Drying contact heating

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

Further, because of the nature of sohds-gas contacting, which is usually by parallel flow and rarely by through circulation, heat transfer and mass transfer are comparatively inefficient. For this reason, use of tray and compartment equipment is restricted primarily to ordinaiy drying and heat-treating operations. Despite these harsh hmitations, when the listed situations do exist, economical alternatives are difficult to develop. 

If the heat flux from friction or viscous shear is properly estimated, the surface temperature, which is of interest in most engineering problems, can be determined through integrating an analytical solution of temperature rise caused by a moving point heat source, without having to solve the energy equation. For two solid bodies with velocity u j and Ui in dry contacts, the temperature rises at the surfaces can be predicted by the formula presented in Ref. [22],... 

Hydroxylamine is a powerful reducant, particularly when anhydrous, and if exposed to air on a fibrous extended surface (filter paper) it rapidly heats by aerobic oxidation. It explodes in contact with air above 70°C [1]. Barium peroxide will ignite aqueous hydroxylamine, while the solid ignites in dry contact with barium oxide, barium peroxide, lead dioxide and potassium permanganate, but with chlorates, bromates and perchlorates only when moistened with sulfuric acid. Contact of the anhydrous base with potassium dichromate or sodium dichromate is violently explosive, but less so with ammonium dichromate or chromium trioxide. Ignition occurs in gaseous chlorine, and vigorous oxidation occurs with hypochlorites. 

ASHRAE, Atlanta (1992)]. Process air stream 6, to be conditioned, passes through the adsorbent wheel, where it is dried. This is a non-isothermal process due to the release of heat of adsorption and transfer of heat from a wheel that may be above ambient temperature. The dry but heated air (7) is cooled in a heat exchanger that can be a thermal wheel. This stream (8) is further cooled, and the humidity adjusted back up to a comfort range by direct contact evaporative cooling to provide supply air. Regeneration air stream 1, which can be ambient air or exhausted air, is evaporatively cooled to provide a heat sink for the hot,... 

Some industrial operations involving bubble and drop formation are extraction, direct contact heat exchange, distillation, absorption, sparger reactors, spray drying and atomization, fluidization, nucleate boiling, air lifts, and flotation. 

Although direct contactive heat transfer can provide high temperatures and neat concentrations and at the same time be smml in size, its use may not always be preferable because of undesired side effects such as drying, contamination, case hardening, shrinkage, off color, and dusting. 

There are three heat transfer mechanisms used to dry textiles. Conduction methods involve direct contact of the wet textile with heated surfaces. These are the most efficient heat transfer methods, but do not allow for control of fabric width during drying. Steam heated cylinders are examples of conduction drying methods... 

Tetraethylene glycol dimethyl ether is the solvent for the preparation. Practical-grade material is purified before use by allowing approximately 2 kg. to remain overnight in contact with about 20 g. of fresh lithium tetrahydroaluminate in a properly vented vessel at 80°. Then it is distilled from lithium tetrahydroaluminate under reduced pressure (b.p. 110° at 0.75 mm.) and kept dry until used. (The checkers note that the solvent can be dried by heating with sodium as an alternate to the somewhat more hazardous tetrahydroaluminate.)... 

Equation (12-112), as it stands, would give an overestimate of the maximum drying rate for the case of contact drying over heated rolls, when there are significant heat losses from the ends of the drum and only part of the drum s surface can be used for drying. In the roller drying arrangements shown in Eig. 12-80, only a fraction a of the drum s periphery is available from the point of pickup to the point where the solids are peeled off... 

Fig. 7.77 shows two schematic representations of a rectangular contact fluidizer which, in this case, also incorporates back-mixed as well as plug flow sections. Therefore, it can be utilized in much the same way as discussed before. Plug flow is achieved with baffles arranged transversely (see also Fig. 7.73). As shown, a rotary distributor disperses the wet feed evenly over the back-mixed section (dry feed and atomized binder liquid could be also used) which, in addition, is equipped with contact heating surfaces that are immersed in the fluidized bed (see also Fig. 7.74b). As shown in Fig. 7.77b, the heating panels can be easily removed for cleaning and maintenance. The supply of thermal energy is selected such that a substantial portion of the required heat is provided by the panels. Therefore, it is possible to reduce both the temperature and the amount of gas through the system significantly which is particularly important if the material to be treated is heat sensitive.

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