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Which drier is the right one?
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A lot of manufacturers and processors of food, chemical and pharmaceutical products face the problem that they have to dry their products to make them storable and to preserve them for trade and the consumer for a time long enough. With a lot of different driers on the market, the decision is not easy.

If the investment costs for a drier are to keep within reasonable bounds, normally a convection drier is the only one to take into consideration. The function principle of the convection driers is based on hot, dry air flowing around the product. The air temperature, on the one hand, should be high enough so that the water absorption rate is as high as possible, on the other hand, however, the product to be dried must not be damaged.

Generally, the following applies: the lower the temperature must be, the higher is the necessary air-flow. As the air speed through the product can only be so high that the product is not carried along by the air flow, the whole drying time is extended and thus the size of the drier and consequently the investment costs are increased.

In general, there are four different types of drying plants on the market: The classic flatbed drier, the belt drier, the duct drier with transverse ventilation or as cascade drier and the rotary drier. We do not go into the tray drier, which is mainly used for smaller batches.

The flatbed or feed-and-turn drier essentially consists of a fixed screen deck, on which the product is fed in a certain bulk height. The air is pressed through the screen deck and through the product from below and escapes freely into the surrounding air after having flown through the complete bulk height or the air is exhausted and lead off by an exhaust fan.The principle is taken over from the well-tried wagon drying system, i. e. drying the product in one batch without motion and mixing.

A flatbed drier has additionally a feed-and-turn device moving the product continuously forward and mixing it. Thus the feed-and-turn drier can be used as a batch drier on the one hand, drying one drier filling completely to the required final moisture; subsequently, the drier is emptied completely and filled with product again. Hereby, the turn device serves for mixing and emptying only. On the other hand, a feed-and-turn drier can be used as continuous mixed flow drier. At one side, fresh product is fed onto the screen, at the other side, the product is discharged. The feed-and-turn device conveyes and mixes the product.

The flatbed drier was nearly forgotten in the eighties and nineties, as its original function of drying grain was taken over almost completely by the more efficient cascade drier. It has recently, however, gone through a renaissance in drying non-trickling products like e. g. herbs, pumpkin seed etc. Here it is nearly unrivalled for smaller up to medium capacities when it comes to costs and flexibility.

The belt drier is unmatched in flexibility. There is virtually no product that cannot be dried on a belt drier, also non-trickling, bulky, sticky or sensitive products.
Similar to the flatbed drier, the product is ventilated through the trellis or perforated plate of the belt. In contrast to the flatbed drier, the product is not moved over the screen, but rests on the belt, which carries the product through the whole drier. Depending on the capacity of the plant and the available space, a belt drier can be designed as one, two or multi belt drier. The belt speed and the bulk heights can be adjusted individually depending on the kind of product. The grain size of the product is basically the only technical limitation.

As a rule, the grain has to be bigger than the mesh width or the hole diameter of the belt. Even if the product agglomerates easily, i. e. sticks together, drying is possible. Some special chemical products having this characteristic can even be dried with a grain size of > 3 m!

A belt drier is flexible in air adjustment and ducting. In only one plant, many things can be combined, to start with a various temperature and air flow up to the alternating upper and lower ventilation of various belt zones. The fields of application of the belt drier include food and petfood industry as well as the chemical, pharmaceutical and recycling industry.

The drier used most for trickling bulk goods is the tunnel or vertical drier. Its main field of application is the drying of grain, corn, oil seed etc. The product runs through the drier from above downwards, the air flowing through the bulk material. Simple tunnel driers consist of vertical, parallel perforated plates with the product between them. The air flows through the perforated plate on the hot air side, then through the product and escapes through the perforated plate on the exhaust side.

The cascade drier dries the product more carefully and consistently; the air is distributed in the product by roof-shaped air ducts. The cascades are arranged in such a way that a virtual “bulk height” develops between inlet and exhaust air roof; this “bulk height” is constant throughout the drier. In the course of the drying process, every single grain is dried constantly and carefully because of the product passage and the permanent change of the ventilation direction. Usually, cascade driers are installed outside because of their size. Therefore, great importance has to be attached to high-quality material. Aluminium has become technical standard, it resists corrosions or mechanical stress when it is high-quality alloyed.

For very high capacities of water evaporation, a drum drier is used in many cases. The product is transported in the rotating drum mainly by paddle installations, the free product overflow to the discharge and the intake air. The drier the product becomes, the further the air flow carries the product to the drier discharge.

A drum drier can dry large product quantities with higher temperatures in a relatively short time. Basically, drum driers, similar to belt driers, are suitable for a vast number of products. Related to the capacity, the purchase costs are approximately identical with those of a belt drier. There is, however, a relatively great amount of energy involved. Prior to the final decision for a drum drier, the product to be processed and its precise parameters have to be considered exactly.


The following table should give an overview of the various drying plants and their fields of application.

  
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