Drying Chambers and Forms of Trays

Drying chambers for freeze drying are built in three basic configurations (Fig. 2.9)  

temperature controlled shelves 1, temperature controlled plate, to expose the product on the upper shelf to the same condition as on the other shelves 2, trays or vials 3, transport system for trays. 

For granulated luxury products and food, two basic forms of tray are used (Fig. 2.14)  

Trays of type (a) are pushed through the plant between the heated shelves without contact (System Atlas Industries A/s, DK 2750 Ballerup, Danmark). The ribbed trays (b) are made 

Rolfgaard [2.2] compares the types of trays and heating systems The ribbed trays are said to have an uneven temperature distribution, because the distances between shelf and tray vary between 0.1 mm and 1 mm. The ribs could compensate this only partially. The variation in distances is correct, but Rolfgaard overlooks that the thermal conductivity in the bottom of the tray is so effective that practically no temperature differences are established in the bottom. Even with an evaporation of 3 kg ice/m2 h and the assumption that all heat is Transmitted only in the center of the tray (8 cm from the border of the tray), the temperature difference between border and center is approx. 5 °C. During the drying under actual conditions, no measurable temperature differences can exist. 

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A filter press (Figure 11.9) consists of a set of trays, generally made of polypropylene, set in a steel (or, preferably, stainless steel) frame, held tightly together by a hydraulic jack. These trays are covered with cloth and designed to form filtration chambers between the trays that receive the turbid liquid, thus making it possible to collect the filtered liquid. The filter is fed by a high-pressure piston pump. At the end of the operation, a compressed air circuit dries the filtration residues. 

The wet product in the form of lumpy solids, pieces, or a paste is spread out on wire gauze or sieve plates/trays, etc. A warmed drying gas flows over and passes through the stationary product surface. The gas is circulated by means of a blower, in which adjustable guide blades ensure an even gas distribution to each product layer. Chamber or tray dryers [0.6, 5.1, 5.2, 5.53] are operated discontinuously usually in gas circulation mode and are suited for small amounts of wet product. For drying, the required total drying area A corresponds to the total support area of the trays, which is... 

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