Freeze-dryers tray-type

Sublimation temperatures are in the range of —10 to —40°C and corresponding vapor pressures of water are 2.6-0.13 mbar. Cabinet tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 lb/sqft is usual for foodstuffs. Drying capacity of shelf-type freeze dryers is 0.1-1.0kg/(hr)(m2 exposed surface). Another estimate is 0.5-1.61b/(hr)(sqft). The ice surface has been found to recede at the rate of 1 mm/hr. Freeze drying also is carried out to a limited extent in vacuum pans, vibrating conveyors, and fluidized beds. Condensers operate as low as —70°C. 

W. L. Gore Associates (Figure 2.17) has developed a type of tray from which product particles cannot escape during the drying process - the Lyoguard tray. When handling cytostatic products, persons, the environment and the freeze-dryer are protected from contamination. 

The thermostatically controlled heaters formerly used in tray-based freeze-dryers were gradually abandoned as they resulted in inconsistent product temperatures. Most freezers of this type currently use hollow trays through which a heat transfer liquid is passed at a controlled temperature. 

Depending on the number of containers to be used at once, some freezer-dryers use a multiport turret (a manifold) or a tray module. The former is suitable for containers of varying shape or size that need not be processed simultaneously the latter is to be preferred when a large amount of product or number of containers is to be processed in a simultaneous manner. The tray module can be equipped with an electromechanical device to close the containers after the products are freeze-dried in order to avoid external contamination. One example of this type of freeze-dryer is the model from FTS Systems. 

By far the largest number of the industrial freeze dryers in operation is of the vacuum batch type with freeze drying of the product in trays. There are two main types, depending on the type of condenser used. In the first type, the condenser plates are alongside the tray-heater assembly and in the same chamber in the second type the condenser is in a separate chamber joined to the first by a wide, in general, butterfly valve. This latter type of plant is always used in pharmaceutical industries, but it can also be used for the freeze drying of foods. Because of the wide variety and complexity of the... 

In the tunnel type of freeze dryer (Figure 12.6), the process takes place in a large vacuum cabinet into which the tray-carrying trolleys are loaded at intervals through a large vacuum lock at one end of the tunnel and discharged similarly at the... 

The Conrad system [5] is a commonly used continuous freeze dryer for treating product in trays. The success of this type of plant is based on the simplicity and reliability of each component that goes to form the total system. The details of this and other continuous systems are given in Refs. [5,8]. 

Fig. 2.9. Basic types of freeze drying chambers. A, bell jar or vertical cylinder b, rectangular or cylindrical chamber with one (or 2) door(s) c, tunnel dryer, in which the trays are transported in and out by a system (shown as a carrier on a monorail).

For the first group the following types of dryers are generally used convective (tray, band, fluid bed, flash dryers, and their modifications) or contact (vacuum dryers such as double-cone dryer-blender, conical dryer with rotating helical mixer, paddle dryer). Pastelike materials are dried in tray dryers, band dryers equipped with extruding devices, and spin-flash dryers. Finally, thin pastes can be dried in spray dryers or on fluid beds or spouted beds of inert particles. Small amounts of solutions and suspensions are generally freeze-dried, especially if the product is thermolabile. 

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