Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
Cabinet or tray driers consist of an insulated cabinet fitted with a shallow mesh or perforated
trays, each of which contains a thin layer of food. Hot air is circulated through the cabinet. A
system of ducts and baffles are used to direct air over and/or through each tray to promote a
uniform air distribution. These driers are used for small scale production and have low capital
and maintenance costs. They are relatively difficult to control and produce variable product
quality.
Continuous conveyor or belt driers are up to 20 metres long and 3 metres wide. The food is
dried on a mesh belt. The airflow is initially directed upwards through the bed of food and then
downwards in later stages to prevent dried food from blowing out of the bed.
In pneumatic, flash and/or ring driers, powders or particulate foods are continuously dried in
vertical or horizontal metal ducts. A cyclone separator or a bag filter is used to remove the dried
product. The moist food is metered into the ducting and suspended in hot air. They have
relatively low capital costs, high drying rates and thermal efficiencies, and good control over the
drying conditions.
Rotary driers are slightly inclined rotating metal cylinders fitted internally with flights that
cause the food to cascade through a stream of hot air as it moves through the drier. Airflow may
be parallel or countercurrent. The agitation of the food and the large area of food exposed to the
air produces high drying rates and a uniformly dried product. The method is especially suitable
for foods that tend to mat or stick together in belt or tray driers. It is used on a large scale in the
sugar industry for sugar and beet pulp drying. In the case of pulp, exhaust gases from the
combustion plant may be used as a heat source when the physical plant and the flue-gas
temperature allows. Some combustion plants can produce flue-gas temperatures of
around 120 ºC.
In tunnel driers, thin layers of food are dried on trays, which are stacked on trucks programmed
to move semi-continuously through an insulated tunnel in which hot air is circulated.
In steam bundle driers, the heating medium or steam is not in contact with the wet product. A
heat transfer surface is used to transfer the heat to the product’s surface for drying. The steam
passes through the drier, through cylindrical tubes/bundles which rotate, to avoid local
overheating and to improve uniform drying. This drier uses less air volume and subsequently
emissions into the air are limited.
Steam drying is a special drier design that uses superheated steam produced via a heatexchanger.
The drier consists of a pressure vessel in which the water from the product is driven
off, turned into steam and then used to dry more product. This system is used in the sugar
industry, on a limited scale, for drying beet pulp. One advantage is the low energy consumption
for drying.
Drying in kilns is used for malting and is described in Section 2.2.15.
Vacuum drying is used for temperature sensitive products. The external pressure is lowered to
avoid drying at a high temperature. One simple type of vacuum drying is vacuum roller drying.
In this method, either one or two rollers are installed in a vacuum housing. The resulting vapour
precipitates in a condenser located between the vacuum chamber and the pump. The product is
removed by a screw conveyor.
44 January 2006 RHC/EIPPCB/FDM_BREF_FINAL