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This is to certify that the

dissertation entitled

Design of a Mixed-Flow Grain Dryer

presented by

James Edward Montross

has been accepted towards fulfillment

of the requirements for
Agricultural

M. S . degree in Engineering

mm

Major professor

 

Date December 1998

MSU is an Affirmative Action/Equal Opportunity Institution 0-12771

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOiD FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE

DATE DUE

DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1!” WWW“

DESIGN OF A MIXED-FLOW GRAIN DRYER
By

James Edward Montross

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
in
Agricultural Engineering

Department of Agricultural Engineering

1998

ABSTRACT
DESIGN or A MIXED-FLOW GRAIN DRYER
By

James Edward Montross

The design of a mixed-flow grain dryer was accomplished using the two-
dimensional MSU mixed-flow grain drying model. The dryer was constructed and tested
at an elevator in mid-Michigan. The mixed-flow dryer performance was measured and
the outlet grain quality characteristics were examined and compared to a cylindrical-
tower crossflow dryer located at the elevator.

To improve moisture uniformity a technique similar to the grain tum-flow in
crossflow dryers was examined. The technique involved reversing modules or turning a
module 180°. The effect of modified exhaust ducts on the reduction of dust emissions
was also examined.

Stress-crack percentages for the mixed-flow dryer were similar to the crossflow
dryer contradicting previous research. The outlet grain moisture uniformity was greater
for the mixed-flow dryer with an average standard deviation of 2.2 %w.b compared to 4.1
%w.b. for the crossflow dryer.

Dust emissions were reduced with each exhaust-duct modification. Louvers
provided the greatest reduction (42 %) while baffles the least (29 %).

The MSU mixed-flow grain drying was validated and proven to be an effective

design tool.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to Professor Fred W. Bakker-Arkema
for his academic guidance and friendship.

The financial support of ffi Corporation is greatly appreciated along with the
partial financial support of the Crop and Food Bio-processing Center at Michigan State
University.

Appreciation is expressed to Dr. W. Bickert and Dr. L. Copeland for serving on
the guidance committee.

Many thanks expressed to Qiang Liu for his friendship and mixed-flow grain
drying expertise. Additional thanks to Kyle Weidmayer for his help during the
construction and testing of the dryer.

I would also like to thank every fi‘iend that I have made within the Agricultural
Engineering Department.

Last but not least, special thanks to my family—for everything.

iii

TABLE OF CONTENTS

LIST OF TABLES .............................................................................................................................. vi
LIST OF FIGURES ............................................................................................................................ vii
LIST OF SYMBOLS ........................................................................................................................... ix
1. INTRODUCTION ............................................................................................................................ I
2. OBJECTiVES ................................................................................................................................. 4
3. LITERATURE REVIEW .................................................................................................................. 5
3.1 MIXED-FLOW GRAIN DRYERS ............................................................................................... 5
3.2 QUALITY OF DRIED MAIZE .................................................................................................... 8
3.3 AIR POLLUTION AND GRAIN DRYING ................................................................................... 9

4. DRYER DESIGN .......................................................................................................................... 12
4.1 Two-DIMENSIONAL MIXED-FLOW SIMULATION MODEL .................................................... 12
4.2 DESIGN CRITERIA ................................................................................................................ 14
4.3 MODULE DESIGN ................................................................................................................. 15
4.4 DUCT DESIGN ...................................................................................................................... 18
4.5 OVERALL DRYER DESIGN ................................................................................................... 21
4.5.1 MODULE REVERSAL ..................................................................................................... 21
4.5.2 ADDITION OF COOLING SECTION ................................................................................. 21
4.5.3 FAN SIZE ...................................................................................................................... 25
4.5.4 MISCELLANEOUS ......................................................................................................... 25

4.6 DRYER SPECIFICATIONS ...................................................................................................... 26

5. EXPERIMENTAL INVESTIGATION ............................................................................................... 29
5.1 EXPERIMENTAL TESTS ........................................................................................................ 29
5.2 INSTRUMENTATION ............................................................................................................. 30
5.2.1 FIELD MEASUREMENTS ................................................................................................ 30
5.2.2 LABORATORY MEASUREMENTS .................................................................................. 31

5.3 DRYER SETUP ...................................................................................................................... 33
5.4 EXPERIMENTAL RESULTS .................................................................................................... 33
5.4.1 GRAIN QUALITY CHARACTERISTICS ............................................................................ 36
5.4.2 MOISTURE CONTENT PROFILES ................................................................................... 39
5.4.3 DUST EMISSIONS .......................................................................................................... 40
5.4.4 EXHAUST-AIR TEMPERATURE AND VELOCITY PROFILES ............................................ 43
5.4.5 PRESSURE LOSS IN GRAIN COLUMN ............................................................................ 45

iv

6. SIMULATION ANALYSIS ............................................................................................................. 46

6.1 MODEL VALIDATION ........................................................................................................... 46
6.2 VALIDATION CONCLUSION .................................................................................................. 48
6.3 INFLUENCE OF DESIGN PARAMETERS ................................................................................. 48
6.3.1 EFFECT OF DRYING-AIR TEMPERATURE AND AIRFLOW .............................................. 49

6.3.2 EFFECT OF AMBIENT TEMPERATURE AND RELATIVE HUMIDITY ................................ 52

6.3.3 EFFECT OF INLET MOISTURE CONTENT AND GRAIN TEMPERATURE .......................... 52

7. SUMMARY AND CONCLUSIONS .................................................................................................. 56
8. RECOMMENDATIONS FOR FUTURE STUDY ................................................................................ 58
9. LIST OF REFERENCES ................................................................................................................. 59
APPENDICES ................................................................................................................................... 61
A. PRESSURE LOSS CALCULATIONS .......................................................................................... 62

B. MOISTURE CONTENT DATA ................................................................................................. 66

C. STRESS-CRACK DATA .......................................................................................................... 77
D. MISCELLANEOUS DATA ....................................................................................................... 81

E. DESIGN DRAWINGS .............................................................................................................. 89

TABLE 4.1.
TABLE 4.2.

TABLE 4.3.

TABLE 4.4.

TABLE 4.5.

TABLE 5.1.
TABLE 5.2.

TABLE 5.3.

TABLE 5.4.

TABLE 5.5.
TABLE 5.6.

TABLE 5.7.

TABLE 6.].

TABLE 6.2.

TABLE 6.3.

TABLE 6.4.

LIST OF TABLES

PARAMETERS FOR THE DESIGN OF THE PILOT-SCALE MIXED-FLOW DRYER MODULE. ............. 15
ENERGY EFFICIENCY RANGES FOR 5-POINT MOISTURE REMOVAL OF SEVERAL COMMERCIAL
CONTINUOUS-FLOW DRYERS IN 1998' .................................................................................... 15
RANGE OF THE MODULE PARAMETERS USED WITH THE MSU MIXED-FLOW SIMULATION
MODEL WITH INCREMENTS FOR THE INITIAL SIMULATION TRIALS ........................................... 17
RANGE OF THE MODULE PARAMETERS ANALYZED WITH THE MSU MIXED-FLOW SIMULATION
MODEL [THE INCREMENTS FOR THE DETAILED SIMULATION TRIALS]. ..................................... I7
DIMENSIONS AND PREDICTED PERFORMANCE OF THE PILOT-SCALE MIXED-FLOW GRAIN
DRYER. .................................................................................................................................. 26
SUMMARY OF THE PILOT-SCALE MIXED-FLOW DRYER TESTS. ................................................ 35
RANGE OF AMBIENT AND DRYER-OPERATING CONDITIONS FOR THE TEST OF THE PILOT-SCALE
MIXED—FLOW DRYER (MF) AND THE ZIMMERMAN MODEL VT4036 (CF) .............................. 36
COMPARISON OF GRAIN MOISTURE CONTENT AND STANDARD DEVIATION ENTERING AND
EXITING THE MIXED-FLOW DRYER (MF) AND CROSSFLOW DRYER (CF). ............................... 37

AVERAGE OUTLET STRESS-CRACK PERCENTAGE AND SCI' FOR THE MIXED-FLOW DRYER (MF)

 

 

AND CROSSFLOW DRYER (CF). .......................................................................... 37
EFFECT OF MIXED-FLOW DRYING ON THE TEST WEIGHT ......................................................... 39
SUMMARY OF PERCENTAGE OF DUST REDUCTION FOR THE THREE MODIFIED EXHAUST DUCTS.
.............................................................................................................................................. 40
AVERAGE PRESSURE LOSS (PA) THROUGH GRAIN COLUMN ALONG THE LENGTH OF THE DUCT
IN MIXED-FLOW DRYER [ZERO POINT AT EXHAUST SIDE OF DRYER]... ...... 45
COMPARISON OF EXPERIMENTAL AND SIMULATED CAPACITY AND ENERGY EFFICIENCY
RESULTS ................................................................................................................................ 47
COMPARISON OF EXPERIMENTAL AND SIMULATED OUTLET STANDARD DEVIATION AND GRAIN
TEMPERATURE RESULTS ........................................................................................................ 47
STANDARD CONDITIONS FOR THE DRYER PERFORMANCE ANALYSIS OF THE PILOT-SCALE
MIXED-FLOW DRYER. ............................................................................................................. 49

SIMULATED EFFECTS OF AMBIENT AIR TEMPERATURE ON OUTLET GRAIN TEMPERATURE ...... 52

vi

FIGURE 1.]

FIGURE 3.]

FIGURE 3.2

FIGURE 3.3

FIGURE 4.].

FIGURE 4.2.
FIGURE 4.3.

FIGURE 4.4.
FIGURE 4.5.
FIGURE 4.6.
FIGURE 4.7.
FIGURE 4.8.

FIGURE 4.9.
FIGURE 5.1.

FIGURE 5.2.
FIGURE 5.3.

FIGURE 5.4.
FIGURE 5.5.
FIGURE 5.6.
FIGURE 5.7.
FIGURE 5.8.
FIGURE 6.1.
FIGURE 6.2.
FIGURE 6.3.
FIGURE 6.4.

FIGURE 6.5.

FIGURE 6.6.

LIST OF FIGURES

SCHEMATICS OF THE FOUR MAJOR TYPES OF HIGH-TEMPERATURE GRAIN DRYERS:

CROSSFLOW, COUNTERFLOW, CONCURRENT-FLOW, AND MIXED—FLOW. .................................. 2
DUCT SYSTEM, AND GRAINFLOW AND AIRFLOW DIRECTIONS WITHIN A MIXED-FLOW GRAIN
DRYER (BROOKER ET AL., 1992). ............................................................................................ 6
MOISTURE AND TEMPERATURE CHANGES OF INDIVIDUAL KERNELS DURING: (A) CROSSFLOW
DRYING AND (B) MIXED-FLOW DRYING (BROOKER ET AL., 1992). ........................................... 6
STRESS-CRACK TYPES IN DRIED MAIZE KERNELS: (A) NONE, (B) SINGLE STRESS-CRACK, (C)
MULTIPLE STRESS-CRACKS, (D) CHECKED KERNELS (THOMPSON AND FOSTER, I963). ......... 10
ILLUSTRATION OF THE CONTROL VOLUMES RESULTING FROM THE INTERSECTION OF THE
AIRFLOW AND GRAINFLOW STREAMLINES (LIU ET AL., 1997). .............................................. I3
TYPICAL COMPUTER OUTPUT OF THE MSU MIXED-FLOW DRYER SIMULATION MODEL .......... 16
PLOT FOR THE SELECTION OF THE MODULE DESIGN FOR THE PILOT-SCALE MIXED-FLOW DRYER
MODULE DESIGN. ................................................................................................................... I9
EXAMPLES OF MIXED-FLOW DUCT DESIGN: (A) INVERTED-V WITH SIDEWALLS, (B)
INVERTED-V WITH TABS, (C) INVERTED-V, (D) NON-TAPERED, (E) TAPERED ........................ 20
AIR-DUCT DESIGN FOR THE PILOT-SCALE MIXED-FLOW DRYER (INCHES) ............................... 22
MODULE DESIGN FOR THE PILOT-SCALE MIXED-FLOW DRYER (INCHES). ............................... 23
ILLUSTRATION OF A GRAIN TURN-FLOW FOR THE MIXED-FLOW DRYER. ................................ 24
FAN CURVES FOR THE DRYING-AIR AND COOLING-AIR FANS ON THE PILOT-SCALE MIXED-FLOW
DRYER. .................................................................................................................................. 27
DESIGN SCHEMATIC OF THE PILOT-SCALE MIXED-FLOW GRAIN DRYER (INCHES). .................. 28
SAMPLING LOCATIONS FOR MOISTURE CONTENT PROFILES: (A) ALONG THE TOP OF THF DUCTS
AND (B) BETWEEN THE DUCTS. .............................................................................................. 32
OVERHEAD VIEW OF THE MIXED-FLOW GRAIN DRYING SYSTEM. ........................................... 34
CORRELATION BETWEEN SELECT DRYING PARAMETERS AND OUTLET STRESS-CRACK
PERCENTAGES ........................................................................................................................ 38
MOISTURE CONTENT PROFILE ALONG THE TOP OF THE DUCTS AT FOUR DIFFERENT LOCATIONS
IN THE LAST MODULE OF THE DRYING SECTION. .................................................................... 41
MOISTURE CONTENT PROFILE BETWEEN THE DUCTS IN THE LAST ROW IN THE COOLING
SECTION ................................................................................................................................. 41
MOISTURE CONTENT PROFILE OF DISCHARGE ALONG EACH OF THE THREE METERING ROLLS.
.............................................................................................................................................. 42
EXHAUST AIR TEMPERATURE PROFILE FOR THE DRYING SECTION .......................................... 44
EXHAUST VELOCITY PROFILE FOR THE DRYING SECTION. ...................................................... 44
SIMULATED EFFECT OF DRYING-AIR TEMPERATURE ON DRYER CAPACITY AND ENERGY
EFFICIENCY ............................................................................................................................ 50
SIMULATED EFFECT OF DRYING AIRFLOW RATE ON DRYER CAPACITY AND ENERGY

EFFICIENCY ............................................................................................................................ 50
SIMULATED EFFECT OF DRYING-AIR TEMPERATURE AND DRYING AIRFLOW RATE ON OUTLET
STANDARD DEVIATION ........................................................................................................... 51
SIMULATED EFFECT OF AMBIENT AIR TEMPERATURE ON THE DRYER CAPACITY AND ENERGY
EFFICIENCY ............................................................................................................................ 54
SIMULATED EFFECT OF AMBIENT RELATIVE HUMIDITY ON THE DRYER CAPACITY AND ENERGY
EFFICIENCY ............................................................................................................................ 54
SIMULATED EFFECT OF INLET MOISTURE CONTENT ON THE DRYFR CAPACITY AND ENERGY
EFFICIENCY ............................................................................................................................ 55

vii

FIGURE 6.7.

FIGURE AJ.

SIMULATED EFFECT OF INLET GRAIN TEMPERATURE ()N THE DRYER CAPACITY AND ENERGY
EFFICIENCY ............................................................................................................................ 55
FIGURE A.2.

SCHEMATIC OF VARIABLES AFFECTING THE STATIC PRESSURE LOSS THROUGH THE GRAIN. .. 64
SCHEMATIC SHOWING LOCATIONS OF PRESSURE LOSS IN THE MODULE DESIGN. ................... 64

viii

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05

MC
SD
SC
SCI

LIST OF SYMBOLS

pressure drop [Pa]

grain height [m]

cross-sectional area [m2]

airflow rate per unit area [m3/S/m2]
horizontal duct Spacing [m]

duct open-bottom width [In]

duct length [m]

hydraulic radius [m]

density of air [kg/m3]

air velocity [m/s]

kinematic viscosity of air [mz/s]
roughness height [m]

friction factor

relative roughness

loss coefficient

grain moisture content [%w.b.]
outlet grain moisture standard deviation [%w.b]
stress-crack percentage [%]

stress-crack index

ix

CHAPTER 1

INTRODUCTION

The quality of grains and oilseeds is affected by both pre- and post-harvest
conditions and operations. Environmental conditions during the growing period and
method and time of harvest are pre-harvest factors that can influence grain quality. More
important are the post-harvest factors related to transportation methods, storage
management, and grain treatments (i.e. chemicals). While each of these factors can have a
significant affect on the overall quality of grains, the artificial drying of grains is the
process most detrimental to grain quality (Brooker et al., 1992).

Artificial grain drying is classified as either high-temperature or low-
temperature/in—bin drying. The drying-air temperature employed and the drying capacity
differ between each class. In general, high-temperature grain drying results in a lower
level of grain quality; despite the lower grain quality, the high drying capacities make
high-temperature dryers the most popular dryer option for grain elevator managers and
farmers.

The four major high-temperature dryer-types are the crossflow, counterflow.
concurrent-flow, and mixed-flow (Figure 1.1). The crossflow is the most prevalent in the
US, the mixed-flow in the rest of the world. The mixed-flow is more expensive than the
crossflow dryer because of the amount of steel required for construction. Field tests on
commercial dryers of each type Show the crossflow dryer-type has the lowest quality,

followed by the mixed-flow and concurrent-flow, if based on the quality criteria

CROSSFLOW COUNTERFLOW

f Moist
g exhaust
air

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.............. > mm"...
Warm Moist
drying exhaust
air air
............' ............... ’
7 ‘ Warm
g drying
3 air
CONCURRENTFLOW MIXED - FLOW
L i wan“ l
drying
& air
I ...... ’
Warm ,. Q Moist
drying "' . exhaust
air . V air
‘ + ....... p
i Moist
é exhaust
V air
-—> GRAIN

................ > AIR

FIGURE 1.1 Schematics of the four major types of high-temperature grain dryers:
crossflow, counterflow, concurrent-flow, and mixed-flow.

of stress-cracking and grain moisture uniformity (Montross et al., 1997, Montross et al.,
1994). Numerous design modifications, such as grain tum-flows, tempering zones, and
variable-Speed discharge, have been applied to the basic crossflow dryer design with
limited success in improving grain quality.

Limited research has been performed concerning the concurrent-flow dryer
because of the lack of popularity among farmers.

The environmental concerns related to dust emissions (i.e. particulate matter) has
led to governmental regulations on the rate of emissions from grain handling operations,
including grain drying. The mixed-flow dryer type, due to high air velocities in the duct,
emits significant amounts of dust in relation to the crossflow dryer type. Large amounts of
money can be spent at a grain facility for dust-control equipment. Modifications to the
dryer design for improved dust-control would be highly beneficial.

This research investigates design modifications to the mixed-flow dryer type for
improving grain quality. A computer simulation model of the mixed-flow drying process
will be used to examine the affects of various design parameters on dryer performance
and outlet grain quality. A pilot-scale mixed-flow model will be designed (using the

mixed-flow simulation model) and constructed for field-testing.

CHAPTER 2

OBJECTIVES

The objectives of this study are:

(1) To design a mixed-flow dryer module using a computer simulation model.

(2) To determine the influence of modified exhaust-duct designs on air pollution.

(3) To examine the effect of module reversal on the standard deviation of the
moisture content of individual maize kernels exiting the dryer type.

(4) To experimentally determine capacity, energy efficiency and grain quality.

(5) To validate the mixed-flow dryer model.

CHAPTER 3

LITERATURE REVIEW

3.1 MIXED-FLOW GRAIN DRYERS

High-temperature grain dryer designs differ in their directions of grainflow and
airflow. As the name implies, mixed-flow is a combination of a counter-flow, concurrent-
flow, and crossflow air patterns. The grain flows by gravity over a series of alternate inlet
and exhaust air ducts (Figure 3.1). The duct cross-sectional area and spacing are the major
differences between commercial mixed-flow dryer designs (Cao, 1993).

The design parameters for mixed-flow dryers, such as drying-air temperature and
airflow rates, differ from the crossflow and concurrent-flow dryer types. Drying-air
temperatures typical of mixed-flow dryers range from 65°C to 130°C (150-265°F) and
depends on the end-use of the grain. Airflow rates of 45-78 m3/min-t (40-70 cfm/bu) are
common.

The drying-air temperature is a parameter that affects the quality of dried maize
(Figure 3.2). Although the temperatures used in mixed-flow dryers are higher than
crossflow, the individual kernel exposure time to the high temperatures is a period of
seconds (Spikes in Figure 3.2b); the crossflow dryer exposure time is a period of hours.

Although it has been accepted that mixed-flow grain drying results in uniform
grain moisture (Brooker et al., 1992), experimental and Simulated analysis has shown

otherwise. A comprehensive study of the mixed-flow drying process concluded that non-

FIGURE 3.1

FIGURE 3.2

    

     
 
 

‘00-..-00.‘

.00. 0... o... oo-

 
 
 

5"? "

‘c—
...-I‘ ...—ooo-do

\o‘oooooco-

Grain path ......
Air path “"-

Duct system, and grainflow and airflow directions within a mixed-flow

grain dryer (Brooker et al., 1992).

GRAIN

MOISTURE CONTENT

 

 

 

 

 

 

GRAIN TEMP

 

 

 

DISTANCE A

a

GRAIN

GRAIN 8: AIR TEN MOISTURE CONTENT

H = Hot air inlet duct

C = Cold air exhaust duct

1 3 Area of crossllow

2 . Area of conwrrent-flew
3 = Area 01 counter-flow

 

 

 

 

 

 

Grain term.

 

 

DISTANCE __
b

 

Moisture and temperature changes of individual kernels during:

(a) crossflow drying and (b) mixed-flow drying (Brooker et al., 1992).

uniform drying of the grain results due to the non-uniform airflow and grainflow patterns
(Liu, 1993).

Duct shape (i.e. cross-section) and spacing influence the drying uniformity.
Lasseran (1987) found that a 200 to 300% difference in individual—kemel retention time
exists within the mixed-flow dryer type. POBHBIH (1987) examined the grain flow patterns
in a mixed-flow dryer using a high-speed camera. It was found that the top angle of the
ducts significantly effects the uniformity of grain flow. Angles of 70° or less are
recommended to avoid a zero-grain-flow region at the top of the duct. HBaHOB (1987)
studied the performance of mixed-flow dryers with different air duct spacing. Highest
capacities were observed with horizontal and vertical duct spacings of 0.25 m and 0.2 m,
respectively. The lowest fuel consumption was achieved with a horizontal spacing of 0.2
m and a vertical spacing of 0.25 m.

Simulation models have been developed to represent mixed-flow as a one—
dimensional process, thus neglecting the crossflow air patterns (Bruce, 1984; Courtois
and Lasseran, 1993). Miller and Whitfield (1984) used a one-dimensional model to
examine the performance of a mixed-flow dryer. Increased energy inputs (e.g. higher
temperature or airflow rate) increased the dryer capacity; higher air-temperatures resulted
in a greater efficiency, higher airflow rates a lower efficiency.

These simplified one-dimensional models predict drying capacity and fuel
consumption adequately, but are not able to simulate the non-uniformity of the mixed-
flow drying process. More recently, a two-dimensional model has been developed which

correctly predicts the non-uniform drying characteristic of the mixed-flow dryer-type (Liu

et al., 1997). This model is used for the analysis and design of the mixed-flow dryer in

this study.

3.2 QUALITY OF DRIED MAIZE

The factors that determine the quality of maize depend on the end-use. Dry-
milling quality criteria differ from feed grain. In general, the properties that affect the
overall quality of maize include an appropriately low and uniform moisture content, low
percentage of denatured protein, low percentage of stress-cracks, low percentage of
broken maize and foreign material (Brooker et al., 1992). In addition, specialty markets
desire maize with high test weight, hard endosperm, no mold, and large sound kernels.
Regardless of the quality criterion, each is affected by artificial grain drying.

The moisture content of a lot of maize is expressed as an average value and
neglects the moisture content difference between single kernels. The drying process,
regardless of dryer-type, induces a moisture content standard deviation of 3-5%
(Montross et al., 1994). This is significant with stress-cracks likely to occur in over-dried
kernels (Montross et al., 1997).

The implementation of a grain tum-flow or grain inverter is a common method to
reduce the standard deviation of the outlet moisture content of grain in crossflow dryers.
The tum-flow is generally placed at the mid-point of crossflow drying section to turn or
invert the drier grain at the inlet-air side of the column to the exhaust-side and vice versa
for the wetter grain at the exhaust-air side of the column. The objective of a grain tum-
flow is to decrease the moisture differential of the grain across the column. This tum-flow

concept will be applied to the module-type mixed-flow dryer in this study.

Stress-cracks in kernels of maize result from moisture and temperature gradients
induced by a rapid rate of drying and cooling. The number of cracks within the kernel
(Figure 3.3) determines stress-crack classification. Stress-cracks in kernels are a concern
for the maize dry-milling industry; typically a stress-crack percentage less than 30% for a
lot of maize is desired (Maier, 1995). A direct relationship between stress-crack
percentage and breakage susceptibility has been established (Paulsen and Hill, 1985).

Drying-air temperature has a negative effect on the percentage of stress-cracked
kernels, germination percentage, test weight, oil content, percentage of denatured protein,
and starch recovery rate in a lot of dried maize (Peplinski et al., 1994; Weller et al., 1988;

Weller et al., 1989).

3.3 AIR POLLUTION AND GRAIN DRYING

The control of dust among grain-handling operations has become a major concern
for the industry. In addition to the negative effects of dust to the human respiratory system
(Wrigley et al., 1979), grain dust can be explosive (Lesikar et al., 1991; Wolanski, 1979).
Dust is emitted during every operation of grain handling. A significant contributor to the
dust pollution problem is the drying of grains. Large volumes of dust-laden air are
exhausted from grain dryers at facilities that can operate 24-hours-a-day for weeks at a

time.

   

\7

 

‘7

C

FIGURE 3.3 Stress-crack types in dried maize kernels: (a) none, (b) single stress-crack,
(c) multiple stress-cracks, (d) checked kernels (Thompson and Foster,
1963).

10

To combat the dust problem at grain-handling facilities the United States
government under actions of the Environmental Protection Agency (EPA) has developed
mandates for emission rates from grain-handling operations; included in these mandates
are regulations for grain drying operations. The rate of dust emission from a grain dryer is
dependent on the dryer type, the type of grain, and the concentration of dust within the
grain (AP-42 Handbook).

The EPA’s New Source Performance Standards has established a 0% opacity limit
for grain dryer emissions. [N ote: Opacity is a measure of the degree to which particulate
matter, such as grain dust, reduces the view of an object.] The standard requires the
screen perforations for crossflow-type dryers not to exceed 2.4 mm in diameter and
mixed-flow type dryers with a screen filter not to exceed 50 mesh (wires per inch)
openings (AP-42 Handbook).

The air in the exhaust-air duct flows across the grain collecting the lighter dust
particles. In order to reduce dust pollution, the cross-sectional area of the duct is chosen

so that the air velocity does not exceed 7.5 m/s (Brooker et al., 1992).

11

CHAPTER 4

DRYER DESIGN

The design of the pilot-scale mixed-flow grain dryer is based on the module
design concept. A single module incorporates both air inlet and exhaust ducts into one
unit. Thus, multiple modules are stacked for various dryer sizes (i.e. grain throughput).
The “optimal” shape and spacing of the ducts within a module can be analyzed via
computer simulation. The MSU mixed-flow dryer simulation model is the tool used for

the design of the pilot-scale module.

4.1 Two-DIMENSIONAL MIXED-FLOW SIMULATION MODEL

A two-dimensional mixed-flow drying model has recently been developed (Liu et
al., 1997). The model is unique because it accounts for the concurrent, counterflow and
crossflow drying patterns that occur during the mixed-flow drying process. The drying
model includes the modeling of the airflow and grainflow patterns for a mixed-flow
dryer. The intersection of the flow streamlines results in control volumes for which
energy and mass balances are made (Figure 4.1).

The model is based on four partial differential equations that represent the mass
and energy balances for the grain drying process. The equations represent the change in
the air temperature and humidity, and the change in the grain temperature; a thin-layer

drying equation (Li and Morey, 1984) is used to determine the grain moisture change in

12

 

 

 

--~'-—- V

FIGURE 4.1. Illustration of the control volumes resulting from the intersection of the
airflow and grainflow streamlines (Liu et al., 1997).

each control volume. The model also accounts for the stochastic nature of a lot of maize,
thus the outlet standard deviation of individual kernel moistures can be determined. The
reader is referred to the paper by Liu et a1. (1997) for a detailed description of the
simulation model.

An additional subroutine was written to permit a search for the average outlet
grain moisture content (i.e. 151005 %w.b). The program was executed on a Pentium 90

MHz computer.

4.2 DESIGN CRITERIA

A four-module stack, full-heat dryer was modeled to determine the "optimal”
design parameters of the module. This provides a 5-point moisture removal (i.e. 20 to 15
%w.b.) for maize with an approximate drying capacity of 4-6 t/h (160-240 bu/hr) at
relatively moderate temperatures and airflow rates. To minimize the overall size of the
dryer, three ducts per row were chosen in the module design, with four air duct layers per
module, i.e. two inlet and two exhaust. The overall dryer size provides the means for
effective field testing of the capacity, energy efficiency and grain quality characteristics of
the mixed-flow dryer.

Table 4.1 contains a summary of the selected simulation parameter values for the
design of the pilot scale mixed-flow grain dryer. The primary objective of the simulation
process was to maximize the drying capacity per unit volume of dryer module, while
maintaining an energy efficiency of less than 5,000 kJ/kg (2,150 Btu/lb) for the drying
section. The energy efficiency is the rate of energy consumption for each unit mass of

water removed from the grain. A secondary objective was to maintain an air duct velocity

14

of less than 5 m/s (985 ft/min) in order to minimize air pollution. The energy efficiency
limit was selected after a survey of values reported for various commercial continuous-

flow dryers (Table 4.2).

4.3 MODULE DESIGN

The vertical and horizontal spacing and the shape (i.e., cross-sectional area) of the
duct affect the dryer performance, i.e. the dryer capacity and energy efficiency, and the
outlet grain moisture uniformity and quality. Since the shape and spacing of the ducts also
influences the volume of grain within a module, a new parameter, the drying density, is
defined as the ratio of the capacity or volume flowrate to the dimensional volume of the
dryer.

TABLE 4.1. Parameters for the design of the pilot-scale mixed-flow dryer module.

 

grain type maize
initial MC 20 %w.b.
inlet SD. 0 %w.b.
initial grain temperature 15 °C (60 °F)
ambient temperature 15 °C (60 °F)
ambient relative humidity 75%
drying-air temperature 95 °C (200 °F)
final MC 15 %w.b.
duct length 1.0 m
ducts per row 3
air-inlet layers per module 2

 

TABLE 4.2. Energy efficiency ranges for 5-point moisture removal of several
commercial continuous-flow dryers in 1998 .

 

efficiency range

 

comjany kJ/Igg BTU/lb

m" 4,460 — 8,860 1,920 — 3,725
GSI“ 5,815 — 8,100 2,500 — 3,485
Grain Handler USA 4,455 — 6,590 1,915 — 2.830
CIMERIA‘" 5,000 — 5,135 2,100 — 2,200

 

: ffi, GSI — crossflow ; CIMBRIA. OH USA - mixed-flow
; full-heat drying
l0-point moisture removal

15

****** MIXED-FLOW GRAIN DRIER PERFORMANCE ******

HOT AIR FLOW RATE, KG(DRY)/HR 28224.00
COLD AIR FLOW RATE, KG(DRY)/HR 5040.00
AIR TEMPERATURE, C 95.00
GRAIN FLOW RATE, KG(DRY)/HR 6379.30
WET GRAIN FLOW RATE, KG(WET)/HR 7973.00
INLET AVERAGE M.C., % D.B. 25.00
INLET STANDARD DEVIATION, % D.B. 0.00E+00
OUTLET AVERAGE M.C., % W.B 14.98
OUTLET STANDARD DEVIATION, % W.B. 2.21
OUTLET GRAIN TEMP, C 38.77
ENERGY CONSUMPTION, KJ/HR 2307450.00
SPECIFIC ENERGY CONSUMPTION, KJ/KG 4903.85
CROSS-SECTIONAL AREA OF DRY SECTION, M*M 1.33
HEIGHT OF DRYING SECTION, M 6.60
**************** END ******************

FIGURE 4.2. Typical computer output of the MSU mixed-flow dryer simulation model.

16

A large number of simulation trials were executed over a broad range of four
module parameters: (1) airflow rate, (2) vertical duct spacing, (3) horizontal duct spacing,
and (4) cross-sectional duct area. A typical computer output from the MSU mixed-flow
dryer simulation model (using the standard input conditions listed in Table 4.1) is shown
in Figure 4.2. The range of module parameters and the incremental values used in the
simulations are listed in Table 4.3; large increments were used initially to minimize
computer time.

TABLE 4.3. Range of the module parameters used with the MSU mixed-flow
simulation model with increments for the initial Simulation trials.

 

 

parameter range increment
airflow per module (m3/h) 2,000 - 4,000 500
horizontal spacing (m) 0.2 — 0.5 0.05
vertical spacing (m) 0.3 — 0.5 0.05

 

Analysis of the first set of Simulation results provided the optimal performance
range for each module parameter for an air velocity in the ducts of 5 m/s (to minimize air
pollution) and an energy efficiency of less than 5,000 kJ/kg. The optimal ranges are listed
in Table 4.4 along with the increments used for the more detailed simulation trials. Over
1,500 different module configurations were simulated in order to determine the optimal
design.

TABLE 4.4. Range of the module parameters analyzed with the MSU mixed-flow
simulation model [the increments for the detailed Simulation trials].

 

 

parameter range increment
airflow per module (m3/h) 2,400 — 3,000 100
horizontal spacing (m) 0.3 - 0.4 0.01
vertical spacing (m) 0.4 — 0.5 0.01

 

17

The module configurations with an energy efficiency value for the dryer within
the specified limits were plotted to select the optimal design. Figure 4.3 shows the plot
used for the selection process. The design limit for the energy efficiency (5.000 lekg) is
also plotted.

The module dimensions which resulted in an energy efficiency of 4,995 kJ/kg (see
Figure 4.3) were chosen for the design, i.e. an airflow rate of 3,000 m3/h, and vertical and
horizontal spacings of 0.33 m and 0.44 m, respectively. The module dimensions were
converted to English units and rounded to the nearest half-inch for manufacturing

purposes.

4.4 DUCT DESIGN

Ducts for a mixed-flow dryer differ in shape or cross-sectional area (see Figure
4.4). The shape of the duct is significant because the air velocity is a function of the
cross-sectional area. To minimize dust pollution low duct velocities should be employed.
The inverted V-ducts with sidewalls are conventional and are used in the pilot-scale
design.

A duct length of 1.4 m (55 in) was selected for the pilot scale mixed-flow dryer.
Tapered ducts were used because of the increased volume of grain per module as
compared to the standard duct.

Three modified exhaust ducts were placed within the mixed-flow dryer. The goal
of the modifications was to reduce the amount of dust mass exhausting the dryer. The

three modifications were (1) baffles, (2) perforation, and (3) louvers. The perforation was

18

 

650

— drying density
600 . ° elliciency

 

drying density (kg/h-m “) or efficiency (10 x IIJ/kg)

 

 

 

1° . °
0 ° 0 <9 00 $69
0 6%

500 '1 """""""" S>"cr """ 9 """ 9o """ b.6230 9"§b%<§:€d%qb °§6%%£° 3:39 ----------
‘ op °o 0668 0%9 omofiodgodgefioh‘f 3
immomw “M9 499.52
1
4

450 ’ j ' ' ' ' ' ' ‘ ‘ ' ' Y Y 1 T r r f T T

simulation trial

FIGURE 4.3. Plot for the selection of the module design for the pilot-scale mixed-flow
dryer module design.

19

/\ A /\

 

 

 

 

FIGURE 4.4. Examples of mixed-flow duct design: (a) inverted-V with sidewalls,
(b) inverted-V with tabs, (c) inverted-V, (d) non-tapered, (e) tapered.

20

bent to 90° at a width of the duct; the louver plate had two rows of openings (size: 3/8x2).
The baffles were spaced at 14 cm (5.5 in) along the length of the duct and extended into
the grain stream. The upper section of the baffle was bent to the same angle as the angle
of the duct taper.

Figure 4.5 is a schematic of an air duct for the pilot scale dryer module. Figure 4.6

shows a drawing of the module design.

4.5 OVERALL DRYER DESIGN

4.5.1 MODULE REVERSAL

The grain tum-flow concept employed with the crossflow dryer type was
investigated for the mixed-flow dryer. Figure 4.7 illustrates the functioning of the grain
tum-flow in a mixed-flow dryer module. The turning of the grain is achieved by reversing
or turning a module 180°; thus, the inlet air ducts become the exhaust air ducts and vice
versa. Over-drying of individual kernels is reduced because the grain closest to the inlet
air ducts is not in contact with the hotter inlet air ducts throughout the entire drying

section.

4.5.2 ADDITION OF COOLING SECTION

In order to be able to analyze the grain quality characteristics of mixed-flow dryer,
the addition of a cooling section was required on the pilot scale dryer. Therefore. a fifth

module was added to the dryer.

21

 

 

[..—
L .
.\

 

 

 

5500 J

 

FIGURE 4.5. Air—duct design for the pilot-scale mixed-flow dryer (inches).

22

 

 

Flc

 

 

 

 

 

 

 

 

 

 

 

 

 

l ,
l/ , \
// /:’ \\ \\
r I’//\\ /A\
L / \ ,/
r’ \L -+-
l l ___J

l
5:00L
l

. 1 A L
I p " Am A
l l

. ,’ / \.
LLLJ L3
L.————-SE‘ 58

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FIGURE 4.6. Module design for the pilot—scale mixed-flow dryer (inches).

23

//M . (:0
“,ng
//
\_\ 4,00J
\ 7 on
—.H._m J
M”?
Liz/I/t 1; 00
ff”: l
f
a
g “ME—L
\M
55 80

 

STANDARD

 

MODULE
—} DRIER GRAIN
------- > WETTER GRAIN
REVERSED
MODULE

 

 

 

 

FIGURE 4.7. Illustration of a grain turn-flow for the mixed-flow dryer.

24

4.5.3 FAN SIZE

For proper fan selection the pressure loss through the grain must be considered in
addition to the combined pressure losses in each component of the dryer (i.e. plenum.
ducts, burner). Appendix A shows the method of calculating pressure loss in the dryer.

The total calculated pressure loss in the ducts is approximately 60 Pa (0.24 in
H20). Therefore, the overall pressure loss within the mixed-flow dryer at 52 m3/min-t (47
cfm/bu), excluding losses in the burner, is approximately 250 Pa (1.0 in H2O). Assuming
a pressure loss of 125 Pa (0.5 in H2O) in the burner, the total pressure loss for the dryer is
375 Pa (1.51 in H2O).

The drying section is equipped with an AV24-10 axial fan. The cooling module is

vacuum cooled with an AV18-1 axial fan. The fan curves are shown in Figure 4.8.

4.5.4 MISCELLANEOUS

The module design directly determines the physical size of the grain dryer system.
The schematic in Figure 4.9 shows the overall dimensions of the five-module dry-cool
pilot-scale dryer. Engineers at ffi Corporation designed the grain discharge system and
constructed the dryer. Minor dimensional changes were made by the company to the
ductwork and framework to permit practical sheet-metal layout.

The grain discharge system consists of three metering rolls. A 0.25 kW (1/3 hp)
motor drives the rolls. The wet holding bin holds two-thirds the volume of a single dryer
module (approximately 0.9 t (36 bu)). The cross-sectional area of the drying-air plenum is

0.61 m x 1.33 m (24 in x 52.5 in) for an air velocity of6 m/s(1150 ft/min).

25

4.6 DRYER SPECIFICATIONS

The dryer specifications and predicted performance for the five-module dry-cool

mixed-flow dryer are listed in Table 4.5. [Note: The top two modules are reversed]

TABLE 4.5. Dimensions and predicted performance of the pilot-scale mixed-flow grain

 

dryer.
dimensions
overall height 9.6 m (31.5 ft)
height of drying section 6.6 m (21.7 R)
cross-sectional area 1.9 m2 (20.1 fiz)
total holding capacity 8.71 t (343 bu)
5-module holding capacity 6.79 t (267 bu)
maximum capacity 8.10 t (319 bu)
drying-air temperature 95°C (200°F)
airflow
drying 283 m3/min (10,000 cfm)
cooling 85 m3/min (3,000 cfm)
fan power
drying 3.18 kW (4.3 hp)
cooling 0.65 kW (0.9 hp)
drying capacityt
20%-15% 6.15 t/h (242 bu/hr)
25%-15% 3.53 t/h (139 bu/hr)

 

‘ capacities are in terms of dry matter flow at the inlet moisture content

26

 

l 800
-l
1600 4

d

1400 4

d

1200 -

d

1000 -

q

800 5

600 4

400 -

I

200 -

static pressure, Pa

 

 

 

 

() I l I I fl T I I

0 50 100 150 200 250 300 350 400 450

airflow, Ins/min

FIGURE 4.8. Fan curves for the drying-air and cooling-air fans on the pilot-scale mixed-
flow dryer.

27

 

K80 . j far)
\2 \ / \

‘\
\
\
\
\

 

 

~— —-4 %‘m
D
A
‘——g)———.-
u-n——3J\———.~
'0
J

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FIGURE 4.9. Design schematic of the pilot-scale mixed-flow grain dryer (inches).

28

CHAPTER 5

EXPERIMENTAL INVESTIGATION

5.1 EXPERIMENTAL TESTS

The pilot scale mixed-flow dryer was field tested with maize to obtain dryer
performance data. Tests were conducted during the Fall of 1998 at Jorgensen Farms
Elevator, Williamston, MI. The moisture of the maize at harvest was considerably lower
than in normal years due to the uncharacteristically dry growing season in the mid-
Michigan area.

The following parameters were measured or calculated:

(1) the average maize moisture content and standard deviation before and after

drying

(2) the maize moisture content profile between inlet air ducts in the last row of

ducts in the cooling section, along the duct length, and exiting each metering
roll

(3) the average percentage of stress-cracked kernels after drying in the mixed-

flow dryer and a ffi crossflow dryer (i.e. Zimmerman model VT4036).

(4) the dust emitted from four different exhaust duct designs

(5) the drying-air and ambient temperatures, and the ambient relative humidity

(6) the dryer capacity and energy consumption.

29

5.2 INSTRUMENTATION

5.2.1 FIELD MEASUREMENTS

The dryer-related operation parameters, such as drying-air and ambient
temperature, ambient relative humidity, static pressure, and air velocity were measured at
each dryer start-up using an electronic Solomat MPM 500e multi-function device
(Solomat Electronics, Norwalk, CT).

Ambient temperature and relative humidity were measured at the start of every
test. The drying-air temperature was monitored with a thermistor that was included in
control system for the dryer (ffi Corporation). The exhaust air velocity and temperature
were measured with a hotwire anemometer and glass bulb mercury thermometer.

Samples of maize were collected at the dryer inlet and outlet on 20-minute
intervals. Seventy-five individual kernels from each sample were analyzed for moisture
content using the PQ-100 Single Kernel Moisture Tester (Seedburo, Chicago, IL). The
meter determines the moisture content and reports the sample average and standard
deviation. The temperatures of grain entering and exiting the dryer were also recorded.
The samples were placed in a thermos and temperature determined five minutes later
using a glass-bulb thermometer. [Note: Thermos was exposed to a temperature that was
expected from grain.] The dryer operation was controlled based on readings from a
Burrows Digital Moisture Tester (Seedburo, Chicago, IL).

The test weight of a sample was measured using a quart cup filled via the standard

filling hopper and determined with the digital computer grain scale (Seedburo, Chicago,

IL).

30

Moisture profiles were examined between the ducts, along the length of the ducts,
and along the outlet of the three metering rolls. Ten separated grain samples were taken at
four locations above the ducts in the last module of the drying section (Figure 5. 1a) using
a 1.6 m (63 in) brass partitioned sampling probe. Two consecutive partitions were paired
and analyzed for average moisture contents along the duct. Eight separate samples were
taken between the ducts of the last row of the cooling section (Figure 5.1b). Four equally
Spaced samples were collected at the outlet for each metering roll.

Dust emitting from the four exhaust duct configurations (i.e. standard, baffles,
louver, and perforated-bottom) was collected for various time periods during the
operation of the pilot scale mixed-flow dryer. Dust was filtered from the exhausting air
stream using nylon stockings. For each modified duct examined a standard duct in the
same row was examined. The mass of dust collected for each duct was used to determine
the percentage of dust reduction for the modified duct.

The pressure loss through the grain was measured between an inlet and exhaust
duct. Copper tubing (for stiffness) with a 90° bend at one end with tygon tubing inside
was inserted into each duct. Readings were made with the Solomat pressure sensor at five

locations along the duct.

5.2.2 LABORATORY MEASUREMENTS

The samples of maize collected in the field were placed in plastic bags, labeled,
and stored in a 4.4 °C (40 °F) cooler in the MSU Post-harvest Laboratory.
Moisture contents of the maize samples were determined using the ASAE oven

method (ASAE Standards, 1991). The weight of wet and dried maize and the weight of

31

4'" MODULE or DRYING SECTION

AA
\AAA/

AAA

ID= 51mml

 

 

 

 

LAST Row or COOLING SECTION

AA

+ +

 

 

T gram boundary
OD = 38 mm
b

FIGURE 5.1. Sampling locations for moisture content profiles: (a) along the top of the
ducts and (b) between the ducts.

32

 

 

 

(IO

{/1

U0

1h:

lis

the cans used for drying were determined using a Mettler PM400 electronic scale to
0.001 g (Mettler Instrument Corporation, Hightstown, NJ).

Kernels were analyzed for stress cracks using a candling method (Thompson and
Foster, 1963). F ifiy whole kernels were randomly selected from each outlet sample to
determine the percentage of stress cracks. The stress cracks were analyzed and
categorized by severity of cracking (i.e. none, single, multiple, and checked) and the

stress-crack index calculated (Kirleis and Stroshine, 1990).

5.3 DRYER SETUP

Figure 5.2 shows an overview schematic of the mixed-flow drying system. Grain
was fed to the dryer by a portable auger that was connected to a wet holding hopper bin.
An adjustable slide gate permitted inlet grain samples to be collected before entering the
auger. The filling of the dryer was automated through the dryer control system. Another
portable auger was connected to the sampling box at the outlet of the dryer. The outlet

grain was transferred to an existing auger leg at the elevator.

5.4 EXPERIMENTAL RESULTS

Testing of the dryer began on October 25, 1998 afier approximately a month. of
preparation (i.e. construction and troubleshooting); tests were completed November 6,
1998. A total of nine tests were conducted. A summary of the test data is shown in Table
5.1. The dryer was tested at a range of different ambient and drying-air temperatures, but
the inlet moisture content range was limited because of the dry harvest season. Table 5.2
lists the ranges of ambient conditions and dryer Operating conditions for the testing

period.

33

SLIDE GATE

l

OUTLET
AUGER

l

      

     

WET-HOLDING
BIN

FAN HOUSING

DRYING FAN

BURNER

COOLING FAN

FIGURE 5.2. Overhead view of the mixed-flow grain drying system.

34

 

 

 

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35

Classical statistics was not applied to the data sets in this section because of the
limited number Of data points for the dryer performance and grain-quality tests.

Samples were collected from a cylindrical—tower Zimmerman model VT4036
crossflow dryer located at the Jorgensen elevator. Although the Operating conditions of
the mixed-flow and the crossflow dryer were not exactly identical, the samples provide a
basis for a general comparison of the grain quality characteristics exiting each dryer type.

TABLE 5.2. Range of ambient and dryer-operating conditions for the test of the pilot-
scale mixed-flow dryer (MF) and the Zimmerman model VT4036 (CF).

 

 

Jarameter rage
ambient temperature 4—2] °C (39-70 °F)
ambient RH 34-76 %
drying-air temperature
MF 67-102 °C (153-215 °F)
CF 91-102 °C (196-216°F)
inlet moisture content
MF 17.4-23.0 %w.b.
CF 16.0-24.5 %w.b.

 

5.4.1 GRAIN QUALITY CHARACTERISTICS

The two quality characteristics examined for the pilot scale mixed-flow dryer
were the standard deviation of the outlet grain moisture content and the average
percentage of stress-cracked kernels. These characteristics were also examined for the
Zimmerman crossflow dryer model VT4036.

Table 5.3 lists the outlet standard deviation for both dryers. Due to the short

 

drying season the dryer could not be tested without module reversing. The average outlet
standard deviation of the mixed-flow dryer is considerably lower than for the crossflow

dryer.

36

The average stress crack percentages and stress-crack index for both dryers are
listed in Table 5.4. The level of cracking for the pilot scale dryer is approximately the
same as the crossflow dryer; this contradicts experimental results of commercial dryers
(Montross et al., 1994). The moisture content of the outlet grain is lower for the mixed-
flow dryer, but the graphs in Figure 5.3 Show there is no relationship between the outlet
moisture content and stress-crack percentage. The speed of the discharge system (i.e.
rpm’s of the metering rolls) is correlated strongly to the percentage of stress-cracks. There
is also a slight correlation of stress-crack percentage to the average inlet moisture content.

TABLE 5.3. Comparison of grain moisture content and standard deviation entering and
exiting the mixed-flow dryer (MF) and crossflow dryer (CF).

 

 

test ave outlet MC, %w.b. outlet SD, %w.b.
date MF CF MF CF
10/26 14.1 15.3 2.02 4.80
10/27 13.2 14.8 2.29 4.86
10/29 14.1 14.7 1.48 2.89
10/30 13.9 14.7 2.54 3.68

 

TABLE 5.4. Average outlet stress-crack percentage and SCI. for the mixed-flow dryer
(MF) and crossflow dryer (CF).

 

 

test ave outlet MC, %w.b. °/o SC SC]

date MF CF MF CF MF CF
10/26 13.5 14.4 45 67 110 153
10/27 13.1 14.1 78 77 282 230
10/29 13.7 14.2 89 75 296 267
10/30 14.3 14.5 88 86 268 286

 

' SC I = 1x(% single) + 3x(% multiple) + 5x(% checked)
The test weight of the grain entering and exiting the dryer is listed in Table 5.5.
The data Show that an increase of approximately 29 kg/m3 (2.3 lb/bu) occurs during the

mixed-flow drying process.

37

100

 

801

 

60‘

o
40*

stress-crack °/o

20 4 ,
R‘ 0.7968

0 T I I
0 20 40 60 80 I 00
RPM,°/o

 

 

 

100

 

80-4

 

60-

stress—crack %

20 . ,
R‘ * 0.4569

 

 

 

100

. 0
80‘ Q

60‘

 

stress-crack °/o

20‘
R2=0.111

 

 

 

I T T I

50 6O 70 80 90 100 l l 0
drying-air temp, (f

100

 

804 ;— .
60~

40, °

stress-crack °/o

20 .. 2
R —O.()113

 

 

V I I T

12 I3 14 15 I6 1 7 l 8
oudet MC, %w.b.

 

FIGURE 5.3. Correlation between select drying parameters and outlet stress-crack
percentages.

38

TABLE 5.5. Effect of mixed-flow drying on the test weight.

 

 

test ave test weight, kg/m3

date inlet outlet increase
11/4 717 745 28
11/5 698 727 29
11/6 693 722 29

 

5.4.2 MOISTURE CONTENT PROFILES

Figure 5.4 is a plot of the moisture profiles along the top of the ducts in the last
module of the drying section (see Figure 5.1a). Sample ports #1 and #2 are inlet ducts and
#3 and #4 are exhaust ducts. There appears to be no difference between exhaust or inlet-
air ducts. The moisture content of the grain near the inlet-air and exhaust walls of the
module are more dry than the center of the duct. Friction along the wall may slow the
flow, thus resulting in over-drying. [Note: Profile for port #2 was determined with
samples from a test date separate from the other ports, thus the lower average moisture]

Figure 5.5 shows the moisture profiles between the ducts of the last row in the
cooling section (see Figure 5.1b). The moisture profile is relatively horizontal which
indicates even drying, except for a high moisture peak on the right side of duct #2 and a
low moisture peak next to duct #1. A flow uniformity test showed that metering roll #1
(above duct #1) unloaded at a slower rate than the other two rolls, thus explaining the
lower moisture content.

Figure 5.6 shows the moisture profiles of the discharge along each of the three
metering rolls. The figure shows a slower rate of discharge for metering roll #1. Metering
rolls #2 and #3 appear to discharge at the same average moisture content. The figure also

shows the greater moisture loss along the inlet-air and exhaust module walls.

39

5.4.3 DUST EMISSIONS

A summary of the results Of the modified exhaust ducts is listed in table 5.4. The
average percentage Of dust reduction was highest for the louver modification at 42%; the
lowest was the baffle modification at 29%. The results for the perforation were not
expected. The values are slightly misleading because the perforated ducts were located at
the location of the module reversal; therefore, each duct was receiving one-half of the
airflow seen by the other modifications (see section 5.4.2) and thus a lower duct air

velocity.

TABLE 5.6. Summary of percentage of dust reduction for the three modified exhaust

 

 

 

ducts.
test
date baffle louver perforation
10/27 42 - -
10/29 44 62 33
10/29 - 55 33
10/30 20 24 41
10/30 - - 10
10/31 - - 26
10/31 - - 50
1 1/3 28 76 -
1 1/4 6 28 -
1 1/5 33 9 -
average: 29 42 32

 

40

 

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«3 ~B--Por1#2
E’ 13.5 -
9 . A- Port #3
a A “ab-Port #4
12.5-‘
, -.wou-fiazh
q 43" , .— 9 T «D,

 

 

—- u—
o —l
u L»
1 fl.

\

I
.“

J ...—a

 

0.25 0.5 0.75 1 1.25
distance from Inlet side, In

FIGURE 5.4. Moisture content profile along the top of the ducts at four different
locations in the last module of the drying section.

18.0

 

17.0 a

16.0 a

15.0 -

o o o 0
14.0 a

average MC, %w. b.
0

130+ ,

12.0 1

11.0-

 

 

10.0 I I I U I

 

duct duct duct duct
#1 #2 #2 #3

FIGURE 5.5. Moisture content profile between the ducts in the last row in the cooling
section.

41

 

 

 

 

 

 

 

 

17.0J -2 A ‘ xx“
2:3 // "Wang H.“ _,___ 41
' » I 5 —--~—““‘"“"" “‘0 t
4 16.5 4r -. ""
g '_ ./.
.\ ‘ , ,
U 1r” ”2 g
E 160 " ./ ° ;II
0 AJ//' ’ 1‘
M ,,/’ \
a J /, -
3' ,0" '7
' 15.5 'i g
1 “l / I _ ‘
exhaust Side . \
1w .. [:1
:r'”
14.5 I I I I r I f f f I I I I
0 0.2 0.4 0.6 0.8 1 1.2 1.4
distance from inlet side, In

FIGURE 5.6. Moisture content profile of discharge along each of the three metering
rolls.

42

A drawback of the louver and perforated modifications is the possibility of
becoming plugged. A visual inspection of the exhaust modifications revealed a build-up
of “flour dust” (very fine dust) in the perforated ducts and bees-wing (light redish flaky
material) build-up in the louver ducts in the top two modules due to the high relative
humidity of the exhaust air. There were no visible plugging problems for the ducts with

baffles.

5.4.4 EXHAUST-AIR TEMPERATURE AND VELOCITY PROFILES

The temperature and velocity of the air exhausting each individual duct of the
drying section were measured. The velocities were measured without the burner on and
with the grain column stationary. The temperatures were measured on two separate test
dates over a time period of approximately one-hour.

Figure 5.6. shows the average air temperature exiting each row of exhaust ducts in
the drying section. A significant increase in exhaust temperature is seen at the fourth row
of the exhaust ducts. The increase is due to the lack of airflow at the previous row of
exhaust (row 5). Reversing the top two modules of the drying section places two rows of
exhaust ducts in consecutive order; therefore only half of the airflow that enters the inlet
ducts actually exits through these rows of exhaust. Thus the grain maintains a
considerable amount of energy from the previous row on inlet ducts, which is then
exhausted through the ducts of row four.

The average maximum velocity for each row of exhaust ducts in the drying

section is shown in Figure 5.7. The effect of the module reversal at row 5 is apparent.

43

 

top 8

3 U ON
; A l

U
1

exhaust row in drying section

l l 4» 87.8 C
‘4} ,9.-- 76.7 C

0,.

11‘

l

 

 

bottom ]

FIGURE 5.7.

/
I V I Y

I.
l/
T T V Y ' ' V

10 20 30 40 50

ll"

average exhaust air temperature. °(_.‘

Exhaust air temperature profile for the drying section.

 

60

 

top 8

exhaust row In drying section

 

 

bottom I

FIGURE 5.8.

W I v V I I I Y

7.5 10.0
average exhaust duet velocity. m/s

Exhaust velocity profile for the drying section.

44

 

15.0

Both row 4 and row 5 receive half the airflow as the other exhaust ducts (i.e. air from

only one row of inlet ducts rather than two rows).

5.4.5 PRESSURE LOSS IN GRAIN COLUMN

The pressure loss through the grain column in the mixed-flow dryer was
determined. Table 5.7 lists the pressure loss through the grain column. The average
pressure loss through the grain is 247 Pa. There is a decrease in pressure loss along the
duct from the inlet Side to the exhaust side. The pressure loss using the procedure
described in Appendix A with the measured airflow of 300 m3/min (10,600 cfm) is 2] 3
Pa; thus there is good agreement between the calculated and measured Static pressure
values.

TABLE 5.7. Average pressure loss (Pa) through grain column along the length of the
duct in mixed-flow dryer [zero point at exhaust side of dryer].

 

location (m)

 

0 0.28 0.56 0.84 1.12 1.40
204 209 242 258 274 294
average along duct = 247 Pa

 

45

CHAPTER 6

SIMULATION ANALYSIS

6.1 MODEL VALIDATION

The experimental results of the mixed-flow dryer were used to validate the MSU
mixed-flow drying program. The inputs to the program include: drying-air temperature
and airflow, ambient temperature and relative humidity, and the inlet grain moisture
content, standard deviation, and temperature. The reader is referred to the paper of Liu et
a1. (1997) for a description of the model.

Table 6.1 compares the experimental and simulated (predicted) capacities and
efficiencies. Table 6.2 compares the experimental and simulated outlet standard
deviations and outlet grain temperatures. The predicted capacities and energy efficiencies
are in general very close to the experimentally-measured values except for three of the
test dates (i.e. 10/30, 11/5, 11/6). It is noted that the three test dates with the largest error
are also the dates with the lowest drying-air temperatures.

Possible errors in predicted capacities and efficiencies may result from the
moisture content determination and the variety of maize. The ASAE method of moisture
determination was used. The standard specifies the oven temperature, but neglects to
specify a vapor pressure. During this study, moisture contents of several samples were
determined twice and sometimes thrice. A maximum number of cans were placed in the
oven (i.e, three perforated shelves with a single layer of cans) and the oven was

maintained a temperature of 103 °C throughout the required 72 hours of drying.

46

 

 

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7
4

Nevertheless, final moisture values differed up to 1.5 %w.b. for some samples. It is
thought that the relative humidity of the air was too high, thus not permitting the proper
drying rates.

The average predicted outlet grain temperature was approximately 4 °C lower
than the average experimental value. An accurate measurement of the cooling fan airflow
or static pressure could not be made; therefore this iS a likely source of error in the
predicted outlet grain temperature.

The predicted standard deviation of the outlet maize moisture content was in

general smaller than the measured value by an average of 0.98 %w.b.

6.2 VALIDATION CONCLUSION

The MSU mixed-flow drying model shows good agreement in predicting the dryer
capacity, efficiency, outlet grain temperature and standard deviation, and thus is
concluded to be an effective design tool. Therefore, the effects of varying the drying
parameters such as drying-air temperature, inlet moisture content and standard deviation,
and ambient conditions, on dryer performance can be examined. The next section details

such an analysis.

6.3 INFLUENCE OF DESIGN PARAMETERS

The MSU mixed-flow drying model was used to examine the effects of various
design parameters on dryer performance, i.e. capacity, efficiency, and outlet standard
deviation. Table 6.3 lists the standard conditions used in the analysis. The process of

Simulation was to vary one parameter while keeping all other parameters constant.

48

TABLE 6.3. Standard conditions for the dryer performance analysis of the pilot-scale
mixed-flow dryer.

 

inlet MC 20 %w.b.

outlet MC 15 %w.b.

inlet SD. 3 %w.b.

initial grain temperature 15 °C (60 °F)

ambient temperature 15 °C (60 °F)

ambient relative humidity 65%

drying-air temperature 95 °C (200 °F)

drying airflow 4,500 m3/h (2,650 cfm)
or 536 m3/h-m (96 cfm/ft)

cooling airflow 3,825 m3/h (2,250 cfm)

 

6.3.1 EFFECT OF DRYING-AIR TEMPERATURE AND AIRFLOW

Figure 6.1 Shows the effect of drying-air temperature on the capacity and energy
efficiency of the dryer. The capacity is a linear function of the air temperature; the
efficiency increases slightly at increasing temperatures. A capacity of approximately
7,000 dry kg/h (276 dry bulb) is achieved at the recommended upper-temperature
boundary (i.e. 135 °C or 275 °F) for drying feed maize in mixed-flow dryers.

Figure 6.2 Shows the effects of drying airflow on the capacity and energy
efficiency. An increase in airflow decreases the energy efficiency while increasing the
capacity. The capacity increases 44 % with an airflow rate change of 3,500 to 5,500 m3/h.

The effect of drying airflow and temperature on the outlet standard deviation is
Shown in Figure 6.3. An increased airflow has minimal effect on outlet standard
deviation; increasing the drying-air temperature from approximately 40 to 140 °C

increases the outlet standard deviation about 0.7 %w.b.

49

 

8000 6500

    
  

 
    
  

 

 

 

q I
7000 4 - 6000
‘ efficiency _
6000 '-
4 - 5500
a 5000 4 * g
3‘ - 5000 .
‘ .0 E
r U
'3’. 4000 4 . .5
E E
i l - 4500 E‘
a
9 3000 " I- g
0
\ - 4000
2000 ' capacity P
. s
1000 -4 3-00
0 v ‘ r U V I I I I I v 3000
20 40 60 80 100 l20 140

drying-air temperature, C

FIGURE 6.1. Simulated efl‘ect of drying-air temperature on dryer capacity and energy

 

 
 

 

 

 

efficiency.
6000 5200
‘ L 5150
5500 ~ ’
r- 5100
d b-
l' 5050
5000 4 P 3’
g y b 5000 3
"3 4500 « “PM“ F4950 “a
E' \ E
u *- to
§' * ~ 4900 9
° 2
4000 4 ' °
\ - 4850
efficiency _ 4800
3500 4 L
- 4750
F
3000 . , . . . r 4 T 4 . . 4700
3000 3500 4000 4500 5000 5500 6000

drying alrt'low per module, m’m

FIGURE 6.2. Simulated effect of drying airflow rate on dryer capacity and energy
efficiency.

50

 

drying airflow per modde. m3lh
3000 3500 4000 4500 5000 5500 6000
l .9 i A 4% A 4 . A

J

 

     

airflow
1 [cm perature

\

outlet standard deviatlon, %w.b.

 

 

 

20 40 60 80 100 120 140 160
drying-air temperature. C

FIGURE 6.3. Simulated effect of drying-air temperature and drying airflow rate on
outlet standard deviation.

51

 

6.3.2 EFFECT OF AMBIENT TEMPERATURE AND RELATIVE HUMIDITY

The effects of the ambient air conditions on drying capacity and efficiency are
shown in Figures 6.4 and 6.5. The dryer capacity decreases with an increase in ambient
air temperature. The density of air at low temperatures is greater than at high
temperatures, thus a greater mass of airflow is blown through the fan. An increase in
relative humidity, from 35 to 85 %, results in a minimal capacity decrease (4 %). The
efficiency of the dryer worsens with an increase in ambient relative humidity.

Table 6.4 lists the simulated outlet grain temperature with various ambient

temperatures. An outlet grain temperature reduction of 9 °C results by changing the

ambient temperature from 25 to 5 °C.

TABLE 6.4. Simulated effects of ambient air temperature on outlet grain temperature.

 

 

ambient outlet grain
temperature, °C temperature, °C
5 30
10 32
15 34
20 37
25 39

 

6.3.3 EFFECT OF INLET MOISTURE CONTENT AND GRAIN TEMPERATURE

Figure 6.6 shows the effect of average inlet moisture content on the capacity and
efficiency of the dryer. At IO-point removal (i.e. 30 % w.b. inlet moisture) the dryer
capacity is reduced 64 % from the capacity at S-point removal; the efficiency improves by

approximately 970 kJ/kg (420 Btu/lb).

52

The effect of inlet grain temperature on capacity and energy efficiency is shown in
Figure 6.7. The capacity increases approximately 620 dry kg/h (24 dry bu/h) with an
increase in grain temperature of 5 to 25 °C. The increased capacity results in an improved

efficiency (a 13 % lower value).

53

5 200

 

 

     
  

600 0

 

 

 

 

 
     
   

 

 

*fficiency _ »
5100 - I/ ” 5500
I-
- 5000
5000 -l
‘ . s g
g 4-00 3
. 4900 4 l 3'.
E‘ 5
'5'. J - 4000 1;
.E' E
i 4800 . ' E‘
3 / " 3500 a,
" E
. . 0
4700 .. capacn)
- 3000
4
4600 ‘ F 2500
‘ I
4500 . . . . . j . . . f . 2000
0 5 10 15 20 25 30
ambient temperature. C
FIGURE 6.4. Simulated effect of ambient air temperature on the dryer capacity and
energy efficiency.
5000 5100
capacity
4950 ‘ I L 5050
4
4900 « ~ 5000 3‘
5 3
3* 1 >2
‘9 g
"‘1 4850 . . 4950 .3
E' E
35 .°
,
5 l E‘
4800 4 4900 E
4
efficiency
47504 . 4850
‘ l
4700 . . . . . f T . . . . 4800
30 40 50 6O 70 80 90

ambient RH. %

FIGURE 6.5. Simulated effect of ambient relative humidity on the dryer capacity and
energy efficiency.

54

 

  
       
 

 

 

 

8000 6000
* P
7000 -
.. 5000
6000 y / efficiency +
‘ L4000 if
g 5000 < 5
* ;
t' ‘ E
‘3 O
. 4000 - - 3000 "'
e ./ E
'2: ‘ capacny y
a l- .
g. a
9 3000 j E
I- 2000 v
2000 4 .
‘ - 1000
1000 4
J n
0 i T f I I I 0
15 17.5 20 22.5 25 27.5 30 32.5

Inlet nmlsture contem, %w.b.

FIGURE 6.6. Simulated effect of inlet moisture content on the dryer capacity and energy
efficiency.

 

5250 5400

     
 

'- 5300

  

efficiency
L

/

5000 - I- 5200

b

*- 5100

4750 4 - 5000

b

capacity, dry kg/h
energy efficiency. kJ/kg

'- 4900

h

   

\

4500 4 capacity - 4800

F
I- 4700

t

4250 v I l’ 1 t t 1 I v ‘r r 4600
0 5 10 15 20 25 30

 

 

 

Inlet grain temperature. C

FIGURE 6.7. Simulated effect of inlet grain temperature on the dryer capacity and
energy efficiency.

55

CHAPTER 7

SUMMARY AND CONCLUSIONS

A pilot-scale mixed-flow dryer was designed using the two-dimensional MSU
mixed-flow grain drying model. The dryer was constructed at an elevator in mid-
Michigan, and tested for dryer performance and grain quality characteristics. The model
was validated with the experimental data and good agreement was observed. Thus, the
model will be an effective tool for future designs of mixed-flow grain dryers.

The reversing of modules to reduce the moisture content standard deviation of
maize exiting a mixed-flow dryer appears to be effective. Additional testing of the dryer
with non-reversing is needed. The outlet standard deviation is considerably lower in the
mixed-flow dryer (2.2 %w.b.) as compared to the crossflow dryer (4.1 %w.b). Stress-
crack percentages were similar for the mixed-flow and crossflow dryers, contradicting
previous research.

The amount of dust exhausting a mixed-flow grain dryer can be reduced by
modifying the under-side of the duets with baffles, louvers, or perforation; the louver
modification resulted in the greatest reduction and the baffles in the least.

The following conclusions are drawn from this study:

(1) The design of a mixed-flow dryer is feasible using the MSU mixed-flow

drying model; the model was proven to be an effective design tool.

(2) Dust emissions from the mixed-flow dryer type can be reduced with exhaust

duct modifications.

56

(3) Module reversing in the mixed-flow dryer (similar to the function of grain
tum—flow in crossflow dryers) appears to reduce the outlet moisture standard

deviation.

57

CHAPTER 8

RECOMMENDATIONS FOR FUTURE STUDY

The recommendations for future study are:

(1) Test the dryer _w_i_th_ou_t reversing the modules in order to provide a comparison
of the outlet moisture content standard deviation with that of the reversed
configuration.

(2) Examine the effect of insulation of the inlet air ducts in order to evaluate the
possible reduction in the percentage of stress-cracked kernels.

(3) Examine the effect of modifying the exhaust duct by increasing the duct air
velocity (i.e. smaller cross-sectional area).

(4) Determine the optimal drying-air temperature and airflow rate for the
individual modules of the dryer, for different grain inlet moisture contents.

(5) Analyze the dryer performance for other crops (e. g. wheat. rice, barley,

canola).

58

LIST OF REFERENCES

AP-42 Handbook. US Environmental Protection Agency.

ASAE Standards, 38‘h ed. 1991. S3 52.2. Moisture measurement—unground grain and
seed. ASAE, St. Joseph, MI

Brooker, D.B., F.W. Bakker-Arkema, and CW. Hall. 1992. Drying and Storage of
Grains and Oilseeds. Van Nostrand Reinhold, New York, NY.

Bruce, D.M. 1984. Simulation of concurrent-, counter-, and mixed-flow grain dryers.
JAgric Eng Res 30(4):361-372.

Cao, CW. 1993. “Experimental investigation of hot air drying mechanism of corn.” In:
Proc ‘93 Int Symp of Grain Drying and Storage Tech. Beijing, China.

Courtois, F. and J .C. Lasseran. 1993. “A computer-aided design (CAD) software to
improve the efficiency of maize drying and better the quality of grain in
processing industries.” In: The Proc ’93 Int Symp of Grain Drying and Storage
Tech, pp. 235-251.

Kirleis, A.W. and KL. Stroshine. 1990. Effects of hardness and drying air temperature on
breakage susceptibility and dry-milling characteristics of yellow dent corn. Cereal
Chem 67(6):523-528.

Lasseran, J .C. 1987. Grain drying in France. Paper No. 87-6014. ASAE, St. Joseph, MI.

Lesikar, B.J., C.B Parnell, Jr., and A. Garcia. 1991. Determination of grain dust
explosibility parameters. Trans ASAE 34(2):571-576.

Li, H. and R.V. Morey. 1984. Thin-layer drying of yellow dent corn. Trans ASA E
27:581-585.

Liu, Q. 1993. Study on the Drying Mechanism, Simulation and Test of Mixed-flow Grain
Dryer. Ph.D. thesis. Beijing, China: Beijing Agricultural Engineering University.

Liu, Q., C.W. Cao, and F .W. Bakker-Arkema. 1997. Modeling and analysis of mixed-
flow grain dryer. Trans ASAE 40(4):]099-1106.

59

Maier, D.M. 1995. [online] Grain Quality F act Sheet #23: Quality grain needs TLC.
Available at: (http://hermes.ecn.purdue.edu:8001/cgi/convert?GQ-23), October
16, 1997.

Miller, P.C.H. and RD. Whitfield. 1984. The predicted performance of a mixed-flow
grain drier. J Agric Eng Res 30:373-380.

Montross, MD. et al. 1994. Moisture content variation and grain quality of corn dried in
different high-temperature dryers. Paper No. 94-6590. ASAE, St. Joseph, MI.

Montross, J .E. et al. 1997. Moisture content and stress-crack distributions in different
high-temperature dryer types. Paper No. 97-6031. ASAE, St. Joseph, MI.

Olesen, HT. 1987. Grain Drying. Innovation Development Engineering Aps. Lyngby,
Denmark.

Paulsen, MR. and L.D. Hill. 1985. Corn quality factors affecting dry milling
performance. J Agric Eng Res 31 :255—263.

Perry, RH. 1997. Chemical Engineers’ Handbook, 7th edition. McGraw-Hill, New York,
NY.

Thompson, RA. and G.H. Foster. 1963. Stress-cracks and breakage in artificially dried
corn. Market Research Report 631. USDA, Washington, DC.

Weller C.L., M.R. Paulsen, and MP. Steinbergi. 1988. Correlation of starch recovery
with assorted quality factors of four corn hybrids. Cereal Chem 65(5):392-397.

Weller C.L., M.R. Paulsen, and S. Mbuvi. 1989. Germ weight, germ oil content, and
estimated oil yield for wet-milled yellow dent corn as affected by moisture content

at harvest and temperature of drying air. Cereal Chem 66(4):273-275.

Wolanski, P. 1979. “Explosion hazards of agricultural dust.” In: Proc ‘79 Int Symp on
Grain Dust, Manhattan, Kansas, pp. 422-446.

Wrigley, C. W. et al. 1979. “The allergenic and physical characteristics of grain dust as
they affect the health of workers in the industry.” In: Proc ‘79 Int Symp on Grain
Dust, Manhattan, Kansas, pp. 81-90.

I/IBaHOB, II. 1987. Optimal spaces of air ducts in mixed-flow grain dryer. Farm
Machinery 24(4).

POBHBIPI, PA. 1987. Design methods of grain dryer. Farm Machine 1987(11).

60

APPENDIX A

APPENDIX B

APPENDIX C

APPENDIX D

APPENDIX E

APPENDICES

Pressure Loss Calculations
Moisture Content Data
Stress-crack Data
Miscellaneous Data

Design Drawings

APPENDIX A

PRESSURE LOSSES
A) GRAIN COLUMN

Figure A.1 shows a schematic for calculating the pressure drop through the grain.
It is assumed that one-fourth of the total airflow to a single air-inlet duct is passed
vertically through the cross-sectional area shown as a horizontal dashed line. The
following equation is used to calculate the pressure drop through a column of shelled

maize (Brooker et al., 1992):

 

20,700Qa2
= h (A. 1)
1n(1 + 30.4Q,)
The cross-sectional area of the column is defined as:
A - (K E) L A 0
— 2 2 ( ...)

The airflow rate for the pressure calculation is 1/24 of the airflow per module (i.e. six air-
inlet ducts and four possible directions of flow).
The resulting pressure drop through the grain is approximately 191 Pa (0.78 in

H20).

B) DUCTS

Figure A.2 shows the location of the pressure losses within the module duct
system. The following has been considered:

0 air enters and exits the duct at the design velocity of 5 m/s (1 and 6)

o frictional losses are occurring over the total length of duct (2 and 5)
0 a 90° directional change of the air (3 and 4) is occurring

63

P— w ——-
QAIR

+0111

PM

l

 

 

FIGURE A.1. Schematic of variables affecting the static pressure loss through the grain.

 

 

 

 

 

2
5m/s
3 V ‘1 .......
GRAIN
6 :
4 ....... ...,- 4
5

 

FIGURE A.2. Schematic showing locations of pressure loss in the module design.

64

At locations 2 and 5 in Figure A.2, the pressure loss is due to fiiction within the

duct. The Darcy equation is used to determine this pressure loss:

L

AP- [—]EK—2— A3
“f 4R 2 ( ')

A relative roughness value and the Reynold’s number are required to obtain a friction

factor from the Moody Diagram. The Reynold’s number is defined as:

4VR
Re = —— (A.4)
v
The relative roughness is defined as:
e (A 5)
a = —— .
4R

For the other locations, the loss coefficient method is used to determine the

pressure loss. The following equation is used to calculate the pressure loss:

pV2
" 2

 

AP (2 K) (A.6)

At the 90° directional change, the loss coefficient is 1.1, while at the entrance and exit of
the duct the loss coefficients are 0.5 and 1.0, respectively (Perry’s Chemical Engineers’
Handbook, 1997).

The resulting pressure loss is the sum of the loss in the grain, the ductwork, and

the burner (assume 125 Pa (0.5 in H20)) and is approximately 360 Pa (1.5 in H20).

65

APPENDIX B

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76

APPENDIX C

stress—cracks

STRESS-CRACK DATA
MIXED-FLOW
date oven stress-crack class SC
date time examined MC none single multiple checked % SCI
10/25 4:20 PM 11/19 12.1 0 3 28 19 100 364
5:00 PM 11/19 13.5 2 3 24 21 96 360
average: 12.8 98 362
10/26 3:20 PM 10/28 14.1 31 15 2 2 38 62
3:20 PM 11/1 14.1 29 13 7 1 42 78
4:00 PM 11/20 13.4 31 5 9 5 38 114
4:20 PM 11/20 13.0 23 11 11 5 54 138
average: 13.5 45 1 10
10/27 6:40 PM 10/28 13.3 16 7 21 6 68 200
7:20 PM 10/28 13.3 15 5 21 9 70 226
5:40 PM 11/1 13.3 9 7 31 3 82 230
6:40 PM 11/1 13.3 10 8 23 9 80 244
7:20 PM 11/1 13.3 9 8 24 9 82 250
6:20 PM 11/20 12.8 7 3 12 23 76 308
7:20 PM 11/20 13.3 10 8 20 12 80 256
average: 13.1 78 282
10/29 2:20 PM 11/1 14.0 8 6 28 8 84 260
2:40 PM 11/1 14.2 7 0 35 8 86 290
3:00 PM 11/1 13.7 6 7 30 7 88 264
2:20 PM 11/19 14.0 5 6 28 11 90 290
2:40 PM 11/19 14.2 9 5 22 14 82 282
3:00PM 11/19 13.7 4 5 27 14 92 312
4:20 PM 11/19 13.3 5 8 22 15 90 298
average: 13 .8 89 296
10/30 3:40 PM 11/25 14.0 6 12 19 13 88 268
4:20 PM 11/25 13.7 6 8 22 14 88 288
5:00 PM 11/25 15.2 6 11 26 7 88 248
average: 14 . 3 88 268
10/31 3:40 PM III] 14.5 13 3 29 5 74 230
4:20 PM 11/1 14.4 15 9 23 3 70 186
3:00 PM 11/19 14.2 1 1 29 19 98 366
4:00 PM 11/19 14.2 4 5 29 12 92 304
average: 14.2 95 335
11/4 5:20 PM 11/20 15.6 10 10 20 10 80 240
6:00PM 11/20 15.9 10 10 27 3 80 212
average: 15.8 80 226
11/5 2:20 PM ‘? 15.8 15 10 18 7 70 198
2:40 PM ? 16.2 11 8 24 7 78 230
3:00 PM ? 14.7 14 8 21 7 72 212
4:00 PM 11/20 16.1 4 12 29 5 92 248
4:40 PM 11/20 15.1 4 10 27 9 92 272
average: 15.6 92 260

78

stress-cracks

11/6 2:00 PM 11/19 15.4 5 9 24 12 90 282
4:00 PM 11/19 15.3 5 7 24 14 90 298

4:40 PM 11/19 15.6 9 13 20 8 82 226

average: 1 5 .4 87 269

11/2 2:20PM 11/20 5 8 16 21 90 322
3:20PM 11/20 14 14 13 9 72 196

4:00 PM 11/20 29 3 15 3 42 126

79

stress-cracks

CROSSFLOW
date oven stress-crack class SC

date time examined MC none single multiple checked % SCI
10/26 3:00 PM ? 16.3 34 11 4 l 32 56
2:20PM 11/23 14.6 18 12 18 2 64 152

3:40PM 11/23 14.1 15 18 13 4 70 154

average: 14 .4 67 153

10/27 8:00 PM 10/28 13.9 25 5 12 8 50 162
3:20 PM 10/28 13.4 13 8 19 10 74 230

6:00 PM 11/20 12.9 12 4 18 16 76 276

7:00 PM 11/23 15.3 11 13 19 7 78 210

7:40 PM 11/23 14.2 12 16 12 10 76 204

average: 14. l 77 2 30

10/29 1:00 PM 11/25 13.9 12 1 18 19 76 300
3:20PM 11/25 14.4 19 8 12 11 62 198

4:20 PM 11/25 14.4 6 7 20 17 88 304

average: 14.2 75 267

10/30 3:00 PM 11/23 14.4 8 3 27 12 84 288
3:20 PM 11/23 14.6 6 9 21 14 88 284

average: 14.5 86 2 86

11/2 3:20 PM ? 25 13 10 2 50 106
####### 11/24 25 16 8 1 50 90

80

APPENDIX D

fan data

MIXED-FLOW DRYER FAN DATA
1 1/24/98

drying fan: AV24-10 1T1 Corporation
cooling fan: AVIS-l ffi Corporation

 

ENGLISH METRIC
airflow (cfm) SP airflow (ma/min) SP
AV24-10 AVIS-l (in H20) AV24-10 AVl8-l (Pa)
14800 5050 0 419 143 0
14400 4600 0.5 408 130 124
13650 4050 1 386 115 249 it“
13200 3450 1.5 374 98 373 1
12200 2650 2 345 75 498
1 1400 2000 2.5 323 57 622
10600 1500 3 300 42 746
8000 1150 3.5 226 33 871 .
6500 4 184 995 E
5800 4.5 164 1 120 g
5100 5 144 1244 g
4400 5.5 125 1368 Fr"
3600 6 102 1493
3000 6.5 85 1617
AVIS-1.5 85% for lack of venturi
(m3/min) (m3/h)
4000 1 1 13 249 96 5773
3450 1.5 98 373 83 4979
2650 2 75 498 64 3825

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83

DUCT LENGTH PRESSURE DIFFERENCE

 

11/13/98 location: 4th module
0 0.28 0.56 0.84 1.12 1.4 .
data 0 11 22 33 44 55 in "
set exhaust air-inlet . “ —— top row
1 -0.818 -O.827 -0.974 -l.001 -1.07 -1.l68 .
2 ? -O.981 -0.853 -O.972 -1.071 -1.136 -1.192 +
i ‘ ‘
average (in 1120) -0.818 -0.84 -O.973 -1.036 -l.103 -1.18
(Pa) -204 -209 -242 -258 -274 -294
owrall average = -1.0 in 1120 -247 Pa
UNLOADING RATE
11/19/98
holding capacity = 338 bu
full
RPM grain start end time emptying time (min) average capacity RPM
(9.) height time R3 R2 R1 R3 R2 R1 ( mm) (bum) (bu/h)
11'19 29 1/2 12:08 4:14 4:21 4:26 246 253 258 252 80 277
of

 

 

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full
RPM grain time capacity RPM
(°’o) emptied (min) (bu/h) (bum)
10126 70 1.5 modules 22 219 313
80 1.5 modules 18.5 260 325
300
Capacity vs. RPM chart. 250
RPM capacity -° 200 .
(98) (dry bu ‘11) E
. 150 d
0 0 z.
29 80 ‘3 100+ y = 3.1673:
70 219 a o R2 = 0.9956
80 260 50 ‘
0 Y I I I
0 20 4O 60 80 100
RPM. °/o

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air velocity

EXHAUST AIR VELOCITY
1 1r'13/98 measurement locations

‘average of 10 readings from the Solomat meter

B‘— baflle L-‘tloover P=perforated

module velocity (In/s) average
overall of
:11 B averages average maximum
13.06 13.18 16.1
10 9.63 7.72 10.6 9.8 10.1 10.1 14.1
8.69 6.46 6.33
L.
12.26 15.58 0.79
9.65 9.43 3.27 10.4 11.1 3.1 8.2 12.3
9.25 8.41 5.34

‘ maximum = 9.0 a bottom center

n2 B
10.34 10.08 14.11
8.96 9.47 9.16 8.6 8.2 9.6 8.8 11.5
6.52 5.12 5.43
P
5.95 5.57 5.53
4.71 5.14 4.67 5.2 4.8 4.9 5.0 5.7
4.87 3.63 4.6
1 2 1.?2
33 P
5.81 5.91 5.91 6.74
4.23 4.25 3.03 4.04 5.0 4.2 3.9 4.5 4.4 6.1

5.07 2.47 2.86 2.81

B
9.33 10.02 9.66 9.97
6.56 7.11 6.99 7.45 7.8 7.5 7.1 8.5 7.7 9.7
7.6 5.27 4.51 8.18
34 B
9.22 9.24 9.69 9.52
6.61 7.48 6.41 6.66 7.4 7.4 6.5 7.3 7.1 9.4

6.22 5.52 3.35 5.61

L
7.79 8.28 5.57 8.58 7.1 7.4 6.7 7.1 7.0 8.1
6.42 6.43 7.74 5.57
3.5 4.59 7.24 3.49
3'5 6.26 6.97 6.69
cooling 7.36 7.56 7.43 6.9 7.9 7.4 7.4 8.0
6.97 9.31 8.2
3.14 3.48 3.94
3.82 3.76 3.54 3.8 3.8 3.9 3.8 4.2

4.37 4.14 4.15

86

exhaust temp

EXHAUST-AIR TEMPERATURE DISTRIBUTION

date:
time:
air temp:

date:
time:
air temp:

11/4
3:30 PM
190 F

module

#1

#2

#3

#4

11/5
2:45 PM
170 F

module
# l

#2

#3

#4

4.5
22.5
25
26
50
43
37
34

24
25.5
27
49
38
31.5
32

temperature (C)

3 5

25 20

25.5 25

27 25.5

56 ‘ 57 56

45 46.5 44
40.5 " 40 38

36.5 37.5 38

"' 54.5 C 40 min before
" 39.5 C 40 min before

temperature (C)

9 10

25 23

26 25

27 26
51.5 49 50.5
40 40 38
33 34.5 37
33.5 34 38

87

average
(C) (F)
4.2 40
22.5 73
25.2 77
26.2 79
54.8 131
43.8 111
38.3 101
36.5 98

average
(C) (F)
9.3 49
24.0 75
25.5 78
26.7 80
50.0 122
39.0 102
34.0 93
34.4 94

MC profiles

SUMMARY OF MC PROFILES WITHIN THE MIXED-FLOW DRYER

DUCT LENGTH MOISTURE MOISTURE BETWEEN DUCT S
oven
distance from oven MC
sampling inlet side MC ducts location (%)
port holes (In) (%)
10:31 1-2 1 12.9
10.131 1 1-2 1.25 15.5 L 13.7
1 3-4 1 15.6 L 14.2
1 5-6 0.75 15.5 C 14.3
1 7-8 0.5 15.4 C 13.7
1 9-10 0.25 15.1 R 13.7
10531 3 1-2 1.25 15.6 R 13.9
3 3-4 1 16.4 2 13.7
3 5-6 0.75 16.7 10:31 2-3 2 14.8
3 7-8 0.5 16.4 L 15.7
3 9-10 0.25 15.3 L 15.2
2 1-2 1.25 11.5 C 14.5
2 3-4 1 12.0 C 14.2
2 5-6 0.75 12.2 R 14.2
2 7-8 0.5 11.9 R 14.2
2 9-10 0.25 11.4 3 14.2
4 1-2 1.25 15.1
4 3-4 1 15.8
4 5-6 0.75 15.5
4 7-8 0.5 15.8
4 9-10 0.25 15.6

MOISTURE ALONG METERING ROLLS

 

 

 

 

oven
MC
roll side (%)
10:31 1 inlet 12.4
13.8
14.2
13.9
exhaust EXHAUST SIDE
10931 3 inlet 14.3
151 El
15.7
15.2
exhaust 14.6
distance
1 194 l inlet 14.8 from inlet 1 2 3
15.6 0 14.8 16.1 16.5
16.0 0.46 15.6 16.7 17.0
exhaust 15.1 0.93 16.0 16.8 16.7
2 inlet 16.1 1.4 15.1 16.0 15.9
16.7
16.8
exhaust 16.0
3 inlet 16.5
17.0
16.7
exhaust 15.9

88

APPENDIX E

 

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