Direct driers

Driers may be classified by the method of transferring the heat to the wet solid. Direct driers employ a stream of hot gas passing over or through the wet solid which vaporizes the liquid and carries it away, whereas indirect driers rely on the transfer of heat through a metal wall. The choice of drier depends upon many factors which include ... 

Method of supplying the heat necessary for evaporation of the moisture. In direct driers, the heat is supplied entirely by direct contact of the substance with the hot gas into which evaporation takes place. In indirect driers, the heat is supplied quite independently of the gas used to carry away the vaporized moisture. For example, heat may be supplied by conduction through a metal... 

In many of the direct driers to be described, the solid is moved through a drier while in contact with a moving gas stream. The gas and solid may flow in parallel or in countercurrent, or the gas may flow across the path of the solid. If heat is neither supplied within the drier nor lost to the surroundings, operation is adiabatic and the gas will lose sensible heat and cool down as the evaporated moisture absorbs latent heat of vaporization. By supplying heat within the drier, the gas can be maintained at constant temperature. 

Solids which ordinarily can be dried on trays, such as powdery and granular materials, heavy sludges and pastes, beads and crystalline solids, can be continuously dried in a turbo-type drier, a form of direct drier. The simplest of these is shown in Fig. 12.18, The drier is fitted with a series of annular trays arranged in... 

Driers of the type indicated in Fig. 12.22 combine the features of the through-circulation and rotary driers. The direct drier shown, the Roto-Louvre [12], consists of a slowly revolving tapered drum fitted with louvers to support the drying solid and to permit entrance of the hot gas beneath the solid. The hot gas is admitted only to those louvers which are underneath the bed of solid. There is... 

Wood particle and fiber driers are used to dry the raw material for particleboard and similar products (20). Just as with the veneer for plywood, the parhcles must be dried before being mixed with the resins and formed into board. Drying is accomplished in a gas-fired drier, a direct wood-fired drier, or steam coil driers. Many different types of driers are used in the industry. Emissions are fine particles and condensible hydrocarbons, which produce... 

In modem manufacturing methods the oil is sometimes reacted directly with the glycerol to form a monoglyceride and this is then reacted with the acid to form the alkyd resin. When the resulting surface coating is applied to the substrate the molecules are substantially linear. However, in the presence of certain driers such as lead soaps there is oxidative cross-linking via the unsaturated group in the side chain and the resin hardens. 

The drying of the curd is completed by passing through a rotary drier such as the Pillet-Bordeaux. In this machine the curd passes down a chute with hot air running counter to the direction of flow of material. Strict temperature control is essential, the maximum discharge temperature being below 65°C. If such control is not made then the resin will be dark in colour. 

We will cover a simple drying model to examine the radiation drier of coated paper. We assume there are no major temperature or humidity variations in the direction of the paper web thickness, and that temperature T and humidity u are constant in the direction of thickness. This assumption requires that the capillary action be ignored, and the pressure gradient of water is zero on the assumption hu/dx = dT/dx = 0. How is it possible that the humidity distribution remains uniform ... 

Dryers are different in characteristic from distillation columns and evaporators in that the heat is added and rejected over a large range of temperature. Changes to drier design can be directed by the plus-minus principle. 

Research will start towards producing drier manure directly from the battery but also towards a closed mechanically ventilated storage and an odour reducing system. 

A number of driers may be used in series. For example, a cascade of well-mixed driers approximafes fo fhe performance of a plug flow drier. However, of more pracfical significance is the combination of a well-mixed drier followed by a plug flow drier for drying very wet solids which may defluidize if fed directly to a PF unit. The CSTR drier is better able to cope with the high moisture content feed and the plug flow drier allows a low final moisture content to be obtained. 

Sometimes it can be difficult to know if the system has come to a true equilibrium concerning water distribution. It has been noted that water adsorption isotherms sometimes show hysteresis effects, which means that the water content, for example, that bound to the enzyme, depends not only on the water activity, but also on the hydration history [6]. More water is thus bound if a specified water activity is approached from a higher value (dehydration direction) than if the enzyme is hydrated from a drier state. The hysteresis effects might be due to slow conformational changes in the enzyme. 

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