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* .j; American Society of Heating and Ventilating Engineers Guide iq ` TEMPERATURE AMD HUMIDITY CONTROL , Temperature and humidity controlling devices usually act unn : control valves or dampers in the air-conditioning apparatus throu h medium of compressed air or electricity. For larger installations pnemn control practically is universal in use on account of simplicity and T ; bility; and also on account of the fact that it permits delicate gradual of the movement of the dampers or valves in proportion to the temrJ ture or humidity changes. pei- In smaller humidifying devices electric controls are very serviceahl and reliable. Electricity has an advantage, because with it there are auxiliaries, such as air compressors, air tanks, air filters, relief valv- etc., required. In the larger systems such accessories are warranted bv the greater size of equipment and by the increased number of control' operated from the same compressed air source. For additional information on this subject, see Chapter 20, Automatic Heat and Humidity Control. HJSfERENCES Rational Psychromctric Formula, by W. H. Carrier {AJ5.M.E. Transactions, Vo!. 33, 1911), Air Conditioning Apparatus, by W, H. Carrier and F. L. Busey (A.S.M.B. Transactions, Vol. 33, 19ii) The Theory of Atmospheric Evaporation with Special Reference to Compartment Dryers, by W. H. Carrier (Journal of Industrial and Engineering Chemistry, Vol,.13, No. 5, May, 1921). Coffey and Horne (American Society of Refrigerating Engineers, 1916). The Temperature of Evaporation, by W. H. Carrier (A..S.H.V.E. Transactions. 1918). * The Control of Bumidity and Temperature as Applied to Manufacturing Processes and Human Comfort by W, H. Carrier (Heating, Piping and Air Conditioning, November, 1929). The Summer Comfort Zone: Climate and Clothing, by C. P. Yaglou and Philip Drinker (A.S.H.V Transactions, VoJ. 35, 1929). \ Chapter 30 Buying and processing of materials ^jiMli ~ethods of Drying; Types of Dryer Construction; Mechanism of S*he Drying; Control of the Drying Operation; Dryer Design; Dryer 4 Arrangement and Construction; Experimental Technique. Sqf general, drying refers to the removal of water or other volatile liquids il^from gas'eous, liquid or solid materials. The term is used in the case r& solids where the water or other liquid removed is present in a relatively imail amount. In the sense implied in this chapter, drying is the removal Hfwater by vaporization from a non-volatile liquid or solid. llliWhere the solid material to be dried contains large amounts of free llliter, the actual drying process is frequently preceded by the removal of gfpart of the water by filtration, settling, pressing, centrifuging, or other Sm^cfianical means. Removal of as much water as possible by such [methods is usually advisable, as the cost of these operations, per pound JoE'water removed, is in general much less than by vaporization. METHODS OF DRYING HP^Classification of Methods According to Heat Supply $P^?rying methods may be classified basically in accordance with the t^means by which heat is supplied to the material to be dried. Thus, in j'some cases, as in air current dryers, the heat of vaporization is transferred '^toithe material from the atmosphere surrounding it. In natural air dryers, |3jthis heat is merely the sensible heat of the outdoor atmosphere; while in lllgsfccalled artificial air dryers heat is added to the air by means of radiators gfplof' the addition of a heated gas such as superheated steam or products S&qf combustion, the air being almost entirely displaced by such gases in ^Jfrsome drying processes. Where products of combustion can be used, a ^^.particularly high overall efficiency of the dryer is possible. In other types of dryers, known as contact dryers, heat is supplied by i,r^t;direct contact between the material and a heated surface such as a pan r1r such surfaces being heated by products of combustion, steam, y'- .t not water, heated oil, electrical resistance, or latent heat of condensation. both air dryers' and contact dryers, additional heat may be supplied h ^ by direct radiation from hot surfaces which are in sight of the material. jggg^In the sun drying of raisins and other fruits, this radiant heat comes from i/The sun, while in artificial dryers it is derived from heated surfaces such as steam coils. 469