Document 7Oag20w88Vo8YgQndNoVX8Zyj

American Society of Heating and Ventilating Engineers Guide, 1926-27 MEASUREMENT OF AIR FLOW The quantity, velocity and pressure of air discharged by a fan or flowing through a pipe may be determined by various methods. An anemometer is used where accuracy is not required and where air velocities not over 600 ft. per min. are to be measured, as at registers. For the greatest, measure of reliability the anemometer shail have been newly calibrated, and correction shall be made for the error as shown by the calibration. The standard method for measuring air velocity and pressure shall be the pitot tube as described in the A. S. H. & V. E. Standard Code for the Testing of Centrifugal and Disc Fans (Trans., A. S. H. & V. E., Vol. 29, 1923, p. 407.) Installation tests for determination of fan capacity and efficiency shall be under laboratory conditions, in accordance with this Code. 244 Chapter XX AIR WASHERS AND FILTERS THE cleansing of air for ventilation purposes is a very important phase of the art and is accomplished by two different means; washing and filtering. Both methods cleanse the air of solid or liquid matter in the form of dust or spray, while in addition the washer cleanses the air of soluble gases and vapors, and hence of many objectionable odors. Air filters are distinguished from air washers in that they clean the air without the use of water or the addition of water vapor. They are of two types (1) the viscous filter depending upon the dirt impinging on surface covered with a viscous fluid or oil; (2) the true dry filter type which removes the dirt from the air by passing it through cloth or felt screens, the openings in which are too small to allow the passage of dirt. TYPES OF AIR WASHERS The washing of air is done by passing it over a large surface area of water which is accomplished in the various types of washers; (1) by pas sing it through a fine spray of water; (2) by passing it over wet surfaces; (3) by passing it both through a spray and over wet surfaces. After the air is washed it is freed from entrained water. When air is cleansed by washing its humidity or moisture content is usually changed. In passing through the water spray or over the wet surfaces both the dry and wet bulb temperature of the air approaches that of the water at which temperature the air tends to become saturated. The moisture content of the air may, therefore, be controlled by control ling the water temperature. By using water at a very low temperature the washer becomes a dehumidifier or by heating the water the air may be humidified. By raising the dry bulb temperature of the air after leav ing the washer its relative humidity may also be controlled. The humidi fying efficiency of any air washer may be given as i _ 1 _ Final wet bulb depression Initial wet bulb depression for example: With an initial wet bulb depression of 20 deg. and the final wet bulb depression of 6 deg., the humidfying efficiency is E = 1 6 deg- =.0.70 20 deg. Material prepared especially for The Guide by W. H. Carrier. Carrier Engineering Corp., Newark, N. J.' 245