Module 3. Food dehydration

Lesson 32


32.1 Introduction

Cabinet/tray dryers are used for batch drying of solid foods at small to moderate scale (say, 2000 to 20 000 kg per day). They are inexpensive and simple to construct. Cabinet dryers consist of a closed compartment in which trays containing the food to be dried are placed( Fig. 32.1). The trays rest on shelves with adequate spacing between them. Heated dry air circulates between the shelves. Very often, tray bottoms are slotted or perforated, in order to provide some air flow also through the trays. the moisture content of the material, depends on its position on the tray. The material located closest to the entrance of dry air has the lowest moisture content. In order to secure more uniform drying, the direction of air flow may be reversed or the trays may be rotated periodically. The cabinet is usually equipped with movable baffles, adjusted so as to have uniform distribution of the drying air throughout the cabinet. Cabinet driers are frequently found in rural installations where they are used for drying fruits (grapes, dates, apples), vegetables (onion, cabbage) and herbs (parsley, basil, mint, dill). Air inlet temperatures are usually in the range of 60–80°C. Air velocity is a few m.s -1 and must be adjusted according to the size, shape and density of the food particles so as to avoid entrainment of dry particles with the wind. Depending on the product and the conditions, the duration of a batch is typically 2 to 10 hours. Most cabinet dryers feature means for adjustable recirculation of the air. The rate of recirculation is increased as drying progresses, when the air exiting the cabinet is warmer and less humid. Recirculation results in considerable saving in energy cost.

This is a multi-purpose, batch-operated hot air drier. It consists of an insulated cabinet, equipped with a fan, an air heater and space occupied by trays of food. In Cabinet dryers, food may be loaded on trays or pans in comparatively thin layers up to a few centimetres. Fresh air enters the cabinet by the fan through the heater coils, and is then blown across the food trays to exhaust. Here, the air is heated by the indirect method. Screens filter out any dust that may be in the air. The air passes across and between the trays in some designs, whereas some other designs have perforated trays and the air may be directed up through these. The air is exhausted to the atmosphere after one pass rather than being re-circulated within the system. Re-circulation is used to conserve heat energy by reusing part of the warm air. In re-circulating designs, moist air, after evaporating water from the food, may have to be dried before being re-circulated to prevent saturation and slowing down of subsequent drying. In such case, this air could be condensed out by passing the moist air over cold plates or coils. But when the exhaust air is not dried for re-circulation, then the exhaust vent should not be close to fresh air intake area, otherwise the moist exhaust air will be drawn back through the drier and drying efficiency will be lost. Cabinet driers are usually for small operations. They are comparatively inexpensive and easy to set in terms of drying conditions. They may run up to 25 trays high and operate with air temperatures of about 95°C dry bulb and with air velocities of about 2.5-5m/sec across the trays. They commonly are used to dry fruit and vegetable pieces, and depending on the food and the desired final moisture, drying time may be of the order of 10 or even 20 hours.


Last modified: Thursday, 27 September 2012, 7:16 AM