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American Society of Heating and Ventilating Engineers Guide, 1937"'' evaporative pan type of humidifier is usually employed. This consists 0f a container offering as much water surface as possible and equipped with means of heating the water. This heat may be applied either electrically ' by steam, hot water, or by circulation of the water from a heated spa<^ :, through the evaporating pan. Since the humidification is accomplish^ by surface evaporation only, it is essential that the air stream be directed across the surface and that the evaporating surface be large. While this * system eliminates the dusting hazard when hard water is used with a ! spray system, hard water tends to scale the heating surface and results in > loss of capacity and the need for frequent cleaning. The rate of evaporation per unit surface exposed is low, thus it {re. \ quently becomes difficult to provide sufficient surface for adequate' capacity. The higher the temperature of the water, the lower the relative ' humidity of the air; the greater the velocity over the surface, the greater is the rate of humidification. The evaporative pan type of humidification limits the water wastage and is usually supplied with water through a float valve. Due to the collection of salts in this evaporating pan such ; humidifying systems require occasional drainage and cleaning. Other methods of humidification attempted.in air conditioning units are through the use of wetted fabrics, porous eathernware plates, or other capillary surfaces. These methods rely upon the capillary absorption of the moisture up from the liquid level into the portion exposed to the air. They have a tendency to lose their effectiveness due to the resulting deposit of mineral salts at the evaporating surfaces thereby clogging the pores and reducing the contact of the air with the water. Also they frequently become foul and often support bacterial growth. Cooling and Dehumidification Those units that employ recirculated, water sprays will undoubtedly use such sprays as their means of cooling and dehumidification by furnishing refrigeration to the water in circulation. Occasionally where an adequate source of cold well water is available, this may be used as a direct spray and run to waste. Other methods of dehumidification accomplished by direct contact with the transfer medium are by means of the so-called adsorption and absorption systems. (See Chapter 2 on Refrigeration.) It must be recognized that these methods of dehumidification do not in themselves provide cooling. The substance removes the water vapor from the air thereby heating it. This highly dehumidified air may then be cooled either by partial rehumidification or by direct contact with a cooling medium of cold water or direct expansion refrigerant. There are now on the market solid adsorbents such as silica gel, and activated alumina, and liquid adsorbents such as calcium chloride in solution and lithium chloride which is one of the most promising examples. Finally a common direct means of cooling and dehumidification is through the use of ice. In such units the ice is brought into as intimate contact as possible with the air handled. Provision is made for the removal of the moisture as rapidly as it is formed from the melting of the ice. Ice is also used to cool water which is circulated through the sprays. ' In conditioning units, the use of surface cooling is probably more 258 -Unit Heaters, Ventilators, Air Conditioning, Cooling Units on than direct spray or other direct transfer means. The type of COI9m employ^ may, of course,, be cast or fabricated from tubes. In SUr nt day practice finned tubes or plate fins through which tubes are Pre*d form the most generally used cooling surface. The detailed fabripassea of this surface and the arrangement of the tubes will depend ter'ely upon the type of refrigerant for which it is intended. The simplest construction is where chilled water or brine is used as the frieerating medium. With. direct expansion refrigerant it is usually Pessary to provide a special arrangement of headers so that proper j. jpbution of refrigerant through all the surface is obtained. In some (^ses ordinary brine coils can be used when operated as a flooded refrieran't system. In some. units a combination of a direct spray and a refrigerant surface is used, the spray being directed against the surface. Such systems claim the advantage of air washing together with the maintenance of a clean and effective cooling coil. It should be noted that when surface coolers are used, adequate pro tection in the form of filters or at least lint,screens are necessary to prevent fouling of the surface from the air borne dirt; ' Surface not so protected frequently becomes completely matted with lint, grease, and similar dirt. The sources of refrigeration used with these surface type conditioning units are discussed in Chapter 2 on Refrigeration. However, they may be divided into the following groups: 1. Direct expansion refrigerant in which the liquid refrigerant is evaporated within the coils of the unit. The vapor froril these coils may be recompressed in centrifugal, rotary, or reciprocating type compressors, and the refrigerant again returned to the evaporator coil. 2. Indirect refrigeration by means of: o. Cold well water. 1 b. Cold city water. c. Artificial refrigerated water provided by direct expansion of refrigerant in a water cooler, direct steam jet refrigeration, or by the melting of ice. It should be recognized that direct expansion of refrigerant within the coils of an air conditioning unit, represent a certain hazard unless the refrigerant is of the non-irritating, non-inflammable type. Therefore, in many cities rather stringent codes have been set up limiting the use of such direct expansion units. These codes are becoming more and more prevalent and it is advisable to keep informed-of latest restrictions in each locality. In the case of hospitals, or places where a large number of people are present, direct expansion is prohibited and the so-called indirect systems are required. In these systems the refrigerant is used to cool a .............................. . . .. . .1 _ --1:---------------.-i filtering--Air Cleaning A variety of methods are employed as a means of controlling air purity. In unit systems where filtering alone is considered satisfactory, the degree of filtering varies widely and in proportion to the actual needs. If the air is chiefly recirculated with but little outside air used for ventilation, filtering requirements are largely limited to keeping the coils in a clean End operable condition. Thus such units are frequently furnished with simple lint screens of low resistance and formed of moderately close 259