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592 CHAPTER 41 1959 Guide element of a unit humidifier is an atomizer or evaporator. To this may be added a fan, a heater, outlet vanes or diffusers, and a housing to enclose the various parts. Unit humidifiers fall into four general classifications, de pending on the method of causing evaporation. These are as follows: (1) Nozzle Type, (2) Rotary Type, (3) Cascade Type, and (4) Heater Type. In the nozzle type of humidifier water is sprayed into the air and evaporation is effected by adiabatic exchange of en ergy. Unite of this type in simplest form, spray a fine mist of water directly into the air in a space. They are used to a great extent in the textile industry. The rotary type of humidifier generates and distributes by mechanical separation using rotating discs. In all other respects, this type of humidifier is similar to the nozzle type. It has the advantage over the nozzle type of being less likely to become clogged. In the cascade type the humidification takes place by wa- ' ter falling in sheets over a series of baffles or trays. This type is usually furnished with a fan, air heater, and air filter, all enclosed in a housing. In the heater type of humidifier the water is heated either to the bailing point or to a temperature at which the water vapor readily passes into the air stream. In the simplest form the heating element using steam, hot water, gas, oil, or elec tricity is placed in a pan or vessel of water, and the vapor passes from the surface of the water to the stream of air. A modification of this type of humidifier is the combination of spray nozzle and heater type in which the water is sprayed over a hot surface and evaporated. It has the disadvantage of accumulating scale on the surfaces of the vessel or heating sur face. Unit humidifiers are usually controlled by a two-position room humidistat operating a controlled device which func tions to start and stop the humidifying action. This may be a valve on the water supply to the humidifier. The fan, if used, normally is also turned off by the humidistat. The con trolled device operates the source of heat in the heater type of humidifier. See Chapter 43 for a general discussion of hu midity controls. AIR WASHERS Spray-Type Washers A spray-type air washer consists essentially of a chamber or casing containing a spray nozzle system, a tank for col lecting the spray water as it falls, and an eliminator section at the discharge end for removal of entrained drops of water from the air. A pump recirculates water at a rate greatly in excess of the evaporation rate. Intimate contact between the spray water and the flowing air causes heat transfer between tiie air and the water, resulting in either humidification or dehumidification, depending upon the method of operation and the relative temperatures of air and spray water. To prevent backlash of spray ahead of the washer chamber, and to provide more uniform air distribution, perforated in let plates, horizontal louvers or vertical eliminator baffles may be provided in the washer inlet. Inlet diffusion plates may be required when the air and water spray are in the same direction because eddy currents may be present. Elimi nator baffles are used where one or more of the water sprays opposes the air flow and is sufficiently close to the entering end that water might come out. At the outlet end of the washer, suitable eliminator, plates are placed and may or may not be provided with flooding devices. These plates or blades, for the removal of entrained moisture, may cause four to six directional air changes and present as many as four effective hooked edges for trapping the water. Blades usu ally have bends at 30 deg to the direction of air flow. Sharper bends add substantially to the resistance without improving the elimination materially. Vertical type baffles may be pro vided with flooding devices where the air contains large quan tities of particulate matter or lint, as in cotton mills, to pre vent build-up on the baffle blades. Derigoofion Table 1.... Classification of Apparatus Used for Humidification and Evaporative Air Coding HomMtficoKon fvoporofive Air Cooling indirect Direct Primary Wef Surfaces Dry Surface or Secondary Cool Surface Two Stage Residential Commercial or Indus trial 1. Evaporating pans with "wick action" water transport. 2. Externally heated evaporator pans (electric or steam heat). 3. Porous, extended surface pad types. 1. Atomisers or sprays. 1. Drip type, usually nonmetal pads. 2. Slinger type, usu ally metal pads. 3. Rotary type, usu ally metal screens. 1. Blast coils cooled by cool ing tower water. 1. Air washers, spray and fill types. 1. Pneumatic atomisation. 2. Pressure atom isation. 3. Centrifugal atomization. 4. Steam jets. 1. Drip type, usually non-metal pads. 2. Slinger type, usually metal pads. 3. Spray or washer type. 4. Rotary type, usu ally metal screens. 1. Blast coils cooled by cool ing tower water. 1. Secondary cool surface cooling followed by pri mary wet sur face or spray cooling. Evaporative Air Cooling and Humidification PLAN 593 installation. In general, the width and height of an air washer will depend on the space available. Washers of nearly equal height and width are desirable from an air flow and economic standpoint, although these proportions are not necessary. The length of washers varies considerably. A space of ap proximately 2/z to 4Vz ft between spray banka is used, and the first and last banks of sprays are located about 1 to 114 ft from the entering or leaving end of the washer. In addition, air washers may be furnished with heating or cool ing coils within the washer chamber. These coils may affect the overall length of the washer. Spray water requirements Iot spray-type air washers used for washing or evaporative cooling vary from 4 gpm with a single bank to 10 gpm for double banks per 1000 cfm. Pump ing heads will usually be in the range from 55 to 80 ft of water Figs. 1 and 2 show the essential construction features of conventional spray-type air washers. Intimate contact be tween the air and water is secured (1) by breaking the water into fine drops, (2) by passing the air over the surfaces con tinuously wetted by water, or (3) by a combination of the two. Essential requirements in the air washer operation are: uniform distribution of the air across the spray chamber, air velocities of from 400 to 700 fpm in the washer chamber, an adequate amount of spray water broken-up into fine drop lets at pressures of from 15 to 30 peig, good spray distribu tion across the air stream, sufficient length of travel through the spray and wetted surfaces, and the elimination of free moisture from the outlet air. Expected performance, physical size, length, number of sprays, etc., vary greatly depending on the functions of the fig. 2------Typicol Two-Bonk Air Washer depending on spray pressure, height of apparatus, pressure losses in pipe and strainers, etc. The resistance to air flow through an air washer varies with the type and number of baffles, eliminators, and wetted surfaces; number of spray banks and their direction; air velocity; size and type of other resistances such as cooling and heating coils; and other factors such as air density. Re sistance may be as low as V* in. or high as I in. water column. It is therefore necessary that the manufacturer be consulted with regard to the resistance of any particular washer de sign combination. Fill-Type Washers Other air washer types achieve contact between air and water by passing the air through cells packed with glass fibers or sheets of specially designed media. See Fig. 3. These wash ers have the cells arranged in tiers over which water is dis tributed at low pressure. The cells are followed by conven-