APPLYING THE DRYING CURVE TO YOUR DRYING PROCESS
Darren A. Traub
A
Mass
Drying Rate / Temperature
s I explained in my coluct increases from the feed temperaAs the drying process proceeds
umn last month, the initure (most commonly ambient temperthrough to the falling rate period, the
tial and constant drying
atures) to approximately the wet bulb
moisture
content
progressively
rates may affect the
temperature during the constant rate
reduces and the rate of moisfalling rate
ture removal decreases
FIGURE 1. TYPICAL DRYING CURVE
and offer benefits or
markedly. The product
advantages as well as limitemperature starts to
Inlet Temperature
tations. Most commonly,
increase more rapidly
Co
the elapsed time for this
because the effects of
-Cu
rren
r
tD
rye
falling rate period is freevaporative cooling are
D
r
y
t
er
n
T Counter
urre
quently directly dependreduced. As a result,
r-C
e
Drying Rate
t
n
Cou
ent on the drying criteria
temperature-sensitive
Exhaust Temperature
achieved during the conproducts become in danT Counter
Exhaust
stant rate period. That is,
ger of thermal degradaTemperature
if materials are dried raption. The falling rate
Product
idly in the high moisture
period is dependent on
Temperature
% Mo
range, they may continue
the desired final moisture
isture
to dry relatively quickly in
content and is typically
Initial
Constant Rate
Falling Rate
the low moisture range.
longer than the constant
Period
Period
Period
This can be attributed to
rate period. Hygroscopic
Time
the formation of a
products or products
Figure 1. During processing, drying occurs in three different
porous structure (capillar- periods, or phases, which can be clearly defined.
with high bound moisture
ies), which favors more
contents will have an
rapid diffusion and increases the
period and then to almost dry bulb
extended falling rate period to achieve
exposed surface area of the product.
temperatures as it nears the end of the
low final moisture contents.
The levels of initial drying rates also
drying cycle. During the constant rate
Figure 1 also illustrates the relative
may enhance and modify the quality of
period, the product temperature
temperature profiles of the carrier
the final product. For example, rehyremains reasonably constant due to
stream for co- and counter-current
dration rates for products such as
the effects of evaporative cooling.
drying systems. Higher inlet temperainstant coffee or milk tend to follow
tures can be used for co-current sysTHE LEVELS OF INITIAL
the drying rate pattern.
tems, and there is a lower potential for
Certain products skin, crack or
DRYING RATES ALSO MAY thermal damage to the product
shrink at elevated temperatures. In this
because the final product temperature
ENHANCE AND MODIFY
case, high initial drying rates will be
approaches the controlled exhaust
PH
detrimental to the drying process by
temperature.
THE QUALITY OF THE
encapsulating moisture within the
FINAL PRODUCT.
Darren A. Traub is executive vice president
product, retarding movement of moisof Drytech Engineering, Irvine, Calif., a
ture from the inside to outside of the
That is, when evaporating water with a
company specializing in thermal drying
product. In instances where cracking
hot stream of gas, the water film at the
and related system technologies. He has
or shrinking occurs, it also may reduce
surface of the solid will assume the wet
engineered and managed the project exeproduct quality and disfigure certain
bulb temperature of the drying gas,
cution of numerous drying and bulk matematerials.
and the evaporative process can be
rials-handling systems in the United
Product density also relates to dryconsidered to be adiabatic (a process
States, Africa, the Middle East and China.
ing rate and, in general, the faster the
in which the system is changing withHe can be reached at (949) 262-1222 or
drying rate, the lower the density.
out the transfer of heat to or from the
e-mail [email protected].
The temperature curve for the prodenvironment).
DRYING FILES
The Drying Curve,
Part 2
Reprinted with permission from Process Heating magazine, © October 2002 by BNP Media.
www.process-heating.com O C T O B E R 2 0 0 2
All rights reserved. For noncommercial, personal use only.
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