CONDUCTION DRYING 0F SHELLED CORN

by
GLENN E. HALL

AN ABSTRACT

Submitted to the College of Agriculture of Michigan State
University of Agriculture and Applied Sciences in
partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Agricultural Engineering

Approved byi Ea M M

May 1960

 

ABSTRACT

The drying characteristics obtained by conduction
heating in comparison with convection heating of a thin
layer of shelled corn were obtained. Kernels of corn were
cut up before and during drying to determine where the
moisture was concentrated and the direction of movement
during drying.

A heated plate with temperatures of 229, 189, 144 and
104°F was used for conduction drying and air flows of 25, 50
and 100 fpm were used over the heated plate in addition to
the natural air current to carry moisture away.

The rate of drying was higher during the first part
of drying by convection, but the conduction drying rate
was higher during the later part of drying. Drying from 56
percent, d.b., to 15 percent, d.b., moisture content was
faster for all four plate temperatures. Conduction drying
from 56 to 15 percent, d.b., at 229°F required 49 minutes
while convection drying required 54 minutes, or a saving of
10.2 percent in time. Conduction drying from 56 to 15 per-
cent, d.b., at 189°F required 96% minutes while convection
drying required 111 minutes, or a saving of 15.1 percent in
time.

The drying rate, which was directly related to the

plate temperature, of the kernel was the same whether the

white or yellow side was against the heated plate. The tip

of the kernel contained the highest moisture content of any

part of the kernel, while the white side had a higher mois-

ture content than the yellow side. Moisture movement was

mainly from the head of the kernel to the tip. The drying

period, which was divided into four falling rate periods,
-k9

was defined by the equation M-Me : e .
Ir M
"AC -A’A e

CONDUCTION DRYING OF SHELLED CORN

by
GLENN E. HALL

A THESIS

Submitted to the College of Agriculture of Michigan State
University of Agriculture and Applied Sciences in
partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Agricultural Engineering

Approved by flM “1M. _

May 1960

ACKNOWLEDGEMENT

The author wishes to eXpress his appreciation to Dr.
Carl W. Hall, whose guidance and encouragement made the in-
vestigation rewarding and enjoyable.

Grateful acknowledgement is also due to Dr. Fred H.
Buelow for his assistance in the experimental set-up and
guidance during the absence of Dr. Hall.

The author feels deeply indebted to Dr. Arthur W.
Farrall, Head of the Department of Agricultural Engineering,
for the graduate research assistantship that made the under-
taking of the investigation possible.

Mr. James Cawood and Mr. Glenn Shiffer of the re-
search laboracmzrare thanked for assistance and c00peration
during the investigation.

The author would also like to express his apprecia-
tion to his wife, Rose, for her patience and assistance
which helped make the completion of the investigation

possible.

11

CONTENTS

PAGE

Summary . . . . . . . . . . . . . . . . . . . . .
List of Figures . . . . . . . . . . . . . . . . .
List of TableS. . . . . . . . . . . . . . . . . .

IntrOduCtion. O O O O O O O O O O O O O O O O O O

ODO\U‘IKNl-'

Review of Literature. . . . . . . . >/. . . . . .
Equipment . . ... ... . . . . . . . . . . . . . . '19
Procedure . . . . . . . . . . . . . . . . . . . . 22
Discussion and Results. . . . . . . . . . . . . . 27
Conclusions . . . . . . . . . . . . . . . . . . . 52
Recommendations for Future Study. . . . . . . . . 53
Bibliography. . . . . . . . . . . . . . . . . . . 54

Appendix 0 O O O O O O O O O O O O O O O O O O I O 56

iii

 

 

 

SUMMARY

The drying rate was directly related to the plate
temperature, with the faster drying rates on the higher
plate temperatures. The drying rate was the same whether
the kernel was lying with its white or yellow side against
the heated plate.

The natural air current rising from the plate due to
the heat was sufficient to carry away the moisture as it
was evaporated from the kernel since added forced air did
not increase the drying rate.

Moisture movement was mainly from the head of the
kernel to the tip.

The tip of the kernel contained the highest moisture
content, 85 percent d.b., of any part of the kernel. The
white side of the kernel had a higher moisture content, 68
percent d.b., than the yellow side, which was about 45 per-
cent d.b.

The drying period could be divided into four falling
rate periods with each successive period having a slower
rate of drying.

Conduction drying is faster than convection drying
when drying from 56 percent d.b. to 18.3 and 15 percent
d.b. for the 229°F, 189°F and 144°F temperatures and to
18.3 percent d.b. for the 105°F temperature. Drying from

1

56 percent d.b. to 15 percent d.b., with conduction results
in a time saving over convection of 10.2 percent for 229°F,
15.1 percent for 189°F, 16.5 percent for 144°F and 3.2 per-
cent for lOAOF.

The equation which can be used to define drying of
shelled corn by conduction drying is as follows:

M-Me = e-ko
Mo-Me

9.
10.

11.

LIST OF FIGURES

Sketch of eXperimental set-up used for con-
ducting the investigation. . . . . . . . . . .

Sketch of kernel as divided into parts dur-
ing the investigation. . . . . . . . . . . . .

Semi-logarithmic plot comparing conduction and
convection drying of shelled corn. . . . . . .

Comparison of conduction and convection drying
of shelled corn. . . . . . . . . . . . . . . .

Drying shelled corn with 229°F conduction
heating and O, 25, 50 and 100 fpm air. . . . .

Drying shelled corn with 189°F conduction
heating and O, 25, 50 and 100 fpm air. . . . .

Drying shelled corn with 144°F conduction
heating and O, 25 and 50 fpm air . . . . . . .

Drying shelled corn with lO4°F conduction
heating and O and 25 fpm air . . . . . . . . .

Drying shelled corn in 189°F conduction
heating plate with additional plate above
kemelsoooooooooooooooooooo

' o
Drying shelled corn on 189 F conduction heat-
ing plate with white or yellow side of kernel
do wnward O O O O O O I C 0 O O O O O O ‘0 O O O

Drying shelled corn on 189°F conduction heat-
ing plate, white or yellow side of kernel
downward, with average moisture contents of
the various sections of the kernel deter-

mined...00.000000009000000

~ 0
Drying shelled corn on 189 F conduction heat-
ing plate, white side of kernel downward,
with the moisture content of W,'C and Y
sections of the kernel determined. . . . . . .

\Jl

PAG'

21

26

39

40

41

42

43

44

45

46

47

48

F1

”'1'?
JV-“

13.

15.

LIST OF FIGURES CONTINUE

Drying shelled corn on 189OF conduction heat-

ing plate, yellow side of kernel downward,

with the moisture content of W, C and Y

sections of the kernel determined. . . . . . . 49

Drying shelled corn on 189°F conduction heat-

ing plate, white side of kernel downward, with

the moisture content of H, M, T and T sections

of the kernel determined . . . . . . . . . . . 50

Drying shelled corn on 189°F conduction heat-

ing plate, yellow side of kernel downward,

with the moisture content of H, M, U and T

sections of the kernel determined. . . . . . . 51

TABLE
1.

LIST OF TABLES

Range of applicability of the k-values for
conduction drying. . . . . . . . . . . . . . .

Summarization of moisture contents from the
229OF data C O - O O O O O O O O O O O O O 0 O O

Summarization of moisture contents from the
1890F data I O O O O O O O O O 0 O O O O O O O

Summarization of moisture contents from the
lMOF data 0 O O 0 0 O O O O O O O O O O O O 0
Summarization of moisture contents from the
lOAOF data C O O O O O O O O O O O O O O I O O

Summarization of the reduction of drying time
by using conduction drying to replace con-
vection drying . . . . . . . . . . . . . . . .

Summarization of drying runs conducted for the
investigation . . . . . . . . . . . . . . . .

PAGE

32

34

35

36

37

57

 

INTRODUCTION

The major portion of drying farm crOps in the past
has been with the use of forced air through the product.
There are a few isolated cases of drying grain with the use
of conduction heating of the product, but the studies were
so limited that conclusive results were not reached. A
considerable amount of information is available on the use
of conduction heating for drying non-hygroscOpic material
such as sand.

According to Hall (1957) 10 percent of the grain
crops in the United States are lost between harvesting and
consumption. If better and more convenient means are made
available to dry grains the practice of drying may become
more widespread and result in a large saving by less spoil-
age and earlier harvest. With a suitable means of drying,
crops could be harvested earlier with less shattering, shell-
ing and quality deterioration.

A machine which harvests and dries in one operation
would be the ideal situation and might be a reality when a
means is available to dry grain rapidly and safely.

The aim of this investigation was to determine the
rate of drying obtained by conduction heating of shelled
corn in comparison with convection air drying considering,
(1) the effect of the temperature and air flow on the rate

6

7

of drying, and (2) the development of an equation which will

apply to conduction drying.

REVIEW OF LITERATURE

The process of drying occurs when the vapor pressure
of the product to be dried is higher than the vapor pressure
of the surroundings, causing the moisture to move outward.
The vapor pressure of the product is increased by heating
the product by conduction, convection and/or radiation.
Jakob (1949) defined heat conduction as a process of heat
transmission due to the elastic impacts of molecules of
gases, to longitudional oscillations in solid non-conductors
of electricity, and to the motion of electrons in metals.

He defined heat convection as the transportation and ex-
change of heat due to the mixing motion of different parts
of a fluid. He defined heat radiation as being identical
with light radiation.

Hukill (1948) wrote that in evaporating moisture from
grain it is necessary to supply the heat necessary for
evaporation. Evaporation from a free water surface requires
less heat per pound of water evaporated than evaporation of
moisture from grain. The amount of moisture evaporated
from grain is proportional to the amount of heat delivered
to the grain. The higher the temperature the more readily
the moisture leaves the grain. To dry at a faster rate it
is necessary to heat the grain to a higher temperature since
the drying rate is directly dependent on the temperature of

the grain.

Robertson (1959) compared conduction drying with con-
vection drying, both at 135°F, and found that after 10
minutes the heated surface dried shelled corn at a faster
rate than the forced heated air. Any moisture content de-
sired which required more than 30 minutes drying time would
be obtained in less time by using conduction rather than
convection drying since the two curves crossed at 30 minutes.

Kelly (1941) heated wheat by conduction in a cement
mixer, placed the wheat in other containers and forced air
through the wheat. Fifty-seven percent to 60 percent of the
moisture loss took place in the first minute of forced air
drying with 43 percent of the temperature drop. Four minutes
after the start of forced air through the wheat the rate of
drying was very slow. An additional eight minutes was used
to reduce the temperature to a level safe for storage.
Approximately 18 percent moisture content w.b. wheat was
used with a moisture loss of about two percent when heating
the wheat to 150°F and exposing it to 12 minutes of forced
unheated air in six inch layers. He found that the thinner
the layer during the air drying and the higher the tempera-
ture to which the grain was heated, the greater the drying
rate for a given air flow.

Information concerning the use of conduction heating
for drying porous substances such as grain is limited.’
There is extensive data on the use of conduction heating
to dry non-porous products such as sand and pulp. This

material is presented since it is related to the problem

10

under investigation by the method of heat transfer to the pro-
duct. The difference between the drying of porous and non-
porous products is related in the articles by Corbin and
Newith (1955), Hall (1957), and Sherwood (1936).

Haines (1927) explanation of moisture movement stages
in a solid started with a dry bed of soil or sand. To this
he added a small amount of water so that the points of con-
tact of the particles held a small volume of moisture. This
was called the pendular stage. Next, more water was added
until each particle had a complete film of moisture around
it and the film was continuous through the bed. This was
called the funicular stage. Finally, water was added until
all the pore Space between the particles was filled. This
was called the capillary stage.

Corbin and Newith (1955) showed that the drying
characteristics of beds of moist porous granular material
are consistent with the capillary theory of drying, the
movement of moisture to the drying surface being governed
by the surface tension, gravity and friction forces acting
on the liquid in the pores and voids of the bed. The form
of drying rate curves differed from those for corresponding
beds of non-porous granular materials mainly through the
capillary action of the internal pores of the granules in
conveying water to the surface during the pendular state.
Thus the rate of drying during the constant rate period is
higher for porous than for non-porous material, and after

the critical moisture content is reached there follows*an

11

initial period having a very rapid reduction in drying rates
and a prolonged second period in which the drying rate falls
slowly. Other differences in drying during the various
stages can be attributed to the water carrying capacity of
the porous granules.

Hall (1957) wrote that there are two major drying
periods, the constant rate period and the falling rate period.
In the constant rate period the drying takes place on the
surface and is influenced by the surrounding conditions.

The constant rate period continues as long as the internal
moisture moves to the surface as fast as surface moisture

is evaporated. The constant rate period is short in dura-
tion and is dependent upon the area exposed, differences in
humidity between the air stream and wet surface, coefficient
of mass transfer and velocity of the drying air. The fall-
ing rate period, which is generally the whole range of dry-
ing, is influenced by the product, diffusion of moisture to
the surface and removal of moisture from the surface. The
falling rate period for shelled corn is divided into four
periods for corn above 25 percent d.b. This can be eXplained
as follows: the surface area of the micrOpores of wet corn
are covered with water; the thickness of the water is
directly proportional to the moisture content. The thick-
ness of a water molecule is about 5x10‘8cm. With four.
layers of water molecules there would be four falling rate
drying periods because the drying constant changes as each

layer is removed. Drying of corn involves two fundamental

12

processes: (1) there is a transfer of heat to evaporate

the kernel moisture and (2) there is a movement of water as
a liquid.and/or vapor from the kernel. The equation repre-
senting moisture movement during the falling rate period is

based on Newton's cooling equation, which, in integrated

t-te ecu-k0
form is t5-te = . Substituting M for t results in
M-Me e'ko M-Me
Ho-fie a When Mo-Me is plotted against 0 on semi-

logarithmic paper, k is the slope of the line. The rate of
M-Me -k0

drying can be found by differentiating fi3:fie = e with

reapect to 0 or by multiplying the mass transfer coefficient

by the effective area and the vapor pressure driving force.

The vapor pressure driving force is the vapor pressure of

the grain minus the vapor pressure of the air.

Sherwood (1936) developed a theory which suggested
three drying periods in the drying process. These three
periods were: (1) a constant rate drying period during
which the surface remains completely wet, the rate of evapora-
tion being the same as that from a constant area free water
surface; (2) a first falling-rate period during which there
is a decrease of wetted surface with the drying rate being
directly proportional to the percent of surface that is
wet; and (3) a second falling rate period during which the
drying rate is proportional to the rate of water transfer
to the surface. He further designated the moisture content
at the end of the constant rate period as the critical mois-

ture content. If the critical moisture content is less than

the required moisture content all the drying will be in the

13

constant rate period, and by the same token, if the critical
moisture is greater than the initial moisture content the
whole drying process will occur in the falling rate period.

Tambling (1953) in drying 0.75 inch thicknesses of
sand on a hot surface, reported four periods during the dry-
ing process: (1) a heating up period during which the tem-
perature of the sand reached a maximum with flash vaporiza-
tion taking place when the sand slab was placed on the plate;
(2) a constant rate period during which the temperature
gradient was constant and the moisture was distributed evenly
through the slab; (3) a transition period during which the
zone of vaporization moved toward the top surface of the
slab; and (4) a falling rate period during which the tempera-
ture started to increase after reaching a minimum and the
moisture was distributed in a bell shaped curve. He found
that the higher the plate temperature the greater the dry-
ing rate and that there were two zones of vaporization, one
starting at the hot plate interface and the other starting
at the open face. The water movement was mainly toward the
heated plate.

Ludt (1957) in drying sand on a hot surface, found
that three drying periods were present, namely, (1) constant
drying-rate period, (2) first falling rate period and (3)
second falling rate period. Constant layer moisture at
the hot surface was instrumental in maintaining the constant
drying rate. The first falling rate begins when a constant

hot surface moisture can no longer be maintained. The

14

second falling rate begins when the pendular state of water
is reached. He found that the constant rate period was from
23 to 10 percent moisture content, d.b., with the falling
rate starting between 10 and 6 percent moisture.

Retford (1957) dried sand on a 220°? heated surface
and found that maximum drying rate and heat transfer coeffic-
ients were obtained with a one-half inch layer as compared
with one or one and one-half inch layers. The heat trans-
fer coefficient was correlated with the drying rate between
moisture contents of two to ten percent, d.b. The drying
rate of the one-half inch layer of sand was 6.9 pounds water
per hour sq. ft., while that for a one inch layer was 6.2
pounds water per hour sq. ft. The one and one-half inch
layer had the same rate as the one inch layer. The change
in the heat transfer coefficient seemed to be controlled by
the area of wetted surface.

Ceaglske and Hougen (1937) showed that moisture flow
in granular non-hygroscOpic solids such as sand was due
mainly to capillary forces. They showed data which indi-
cated movement of moisture from an area of low concentra-
tion to an area of high concentration, which was not possi-
ble considering only diffusion forces.

McCready (1935) has shown that pulp slabs in contact
with a hot surface dried at a constant rate for an interval
of time and then experienced a falling rate period. A
constant temperature occurred in the slab until the critical

moisture was reached and then the temperature decreased. IIn

15

the first period of falling-rate drying, the rate of drying,
heat transfer from plate to slab and the temperature of the
slab decreased. This indicated an increased resistance be-
tween the slab and hot surface due to the drying at the hot
surface. In later parts of the falling-rate period there
was an increasing temperature difference across the zone of
vaporization with a decreasing temperature difference be-
tween the hot plate and slab. He showed that the tempera-
ture and relative humidity of supplementary air had very
little effect on the drying rate as the heat of vaporization
is supplied by the hot plate. The thickness of the pulp
slabs and the temperature of the hot surface were the major
factors which influenced the drying rate.

Shroff (1949) in drying 0.17 to 0.20 inch thick
laminations of paper pulp found that flash vaporization took
place on the 240°F heated surface. The temperature of the
sheet rose to the boiling point of water while the tempera-
ture of the plate decreased. Both temperatures then de-
creased to a minimum, then continually increased with the
plate temperature attaining its original value. The period
of falling temperatures includes the falling rate period
during which drying begins. The interval of increasing
temperature constitutes the rest of the falling rate period
which is controlled by internal diffusion. The moisture
content of the sheets decreased very rapidly, with the
thinner sheets drying at a faster rate than thicker sheets.
The controlling mechanisms during drying were (1) unsaturated

16

surface drying, (2) internal liquid diffusion and (3) dif-
fusion from the interior of the fiber. The rate of drying
depends on the amount of heat transferred to the pulp which
in turn is controlled by the coefficient of heat transfer
and the resistance of heat flow in the material. As the
sheet dries the resistance to heat flow is increased and the
drying rate decreases. The coefficient of heat transfer be-
tween the plate and sheet varied from 122 Btu/hr. sq. ft. °F
at time zero to 0.9 Btu/hr. sq. ft. 0F at the end of seven
minutes, at which time the diffusion of moisture from within.
the fibers seemed to be the controlling factor.

Ernst, Ardern, Schmiel, Tiller (1938) found that in
drying Prussian Blue, a gelatinous solid that adheres firmly
to the drying plate, the drying rate proceeded normally at
first and at the end of the initial period suddenly decreased.
The rate then rose sharply and fell off gradually as during
a normal falling-rate period. Drying proceeded normally at
first while surface moisture and internal reservoirs were
being emptied. After this the only moisture being evaporated
was held in the capillaries themselves. The liquid then be-
gan to rise in the capillaries causing a vacuum since the
Prussian Blue adhered to the plate and prevented the water
from rising to the surface, thus the sudden decrease in dry-
ing rate. The solid, which had been drying at the heated
surface finally broke away from the surface and air entered

the capillaries. -Drying then resumed as normally eXpected.

17

King and Newitt (1955) dried non-porous granular
glass beads on a heated plate under various conditions and
found that there was an initial heating period during which
the drying rate was constant and the conductivity of the
drying bed did not change. This was followed by a pseudo-
constant rate period of a slowly decreasing drying rate which
is similar in duration to the constant rate period for normal
convection air drying and in the period during which the sur-
face of the bed is sufficiently wet to maintain the air film I
in contact with it in a saturated condition. The constant
rate period was followed by a first falling rate period during
.which the rate fell rapidly and a second falling rate period
during which the rate decreased slowly. The heat conductive
ity of the bed decreased as the bed dried, since the con-
ductivity is a function of the moisture content, resulting
in a decreasing drying rate.

They also used forced air of varying velocities and
temperatures across the heated plate and found that the
higher the temperature and velocity of the air the faster
the drying rate. Also, the higher the plate temperature
the faster the drying rate.

Ernst, Ridgway and Tiller (1938) stated that drying
in a vacuum could be used to dry substances that decompose
or undergo undesirable physical changes at high temperature.
Their results indicated that there was little change in the

rate of drying when the vacuum was changed, when there is a

large difference between the boiling point of the water'at

18

the drier pressure and the plate temperature. The rate of
drying depended more on the plate temperature than on the

vacuum 0

EQUIPMENT

The shelled corn was dried on a 0.040 inch thick
steel plate suspended above a 220 volt hi-speed calrod burner
hot plate. The plate temperature was regulated by changing
the height of the heated plate above the burner. Five light
bulbs were connected in parallel and Operated through an
electrical volt meter to control the voltage to the hot
plate and thus provide a control of the temperature by
changing the size of the light bulbs. —

The temperature was measured by a thermometer set in
a 1/4 inch piece of steel drilled out to the size of the
thermometer bulb and placed on the heated plate.

The samples removed during drying were weighed be-
fore and after oven drying on a Mettler Analytical Balance.

After the initial weighing the samples were dried in
a Precision Scientific Air-oven.

Air from a compressed air line was dispersed in a
9 inch x 16 inch x 20 inch plenum chamber before being
blown 2 inches above and parallel to the corn for several of
the runs. The air velocity was controlled by a valve in the
line and measured with a Hastings Air Meter.

The experimental set-up is shown in Figure 1.

19

2O
EQUIPMENT LIST

Hot plate (to heat the drying plate)
Hi-speed calrod burner
Size: 220 volt

Plenum Chamber (to get uniform air flow)
9 inches x 16 inches x 20 inches

Air Meter (to measure rate of air flow)
Make: Hastings
Model: G-5
Manufacturer: Hastings Instrument 00., Hampton,

Virginia
Volt-Amp Meter (to maintain constant voltage to hot
plate)
Make: Weston Industrial Analyzer
Model: 639
Type: 2

Number: 4161
Manufacturer: Weston Electrical Instrument Corpora-
tion, Newark, New Jersey

Analytical Balance (to weigh samples)
Make: Mettler
Accuracy: 0.00005 gram
Manufacturer: Mettler Instrument Corporation,
Highstown, New Jersey

Drying Air-oven (for removal of moisture in samples)
Type: Air
Manufacturer: Precision Scientific Co.,
Chicago, Illinois

Variac (to regulate heating for convection drying)
Size: 0-130 Volts

Hair Dryer (for convection drying)
Make: Kenmore
Model: 559830

Psychrometer (to obtain relative humidity)
Type: sling

Thermometers (for temperature measurement)
Size: 0-2300F

21

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

"“w—WJU

 

 

 

 

Lime 1. SketE 0? experimental sit-1n: need for conducting the investigation

 

 

 

 

PROCEDURE

All corn used in the drying portions of this experi-
ment was hand shelled Pfister's Hybrid 244P yellow dent corn.
It was shelled on 12 September 1959, when it contained ap-
proximately 56 percent moisture, d.b., and placed in a zero
degree Fahrenheit cold storage box. One week prior to being
used in the eXperiments, the corn was placed in the 40°F cold
storage box.

The corn was removed from the 40°F box and placed on
the heated plate. A 10 second time period was allowed be-
fore starting the time for drying during which the kernels
were flattened on the hot plate in a single layer. At each
time interval a sample of six kernels selected randomly were
removed. The sample was placed in a closed metal container
and allowed to cool 10 minutes before weighing on the analyti-
cal balance. After weighing, the sample was placed in a
212°F drying oven for 72 hours. After oven drying, 10 minutes
were allowed to permit the sample to cool before reweighing
to get the dry matter weight. This procedure was repeated
for each sample. Four runs were made for each drying condi-
tion.

Air from a compressed air line was used to increase
the velocity across the shelled corn while drying for some

of the runs.

23

Samples from several of the runs were cut into various
parts after drying to determine the drying effect throughout
the kernel. The notation for each part of the kernel is
shown in.Figure 2. Runs were also made to determine if the
drying rate was the same whether the kernel was lying with
its white or yellow side against the heated plate.

The equilibrum moisture contents for the four plate
temperatures were found by drying the shelled corn on the
heated plate until no further significant amount of moisture
was lost by the sample.

In order to obtain the drying rates the data obtained
from the drying runs was plotted on semi-logarithmic graph
paper. Values of moisture content ratio, (M-Me)/(Mo-Me),
were plotted as the ordinate on the logarithmic scale against
drying time on the arithmetic scale. The plots resulted in
straight lines over limited portions of the curve with sharp
changes in the slaps between the straight line portions of
the curve. The results show that the drying of shelled corn
by conduction heating obeys the equation (M-Me)/(Mo-Me)=e‘k°,
which is analogous to Newton's law of cooling, where k is
valid over limited ranges of moisture content.

The drying rate expression is obtained by differenti-
ating with respect to time the equation:

(M-Me)/(Mo-Me) = e‘k0 (1)
(M—Me) = (Mo-Me)e"k0 (2)

M = (Mo-Me)e"’k0 + Me ’ (3)

dM = -k (Mo-Me) e'"k0 (4)

33

24

Substituting equation (2) into equation (4) results

dM = -k M-M
35 ( e) (5)

which states that the drying rate is directly related to the
free moisture content and the drying constant k, where k is
the slape of the straight line portions of the semi-logarith-
mic plots. The values of k were calculated for the four
plate temperatures and are listed in figure 3. The values

of k were generally higher for the higher plate temperatures.
The discovery of four falling rate periods, corresponding to
the four k values agreed with Hall (1957) who first reported
that corn above 25 percent d.b. moisture content has a fall-
ing rate period of drying subdivided into four periods.

To compare conduction drying with convection drying,
corn of the same variety and moisture content was dried in
thin layers by the use of convection heating in runs 88-95.
This was accomplished‘by forcing air at the rate of 235 fpm
through the single layer of corn with a hair dryer. The
temperature was controlled by a variac controlling the volt-
age to a heating element which heated the inlet air to the
dryer to get the 229°F and 189°F air temperatures. The
144°F and 104°F air temperatures were obtained by controlling
the hair dryer heating element with a varies. A sample of
approximately 10 grams was placed in a wire basket suspended
in the heated air stream. The basket was removed from the

air stream and weighed periodically during the drying process.

25

After the heated air drying process the samples were placed
in the air oven for 72 hours so the weight of dry matter of
the sample could be determined. The times for drying by
convection were the same as those used for the conduction

drying.

DISCUSSION AND RESULTS

The drying rate periods consisted of four falling
rate periods. This was in agreement with Hall (1957). who
stated that the initial moisture content is usually less
than the critical moisture content, therefore, all the dry-
ing would take place in the falling rate period.

Runs one through four at 229°F plate, runs five
through nine at 1890F plate, runs 10-13 at 144°F plate and
runs 14-17 at 104°F plate gave quite consistent results with-
in each group.

Runs 18-21 had 100 fpm forced air of about 35 percent
relative humidity parallel and two inches above the 229°F
plate and did not dry as rapidly as runs 22-25 with 50 fpm
or runs 26-29 with 25 fpm. This was caused by the high ve-
locity air cooling the plat off and llowing less heat to he
transferred to the kernel. The 229°F plate runs with forced
air at 100 fpm raver came back up to 229°F after the initial
drop to 15 O? in plate temperature, but attained a tempera-
ture of approximately 186OF. The 229OF plate temperature
after the initial drOp in plate temperature to approximately
173OF which was the same as for the zero fpm runs. Using
forced air at 100 fpm was, in effect, the same as using
a lower plate temperature. The work done by King and Newitt

(1955) showed that a faster rate of drying was obtained by
27

(‘0
CD

using forced air across the product in combination with the
conduction heating. This occurred because they maintained a
constant temperature on the plate by supplying more energy
during the drying periods which would cause the plate tempera-
ture to decrease. The runs without any forced air across

the 229°F plate dried as fast as those with air. This would
indicate that the convection current set up by the 229°F

plate was sufficient to carry away the moisture as fast as

it was evaporated from the kernel.

Runs 30-33 with forced air at 25 fpm and runs 35-38
with forced air at 50 fpm dried at the same rate as runs
five-nine which didn't have forced air across the 189°F
plate. This indicated the moisture was carried away rapidly
enough by the natural convection current at 189°F. Runs 34
and 39 with forced air at 100 fpm dried at a slower rate
than those without forced air: here again, the additional
air flow did not have the drying capacity to make up for the
loss of heat from the plate. The 1890F plate runs with
forced air at 100 fpm had an initial drop in plate tempera-
ture to 143°? and then attained a maximum of 171°F. The so
fpm runs cooled the plate below the 189°F set point, but
still dried as fast as the 25 fpm and natural convection
runs at 189°F plate. The 189°F plate runs with forced air
at 50 fpm had an initial drop in plate temperature to 149°F
and then attained a maximum of 183°F.‘ The 189°F plate with
forced air at 25 fpm had an initial drop to 154°F and then

attained the 189OF temperature.

29

Using the 1440F plate, the runs from 40 through 43
with 25 fpm forced air dried at the same rate as the runs
without forced air. The 144°F plate runs with forced air
at 25 fpm had an initial drop in plate temperature to 120°?
and then attained the 144°F temperature. The 144°? plate
runs with forced air at 50 fpm had an initial drop in plate
temperature to 112°F and then attained a plate temperature
of 131°F with the drying rate less than that obtained with
natural convection.

With the 104°F plate it was not necessary to use
forced air as the runs 48 through 51 with 25 fpm forced air
dried at a slower rate than runs 14 through 17 in which the
natural convection was not supplemented with forced air.

The 104°F plate runs with forced air at 25 fpm had an initial
drop in plate temperature to 91°F and then attained the 104°F
temperature.

In all cases, the natural convection currents rising
from the heated plate were sufficient to carry the moisture
away as fast as it,was being evaporated from the kernel.

When the corn was placed on the plate there was an immediate
drop in the temperature of the plate to a minimum and then

a continuous rise till the original plate temperature, or a
lesser value in some of the forced air runs, was reached with
the temperature remaining constant during the remainder of
the drying period. Shroff (1949) and Tambling (1953) exper-
ienced the falling temperature and then the rising tempera-
ture, after drying had progressed, that was experienced in

all the drying runs.

30

The question of whether the drying rate was affected
by the position of the kernel was investigated next. Using
whole kernel samples in runs 52, 53, 56 and 57, it was found
that the drying rate was not affected whether the W or Y side
was against the heated surface. This promoted the theory
that the moisture moved from head end to tip end to get out
of the kernel during the drying process. This theory was
substantiated visibly by putting hot kernels in colodium;
moisture bubbles could be seen leaving the kernel from the
tip end.

An additional plate was placed on tap of the kernels
being dried in runs 60-61 to try to utilize more of the
heat from the lower plate. The drying rate was increased,
but excessive discoloration of the kernels resulted. In
runs 62-63, a screen with 50 percent openings was placed on
top of the kernels which resulted in a small increase in
drying rate over runs 5, 6, 7, and 8, but there was still
excessive discoloration of the kernels.

During the course of the investigation an attempt was
made to determine where the moisture was concentrated in the
kernel and the effect of this on the drying rate. Undried
kernels were cut into three parts, W, C, and Y, as shown in
Figure 2. It was found that the W side contained the high-
est moisture content with about 68 percent moisture content
d.b. and the Y side the least with 45 percent d.b. The 0
portion contained approximately 55 percent d.b. Other un-

dried kernels were cut into four parts, H, M, U, and T, as

31

shown in Figure 2. These results showed that the T portion
contained the highest percentage of moisture with U, H, and
M, following in decreasing order. The T part contained 85
percent d.b., the U had approximately 60 percent d.b., H
had approximately 46 percent d.b., and M had approximately
42 percent d.b. Several other varieties of yellow dent corn
were examined by the same methods with similar results.
When the kernels were cut into portions D, E, and F, (figure
2) the E portion had a considerable higher moisture content
than the D and F sections. This was expected since the E
portion contained most of the T part and the wetter part of
the W side. For approximately 56 percent d.b. corn, side
D and F contained about 40 percent moisture content and the
E portion contained approximately 64 percent d.b. The next
step was to cut up kernels during the drying operation to
see which part of the kernel dried the fastest and how the
moisture moved through the kernel, whether from the heated
side upward or from end to end.

Runs 54, 55, 58, 59, 65, 74, 75. 78, 80 and 82 through
87 were to determine the drying_rate of the various parts of
the kernel. The drying rates of the H, M, U, and T, parts
were uneffected regardless of whether the white or yellow
side was against the heated plate. With the white side in
contact with the heated plate, the white side, which had
the highest moisture content originally, had a faster dry-
ing rate and dried to a lower moisture content than the
yellow side by approximately 5 percent, d.b. When the

32

drier yellow sideymm in contact with the heated plate the
drying rate of the two sideswere almost equal with the yellow
side being the driest, at the end of the drying period, by
about 1 percent, d.b.

From the drying curve of the parts of the kernel it
would seem that the moisture moves from the H section to the
T section. On the composite plot of the H, M, U, and T,
sections of runs 75, 79, 83, and 87, it can be seen that at
the end of 20 minutes the H portion had dried at a faster
rate than the U and M portions and is the driest of all 4
portions of the kernel. The T portion, containing material
more conducive to drying, has the lowest moisture content of
the sections after about 1 hour. The H section is drier
than the M and U section until 2 hours, after which the 3
portions are at approximately the same moisture content.

Figure 3 shows the semi-logarithmic plot of conduction
versus convection drying and lists the k-values. The range
of applicability of the k-value for conduction drying are
as follOws: I

Table 1. Range of applicability (percent, d.b.)
of the k-value for conduction drying.

 

229°r 189°F 144°? 104°F
k1 54.9-43.12 55.42-45.36 55.88-48.09 .56.03-49.50
k2 43.12-26.59 45.36-29.34 48.09-31.87 49.50-31.85
k3 26.59-16.68 29.34-16.76 31.87-20.70 31.85-13.71 ——'

 

 

 

k4 16068-8051

16076-9016 20.70-10.90

_—.~—

13 071-8 0 88

          

33

Figure 4 shows that convection drying was faster, at
the beginning of drying, than conduction, but was slower
after a certain time had elapsed. For the 229°F temperature,
conduction drying was faster than convection drying after 20
minutes, for 199°F after 64 minutes, for 144°F after 116
minutes and for 105°F after 10 hours. The saving of time
for drying to 18.3 percent, d.b. and 15 percent, d.b., which
are the moisture contents generally accepted for selling and
storage, respectively, are listed in Table 6. With the ex-
ception of drying to 18.3 percent, d.b. from approximately
56 percent, d.b. at 104°F, conduction drying is faster than
convection drying at the same temperature. Robertson (1959)
experienced the same crossing of the conduction and convec-
tion drying curves at a temperature of 135°F.

Tables 2 through 5 summarize the data obtained with
the four plate temperatures at zero fpm. additional air
flow.

Figures 5 through 15 are a graphical representation
of the data taken during the conduction drying part of the
investigation.

 

 

 

 

Table 2. Summarization of Moisture Contents from the Data
229°F Plate No Forced Air

Time Plate Rin 1 ‘fiun 2 —REH—3_fi Run 4 Average
min. °F .dih. 5gpb. d.b. d.p, d.b.
0 229 54.16 54.96 54.92 55.92 54.99
1 172 51.31 48.59 48.09 52.65 50.16
3 185 53.21 49.59 47.21 46.96 49.24
5 190 43.06 44.31 44.66 41.68 43.43
7 201 43.14 42.01 42.95 36.99 41.27
10 210 36.42 33.52 37.05 39.92 36.73
15 219 32.49 31.93 29.02 31.35 31.20
20 225 28.00 25.68 26.77 27.66 27.03
25 227 28.04 26.29 22.11 26.67 25.78
30 229 23.18 22.72 20.34 22.46 22.18
40 230 17.43 19.51 15.88 16.39 17.30
50 228 15.25 16.09 13.87 14.00 14.80
60 229 13.47 11.38 11.59 11.54 12.00
80 230 10.60 8.47 10.25 13.20 10.63
100 229 10.01 8.85 6.83 8.35 8.51

 

Me : .17 percent d.b.

 

 

35

 

 

 

Table 3. Summarization of Moisture Contents from the Data
189°F Plate No Forced Air
313:? 8282': 8th? 8:88 883:
O 189 54.65 55.47 55.95 54.61 55.42
1 140 53.66 55.57 51.88 50.52 52.91
3 146 51.62 50.37 52.65 49.29 51.98
5 154 50.83 51.77 50.27 51.05 50.98
7 171 47.75 45.60 43.99 49.57 46.73
10 176 47.75 50.93 41.78 42.07 43.13
15 183 41.73 36.89 37.10 38.52 38.56
20 187 39.11 34.3 35.77 35.07 36.08
25 189 33.88 31.35 31.43 ----- 32.22
30 189 32.09 28.57 27.09 27.93 28.95
40 189 28.94 26.49 24.84 25.07 26.39
50 190 25.68 22.22 22.03 ----- 23.31
60 189 24.23 20.23 20.79 20.88 21.53
80 188 17.71 18.61 15.42 15.73 16.87
100 189 15.56 15.82 14.62 ----- 15.33
120 190 ---------- 12.94 12.37 12.65
160 188 --------------- 10.07 10.07
200 189 7.76 8.33 8.37 9.67 9.16
Me 1.06 percent d.b.

 

 

36

 

 

 

Table 4. Summarization of Moisture Contents from the Data
144°F Plate 4—8 No Forced Air

$13? P3?“ R319 331.“le ”it? R3533 “31$
0 145 56.07 54.12 56.53 55.79 55.88
2 120 56.09 56.47 53.08 54.72 55.09
5 130 55.52 54.89 56.37 52.87 54.91
10 134 49.42 51.47 48.76 48.59 49.56
15 136 48.06 49.19 46.31 49.06 48.16
30 144 41.78 40.86 37.30 42.79 40.68
45 145 37.77 38.11 33.75 32.94 35.64
60 143 30.05 34.61 32.75 29.59 31.78
75 145 28.55 28.02 30.97 26.49 28.51
90 144 24.60 24.57 27.39 24.97 25.38
120 144 19.34 23.17 20.79 20.60 20.98
150 145 18.37 18.76 17.59 16.59 17.83
180 143 16.07 17.11 16.11 15.42 16.18
210 144 17.64 14.42 13.83 14.67 15.14
240 144 11.89 12.76 13.39 13.31 12.84
310 144 ---------- 11.62 10.18 10.90

Me 2.39 percent d.b.

 

 

 

 

 

Table 5. Summarization of Moisture Contents from the Data
104°F Plate No Forced Air

312‘? P888 8383 R3885 838.5 83887 “if?
0 104 55.74 54.99 55.80 57.59 56.03

5 94 54.35 51.08 59.87 56.41 55.43

10 99 50.10 54.32 51.33 53.45 52.30
20 101 52.09 52.54 51.30 53.24 52.29
30 104 48.09 50.11 49.44 52.27 47.48
60 103 46.28 48.43 46.72 47.72 47.29
90 105 42.59 39.85 40.22 42.44 41.28
120 104 36.54 38.19 36.32 35.71 36.69
150 105 33.29 34.04 36.06 37.57 35.24
180 104 31.89 31.68 29.76 32.92 31.56
240 103 30.84 27.04 27.42 28.88 28.55
300 104 24.41 24.54 25.82 26.10 25.22
360 104 23.77 20.77 24.03 24.43 23.25
480 105 17.63 20.93 17.07 19.16 18.70
600 105 15.57 14.50 15.10 17.09 15.57
720 104 13.62 11.93 13.25 15.67 13.62
840 103 12.39 10.24 12.30 14.30 12.31
1190 104 12.19 ---------- ----- 12.19
1440 104 ----- 10.37 ---------- 10.37
1560 103 --------------- 9.04 9.04
1620 104 ---------- 8.88 ----- 8.88

 

 

Me = 4.30 percent d.b.

 

 

Table 6.

Summarization of the Reduction of Drying

Time by Using Conduction Drying to Re-
place Convection Drying

 

 

 

Temp '—:E¥5m756%'d.b. TEES———Reduction in Time
0F Condition tog%, min. Percent
229 Convection 18.3 39 -
229 Convection 15.0 54 -
189 Convection 18.3 79 -
189 Convection 15.0 111 -
144 Convection 18.3 172 -
144 Convection 15.0 226 -
104 Convection 18.3 420 -
104 Convection 15.0 655 -
229 Conduction 18.3 36 8.5
229 Conduction 15.0 49 10.2
189 Conduction 18.3 74 6.7
189 Conduction 15.0 96% 15.1
144 Conduction 18.3 160 7.5
144 Conduction 15.0 194 16.5
104 Conduction 18.3 492 -14.6
104 Conduction 15.0 635 3.2
189 Cond. with

plate above 18.3 64 23.4
189 Cond. with

plate above 15.0 73 52.1
189 Cond. with

screen above 18.3 68 16.2
189 Cond. with

screen above 15.0 83 45.8

 

 

 

 

 

 

MOISTURE RATIO

C NDUCTION CONVQTION-u.--
I.O ’ 229°F I89°F I44°F I04°F 229°F l89°F I44°F I04°F
\k' k. 2.65 2.30 .66 .29 3.97 2.78 1.09 .44
9 \ 1 k2 I33 I72 .58 .20 2.!8 I89 .55 .24
8 I“ \ k3 L38 1.03 .47 .I3 mo .6: .32 .IO
' 3\\ \\ ((41.75 .57 .24 .04L.63 Al ’17 .03
.7--1 \ \ \\ .
1‘ \ \ ,2
.1 I \ \
.6-1 I\ \ \
3 II \ \\
- \ \ \
'53 \\ \
2 \
3 \ \\ k,
I \\
'4": \ \\ \
: \ .
: \ \
.3; \ \
I \ \ \\
i \ \ \
j \ \ \
. \ \ \
. \ \ \
.2- \ \ \
. \ \
q \ \ \
- \ \ |44°F
\ .
’ 229°F |89°F
.l5 I I T I I
' 2 3 4 5 6

39

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TIME (HOURS)

Figure 3. Semi-logarithmic plot comparing

conducflon and

convection drying

4O

58 02.9.6 .6 0:36 8:328 nco cozonocoo .6 53.3800 so 2:9“.

Ammsorv 22:.
m

 

 

 

 

— P b b T L F r _ p — L O
I “— omw_ III momNN I [I I0_
mo¢¢_ Ill, /
III!- I, / l m
III, l/ MN
I /
// 18%
I, 3
momo_ III], / / I 3
I, / nNU
I, /
I/ / emu
/ /
l/ / . Hm
/ / Mu.
// / / teen
/ a
/ N
/ I I.
x r P
llll ezie 225.528 C. at
oz_>mo 29.530200 /

 

 

 

 

 

° RUN, l
" 1' RUN 2
3 D RUN 3
‘2 ' A RUN 4
£40- 229'F, 0 fpm
3 1h
5
. 80"
2 .
3 a
O I ' I ' I I I
20 40 60 80
Tune (Mimics)
60
0 RUN 22
8_ x RUN 23
5° ‘ a RUN 24
‘ . A RUN 25
404 229'F, 50 (pm

Moisture Content (Perm! (it)
8

 

 

 

20...
I
lO-y
o I ' I I I f I
20 40 60 80
Tune (Minutes)

1
I00

41

 

 

 

 
  
  
   

 

 

60 0 RUN 26
I RUN 27
3.; 508 a RUN 28
‘5 - m A RUN 29
'§' 404 8. 229°F, 25 fpm
g; .
a 30‘
g
g .
20..
5 .
3 nos
0 v I v I T I ' I I I
20 4O 60 so I00
Time (Minutes)
'- ° RUN IS
I RUN l9
3 a RUN 20
" A RUN 2|
Ew- 229° F, IOO fpm
E 30‘
2
8
U
2 2°“
F; 4
z (0..
O ' I ' ' ' I ' I ' 1
20 4O 60 80 I00
Time (Minutes)

noun: 5. mm suntan CORN mm 229 or common
name no 0. 25. so m 100 m m

Moisture Content (Noon! db.)
. e . '9 . 8F . e .

 

 
   
 
   

RUN 5
RUN 6
RUN 8
A RUN 9
I89“ F, 0 fpm

Eli-0

 

 
 
   
    

Moisture Content (Plum d.b.)

 

‘36'20'60'260
Tune(MinuIesI

0 RUN 35
I RUN 36
0 RUN 37
A RUN 38
I89°F, 50 fpm

 

e.e-§-§e§.

. 46 T 60 T I20
Time (Minutes)

I60 200

42

 

 

 

     

 

 

60
.' 0 RUN 3O
‘ . RUN 3|
:50 " a RUN 32
j . x ARUN 33
£405 I89° F, 25 fpm
530‘
5 4
£20~
g .
=3 not
0 f I r I I I ' I I I
40 80 l20 I60 200
Time (Minutes)
._ °° 0 RUN 34
3 y ° ‘ RUN 39
O
E40 I89 F, mom
.5: 30"
‘5' I
Q
o 20*
3 I0«
0 I I I I I I I I . 1
4O 80 (20 ISO 200
Tune (Minutes)

mu: 0. name memo CORN mu m 0r oououcnou
name m o. 25. so new m m

Moisture Content (Pemnt d.b.)

 

 

45

 

 

 

 

60- 60
° RUN l0 0 RUN 40
I RUN ll " I RUN 4|
50‘ a RUN I2 350‘ ‘3 RUN 42
< A RUN l3 3 A RUN 43
40+ I44“ F, O fpm Ewe I44“ F, 25 fpm
30- ~ «
1 S w
.§
20- . g 20«
. ,5 ,
IO- 0 3 ”-4
I 1
0 ' T ' I ' I ' I O I I T I I I r I I I
60 I20 I80 240 300 60 l20 I80 240 300
Time (Minutes) Time (Minutes)
' -J o RUN 44
.. ll RUN 45
35°“ . a RUN 46
j: + A RUN 47
gm I44'F, so fpm
530-
g 1
.20-I
g ,
3 I0-
0 v I

 
   
 
  

 

  

 

60

'iz‘oTiéo'zio'soo

Time (Minutes)

FIGllE 1. URN“ SNELL“ MI IITII 144 OF MUN
Natl“ m 0. 25 AU 50 FF. All

44

RUN l4
RUN l5
RUN l6
RUN I7
40-I l04°F, Otpm

.s
[>on

MoisIure Content (Percent db.)

 

 

 

 
   
 
   

60;:
' ° RUN 48
I RUN 49

2; A a RUN 50
j ‘ , A RUN 5|
§401 ' I04°F, 25 fpm
g .. .
3330‘
5', I
020‘
s
E I
5 I04

0

 

 

Time (Hours)

FIGIRE 8. DRYIK SIIELLED CORN IITII 104 0F 00”!”le
NEATIK In 0 AND 25 FPN AIR

45

  

A RUN 60
I RUN 6|
|89° F, 0 fpm

Moisture Content (Percent d.b.)

 

 

0 I I I I I I f I I fi
40 80 I20 I60 200
Time (Minutes)
60
50‘ A RUN 62
" RUN 63
0
4O_ l89 F, 0 rpm

Moisture Content (Percent db.)

 

 

 

o - r A r r T v u A 1
4O 8O l20 IGO 200
Time (Minutes)

mu»: 9. mm SNELLED cone orr m 0r oorroucrlorr name
PLATE mu euoirrom Pure ABOVE uterus

46

. 0 RUN 56 YELLOW SIDE DOWN
50* x RUN 57 WHITE SIDE DOWN
. I89° F, 0 (pm

Content (Percent d. b.)
A
C?

Moisture
N
C?

 

 

 

' I j I V T ' I
40 80 l20 I60 200
Tune (Minutes)

A RUN 52' YELLOW SIDE DOWN
x RUN 53 WHITE SIDE DOWN
I89° F, O fpm

 

Content (Percent d. b.)

MDIsture
N
o
I
/

I04 “‘-6

 

 

T T

O . r v I r
40 80 I20
Time (Minutes)
FIGURE 10. DRYING SHELLED CORN ON 188 OF CONDUCTION HEATING
PLATE NITN UNITE 0R TELLON SIDE OF KERNEL DUNN

Iéo ' 260

 

4'7

 

Moisture Content (Percent d. b.)

    
 
 

Moisture Content (Percent d. b.)

 

 

 

 

 

  

 

 

 

 

 

60..
I RUNS 55,59,65 885 II RUNS 54.58.80334
50_ WHITE SIDE DOWN 504) YELLOW SIDE DOWN
WHITE SIDE 3 II WHITE SIDE
I ‘, CENTER ------- j \ CENTER -------
4m ‘, YELLOW SIDE—-— 54m . YELLOW SIDE—"—
I I89° r, o fpm é; . \ I89° F, o fpm
30‘ E 30-I '
4 :5; I
204 e 204
.0. E .0-
I 'I
O I I I I I T I I I 1 f I I # v v 1
4o 80 I20 I60 200 0 43 so I27) I60 200
Time (Minutes) Time (Minutes)
‘72:) ‘83
RUNS 75.79. 83 8 87 I RUNS 74,78,82 a 86
\ WHITE SIDE DOWN 60 “ YELLOW SIDE DOWN
H —— ., . H .—
50. 3 50 I, M ----
4 E ' U — '—
s ‘I. .
40A g4o- -
J ‘- d
5
soA 5 so-
, 2 ,
20~ g zo-
§ I
IO-I \\‘ I0«
0 I T I I r I ' I ' I 0 v T v I 1 I v ' I I
40 30 I20 I60 200 40 90 I20 I60 zoo

Tune (Minutes) Time (Minutes)

FIGINIE 11. mm SIIELLED CORN on m 0r oououcIIoII HEATING
PLATE, IIIIIIE on rELLoII SIDE or IIERIIEL 0M. IrIrII-
IIERAeE IIDISIURE comers or “IE muous SEcrIoIIs
or IIIE IIEIIIIEL DEIEIIIIIIIED

 
  
  

 
  
  

48

 
   
 
  
 
 
 
  

 
   

 

 

 

 

RUN 55 RUN 59
3 WHITE SIDE DOWN WHITE SIDE DOWN
3 -° A WHITE SIDE— A WHITE SIDE—
, . a CENTER ------ a CENTER ------
E . YELLOW SIDE-'- .. YELLOW SIDE-~-
JO‘ u I89“ F, o fpm I89° F, o fpm
E .
330A
£20-
3 I
IO- 0
OIIIIIIIfiIIOIIIIIIIIII
IoeoIzoIsozoo 4080I20I60200
Time (Minutes) Time (Minutes)

 

 

 

 

 

RUN 65 . RUN 85
_. WIIITE SIDE DOWN _. WHITE SIDE DOWN
f; 0 WHITE SIDE— 3501 0 WHITE 5mg—
.. :3 CENTER ------ - ' a CENTER ------
E x YELLOW SIDE--- g i I .. YELLOW SIDE—-
_, l89’ F, o fpm ~40- E I89“ F, o fpm
'5 § ‘ .
3 5w
5 320A
3 § .
ID-
0 ' I ' I I I I I I I 0 I I I I T T ' i ' I
. 40 so I20 ((0 200 40 so I20 I60 zoo

Tune (Minutes) Time (Minutes)

FIGIRE 12. DRYING SNELLED CORN ON 188 0F WNOUCTIM NEATIK
PLATE, NHITE SIDE OF NERNEI. DONN, NITN NOISTINIE
CONTENT OF N. C AND T SECTIONS OF THE NERNEI.
DETERMINED

Moisture Content (Percent db)

Moisture Content (PeIoent out

 
   
       
  
 
 

 

30-
ZO-I
IO-I
O I I I I I I I r I 1
40 so I20 I60 200
Time (Minutes)
RUN 80
YELLOW SIDE DOWN
0 WHITE SIDE—
. aCENIER ------
I YELLOW SIDE—‘—
40" I89° F, 0 tom

.3

 

RUN 54
YELLOW SIDE DOWN
A WHITE SIDE—
a CENTER ------
. YELLOW SIDE-—
I89' I, O fpm

 

 

 

14'078TOVIEOYIGIOVZOO
Time (Minutes)

49

RUN 58
YELLOW SIDE DOWN
0 WHITE SIDE ——
E! CENTER ------
. YELLOW SIDE ---
(89‘ F, 0 (pm

 
 

     
 
  
   
 
  

 

 

E'eb'tz'o‘ Iéo'zdo
TImeIMinutes)

  
  
  
 
   

RUN 84
YELLOW SIDE DOWN
0 WHITE SIDE —
E! CENTER ------
I YELLOW SIDE---
I89“ F, O fpm

Moisture Content (grunt db)

.‘PLS‘KP.

 

 

V W

I ' T I ' I ' 1
40 SD I20 I60 200
Tune (Minutes)

FIGIRE IS. DRYING SIELLED CORN ON 188 0F WUCTION HEATING
PLATE. YELLON SIDE OF NERNEL DONN, NITI'I NOISTIRE '
CONTENT OF N, C AND T SECTIONS OF THE NEINEI.

DETERN I NED

A RUN 75
- ,. WHITE SIDE DOWN

Moisture Content (Percent db.)

 

 

 

° ""'Izo'Iéo'

 

 

 

 

40 so
Time (Minutes)
'A RUN 83
I, WHITE SIDE DOWN
2 I‘ o H
-equ an -—--
‘ | ,, U _.._
g .b A T _ .....
:4“ I89° F, 0 (pm
.5; I
330—
% zo-
é’
)O-I
o T I 7' T I T v I
40 so I20 I60 zoo
Time (Minutes)

 
   
   
 
 
  
   

 

50

 

  

 

 

 

 

 

,94
!
I
! RUN 79
° .. I, WHITE SIDE DOWN
:5 ° H
1550., a M ----
g I L‘,’ I:
:40“ . I89° F, 0 (pm
5 30~
fg' ZO-I ‘52. 5x
2 \.\
~ . -\ O
m ‘3?
'4 A
0 I ' I ' I T N f 1
40 so I20 Iso 200
Time (Minutes)
,88
I
g RUN 87
° I WHITE SIDE DOWN
3 .. 0 H
g N U —.—
A T _._._.
:40‘ g I89° F, O (pm
330—
o 4
.3204
§
ID
0 I I 1

8O . Izo ISO ‘
Time (Minutes)

'30

FIGURE 14. DRYING SHELLED CORN ON 189 0F CONDUCTION HEATING
PLATE. NHITE SIDE OF NERNEL DONN, NITH MOISTURE
CONTENT OF H, N, U AND T SECTIONS OF THE KERNEL

DETERMINED

Moisture Content (Percent db.)

Content (Percent d. b.)

Moisture

 

 
  
 
 
  
   

   
 
 
  
   

 

 

 

 

 

.78
I“ l92
as
RUN 74 .. ' RUN 78
YELLOW SIDE DOWN YELLOW SIDE DOWN
' o H —— 3 e H _—
50- a M ---- ‘5 -A a M ----
1 x U _._ E i, U _._
A T ._ ..... a A T _ .....
40‘ l89° F, 0 (pm :40“ l89° F, O fpm
so~ 53m
. g .
20* 320‘
. S
Io- ~ ...... - IO«
4 .-1 ..
Ovtvuvuvrvwovsv.vsvsfifi
4o 30 I20 I60 200 40 80 I20 I60 200
Time (Minutes) TIme (Minutes)
#582
7 AI
"4‘ RUN 82 RUN 86
6 H YELLOW SIDE DOWN 50" ‘! YELLOW SIDE DOWN
A ° H 3 ‘I ° H _
50! BM---- Iso-‘I [IN-"-
‘I E I .3. ' U _'_
°\\o A T — .....
£40“ '\,\ l89° F, 0 (pm
Sm -

 

 

 

 

 

~220-
'6
2
l0-
0 v r r 1 r T fl I v j o I T I T I I V I ' Y
40 80 I20 I60 200 40 so I20 I60 200
Time (Minutes) Time (Minutes)

FIGURE 15. DRYING SHELLED CORN ON 188 0F CONDUCTION HEATING
PLATE, YELLON SIDE OF NERNEL DOW. NITII MOISTURE
CONTENT OF H, M. U AND T SECTIONS OF THE KERNEL
DETERMINED

CONCLUSIONS

The moisture moves mainly from the head to tip sections
of the kernel.

Natural convection was sufficient to carry away the mois-
ture as vaporization occurred, for conduction drying.

The drying rate is the same regardleSS of which side,
white or yellow, of the kernel is in contact with the
heated plate.

Drying occurred in four falling rate periods for shelled
corn at 56 percent d.b., dried to approximately 8 per-
cent d.b.

The tip of the kernel contained the highest moisture
content of any part of the kernel at the beginning of
drying and the lowest after drying to below 22 percent
d.b.

The higher_the plate temperature the faster the rate of
drying.

The rate of drying by conduction can be defined by the
differentiated form of the equation M-Me
Mo-He : e"k0

as is used for convection drying.

Conduction drying is faster than convection drying for
drying 56 percent d.b. shelled corn to less than 26 per-
cent d.b. at 229°F, to less than 20 percent d.b. at 189°F,
to less than 22 percent d.b. at 144°F and to less than
15.5 percent d.b. at 104°F.

Reducing the plate temperature 40° from 229°F approxi-
mately doubled the time required to dry 56 percent d.b.
shelled corn to 15 percegt d.b. Reducing the plate
temperature 85° from 229 F, approximately quadruples
the time required to dry shelled corn from 56 percent
d.b. to 15 percent d.b. Reducing the plate temperature
125° from 229°F increased the drying time approximately
13 times to dry from 56 to 15 percent d.b.

RECOMMENDATIONS FOR FUTURE STUDY
Compare the economics of conduction and convection dry-
ing.

Determine the effect of conduction drying on milling,
flavor, etc.

Determine the effect of conduction drying on germination.

Determine the drying rates of other products using con-
duction drying.

53

BIBLIOGRAPHY

Ceaglske, N. H. and Hougen, O. A. (1937) The Drying of
Granular Solids, American Institute of Chemical Engi-
neers, Transactions. 33:283-311.

Corben, R. W. and Newitt, D. M. (1955) The Drying Charac-
teristics of Porous Granular Material, American Insti-
tute of Chemical Engineers, Transactions, 33:52-63.

Ellenwood, Frank O. and Mackey, Charles 0. (1944) Thermo-
dynamic Charts, Second Bulletin, John Wiley and Sons,
Inc., New York, New Ybrk.

Ernst, R. C., Ardern, D. B., Schmiel, O.K., and Tiller,
F. M. (1938) Drying Prussion Blue on Heated Shelves,
Industrial and Engineering Chemistry, 30:1119-1121.

Ernst, R. C., Ridgway, J. W., and Tiller, F. M. (1938)
Vacuum Drying of Solids, Industrial and Engineering
Chemistry, 30:1122-112A.

Haines, W. B. (1927) Studies in the Physical Properties
of Soil, IV, Journal of Agricultural Science, 17:264-
290.

Hall, Carl W. (1957) Drying Farm CrOps, First Edition,
Edwards Brothers, Inc., Ann Arbor, Michigan.

Hukill, W. V. (1948) Types and Performances of Farm Grain
Dryers, Agricultural Engineering, 29 (2):53-54. 59.

Jakob, Max (1949) Heat Transfer, Volume One, John Wiley and
Sons, Inc., New York, New Ybrk.

Kelly, c. F. (1941) Drying Artificially Heated Wheat With
Unheated Air, Agricultural Engineering, 22(9):}16-320.

_{Eing, A. R. and Newitt, D. M. (1955) The Drying of Solids
-«/ With Heat Transfer by Conduction, Transactions of
Institution of Chemical Engineers, 33:64-69.

Ludt, R. W. (1957) Drying of Sand on a Hot Surface, American
Institute of Chemical Engineers Journal, (3):}43-147.

McCready, D. W. (1935) Dr ing of Paper and Pulp, Paper
Trade Journal, 101(13 :66-71.

54

55

Retford, David Thomas (1957) Drying Rates and Heat Transfer
Coefficients on a Heated Surface for the Drying of Sand,
Unpublished Thesis for M.S. Degree, Michigan State Univer-
sity, East Lansing, Michigan.

Robertson, Keith (1959) Drying Single Layers of Shelled Corn
on a Heated Surface, Unpublished Report of Special Pro-
blem, Michigan State University, East Lansing, Michigan.

Sherwood, T. K. (1936) Air Drying of Solids, American Insti-
tute of Chemical Engineers, Transactions, 32:150-68.

Shroff, Phiroze Dhunjishaw (1949) The Drying of Paper Pulp
on a Heated Surface, Unpublished Thesis for M.S. Degree,
Michigan State University, East Lansing, Michigan.

Tambling, Thomas Noel (1953) A Study of the Mechanism of
Drying Sand on a Heated Surface, Unpublished Thesis for
M.S. Degree, Michigan State University, East Lansing,
Michigan.

APPENDIX

57

Table 7. Summary of Drying Runs for the Investigation

 

 

Run. ‘Temperature ‘fiType of ‘AirfFlow
Number °F Heajé§g_» fpmg_fi Misc.* __
1-4 229 Conduction 0
5-9 189 Conduction O
lO-l3 144 Conduction 0
14-17 104 Conduction 0
18-21 229 Conduction 100
22-25 229 Conduction SO
26-29 229 Conduction 25
30-33 189 Conduction 25
34,39 189 Conduction 100
35-38 189 Conduction 50
#0-43 144 Conduction 25
44-47 144 Conduction 50
48-51 104 Conduction 25
52,56 189 Conduction 0 Yellow down
53.57 189 Conduction 0 White down
60—61 189 Conduction 0 Plate on top
62-63 189 Conduction 0 Screen on top
54,58, 189 Conduction O Yellow down
80,84 W,C,Y parts
55,59, 189 Conduction O White down
55.85 W,C,Y parts
74,78 189 Conduction O Yellow down
82,86 H,M,U,T parts
75,79, 189 Conduction 0 White down
83,87 H,M,U,T parts

Table 7. Continued

 

 

 

Run Temperature‘fi:Type of Air Flow
__Number OF Heating, fpm _fifi Misc.

88-89 229 Convection 235

90-91 189 Convection 235

92-93 144 Convection 235

94-95 104 Convection 235

W

Btu

d.b.

hr

M
Me

Mo

to
w.b.

X

59
LIST OF SYMBOLS

British Thermal Units

Dry basis

2.7128, natural logarithm base

Number which is represented from the origin on
the abscissa equal to the width of one cycle on
the ordinate on semi-logarithmic graph paper
Feet per minute

Degrees Fahrenheit

Heat transfer coefficient, Btu/hr sq. ft. °F
Hour

Drying constant, l/hours

Moisture content at any time, percent, dry basis
Equilibrium moisture content, percent, dry basis
Initial moisture content, percent, dry basis
Constant

Heat flow, Btu/hr.

Square feet

Temperature, degrees Fahrenheit

Equilibrium temperature, degrees Fahrenheit
Initial temperature, degrees Fahrenheit

Wet basis

Actual distance measured on abscissa

Actual distance measured on ordinate

Time, hours

60
SAMPLE CALCULATIONS

Calculation of Moisture Content:

: Weight after plate drying:weight after oven drying
Weight after oven drying

2 0211 rams-l 6804 rams
1.5805 grams 100

= 20.27 percent d.b.

Calculation of Moisture Ratio:

M-Me
Mo-Me

= 20.22 percent d.b.-1.06 percent d.b.
55.27 percent d.b.-1.06 percent d.b.

a—a

 

Calculation of Slope of Line on Semi-log Plot:

kz-ll .1.
x . 3 3 f

= - 25 1 1
l3 .Z3E3 1.557

R: '20 65

  

 

Tutu“. (I iviJflm t... I" .07 1'.“- ‘VWMW.WG"‘ W”. I“.

:25: 31.?”5’” 5W” ”4

 

 

 

 

 

MICHIG

A

 

in!

N STATE UNIVERSITY LIBRARIES

1293 03062 1753