Food Engineering

Lesson 33. TUNNEL DRYING

Module 3. Food dehydration

Lesson 33

TUNNEL DRYING

33.1 Introduction

These type of driers consist of long insulated tunnel either square or rectangular. Tray loads of the wet material are assembled on trolleys which enter the tunnel at one end. The trolleys travel the length of the tunnel and exit at the other end. Heated air also flows through the tunnel, passing between the trays of food and/or through perforated trays and the layers of food. The air may flow parallel to and in the same direction as the trolleys. This is know as concurrent tunnel. Other designs featuring counter current, co-current and cross flow of air are available. For large operations, tunnel driers with elongated cabinets, through which carts pass, are used. A main construction feature by which tunnel driers differ has to do with the direction of airflow relative to tray movement. In a counter flow, or countercurrent pattern the hottest and driest air contacts the nearly dry product, whereas the initial drying of entering carts gets cooler, moister air that has cooled and picked up moisture going through the tunnel. This means that the initial temperature and moisture gradient will not be as great, and the product is less likely to undergo case hardening or other surface shrinkage, leaving wet centers. Further, lower final moisture can be reached because the driest product encounters the driest air. In contrast, co-current flow tunnels have the incoming trays and incoming hottest driest air travelling in the same direction. In this case, rapid initial drying and slow final drying can cause case hardening and internal splits and porosity at centers, which sometimes is desirable in special products.

Tunnel dryers consist of long tunnels through which trucks carrying stacks of trays travel with or against a stream of drying air ( Fig. 33.1). The material to be dried is evenly spread on the trays. Typical tray loading for wet vegetables is in the order of 10–30 kg per m². As one truck with wet material is introduced into the tunnel at one end, another truck, carrying dehydrated product, exits at the other end. Depending on the size of the trucks and the tunnel, the trucks are moved manually or mechanically, e.g. with the help of chains. With respect to the relative direction of movement of the air and the trucks, tunnel dryers operate in co-current, counter current or mixed current fashion.

n the case of the co-current tunnel, air with the highest temperature and lowest humidity meets the food with the highest humidity and lowest temperature. This provides the highest ‘driving force’ for drying and therefore the most rapid rate of water transfer at the entrance to the tunnel. If the feed material is sufficiently humid, its temperature remains low despite the contact with hot air. The ‘driving force’ , however, diminishes as the food travels towards the exit. The air at the exit end of the tunnel is the most humid and the coolest. Consequently, the final residual moisture content of the product may not be as low as desired. The contrary occurs in the case of counter current tunnels. The starting rate of drying is lower but it is possible to dehydrate the product to the lower final moisture content. The mixed flow, central exhaust tunnel functions as two tunnels in series. Its first portion is co-current and provides the desired high initial drying rate. Its last portion is countercurrent and gives the desired finishing effect. Unlike cabinet drying, tunnel drying provides the possibility of exposing the product to a changing profile of external conditions. In addition to air temperature and humidity, it is possible to vary air velocity by reducing the cross sectional area normal to the direction of flow (Fig. 32.2).