Driers circulation

Panels then move into a cooling device, normally a wheel or rack, where they are held individually and air is circulated between them to remove the majority of heat remaining in the boards after pressing. It is desirable to reduce the average board surface temperature to about 55°C. This temperature is sufficient to complete the cure of adhesive in the core of the board. The heat also helps to redistribute moisture uniformly within the boards, because the board surfaces are drier than the core when the boards come out of the press. Warm boards are normally stacked for several hours to a day to allow for resin cure and moisture equalization. 

Compartment driers are closets or even rooms in which the material is spread on trays or in shallow pans. The operation is often discontinuous in order to make it continuous, the pans may be placed on trucks traveling on rails, and these are pushed through a tunnel in which warm air circulates. Belt driers and chain driers have similar goals. 

Drying Rooms For drydng materials that cannot be agitated, the drying room, and tray drier (which there are many varieties) can be used. In the former, the material may be spread on pans and these stacked in a rack, which leaves a space between the trays for air circulation. The room has loosely fitted side walls of plaster board, or similar material, or even canvas, and contains a steam radiator. The natural circulation of the warm air removes the moisture the drying period is generally long, for example, one or two days. 

Through-circulation dryers, 9 120, 148 Through driers, 2 149 Throwing power, 9 773 Thujopsene, 24 543-544 Thulium (Tm), 14 631t, 635t electronic configuration, l 474t Thyme, 23 171... 

The well-mixed drier has fhe disfinct disadvanfage of nof being able to dry particles to very low moisture contents because of fhe wide particle residence time distribution. In other words, some particles leave the bed immediately, having had no opportunity to dry and effectively bypass the bed some circulate within the bed for a long... 

Figure 9.4. Types of dryers cited in Tables 9.1 and 9.2. (a) Tray or compartment, (b) Vacuum tray, (c) Vertical agitated batch vacuum drier, (d) Continuous agitated tray vertical turbo, (e) Continuous through circulation, (f) Direct rotary, (g) Indirect rotary, (h) Agitated batch rotary (atmos or vacuum), (i) Horizontal agitated batch vacuum drier, (j) Tumble batch dryer, (k) Splash dryer. (I) Single drum, (m) Spray, (n) Fluidized bed dryer, (o) Pneumatic conveying (mostly after Nonhebel and Moss, 1971).

Continuous hoi air driers are used to prepare most of the high quality, dried, piece-furm fruits and vegetables produced in the United States. Liquids and pastes arc commonly dried in spray, drum, or feeze dryers. Particulate foods can be dried in batch or continuous air-fluidized beds or freeze dryers. Many agricultural commodities are sun-dried when weather conditions at harvest provide low humidity, warm temperatures, and good air circulation. 

The web passes the screen vertically into the drier above the drying or pre-gelling of the print is by means of hot air blown on both sides of the web, the air being heated by thermal oil or, more usually, by direct gas firing. The volume of air circulated and its temperature are controlled and can be varied. After drying or pre-gelling the web must be cooled so that it will not carry heat to the next screen, and this is done by passing it round two water-cooled cylinders. 

If the material is of suitable granular form, drying times may be reduced to 1 h or less by passing the air downward through the material laid on mesh trays. The oven in this form is called a batch through-circulation drier. 

Forced air-driers merely require a roof and fans to circulate air (1-2 m s ). A cut-out switch turns the fans off when the relative humidity is exceeds 85-90% (to save power when drying will be ineffectual) or when the relative humidity falls below about 40% (when surface checking is likely). 

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