HIGH TEMPERATURE CONTINUOUS FLOW
GRAIN DRYING WITH CONCURRENT DRYING
AND COUNTER- CURRENT COOLING

PART I: DESIGN AND TESTING

Thesis for the Degree of M. SI
MICHIGAN STATE UNIVERSITY
JAMES ALFRED CARRANO
1970

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ABSTRACT
HIGH TEMPERATURE CONTINUOUS FLOW GRAIN DRYING
WITH CONCURRENT DRYING AND COUNTER-CURRENT COOLING

PART I: DESIGN AND TESTING

By

James Alfred Carrano

A working model of a high temperature continuous flow grain
dryer using concurrent flow heating and counter-current flow cooling
was designed, constructed and tested. The dryer's performance was
affected by the hot air inlet design, grain spreading at the inlet, and
unloader design and position. Problems in these areas were resolved
and satisfactory performance was achieved. Grain dryers of the type
used in these tests will be capable of maximized output when temper-

ature-time relationships for grain quality have been established.

Approved

 

    

Approved

   

D partment Chairman

HIGH TEMPERATURE CONTINUOUS FLOW GRAIN DRYING
WITH CONCURRENT DRYING AND COUNTER-CURRENT COOLING

PART I*: DESIGN AND TESTING

by

James Alfred Carrano

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Agricultural Engineering
1970
*PART II: SIMULATION AND OPTIMIZATION

by
Daniel G. Elzinga

ACKNOWLEDGEMENTS

My deepest appreciation is extended to Dr. W. G. Bickert
(Agricultural Engineering) whose counsel and guidance led to the com-
pletion of this study. He has been both a friend and advisor to me
during the course of this study; -- my most sincere thanks to him.

I would also like to thank Dr. F. W. Bakker—Arkema (Agri-
cultural Engineering) and Dr. R. T. Hinkle (Mechanical Engineering)
for serving on my guidance committee and for the education I received
from them during the completion of my program.

It was also a pleasure to work with Daniel Elzinga and I wish
to thank him for the help he supplied on the portions of this project
which were done jointly.

To the memory of my deceased father, Alfred, who would have

greatly appreciated it, I dedicate this work.

James A. Carrano

ii

Also To:

Mrs. A. Carrano
Mrs. Minnie Wilson
Mr. And Mrs. H. J. Thomas

iii

TABLE OF CONTENTS

I. INTRODUCTION
1.1 Background on Continuous Flow Dryers

1.2 Principles of Air and Grain MOvement
1.3 Objectives

II. EXPERIMENTAL
2.1 Design Criteria
2.2 Description of Apparatus
2.21 Dryer Construction
2.22 Control and MOnitoring Equipment

2.23 Summary of Control and Monitoring Equipment
2.3 Testing Proceedure

III. RESULTS AND DISCUSSION

3.1 Output of Dryer

3.11 Example of Mass Balance for Test 5A
3.2 Quality
3.3 Computer Simulation

IV. SUMMARY AND CONCLUSIONS

4.1 Summary
4.2 Conclusions

V. RECOMMENDATIONS FOR FUTURE STUDY

LIST OF REFERENCES

APPENDIX

iv

Page

U)

10
10
18
22
24

27
27
34

37
38

39

39
4O

41

42

43

Table

3.1A

3.13

3.1C

3.1D

3.1E

3.1F

3.1C

3.1H

LIST OF TABLES

Grain Parameters for Tests 2 Through 6B.
Heating Air Parameters.
Cooling Air Parameters.

Exhaust Air Parameters.

Effect of Spreading on Drying Air Temperatures.

Drying Air Temperatures for Tests 6A and 6B.
Air Temperatures in Cooling Section.

Drying Air Temperatures for Test 3A.

Page
43
44
45
46
47
48
49

50

Figure

1.1A

2.1A

2.1B

2.1C

2.1D

2.1E

2.1F

2.21A

2.21B

2.21C

2.21D

2.21B

2.21F

2.21G

2.21B

2.211

2.21J

2.21K

2.23A

301A

3.13

3.10

3.lD

3.1E

LIST OF FIGURES

DRYER COMPONENTS SHOWING AIR AND GRAIN MOVEMENT
GRAVITY FORMED RIDGES AND TROUGHS

PLASTIC MODEL

LAYER OF GRAIN ENTERING CHAMBER

LAYER EXITING CHAMBER

OFF CENTER UNLOADING

STALL AREA ON LEFT

ASSEMBLED DRYER WITH INSULATION REMOVED

BASIC SECTION

GRAIN STORAGE TANK

HOT AIR INLET WITH ADAPTER REMOVED

HOT AIR INLET WITH SPREADER, VALVE AND CYLINDER
DRYING SECTION

EXHAUST SECTION

COOL AIR INLET

AIR FILTERS

COOL AIR INLET AND UNLOADER

UNLOADER CONSTRUCTION

CONTROL AND MONITORING EQUIPMENT

EFFECT OF SPREADING ON DRYING AIR TEMPERATURES
DRYING AIR TEMPERATURE VERSUS DEPTH FOR SEVERAL TESTS

THEORETICAL AIR AND GRAIN TEMPERATURES

COOL AIR TEMPERATURES VERSUS POSITION

COOL AIR TEMPERATURE VERSUS POSITION

Page

11

ll

ll

l3

l3

l6

16

16

16

16

16

23

3O

31

32

33

33

out

LIST OF SYMBOLS

Abbreviation for dry air (no water vapor present).
Rate of dry air movement, pounds of dry air per hour.
Rate of wet air movement, pounds of moist air per hour.
Rate of grain movement, pounds per hour.

Specific humidity of air, pounds of water vapor per
pound of dry air.

Moisture content, initial, Z dry basis.
Moisture content, outlet, Z dry basis.
Relative humidity, percent.

Temperature, degrees Fahrenheit.

Initial temperature, degrees Fahrenheit.

Abbreviation for wet or moist air.

Concurrent flow

Cooling air

Counter—current flow

Cross flow

Heating air

Hot air

LIST OF TERMS

Condition when the air and grain have
flow velocities in the same direction.

Air from the air conditioning unit when
used to cool the grain.

Condition when the air and grain have flow
velocities in Opposite directions.

Condition when the air and grain have flow
velocities in perpendicular directions.

Air when used to heat the grain in the
drying section.

Air which has passed through the burner
or heater.

I. INTRODUCTION

Each year, a part of some farmers' time and money is spent on
drying grain. The most popular method for this has been the deep bed
dryer using heated air. With farmer work loads increasing and yields
per acre climbing, more and more time will be spent drying grain in
larger dryers. Shortening of drying and handling time would enable
a farmer to spend more time on other farm management functions. To
shorten drying time, continuous flow dryers which operate with higher
air temperatures have come into use. But, the practice of using
hotter air for drying has an upper limit and exceeding it can cause
losses in grain quality; although the damage may not occur due to
temperature alone but to the length of time at that temperature.

This principle can be demonstrated dramatically by merely passing
one's hand through a flame, first very quickly and again more slowly.
Thus, it seems feasible that if grain were in contact with higher
temperature air for a short time, no damage would result. The answer

is in how high a temperature and how long?

1.1 Background on Continuous Flow Dryers

Much research has been done in the field of layer and batch drying
but very little has been done in continuous flow drying. Continuous
flow drying can be done with a number of different methods including
cross flow, concurrent flow and counter-current flow.

Cross flow continuous dryers and deep bed dryers produce an
overdried condition in the grain passing or positioned near the air
inlet and an underdried condition in the grain near the exit. More uniform
drying results when either concurrent or counter-current air flows are
used. There are a few patents on drying methods using both concurrent and
counter-current flows. On July 19, 1966, Heinrich Tillmanns received
United States Patent Number 3,261,109 entitled, "Apparatus for Drying and
Cooling Particulated Material". This patent describes an apparatus sub-
stantially vertical using an upper drying zone and a lower cooling zone.
The material to be dried is fed in at the top and is discharged at the
bottom. The cooling air is entered at the lower end and is partially
exhausted in the center. The remainder of the air is heated in the central
portion and is exhausted at the top. This system is a counter-current
flow dryer for both heating and cooling. On February 7, 1967, Douglas L.
Graham.was granted United States Patent Number 3,302,299 entitled, "Drying
Apparatus and Method". This system also uses an upper drying section and
a lower cooling section but the drying section uses concurrent flow and
the cooling section counter-current flow. Air, both cooling and heating,
is exhausted in the central portion. The Anderson's in Maumee, Ohio, have
a large dryer which uses this same principle. The M & W Perfect Kern'l

dryer is commercially available and is modeled after Graham's apparatus.

1.2 Principles of Air and Grain Movement

Grain which is dried for wet milling is generally limited to
140° F kernel temperatures. Graham (1967) states, ".... high moisture
corn can stand high drying rates without drying damage; while at
lower moistures the grain can be damaged by high drying rates." Since
concurrent flow drying is characterized by a continuously decreasing
drying rate as drying occurs, it is ideal for higher temperature
drying. Since initially the grain is cool and moist and the drying
air very hot, evaporation of moisture from the grain is at a high
rate. This high evaporation rate keeps the grain temperature below
the air temperature and also drastically decreases the air temperature
at the same time. As moisture is removed from the grain, the air
and the grain theoretically approach the same temperature and the
drying rate decreases. Thus, at the end of the drying section, the grain
and air are ideally in equilibrium and moisture has transferred to the air.

Cracking occurs when warm grain is cooled rapidly. It is there-
fore necessary to cool the grain with air that becomes increasingly
cooler as the grain temperature decreases. Counter-current cooling
provides the proper cooling action for accomplishing this. If a
vertical dryer is used, grain movement will be downward. By providing
a cool air inlet at the lower end of the dryer, the grain temperature
will decrease as the grain approaches the air inlet, optimally reaching
the same temperature as the incoming cooling air at this point. It is
necessary to have the cooling air move vertically upward through the
downcoming grain. This is accomplished easily by providing an exhaust

section immediately above the cooling section. An exhaust fan at this

point would enable the heating air to move downward and the cooling
air to move upward. The dryer constructed for this research uses the

above principles and these are outlined in Figure 1.1A.

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1.3 OBJECTIVES

 

The objectives of this sutdy were to gain an understanding of
the principles of grain drying and apply these principles to the design
of concurrent - counter-current flow grain dryers. Having gained
an appreciation of the above, the next objective was to develop and
build a working dryer capable of demonstrating the principles involved
in concurrent flow heating - counter-current flow cooling grain drying.
This dryer would be used to obtain representative data for such a drying

system.

II. EXPERIMENTAL

2.1 DESIGN CRITERIA

 

It was desired to build a dryer which permitted the entrance
of both product and heated air at the top, exit of product and entrance
of cooling air at the bottom, and exhausting of both heating and cooling
air in the central portion. Such a dryer would have a drying zone in
the upper portion and a cooling zone in the lower portion. It was also
desirable to be able to vary the lengths of these sections with ease.

A third design factor was ease of construction and variability of
functions, i.e., the design must permit the dryer to be used either
completely as a concurrent dryer or completely as a counter—current
dryer. For these reasons, a square cross section,to give simplicity in
symmetry, was decided upon. Thus, modular construction would allow
interchangeability of both vertical position and face orientation.

The necessity of spreading the grain would have to be determined
experimentally, since there are dryers both with and without Spreaders
in use. It was hoped, for simplicity, that the spreaderless type would
be successful. With this in mind, it was decided to place chambers in
the dryer which would split the grain flow into two flow streams, each
of which would have an increased flow velocity due to the restriction
in the cross sectional area. The angle of repose of the grain would
cause it to form two ridges under the flow streams and troughs in the
center and on both sides. (See Figure 2.1A)- Similar chambers could be
used to provide an exhaust section by merely making the chambers from
perforated plates. A hOpper bottom with perforated plates would provide

an entrance for the cooling air. Airtight unloading could be accomplished

7

by use of a rotary airlock feeder mechanism.

A half-scale clear plastic (Plexiglas) model was constructed to
test the design before actual construction of the dryer. Critical design
changes arose from the use of this model. Grain flow studies were carried
out by filling the model dryer with beans. Some of the beans were painted
and placed as layers in the model (See Figure 2.18). While the model
was unloading, pictures were taken at time intervals to give a record of
the grain flow pattern. The grain flow through the restrictions was
very satisfactory (See Figures 2.1C and 2.1D). However, the unloading
was off-center and created a stall area on the front side (closest side
when viewing the unloading grain) (See Figures 2.1E and 2.1F). This
condition was corrected in the full scale dryer and a satisfactory

unloading was achieved.

   

A. GRAVITY FORMED B. PLASTIC MODEL
RIDGES AND TROUGHS

   

C. LAYER 0F GRAIN D. LAYER EXITING CHAMBER
ENTERING CHAMBER

   

E. OFF CENTER UNLOADING F. STALL AREA ON LEFT

FIGURES 2.1A through 2.1F

2.2 Description of Apparatus

 

2.21 Dryer Construction

The dryer has six basic sections. These are, from top to bottom
(See Figure 2.21A), the grain storage tank, the hot air inlet section, the
drying section, the exhaust section, the cooling section, and the cool
air inlet and unloader section. All sections were constructed from 14
gage steel plate bent and welded to form one foot by one foot inside
dimension tubes of varying lengths depending upon the section require—
ments. A square ring made from one inch by one—eighth inch steel bar with
9/32 inch holes for 1/4 inch bolts was welded to each end of a section
for fastening the sections together. The bolt hole spacing was symmetric
so the sections could be assembled in any orientation. A section of
such construction will hereafter be referred to as a basic section
(See Figure 2.21B). All sections were painted with extreme high temper—
ature paint and were assembled with two asbestos gaskets at each connec-
tion.

The grain storage tank was two feet square and three feet in
length. A cover was made 26 1/8 inches by 26 1/8 inches to accomodate
the outer edge of the topmost steel ring. A rubber gasket was glued
to the cover to provide an airtight seal when the cover was fastened
in place. An adapter was welded to the bottom of the storage tank
to provide a smooth transition to the one foot square sections below

(See Figure 2.21C).

10

ll

 

FIGURE 2.21A ASSEMBLED DRYER WITH INSULATION REMOVED

   

FIGURE 2.21B FIGURE 2.21C
BASIC SECTION GRAIN STORAGE TANK

12

The original design called for a storage tank of one foot by one
foot cross section and three feet long. It was found that this did not
give sufficient grain storage for a test. An alternative was to mount
a rotary airlock feeder on top of the storage tank and refill the tank
during testing. The cost of these units proved detrimental to the idea.
Therefore, the larger tank as described above was constructed and was
quite satisfactory.

The hot air inlet was made by modifying a basic section. The
modifications consisted of adding three steel chambers to the inside
so that the grain flow was divided into two streams, each of which had
one sixth the cross sectional area of the basic section. This restric-
tion caused a flow velocity of three times the velocity in the basic
section. The restriction chambers served as an entrance for the heated
air above the grain (See Figure 2.21D). The sides of the chambers were
hinged and could be swung from side to side to spread or level the
grain. It should be noted that these chambers were made of solid steel
sheets (as opposed to perforated) to prevent exposure of the grain to
the hot air. One end of each of the chambers was cut out and an adapter
was made to connect the chambers to a four inch stove pipe. The Spreaders
were moved by a pneumatic cylinder operated by a reversing valve (See
Figure 2.21B) which was triggered by a microswitch and stepping relay.
It was not necessary to operate the Spreaders continuously, so a variable

time delay was incorporated at each half cycle.

l3

 

HOT AIR INLETS 3

 

 

 

 

 

 

 

 

 

SPREADERS (4)

 

 

FIGURE 2.21D HOT AIR INLET WITH ADAPTER REMOVED
(not to scale)

   

FIGURE 2.21E HOT AIR INLET WITH SPREADER, VALVE AND CYLINDER

14

This oscillating spreader was constructed because the first hot
air inlet section was not capable of spreading the grain. The lack
of spreading caused the grain to form a ridge under each of the two
flow streams (See Figure 2.1A). This proved to be undesirable since
a streamline of heated air was observed below the center of the troughs.

The drying section consisted of a one foot long basic section
which was drilled to allow samples to be taken (See Figure 2.21F).
The sample holes were one inch diameter and spaced at two inch intervals
along a vertical line in the center of one side. The drying section
could be made any length by merely adding or removing different length
basic sections.

The exhaust section was a modified basic section very similar
to the first hot air inlet section, the distinction being that the
chambers were constructed of perforated steel plate and were equipped
with perforated bottoms to prevent some of the fine particles from being
exhausted with the air. The flow streams in this section had a cross
sectional area of one fourth the cross sectional area of the drying
section. This resulted in twice the flow velocity in this section
as in the basic section. The perforated plates had l/8 inch holes spaced
1/4 inch center to center in 60 degree rotated planes. The section
was fitted with an adapter similar to the hot air inlet adapter except
that the exhaust adapter was designed for a five inch stove pipe
(See Figure 2.21C).

The cooling section was made from a basic section which was six

inches in length. Sampling holes were not cut into these sections but

15

could have been added at any time if it were necessary to use one
of these sections to lengthen the drying section or if samples
from the cooling section were desired. In Tests 1 through 5C, two
six inch sections were used to form a one foot cooling section.
Test 6 used only one six inch section.

The cool air inlet and unloader section was a six inch long
basic section except that it had only one connecter ring. Two
perforated plates were installed inside at 45 degrees from the
horizon to form a typical hopper bottom with a three inch wide by
one foot long opening. Steel plates were welded into the remaining
space in the horizontal plane containing the opening. A hole was
cut in each of the sides of the section enclosed by a perforated
plate and a bottom plate. This hole was fitted with a four inch
stove pipe connecter (see Figures 2.21H, J and K).

The unloading mechanism was constructed from a one foot long
piece of four inch standard wall steel pipe to which was welded
sixteen steel bars (one half inch by one eighth inch by twelve inch)
equally spaced radially on the outside (see Figure 2.21K). The
fluted wheel just described was turned true axially and both ends
were faced true. A piece of steel plate was rolled to a two and
three quarter inch radius through an arc of 125 degrees to form
the back plate of the rotary airlock. This acted as an airlock
since more than two flutes of the wheel were in contact with the
back plate at all times. The front piece of the airlock was orig—
inally constructed similarly but was later changed to include a
piece of sheet rubber to provide a better seal. Each fluted
segment of the unloading wheel had an approximate volume of four

and one half cubic inches (1/2" x 3/4" x 12").

l6

 

F. DRYING SECTION C. EXHAUST SECTION

 

H. COOL AIR INLET (top View) I, AIR FILTERS

   

J. COOL AIR INLET AND UNLOADER K. UNLOADER CONSTRUCTION

FIGURES 2.21F through 2,21K

l7

Sixteen segments per revolution gave 72 cubic inches or approximately
two pounds of shelled corn per revolution depending on the moisture
content.

The first fluted wheel was made of three and a half inch steel
pipe (4" O.D.) with one inch by one eighth inch steel bars welded radially
to form eight segments. The flow rate of such an unloader was much too
great for this dryer due to the large volume of grain removed per revol-
ution. To obtain the desired flow rates, it was necessary to turn the
wheel very slowly. This slow speed resulted in step flow which could not
be considered continuous. Ready made rotary airlock feeders are available
but only with flow rates above those desired for these tests.

One more problem was encountered with the unloading mechanism.
By studying movies made of the plexiglas half scale model, an off-center
hopper bottom unloading was noted. As mentioned above (see Figure 2.1E),
the flutes on the unloading wheel would fill up with grain from the back
of the dryer (when viewing the unloading grain broadside) (see Figure
2.1F). This caused a stall area along the front of the dryer which was
undesirable. By narrowing the opening from the back side and by using
the sixteen segment fluted wheel described above, the unloading was success-
fully moved to the center of the dryer.

The unloader drive was constructed from a 1963 Ford windshield
wiper motor (12 volt D.C.) which was modified to give rotary motion
instead of reciprocal motion. This was connected to an eleven to one gear

reducer which in turn was connected to the fluted wheel shaft by sprockets

18

and a chain (See Figure 2.21J). This combination gave an overall gear
reduction of forty to one which allowed flow rates up to five pounds per
minute for shelled corn. The direct current motor was capable of
variable speeds by varying the input voltage using a variable D.C. voltage
power supply. This system proved to be very satisfactory and easily
controlled.

The entire dryer was placed on a table so that the outflow could
be collected in a container placed on a scale giving continuous moni—

toring of the flow rate.

2.22 Control and Monitoring Equipment

Constant air flow rates and air temperatures were required at both
the hot air inlet and the cool air inlet. Also, it was necessary to maintain
constant relative humidity of the cooling air at the cooling air inlet.

The grain flow rate and the hot air temperature were the primary variables.
The air flow rates were secondary variables. Both primary and secondary
variables were monitored continuously and were corrected whenever any
variance occurred.

Hot air temperature was controlled by two means -- the gas regul-
ator and the air damper. The heat output of the burner was directly
related to the fuel consumption, ergo the pressure regulator. If a lower
temperature was desired than was available with the smallest orifice and
lowest pressure, the air damper was used. This routed some of the heated

air into the room rather than into the hot air inlet. The five foot length

19

of pipe between the damper and the inlet provided a cooling effect. It
was possible to maintain the inlet temperature within five degrees of
the desired value.

Due to the problems involved with higher temperature air measure-
ment, it was desired to use an orifice plate. To accurately measure
the air flow rate at four or five hundred degrees Fahrenheit, it was
recommended that a six inch steel pipe be used with an orifice plate
containing a three and a half inch bore. The size and weight of such a
system eliminated it as a possible flow measuring device. Therefore,

a pitot tube was used to determine the hot air flow rate. The pitot
tube was calibrated using a laminar flow element and variable speed fan.
Pitot tube differential pressures were measured with a micro-manometer
which could be read to 0.001 inches of water.

Chromel/Alumel thermocouples were placed in the hot air chambers
to give an indication of the air temperature just above the grain. These
thermocouples were connected to a pen recorder to give continuous moni-
toring of the inlet temperature.

Values of cooling air temperature and relative humidity were main-
tained constant by the use of an air conditioning unit and were inter—
mittently recorded during tests as a precautionary measure. The air
conditioning unit was equipped with a variable speed fan to allow increasing
or decreasing the cooling air rates.

The relative humidities of the cooling air and the exhaust air were
monitored continuously with electric hygrometer indicators. Values from

these were periodically recorded in the log book.

20

Two methods were used to determine the cooling air rate. In the
earlier tests (1 through 5) a pitot tube was used to measure the hot air
rate and a laminar flow element was used in the exhaust line so that a
mass balance could be used to calculate the cooling air rate. This pro-
cedure was followed since it was initially desired to use room air for
cooling and, by using a fan on the hot air line and in the exhaust
line, no cooling air fan was necessary. A second method was used in
test 6 and will be employed in the tests to follow. This method required
the laminar flow element to be installed in the cooling air line. This
was a more direct means of obtaining the cooling air rate. Maintaining
the air flow rates was easier with this method since clean air was passed
through the laminar flow element and the filters could be removed.

The air filters were necessary with the earlier method to prevent plugging
of the laminar flow element.

Copper/constantan thermocouples were positioned in the drying
section when the first tests were run. After more than an hour at high
temperatures, the insulation on the thermocouple wires began to melt.
However, the wires remained insulated and true readings were still recorded.
For safety, chromel/alumel thermocouples with a fiberglas and asbestos
insulation were installed in the upper portion of the drying section.

For test 6, the thermocouples were again changed to copper/constantan

to take advantage of the better accuracies. The thermocouples were

located in columns, each column located three inches (one fourth of

the diameter ) from two adjoining sides. With this configuration, and with
more than one thermocouple at each depth, it was possible to average

the thermocouple readings. This was done for both test 6A and 68 (See

21

Table 3.1F). Future tests will also have more than one thermocouple
at each cross section to give a better indication of the air move-
ment through the grain.

Copper-constantan thermocouples were positioned in the center
of flow in the cooling section for all tests. Consecutive readings
were recorded and five of these consecutive readings were averaged
for cooling section data.

The exhaust fan was equipped with a variable diameter pulley
and the fan speed was controlled by changing the distance between
the motor and the fan. The hot air fan was a constant speed fan
of the vane axial type. The cooling air fan was driven by a
variable speed motor. By varying the speeds of the latter two
fans numerous combinations of air flows were available. These
were monitored continuously by use of manometers connected to
the pitot tube and the laminar flow element. The laminar flow
element required clean air and this necessitated the installation
of air filters in the exhaust line. The filters (see Figure 2.211)
were custom made and were in three stages, each successively
smaller up to the third stage, a furnace filter, which eliminated
the fines. It was not necessary to use the filters when the laminar
flow element was used in the cool air line (as in Test 6).

The grain flow rate, as mentioned previously, was controlled
by varying the voltage to the unloader motor. The grain output
was collected in a container which was on a scale, thus, continuous

monitoring of the grain flow was available.

2.23

l.*

10.

ll.

12.

13.

14.

15.

22

Summary of Control and Monitoring_Equipment

 

HEATER: Aerovent supplemental heater model SH 12, equipped with
a 1/16 inch spud for 90,000 btu/hr output.

PITOT TUBE: Meriam model M-l68 with 1/8 inch ram hole.
MANOMETER: Meriam micro-manometer model 34FB2 ( to 0.001 inch H20).
THERMOCOUPLES: Chromel/Alumel (type K. :_ 5°F), 20 gauge.

RECORDER: Texas two pen, model P502W6A, accuracy :_2° F, linearity
+ o.3° F.

THERMOCOUPLES: Copper/constantan (type T, :_O.5% or :_l.5° F),
24 gauge.

RECORDER: Texas twenty-four point, model FMWT6B, accuract :_0.75° F,
linearity : O.3° F.

AIR FILTERS: Custom made at Agricultural Engineering, M.S.U.

LAMINAR FLOW ELEMENT: Meriam model 50MCZ-4P, accuracy :;0.5% of
calibration curve.

MANOMETER: Meriam model 4OGD10WM-6 (to 0.02 inch H20).

EXHAUST FAN: Clarage Fan Co. size 3/4, with 9 inch squirrel cage
rotor, driven by 5 hp General Electric motor by
variable diameter pulley.

RECORDER: Brown twelve point, model 153X65P12-X-2F, accuracy
+ 1.2° F.

AIR CONDITIONING UNIT: Aminco-Aire model 4-5580, capable of
maintaining constant temperature Q: 1° F)
and relative humidity (: 3% r.h.) equipped
with variable speed fan motor.

HYGROMETER: Hygrodynamics electric hygrometer-indicator, model
15-3001, accuracy :_1.S% r.h.

POWER SUPPLY: Electro D.C. power supply model EB, 0 to 32 volts,
4 amp maximum.

See Figure 2.23A

23

 

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mun<u

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24

2.3 TESTING PROCEDURE

 

Tests were based on two heating air temperatures, 450 and 220 degrees
Fahrenheit. Grain flow rates were changed to get different moisture
removal rates. The initial grain temperature was substantially lower
than room temperature because the grain was kept in a 40° F cooler. This
was a desired condition since corn temperatures are almost always below
80° F during the fall harvest.

Pretest procedures included rewetting the shelled corn and allowing
it to temper and running an operational checkout of the dryer. The corn
used in the first series of tests (1 through 4) had been field shelled
in the Fall of 1969. The corn used in the remainder of the tests (5A
through 6B) was picked and crib dried the same harvest year. This corn
was shelled and cleaned less than a week before tests 5 and 6 were run.

It was believed that this latter corn would more closely resemble new corn.
New corn was not available at the time of testing.

The samples to be dried were rewetted by adding water to the corn
and mixing the mixture in a cement mixer. The mixture was then placed
in galvanized steel trash containers. These were covered and placed in
a 40° F cooler for at least 36-42 hours. The contents were dumped and
remixed after 12 hours to give a more uniform moisture content throughout
the sample.

The operational check out of the dryer included an ambient air
check of the thermocouple readings and a check with cooling air moving

through the dryer. All thermocouple outputs were compared to a standard

25

millivolt source at 32° F and at 80° F when first installed in the dryer.
The thermocouples were not rechecked at 32° F unless there was a notice-
able deviation during the operational checkout with ambient and with
cooling air. This procedure was followed because of the extreme difficulty
involved in positioning the thermocouples in the dryer.

The unloading rate was approximated by measuring the rotation
rate of the fluted wheel. The actual unloading rate was slightly less
than this initial approximation due to the increased load on the drive
motor under test conditions. Next, the air filters were cleaned and the
manometers were zeroed. The air conditioning unit was started and allowed
to stabilize at operating conditions. The dryer was then filled with corn.
The lower sections of the dryer were filled with dry corn to provide
more rewetted corn for the test. The wet corn was again mixed prior to
filling the dryer and samples were taken.

After the dryer was filled, the grain storage tank was sealed to
prevent air movement upward through the stored grain. This procedure was
necessary to maintain steady state inlet conditions during testing.

The recorders and the unloader were started prior to lighting the burner.
This was done to get a good representation of the starting conditions.

The cooling air and exhaust air fans were also started at this time.

The final steps in starting a test were lighting the burner, starting

the clock, and adjusting the grain flow rate. Steady state conditions were
reached after approximately 1/2 hour, the exact time depending on the
moisture content of the grain and the unloading rate.

After steady state conditions were reached (indicated by a con-
stant grain temperature at a position over a period of time), there was

a time delay before output samples could be taken. It was necessary to

26

unload at least 130 pounds of grain to allow the grain just entering
the drying section when steady state conditions were reached to pass
completely through the dryer.

After the delay, samples were taken and tested for moisture
content. At this time, if the dryer contained sufficient grain, a
second test could be run as was done in tests 3, 5, and 6. Whenever
the thermocouples in the drying zone began to indicate higher temper-
atures, the burner was shut down since this indicated a lack of grain
in the storage tank. Emptying the dryer and shutdown of equipment con—

cluded the test.

III. RESULTS AND DISCUSSION
3.1 OUTPUT 0F DRYER

A summary of the tests run, including output moisture contents,
is given in Tables 3.1A through 3.1D. The output moisture content
was for grain which had passed through both the drying and cooling
zones. The amount of drying done in the cooling zone was considered
to be only slight compared to the amount done in the drying zone
but no attempt was made to prove this point. Since the dryer was of
a one square foot cross section, grain and air rates can be compared
to other dryers on a square foot basis.

The effect of grain spreading on drying air temperatures can
be seen in Figure 3.1A (values listed in Table 3.1E). The testing
was accomplished by merely stopping the Spreaders at their center
position and noting the air temperatures in the drying zone for a
period of time and then comparing these to the values recorded
during the next period of time when spreading was used. At steady
state conditions, the air temperatures in the t0p eight inches of
the drying zone were lower when spreading was used compared to the
same test with no spreading. The final destination of the grain
will determine whether spreading is essential. For instance, if the
grain were used for seed, the higher air temperatures from not using
spreading could cause considerable seed damage.

The drying air temperatures at various depths in the drying
zone for tests 3A, 6A and 6B are listed in Tables 3.1F and 3.1H and
are shown graphically in Figure 3.1B.

Grain temperatures in a continuous flow dryer are difficult to

measure and, therefore, theoretical temperatures are of significant

27

28

importance. Bakker—Arkema et al.(l970) have obtained theoretical
values of grain and air temperatures for concurrent flow drying (see
Figure 3.1C). The significant point of this figure is the peak
in the grain temperature curve. The overall shape of the curve is
important for high temperature tests since the grain temperatures
appear to be very high if the curve is superimposed onto Figure 3.1B
for test 3A. Also, by superimposing this curve onto Figure 3.1A,
one can see that grain spreading will greatly reduce the peak grain
temperature. This fact is very important in maintaining grain
quality, since the grain temperature approaches but never equals
the heating air temperature. The grain temperature increases at a
high rate and at the same time the air temperature decreases rapidly
in the first few inches of the drying zone. The grain temperature
peak will be influenced by the initial grain moisture content, the
air flow rates, the air humidity, and the initial difference between
the air and grain temperatures. Thus, for a given grain temperature
there is a maximum allowable heating air temperature to keep the
peak grain temperature low. The length of time that the grain is
kept at a given peak temperature will have the greatest effect on
the quality of the grain. Thus, there is also a relationship
between unloading rate at a given heating air temperature and quality.

Cooling air temperatures at various depths in the cooling zone
are given in Table 3.1C. These values are plotted in Figures 3.1D
and 3.1E for both high and low temperature tests, respectively (see
Tables 3.1A through 3.1D for details of tests). Little or no
cooling took place in the cooling section for the low temperature

tests and the need of the section under such conditions is doubtful.

29

The cooling air conditions could, under certain circumstances,
actually cause rewetting of the grain. Under these conditions the
addition of supplemental heat at the cooling air inlet could prove
invaluable if the cooling section were used at all.

Due to the location of the exhaust hygrometer sensor, difficulty
in recording values of the exhaust relative humidity was encountered.
Wide fluctuations in the readings were caused by the fact that the
cooling and heating air exhausts were not mixed when passing the
sensor. Both exhausts were at nearly the same temperature but were
greatly different in humidities. Because of this difficulty, the
exhaust relative humidity was calculated by a water mass balance of
the heating air, fuel water, cooling air and moisture loss of the
grain (see Table 3.1D). An example of such a calculation is outlined
in Section 3.11. The fuel water mentioned above is a product of
combustion and is only a factor when direct fired burners are used
as in these tests. Fuel water amounts will vary depending on the
composition of the fuel, the heating value of the fuel and the air
flow rate. The inclusion of fuel water in the heating air results
in an equivalent specific humidity which is higher than for air

heated indirectly to the same temperature.

DRYING AIR TEMPERATURE, °F

160

150

140

130

120

110

100

90

80

10

1

III

I

 

30

    
   
  
 

WITHOUT SPREADING Q)

WITH SPREADING A

GRAIN RATE 3.2 BU/HR

  

GRAIN TEMPERATURE 54° F

HEATING AIR TEMPERATURE 205° F

HEATING AIR RATE 130 CFM/FT2

2 4 6 8 10 12 14 16 18

DEPTH IN DRYING ZONE, INCHES

Figure 3.1A Effect of Spreading on Drying Air Temperatures.

°F

TEMPERATURE,

500

450

400

350

300

250

200

150

100

50

31

Test 3A C)

Test 6B CI

Test 6A A

 

 

 

 

 

 

I I I I L I I l I I I I I I I I I I
I I I F I I I I I I I I I I I I I I
o 2 4 6 8 10 12 14 16 18

DEPTH IN DRYING ZONE, INCHES

Figure 3.18 Drying Air Temperature Versus Depth for Several
Tests.

 

TEMPERATURE , OF

32

180

  

 

 

 

 

 

 

 

 

 

pRYINGAIR
11.0-—
120
100
.4
80---
10--
DEPTH, feet
.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1
o I I II I I I I II I L I I
I I I I T T l r T I T T T l
1 2 3 4 5 6 7 8 9 10 11 12 13 14

DEPTH, inches

Figure 3.1C Theoretical Air and Grain Temperatures
(from BARKER, 1970)

 

TIT *

 

33

140 '

130 ‘

120 n

110 H

100 1

TEMPERATURE, °F

90 ‘

 

8O ‘

II—

70

IIILI
Iirlr

I 1
10 1112 13 14 15 16

 

db-
-

 

F’
5

.II—
N 1|-

] l
I ‘r
8 9

0
hi
hi
LID-.—
P—e.

POSITION BELOW EXHAUST, INCHES

Figure 3.1D Cool Air Temperatures Versus Position (450° F Tests)

90 a.

80 q

 

 

TEMPERATURE, °F
\1
u:
I

 

 

7O '
65 1
60 I l L I I I I II I I I I
T I I l I T l I I l T I
O 1 2 3 4 5 6 7 8 9 10 ll 12

POSITION BELOW EXHAUST, INCHES

Figure 3.1E Cool Air Temperature Versus Position (220° and 205°F Tests)

3.11

34

Example of Mass Balance for Test 5A

Measured values:

Values

Room (cooling air) temperature
Room (cooling air) relative humidity

Heating air temperature

Heating air rate at 450°F

Exhaust air temperature

Exhaust air rate at 118°F

Grain temperature at inlet

Grain rate at inlet

Grain moisture content at inlet, MC

in

Grain moisture content at outlet, Mcout
dependent on above values:

Density of moist air at 83° F

Density of moist air at 450° F

Density of moist air at 118° F

Specific humidity for cooling air, Hcool

Specific humidity at burner inlet, H

Known values:

Specific heat of air, Cp
Heating value of fuell, HV

Water to fuel ratio (grams water from grams
fuel)

 

1 See

Smith (1952)

83° F

55 Z

450° F
96.5 cfm
118° F

153 cfm
50° F

3.2 bu/hr
37.3% d.b.

26.570 d.b.

.072 1b/ft3

.04324 lb/ft3

.0658 lb/ft3

.0137 lb water/lb d.a.
.0137 lb water/lb d.a.
.24 BTU/lb °F
19,200 BTU/lb

72/44

 

35

Calculated values:
Gawfor heating air (rate x 60 x density) 250.3 lb/hr

Gad for heating air [Saw/(1+H)], excluding fuel water 246.9 lb/hr

 

Water carried in heating air (Gaw-Gad), no fuel water 3.4 lb/hr

 

Heat needed to warm heating air (AT x Gaw x Cp) 30000 BTU/hr

Fuel needed (Heat needed/HV) 1.56 lb/hr #1
Water of combustion (Fuel x water to fuel ratio) 2.56 lb/hr éI E
Total water in heating air 5.96 lb/hr I
Moisture content change (MCin-Mcout) 10.8 % d.b.

Dry matter grain rate [rate x 56 x (l—MCout)] 131.7 lb/hr ;,J
Water removed from grain (d.m. rate x MC change) 14.22 lb/hr

Equivalent specific humidity for heating air 0.024 1b/lb

(Total water in heating air/Gad)

Mass balance:
Two mass balances are needed, one for the dry air and another for
the water vapor. Since the exhaust relative humidity was not a valid
measurement, it was necessary to solve both balances simultaneously to
obtain the exhaust humidity. The dry air balance was:
Gad cool 3 Gad exhaust — Gad hot
The water vapor was a summation as follows:
Waterexhaust = Water in hot air + water in cool air + water from grain
Since the water exhausted can also be expressed as
Waterexhaust = Gad exhaust x Hexhaust

and since

Gad exhaust = Gaw exhaust / (l + Hex)

36

then, by substitution

Water = H x (G

l + H
exhaust exhaust / ( ex)

aw exhaust

Equating the expressions for water exhausted and substituting the
values for the water in the hot air and the water from the grain gives:

H x (saw ex / (1+Hex))= 20.18 + H

ex / (Ii-Hex) - G

cool x [(Gaw ex ad hot]

Substituting in values for G ~Gad hot and H and solving for Hex:

aw ex’ cool

“exhaust = 0.0428 1b H20 / lb d.a. I

With this value, the remainder of the values were calculated:

 

Gad exhaust = 577.3 lb/hr ;
= I

Gad cool 330.4 lb/hr 2:

Water in cool air = 4.52 lb/hr

Gaw cool = 334.92 lb/hr

Cooling air rate = 77.5 CFM/ft2

Relative humidity of exhaust air = 56 %

37

3.2 Quality

A very extensive series of tests is needed to determine the
true value of grain dried at high temperatures for short periods
of time. This value will be determined by the final destination
of the grain, that is, feed, seed, milling, etc. Such tests were
beyond the scope of this study but samples of grain from tests
5A and SC were taken and germination tests were run. Test 5A
exposed the grain to 450°F heating air with the grain travelling
vertically downward at approximately 0.68 inches per minute. The
grain was exposed to the hottest air for at least one half minute.
Test 5C used 2200F heating air but due to the grain velocity of
0.37 inches per minute, the grain was exposed for at least one

minute. The germination test results were:

SAMPLE z GERMINATION
l, undried 59
2, Test 5A (450°F) 28
3, Test 50 (2200F) 51

If the results of Sample 1 are considered 100% then the results

of Samples 2 and 3 are 47.5% and 86.5% respectively. It would have

been invaluable to have had samples which were dried at the same
conditions only without spreading to verify the theoretical grain

temperatures for spreading compared to no spreading.

 

"I .42“-

38

3.3 Computer Simulation

 

A working computer model is a invaluable tool for grain drying
research. Once the model is proved to be trustworthy, parameters can be
changed to find the effect of each separately. Without the computer model
many hours of testing would be necessary to obtain an optimum design.

The type of dryer used in this research requires two computer FR
models, one for the drying zone and another for the cooling zone. Elzinga
(1970) developed a simulation model for the concurrent flow drying portion

of the dryer used in this study. Evans (1970) developed a model for

 

Ilv‘.
I

counter-current flow drying. This model must be modified for cooling
since very little drying takes place in the cooling section of the dryer
used in this research. For this reason the cooling section was ignored
as far as moisture content changes. The model developed by Elzinga

can be used to find the optimum length of drying zone. By modifying

the Evans model, the optimum cooling section length could also be found
with very little testing.

Grain temperatures are difficult to measure during grain drying
tests without interrupting the testing. Air temperatures on the other
hand, are relatively easy to measure. A computer simulation model can be
used to determine both air and grain temperatures. If the air temper-
atures can be verified experimentally, it is safe to assume that the grain
temperatures are also correct.

The use of these models will enable the achievement of the ideal
concurrent - counter-current flow dryer design. Such models will also en—

able the designer to keep the grain temperatures below the established limits

which will result in higher quality grain and the needed speed in drying.

 

IV. SUMMARY AND CONCLUSIONS

4.1 Summary

Continuous flow grain dryers using concurrent flow drying
and counter-current flow cooling are capable of high moisture
content removal at initially high rates which decrease as drying
progresses. A grain Spreading device at the top of the drying
zone is necessary to maintain the grain temperature below the high
limit when high temperature air, e.g. 400°F, is used for drying.
From a moisture content only aspect, these dryers are an answer
to the increasing drying demands. Further tests are necessary
to determine the values of the limiting temperature-time
relationship for grain drying, specifically grain damage limits.
Once this limit is established, continuous flow grain dryers can

be properly designed to do the best possible job.

39

 

 

'4

R.
"L
van

40

4.2 Conclusions

 

l. The use of a grain spreading device is essential to assure
high quality grain when using high temperature concurrent
flow drying.

2. Direct fired burners significantly increase the absolute
humidity of the heating air and thus affect the drying rate
and optimum drying section length.

3. Good mixing and uniform distribution of the heating air above

the drying zone is essential to uniform drying.

 

4. In the dryer used for these tests, exhaust air relative
humidity was difficult to measure due to the high air
flow rates and mixing problems.
5. Counter-current cooling prevents or at least limits the
amount of stress cracks due to cooling warm grain.
6. The use of natural air (room air for these tests) effectively
cooled the dried grain to an acceptable handling temperature.
7. The use of rotary airlock unloaders is satisfactory if the

airlock is properly designed and positioned.

V. RECOMMENDATIONS FOR FUTURE STUDY

The most important recommendation for grain drying research
in general is that tests be run using new field shelled corn to
best approximate farmer and elevator drying conditions. Another
recommendation is the recording 0f the average size of the kernels
to aid in the study of moisture and heat transfer in relation to
grain drying.

Testing is needed to determine the economic effect on
drying rates of adding products of combustion to the heating air.

Recommendations that apply specifically to the type of drying
described in this paper include: (1) providing smoother transitions
at air inlets and exhausts; (2) testing both with and without
spreading to determine its effect on grain quality; (3) testing
concurrent flow drying with no cooling, and; (4) testing the
cooling section knowing the temperature and moisture content of
the grain at the top to determine the amount of drying that occurs
in that section.

Also, the exhaust section should be separated to allow
monitoring the temperature and relative humidity of both the
exhausted cooling air as well as the exhausted heating air instead
of monitoring the mixed exhaust air.

Further testing should be done to determine the exact limits
of the temperature-time relationship needed to assure good quality
grain. Other biological criteria should be established for grains
of various final utilizations so that dryers can be designed for

optimum capacity while maintaining quality standards.

41

LIST OF REFERENCES

 

REFERENCES

Amdur, E. J. and E. 0. Meyer. (1964). Humidity instrumentation.
Paper number 64-904. American Society of Agricultural Engineers.

Bakker-Arkema, F. W., T. W. Evans and D. M. Farmer. (1969) Simula-
tion of multiple-zone grain drying. Paper number 69-835, American
Society of Agricultural Engineers.

Bakker-Arkema, F- W., L. E. Lerew. (1970). MSU Grain Drying Models.
Paper number 70-832. American Society of Agricultural Engineers.

Bakker-Arkema, F. W., R. J. Patterson, and W. G. Bickert. (1967).
Statis pressure airflow relationships in packed beds of granular
biological materials such as cherry pits. Transactions of
American Society of Agricultural Engineers, Vol. 12, No. 1,
134-36 and 140.

 

 

 

Bickert, W. G., and F. W. Bakker-Arkema. (1966). A deep-bed computational
cooling procedure for biological products. Transactions of American
Society of Agricultural Engineers, Vol. 9, No. 6, 834-36, and 845.

 

 

Elzinga, Daniel G. (1971). High temperature continuous flow grain drying
with concurrent drying and counter-current cooling, Part II:
Simulation and optimization. M.S. Thesis, Michigan State University.

Evans, Timothy W. (1970). Simulation of counter-flow drying. M.S.
Thesis, Michigan State University.

Farmer, David, M. (1968). A simulation of the convective deep-bed
drying of biological products during the constant rate and falling
rate drying periods. M.S. Thesis, Michigan State University.

Graham, Douglas L. (1967). Concurrent flow grain dryer design. Paper
number 67-859, American Society of Agricultural Engineers.

Graham, Douglas L. (1967). Drying apparatus and method. United States
Patent Number 3,302,299.

Hall, Carl W. (1957). Dgying Farm Crops. Edwards Bros., Inc., Ann
Arbor, 336 pp.

 

Smith, M. and K. Stinson (1952). Fuels and Combustion. McGraw-Hill
Book Co., Inc., New York.

 

Tillmanns, H. (1966). Apparatus for drying and cooling particulated
material. United States Patent Number 3,261,109.

42

APPENDIX

43

 

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