Document pBKKQERd0BzyMw06kwgRXk2VX

T able 1. D ryers for Evaporation of Water Com p a rtm e n t .. A g ita te d Vacuum Tunnel Rotary ' Drum 1 ! Continuous. j Cylinder Festoon Tower or Colurpn Spray Induction Suspended, T ru c k , , 80 to Steam Coils, A ir, W hen production does not w ar W here headroom is available W here one side cannot come in contact w ith supports u n til dry Hygroscopic- materials dried w ith .vacuum', and packed im m e d ia te ly W here m aterial comes in sheets or rolls, and w ill stand direct contact w ith heating surface Where m aterial w ill stand rough handling and is not subject to balling up 100 Steam Coils, A ir, to E lectricity, Prbducts For, high production 350 of Combustion Cost of operation h ig h, for expensive materials 100 W ater, Steam to Jacketed, m ay have Where dust must be saved 330 Vacuum on top HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 (ft I - g4 i p rant continuous drier 260 . Steam Coiis . A ir, Steam Coils to , Steam inside 350 of D rum Steam, m ay have'Vacuum on Top '6 GO 5 *& . Hfe* lis a* jl 180 E le c tric ity 80 to W ater, 300 Steam A ir, Steam, . Products of C o m b u s tio n 0o0 "og `o'eo to 200 Falls through b y . '125 to A ir, C ontinuous Sheets, ~ Suspended on . M etal Screens. . G ravity.. C ontinuous Sheets, Endless Chain B elt Flowed on Drum , i D ry M aterial Scraped off Cascades through Truck, Tray, Belt !___ ____ _ Chemicals, Explosives, Phar T ra y, Basket, maceuticals, Food Products T um bling D rum Shoveled in to D rum or Pan ______ Grains, Sand. Papert C h e m ic a ls Paper, Textiles, Chemicals Liquids, Slurries :__________ Ceramics, Chemicals, Lum ber, Food Products . ; 1 Chemicals.too sticky for Rotary'- D rie r Yarns, Lumber, Foodstuffs. Tray B u lk Paper, Leather, 3| Js. :ur. . % .. - . -C . '.s' ' ` -- JS 1 : '.EShO""- ~ Solutions over 30% Solids Sprayed into Chamber ,i20 to A ir, Products of | 350 Combustion D ry in g is alm ost instantaneous Metals, fo r removal of traces of W ater Placed in H ig h . Frequency Field to 400 1 E le c tric ity Where heating of metal from in side o u t is im p o rta n t 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 in creases 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 Fig. 2. Section of Continuous Dryer, , Blow-Through Type 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 in 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: r :` 1. Rotary drum dryers, (Fig. 1). 2. Tunnel or oven dryers.^ batch; type. 3. Tunnel dryers, conveyor type, (Figs. 3 and 4). ,\v , 4. Tower or column dryers. 5. Through-circulation dryers, (Fig. 2).- 749