In drum drying, a large amount of thermal energy is
released by the condensing steam in the drum and
conducted through drum wall to the product. During
drying, a product may go through three general periods.
Initial heating period: after wet material is applied onto
the drum surface in a thin layer, intensive heat transfer
takes place due to a great temperature difference between
the drum surface and the wet product. Product temperature
increases rapidly to reach the boiling point of free water
(Fig. 3).
Constant product temperature period: after
reaching the boiling temperature, a large amount of free
water evaporates and product temperature remains
constant. The drum surface temperature, however,
decreases due to an intense evaporative cooling. Rising
product temperature period: after removing most of the
free water, the amount of moisture for evaporation is
dramatically reduced. The heat transferred from the steam
gradually exceeds the energy used for evaporation. As a
result, drum surface temperature increases. The bound
water starts to play a major role in controlling the rate of
evaporation. As bound water has a higher boiling
temperature, product temperature gradually increases as
drying proceeds. This trend continues till it reaches the
knife where the dried product is scraped off. After the
product is removed from the dryer, drum surface
temperature continues to increase until new wet material
is applied.
The evaporation rate of free water can be estimated by
the following relationship
In drum drying, a large amount of thermal energy is
released by the condensing steam in the drum and
conducted through drum wall to the product. During
drying, a product may go through three general periods.
Initial heating period: after wet material is applied onto
the drum surface in a thin layer, intensive heat transfer
takes place due to a great temperature difference between
the drum surface and the wet product. Product temperature
increases rapidly to reach the boiling point of free water
(Fig. 3).
Constant product temperature period: after
reaching the boiling temperature, a large amount of free
water evaporates and product temperature remains
constant. The drum surface temperature, however,
decreases due to an intense evaporative cooling. Rising
product temperature period: after removing most of the
free water, the amount of moisture for evaporation is
dramatically reduced. The heat transferred from the steam
gradually exceeds the energy used for evaporation. As a
result, drum surface temperature increases. The bound
water starts to play a major role in controlling the rate of
evaporation. As bound water has a higher boiling
temperature, product temperature gradually increases as
drying proceeds. This trend continues till it reaches the
knife where the dried product is scraped off. After the
product is removed from the dryer, drum surface
temperature continues to increase until new wet material
is applied.
The evaporation rate of free water can be estimated by
the following relationship
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