dew-point, a two-bankwasher is generally necessarywith large spray nozzles to pass the necessary quantity of water for the temperature riseallowed. Spray nozzles vary in capacity from 0.05 to 0.25 litre/s, and should be easilycleanable and of non-corrodible metal.The air velocity through an air washer is usually 2.5±3 m/s. The length is normallyabout 2.5 m, but is increased with the two or more banks of sprays needed for cooling ±sometimes to as much as 4.0 m. On the inlet side straightening vanes, or a perforatedgrille, are required in order to distribute the air evenly over the whole area. The tankiskept filled by a ball-valve, with a hand valve for quick filling, and there is in addition adrain and overflow pipe. The pump is of normal type, preferably with flexible connectionswhere noise may cause trouble, and arranged to be flooded by the water in the tank, inorder to avoid the need for priming.Apart from removing heavier and gritty material a spray washer has a low air filtrationefficiency, but is effective in adsorbing certain gases, such as sulphur dioxide, from the air.Capillary washersOne arrangement of this type is shown in Figure 17.25. Typically such units comprise oneor a series of cells inclined at an angle, or vertical, and containing corrugated aluminium