Document 71Rk3KDMxKxBVxVQ2YaGpLYbo

T a b l e 1. D ryers for E vapo r atio n of W ater Suspended, Truck, Tray Shoveled in to D rum or Pan T ray, Basket, Tum bling Drum T ru c k ,: Tray, B e lt Cascades through Flowed on' D rum , D ry M aterial Scraped off C ontinuous Sheets, Endless Chain B elt C ontinuous Sheets, Suspended on M e ta l Screens Falls through by G ravity Sprayed into Chamber Placed in H igh Frequency Field W here one side cannot come in contact w ith supports u n til dry W here m aterial comes in sheets or rolls, and w ill stand direct contact w ith heating surface Hygroscopic m aterials dried w ith vacuum, and packed im m ediately Where m aterial w ill stand rough handling and is not subject to balling up Steam Coils, A ir, | For high production E lectricity, Products of Combustion Cost of operation high, for expensive m aterials Too W ater, Steam to Jacketed, m ay have Where dust must be saved 330 Vacuum on top When production does not w ar rant continuous drier HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944 I g ! 120 to A ir, Products of 350 Com bustion 125 to A ir, 250 Steam Coils Steam Coils to Steam inside 350 of D rum Steam, may have Vacuum on Top A ir, Steam, Products of C o m b u stio n 80 to Steam Coils, A ir, 180 E le c tric ity 80 to W ater, 300 Steam co I . g&* in O -o if - -i ad t.o 3a 1 rrt 00 80` to 500 o "So Paper, Leather, Yams, Lumber, Foodstu Chemicals too stieky for Rotary D rier Chemicals, Explosives, PI maceuticals, Food Produc Ceramics, Chemicals, Lum ber, Food Products ' 3 CQ Liquids,Slurries C y lin d e r i Drum R o ta ry Com p a rtm e n t A g ita te d Vacuum Tunnel K in d 'i ;e c 4- 3 Batch or Interi 734 0> - '3 O 3 , C o U Paper, Textiles, . Chemicals Festoon it * Tower or C o lu m n Spray Induction Paper, C h e m ic a ls 1' to A ir, 200 1 Grains, ; t Sand _ , ' i Solutions over 30% Solids E le c tric ity 1 W here headroom is available D ry in g is alm ost instantaneous Where heating of metal from in side o u t is im p o rta n t Metals,' for removal of traces of W ater to 1 400 i CHAPTER 41. DRYING SYSTEMS convection currents, and in low-temperature dryers only about one-third to one-half of the total heat for evaporation is actually supplied to the material by radiation. At high temperatures the radiation output increases rapidly, according to the fourth-power law. The total radiation may be computed by the equations and tables given in Chapter 3. In general fins and irregular surfaces do not increase radiation, hence the area to be used in calculations is the area of a smooth-surface envelope enclosing the radiating elements. A certain amount of air circulation is required through a radiant dryer, in order to carry off the vapor. Conduction or Direct Contact Drying Drying rolls or drums, flat surfaces, open kettles and immersion heaters are examples of the direct-contact method. Intimate contact of the material with the heating surface is important, and in some cases agitation PBf findit Ok I Cg) toy* (H) j i ft, ' (d) (s) Fig. 2. Section of Continuous Dryer, Blow-Through Type Fig. 1. Section of Rotary Dryer is desirable to increase the uniformity of heating or to prevent overheating. Greatest resistance to heat transfer occurs on the air side of the material being dried. The rate of heat transfer from the surface of the heated material to the air, and hence the rate of drying, may be increased by: (a) forced convection or air circulation and (b) vacuum operation to lower the boiling point of the liquid being evaporated. Convection or Air Drying The circulation of heated air or other gases over the material being dried is termed convection drying: Some convection drying occurs m practically all types of dryers, but if the main source of: heat is from the air or gases, the dryer may be called a convection dryer. Typical forms or examples may be enumerated: - 1. Rotary drum dryers, (Fig. 1). 2. Tunnel or oven dryers, batch type. 3. Tunnel dryers, conveyor type, (Figs. 3 and 4). 4. Tower or column dryers. 5. Through-circulation dryers, (Fig. 2). 735 '>