AIR CONDITIONING. The term is derived from the word "conditioning" which was applied to the determination of the moisture content of textiles, and covers the treating of air in an enclosure to regulate its moisture content, temperature, purity and usually its circulation. For increasing human comfort and for the manufacturing or processing of certain materials, an accurate con trol of the moisture content of the air is most important. It is sometimes roughly effected by adding water vapour or mist directly to the enclosed air, but more precisely by using an air conditioning apparatus by which the air is purified and its tem perature and moisture definitely controlled before it is introduced into the space to be conditioned.
Originally only places where equable and moist climatic condi tions prevail, such as Lancashire, England, and the Fall River dis trict in Massachusetts, could be chosen for the building of cotton mills. Then moisture was added to the air by evaporating water in steam pots. This method sufficed for small mills, but was inade quate in the larger mills on account of the heat generated by the great concentration of machinery. In the latter half of the i 9th century the direct introduction into the room of an atomized spray of water both cooled and moistened the air, thus making it prac ticable to spin fine cotton yarns in all climates. At about the same time moist air was supplied to malting floors by passing the air through coke filters over which water trickled, or through a large series of wetted baffle plates. In 1897 Joseph McCreary of Toledo. O., patented an air washer intended primarily for purifying the air, but incidentally it cooled it and increased the moisture content. In 1906 Stuart W. Cramer of Charlotte, N.C., and Willis H. Car rier of Buffalo, N.Y., working independently, made two important contributions to the art. Cramer automatically controlled humid ity by a system which depended upon the relation between the wet and dry bulb temperatures. This was applied to the humidifying spray heads located in the room. Carrier devised the "dew point control," an improved form of air washer which not only brought the air to saturation, but also controlled automatically the tem perature of such saturation, thus determining the moisture content of the air supplied.
In the conditioning chamber it is brought into intimate contact with water finely divided by spraying through special nozzles under moderate pressure. All of the free particles of water are then removed by an "eliminator," which usually consists of a series of specially formed baffle plates on which all free particles of mois ture are caught, leaving the air saturated but free of all suspended water particles.
The spray water may be heated or cooled, and the temperature of the saturated air may be modified before passing it into the room at the theoretic dew point.
Under certain conditions it is desirable to remove, dust and soot from air with dry filters. Two general types of air filters are employed. Closely woven cloth or paper fabric is mounted in frames, accessible for frequent cleaning, through which the air to be filtered is passed. In the viscous type filters, the medium used is fibrous metal, fibrous glass or paper having the surfaces dipped in an oil. The dust or soot particles carried by the air are removed by adhering to the viscous surfaces. In such cases either "cloth filters" or "viscous filters" are em ployed. The cloth filters are large areas of closely woven cot ton cloth held in frames, provision being made for frequent cleaning. The viscous filters are either intricate metal baffles or cells filled with masses of fibre-like metal. These are dipped in a viscous oil which causes the particles of dust or soot to adhere to the metal surfaces.
Usually temperature is automatically controlled by thermostats governing the admission of steam to the air heaters, or regulating the supply of cold dehumidified air. Another form controls the sprays, but is mostly used in the direct spray type humidifiers.
Since the air conditioning of small shops, restaurants and other places has become general, air conditioning units com pletely assembled at the factory have been developed. These units are arranged in sections enclosing the fans, heaters, cooling coils, humidifier and air filters. In conjunction with these, refrigerating units of comparable sizes, embodying safety and simplicity of operation, have been developed. The use of new and safe refrig erants has been an important factor. Self-contained air condition ing units, complete with refrigeration equipment and all other elements built into one enclosure, are (194o) widely used for of fices, stores, restaurants, etc.
The quantity of moisture which may remain in the vapour state in a unit volume of space is dependent only upon its temperature. The saturation pressure of water vapour is approximately doubled for each 2o°F rise in temperature, i.e., the maximum vapour pressure increases approximately in a geometric ratio for equal increments of temperature. The relation of the weight of water vapour in the atmosphere to the weight of air varies approximately in accordance with Dalton's law of partial pressures (see DALTON, JoHN). When water is brought into contact with unsaturated air it evaporates. If the air is the sole source of heat necessary to pro duce evaporation, an eventual equilibrium saturation temperature is established depending on the initial air temperature and its ini tial degree of saturation. The reduction in the sensible heat of the atmosphere is equalled by the latent heat of the acquired water vapour; the total heat remains constant. The equilibrium tem perature of evaporation is observed with practical accuracy by a wet-bulb thermometer. The general formula based upon this heat balance is found in "Rational Psychrometric Formulae," W. H. Carrier, Trans. A.S.M .E., vol. xxxiii.
When unsaturated air is passed through an air washer and brought to saturation without changing the temperature of the spray water, it will be cooled to the temperature of evaporation (substantially, to the wet-bulb temperature of the entering air). For example, when the outside air is at 95°, the average wet-bulb temperature will be approximately 75°, and the air passing through a spray humidifier will be cooled to substantially 75°. This method of cooling is especially valuable in textile mills and tobacco factories where the increased humidity as well as the lower temperature is desirable.
The temperature of evaporation is independent of the actual rate of evaporation. The rate of evaporation of water into air is dependent upon the air velocity over the water surface and upon the difference in vapour pressure existing between the water and the air. The determinations of these relationships have important applications in the drying of materials in air.
Regain.—Theratio of moisture content to the weight of dry material is technically termed regain. The maximum regain for various materials varies widely, the amount depending principally upon the relative humidity of the air and to a slight extent upon \ the temperature. There is, however, no exact constant ratio be tween change of regain and change of relative humidity. Materials such as natural and artificial silk, wool, cotton, yarn and tobacco are sold on the basis of a standard percentage of regain, so in the drying and manufacturing of these materials it is necessary to control the relative humidity.
Comfort.—Therelation of moisture, heat and air motion to human comfort and physiological efficiency has been the subject of much research, which has determined the points of equivalent comfort. Curves based on experimental results have been drawn to show the relation between temperature and humidity which will give the same sensation of comfort or temperature. Such a curve is termed an "effective temperature" line. Temperature by itself, even when automatically controlled, does not determine comfort.
Humidity is equally important. The proper relation between tem perature and humidity can be maintained only by complete condi tioning.
Some instrument of measurement besides the dry-bulb ther mometer has yet to be devised as an indicator of comfort. One instrument attempting to accomplish this, devised by Sir Leonard Hill in England, was known as the "kata-thermometer." Under conditions of high humidity this is a reliable indicator but it does not follow with precision the experimental effective temperature lines under varying humidities.
BIBLIOGRAPHY.-Dr. Leonard Hill, "Measurement of Rate of Heat Bibliography.-Dr. Leonard Hill, "Measurement of Rate of Heat Loss at Body Temperature by Convection, Radiation and Evapora tion," Philosophical Transactions Royal Society, London, ser. B. vol. ccvii. (1916) ; "Electrically Heated Kata-Thermometer and Compari son of Wet Kata-Thermometer with Wet-Bulb," Journal of Physiology (London, 1919-2o) ; W. H. Carrier, "The Theory of Atmospheric Evaporation—with Special Reference to Compartment Dryers," Journal of Ind. & Eng. Chem., vol. xiii., No. 5 (1921) ; W. K. Lewis, "The Evaporation of a Liquid into a Gas," Trans. A.S.M.E., vol. xliv. (1922) ; F. C. Houghton and C. P. Yaglou, "Determining Equal Comfort Lines," Journal A.S.H. and V.E. (March 1923) ; Himus and Hinchley, "Effect of a Current of Air on the Rate of Evaporation of Water below the Boiling Point," Chemistry and Industry Review (Aug. 22, 1924) ; W. J. McConnell and C. P. Yaglou, "The Kata Thermometer—its Value and Defects," U.S. Treasury department, Public Health Reports, vol. xxxix. (Sept. 5, 1924) ; Margaret Ingels, "Value of the Kata-Thermometer in Effective Temperature Studies," Journal A.S.H. and I7.E. (June 1924) ; W. H. Carrier and D. C. Lind say, "Temperature of Evaporation of Water into Air," Trans. A.S.M.E., vol. xxxxvi. (1924) ; Philip Drinker, "Humidity Control in Resi dences," American Journal of Public Health. (Aug. 1925) ; Lionel S. Marks (editor), Mechanical Engineers' Handbook.
(W. H. CA. ; X.)