Document gaxOLkG5DjRKNgbgjKjzvRr83

American Society of Heating and Ventilating Engineers Guide, 1934 value of 5 gpm may be Used. The nozzles are spaced on one foot centers across the top of the washer over the scrubber plates. Water is supplied to spray type units through atomizing nozzles gener ally arranged in banks across the washer. The nozzles spray either in the direction of the air flow, that is, downstream, or against the air flow, or upstream. Nozzle capacities varies with the manufacturer, from 1-J^ to 2 gpm at a water pressure of about 25 lbs per square inch which pressure is required for effective atomization. The spacing of spray nozzles is determined by the water requirements of the particular installation. A spray type washer may contain one, two or three banks of nozzles depend ing upon its application. When an air washer is used for cleaning air it removes impurities and dusts. In general it does not function as efficiently in this service as a filter. For non-microscopic soluble dust its efficiency averages about 50 per cent, unless the concentration of dust is high. Its effectiveness in removing greasy microscopic dust is practically negligible as is also its deodorizing ability. When a washer is used to regulate the moisture content of air it adds moisture to (humidifiers) or removes moisture from (dehumidifiers) the air to achieve the desired moisture content. (See also Chapter 3). When air passes through a washer wherein water is circulated without the addition or removal of heat, the air tends to become saturated at its entering wet-bulb temperature. What occurs here is.partial or complete adiabatic saturation. The total heat content of the air is unchanged, inasmuch as the dry-bulb temperature of the air drops in proportion to the amount of additional water evaporated. This action is also known as evaporative cooling. A measure of the washer's effectiveness under these conditions is its saturating efficiency which is equal to the drop in drybulb temperature in per cent of the entering wet-bulb depression. Other things being equal, the saturating efficiency of a spray type washer is a function of the number of spray banks and the direction in which they spray. The following table gives a general comparison: 3 banks--2 upstream--1 downstream.-........ ..... 100% saturation efficiency 2 banks--2 upstream............................................ . 95% saturation efficiency 2 banks--1 upstream--1 downstream--.............. 85% saturation efficiency 1 bank--upstream...... .............................................. 80% saturation efficiency 1 bank --downstream............................................... 65% saturation efficiency When air passes through a washer wherein the circulated water is either cooled or heated before being returned to the spray chamber,. a heat interchange between the air and water occurs, and the air tends to become saturated at the temperature of the leaving water. The extent to which the leaving air and leaving water temperatures approach each other is an index to the effectiveness of the washer under the operating conditions. The total heat absorbed by the water in the process equals the total heat given up by the air or the heat given up by the water equals the heat absorbed by the air. Depending on whether the moisture con tent of the air is increased or decreased during the operation, humidifi cation or dehumidification occurs. Heat will be added to or removed from the air as the water supplied is of a higher or a lower temperature than the wet-bulb temperature of the entering air. 148 Chapter 11--Humidifying and' Dehumidifying Equipment For dehumidifiers the ratio of the difference between the leaving wetbulb and the leaving water to the difference between the entering wetbulb and the entering water may be figured as follows: 3 banks--1 downstream--2 upstream------- 1 bank --upstream.--...................................... 1 bank --downstream...................................... .......................... 0% .......................... 5% -........................15% ..........................20% ......................... 35% Humidifiers may be figured on the same basis as dehumidifiers; the leaving water temperature, of course, will be higher than the wet-bulb temperature of the leaving air. note that heating is generally accomplished by passing the water through closed hot water heaters or by injecting steam into the water circuit; cooling, by passing the water through closed coolers or over refrigerating coils in a baudelot chamber. Often in a cooling and dehumidifiying application, the refrigerating coils are located within the washer chamber. Washers are sometimes arranged in two or more stages to cool through long ranges or to increase the overall efficiency of heat transfer between air and the cooling or heating medium (water, brine, etc.) A multi-stage washer is equivalent to a number of washers in series arrangement. Each stage is in effect a separate washer. Usually the catalog capacity of a washer is expressed in cubic feet of air per minute and is based upon an air velocity of 500 feet per minute through the gross cross sectional area of the unit above the water level in its tank. At this rating spray type washers handle about 2-j/<> gpm of 149