1) Drying curves plot drying rate versus remaining water content and can describe the drying process in steps with changing drying rates. 2) There is an initial constant rate period where unbound surface water is removed, followed by a falling rate period where the drying rate decreases as water moves internally through the material. 3) Drying rates determined experimentally can be used to calculate drying times to design drying equipment and operations. Simplified calculations can provide useful estimates by assuming constant temperature and humidity.

1. DRYING RATE
Mamta Sahurkar

2. Drying curve
• Drying curve is the plot of the drying rate Φ or N
versus the remaining water content X. Water content
X is expressed as kg of water per kg of dry matter.
Drying process of a material can be described as a
series of steps in which drying rate plays a key role.
• Following figure shows typical drying rate curve for a
constant drying conditions.

4. • Point B represents equilibrium temperature conditions of
the product surface.
• Section B to C of the curve, known as the constant rate
periods, represents removal of unbound water from the
product.
• The water acts as if the solid is not present.The surface of
the product is very wet and water. And the water activity
is equal to one.
• The constant rate period continues as long as the amount
Constant rate
period

5. In the constant-rate period, the water is being evaporated from
what is effectively a free water surface.The rate of removal of
water can then be related to the rate of heat transfer, if there is
no change in the temperature of the material and therefore all
heat energy transferred to it must result in evaporation of
water.The rate of removal of the water is also the rate of mass
transfer, from the solid to the ambient air.These two - mass
and heat transfer - must predict the same rate of drying for a
given set of circumstances.

6. • The falling rate period is reached when the drying rate starts
to decrease, and the surface water activity falls to less than
one.
• The rate of drying is governed by the internal flow of liquid
or vapor.This point is represented by C in the figure.
• At this point there is not enough water on the surface to
maintain a water activity value of one.
Falling rate period

7. • The falling rate period can be divided
into two steps.
• First falling drying rate
• Second falling drying rate

9. A first falling drying rate occurs
when wetted spots in the surface
continually diminish until the
surface is dried (Point D).
First falling drying ra

10. • Second falling rate period begins at point D
when the surface is completely dry.The plane of
evaporation recedes from the surface.
• Heat required for moisture removal is
transferred through
the solid to the vaporization of moisture in the
solid and the vapor moves through the solid into
Second falling drying
rate

11. • The amount water removed in this period
can be relatively small compared to the
constant rate and first falling rate period.
• However this period may take much
longer than constant rate period because
the drying is slow.

12. Drying rate experiments can be simply done
by measuring the weight change during
drying.The material to be dried is placed on
the tray.The tray is suspended from a
balance and exposed to air flow in drying
tunnel.
Drying rate experimen

13. Water removed/time can be easily
determined

17. Drying time

18. Drying rates, once determined experimentally or
predicted from theory, can then be used to
calculate drying times so that drying equipment
and operations can be designed. In the most
general cases, the drying rates vary throughout
the dryer with time as drying proceeds, and with
the changing moisture content of the material.
So the situation is complicated.
Calculation of DryingTimes

19. However, in many cases a simplified approach can provide
useful results. One simplification is to assume that the
temperature and RH of the drying air are constant.
In this case, for the constant-rate period, the time needed to
remove the quantity of water which will reduce the food
material to the critical moisture content Xc (that
corresponding to the end of the constant-rate period and
below which the drying rate falls) can be calculated by
dividing this quantity of moisture by the rate.

20. In this case, for the constant-rate period, the time
needed to remove the quantity of water which will
reduce the food material to the critical moisture content
Xc (that corresponding to the end of the constant-rate
period and below which the drying rate falls) can be
calculated by dividing this quantity of moisture by the
rate.

21. and Xo is the initial moisture content and Xc the final
moisture content (the critical moisture content in this
case) both on a dry basis, w is the amount of dry
material in the food and (dw/dt) const is the constant-
drying rate.

22. Where the drying rate is reduced by a factor f then this can be
incorporated to give:
Dt = w (DX)/ f(dw /dt)const.
and this has to be integrated piecemeal down to X = Xf where
subscript f denotes the final water content, and f expresses the
ratio of the actual drying rate to the maximum drying rate
corresponding to the free surface-moisture situation.

23. THANK YOU