Shelf dryer

Manufacture. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother Hquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide (341,342). In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkaH. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible. 

A pseudo-convective heat-transfer operation is one in which the heating gas (generally air) is passed over a bed of solids. Its nse is almost exchisively limited to drying operations (see Sec. 12, tray and shelf dryers). The operation, sometimes termed direct, is more aldu to the coudnctive mechanism. For this operation, Tsao and Wheelock [Chem. Eng., 74(13), 201 (1967)] predict the heat-transfer coefficient when radiative and conductive effects are absent by... 

Radiative Heat Transfer Heat-transfer equipment using the radiative mechanism for divided solids is constructed as a table which is stationary, as with trays, or moving, as with a belt, and/or agitated, as with a vibrated pan, to distribute and expose the burden in a plane parallel to (but not in contacl with) the plane of the radiant-heat sources. Presence of air is not necessary (see Sec. 12 for vacuum-shelf dryers and Sec. 22 for resubhmation). In fact, if air in the intervening space has a high humidity or CO9 content, it acts as an energy absorber, thereby depressing the performance. 

Vacuum-shelf dryers require auxiliary stream jets or other vacuum-producing devices, intercondensers for vapor removal, and occasionally wet scrubbers or (heated) bag-type dust collectors. 

Vacuum-Shelf Dryers Vacuum-shelf diyers are indirec t-heated batch diyers consisting of avacuumtight chamber usually constructed of cast iron or steel plate, heated, supporting shelves within the chamber, a vacuum source, and usually a condenser. One or two doors are provided, depending on the size of the chamber. The doors are sealed with resihent gaskets of rubber or similar material (Fig. 12-49). 

Design Methods for Vactium-Shelf Dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying [Ernst, Ridgway, and Tiller, Jnd. Eng. Chem., 30, 1122 (1938)]. 

Performance Data for Vacttum-Shelf Dryers The purchase price of a vacuum-shelf dryer depends upon the cabinet size and number of shelves per cabinet. For estimating purposes, typical prices (1985) and auxiliai y-emiipment requirements are given in Table 12-12. Installed cost of the equipment will be roughly 100 percent of the carbon steel purchase cost. 

The thermal efficiency of a vacuum-shelf dryer is usually on the order of 60 to 60 percent. Table 12-13 gives operating data for one... 

TABLE 12-13 Performance Data of Vacuum-Shelf Dryers... 

The maximum temperature at which the drying material may be held is controlled by the thermal sensitivity of the product and this varies inversely with the time of retention. Where lengthy drying times are employed, as for example in a batch shelf dryer, it is necessary to operate under vacuum in order to maintain evaporative temperatures at acceptable levels. In most continuous dryers, the retention time is very low, however, and operation at atmospheric pressure is usually satisfactory. As noted previously, dryer selection is considered in some detail in Volume 6. 

Tray or shelf dryers are commonly used for granular materials and for individual articles. The material is placed on a series of trays which may be heated from below by steam coils and drying is carried out by the circulation of air over the material. In some cases, the air is heated and then passed once through the oven, although, in the majority of dryers, some recirculation of air takes place, and the air is reheated before it is passed over each shelf. As air is passed over the wet material, both its temperature and its humidity change. This process of air humidification is discussed in Volume 1, Chapter 13. 

No indirect heating, static solids bed shelf dryer subclasses have been identified. 

Vacuum-shelf dryers may vary in size from 1 to 24 shelves, the largest chambers having overall dimensions of 6 m wide, 3 m long, and 2.5 m high. 

The most suitable equipment for the production of dried fruit is the chamber or shelf dryer. Smaller types have strips inside, along which the drying screens (shelves) are moved like shutters. Bigger dryers are designed in such a way that a mobile stand is filled with these shelves. The stand is then pushed into the dryer. Electrical heating elements are usually employed to heat the dryers. Built-in fans provide the necessary air circulation. The better dryers are fitted with air baffle plates which distribute the warm air evenly throughout the dryer. 

Drying Under Vacuum,—The most common type is the standard shelf dryer arranged very similarly to the apparatus shown in Fig. 31. However, the housings have to be built substantially in order to withstand the outside pressure. The vapors are condensed in either surface or barometric condensers. This type... 

Rubber and electrical insulating material should be treated in standard chamber dryers or vacuum-shelf dryers, with or without the recovery of the so Ivents. Shel dryers are more expensive to install, but the saving in steam quickly pays for the extra expense. Explos ves are handled in vacuum-shelf dryers, especially built for this purpose, with and without recovery of the solvents. The drying housing is equipped with large automatic doors which will open in case of explosion. 

Inorganic Products.—General rules for the application of certain types of dryers cannot be given as there are too many variations in the nature and quantity of the products to be handled. Usually rotary vacuum and drum dryers are used for delicate products, and direct-heated or steam-heated cylindrical rotary dryers are used for bulky materials that are not affected by higher temperatures. Chamber and shelf dryers (either atmospheric or vacuum) should be used for all delicate and sticky materials, also pastes, and in cases where the material is strongly acid or alkaline, as the trays that are placed on top of the shelves can be made of almost any kind of material which will prevent contamination of the product. Sticky material and paste can also be handled successfully on either a single or double roll drum dryer. 

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