Home / EMc Downloads/PDF Viewer

EMc Downloads/PDF Viewer

University of Arkansas "On-Farm Grain Drying Methods"

Read page text

Logo of University of Ar
k a n s a s D i v i s i o n o f A g
riculture, Research and Extension, University of Arkansas System
UJA--
Agriculture and Natural Resources
FSA1072
OnFarm
Grain Drying Methods
Sammy Sadaka,
Ph.D., P.E.
Assistant Professor
Extension Engineer
Karl VanDevender,
Ph.D., P.E.
Professor Extension
Engineer
Griffiths Atungulu, Ph.D.
Assistant Professor
Grain Processing
Engineering
Arkansas Is
Our Campus
Visit our web site at:
https://www.uaex.uada.edu
University of Arkansas, United States Department of Agriculture, and County Governments Cooperating
Introduction
When grain is harvested from the
field, it contains dry matter and water.
While water is necessary for
plant growth and grain production,
excess moisture after grain maturity
can lead to storagerelated problems.
Grain moisture content is expressed
as a percent of the grain weight.
For example, 100 pounds of 13%
moisture content rice contains 13
pounds of water and 87 pounds of dry
matter rice. Grain moisture con
tent and temperature play a key role
in determining safe storage life (see
https://www.uaex.uada.edu/publications/
pdf/FSA1058.pdf). As a rule, dryer grain
and cooler temperatures increase safe
storage durations. In contrast, wetter
grain and warmer tempera
tures increase the potential for pests,
insects, mold and fungi to reduce
grain quality and market value.
Therefore, the primary objective of
grain drying and storage is to manage
the temperature and moisture of the
air around the grain to minimize
grain quality and market value losses
while holding grain for better market
opportunities. Maintaining grain qual
ity requires drying the grain to safe
moisture content levels after harvest
followed by lowering and maintaining
the grain temperature within a few
degrees of ambient air temperatures.
Traditionally, onfarm grain
drying and storage has seen limited
use in Arkansas. However, recent
changes in agricultural markets and
technological advances have made
grain production more attractive
resulting in more producers and
more production. This increased sup
ply is associated with a larger grain
price swing between harvest and
nonharvest periods. Therefore, in
addition to more control of harvest
timing, there are potential economic
advantages to onfarm drying and
storage. Accordingly, several Arkansas
producers indicated their interest in
exploring the possible techniques of
onfarm grain drying and storage.
This fact sheet provides an overview
of the basics of onfarm grain drying
and storage methods.
Grain Drying Basics
Storage Moisture Content
The first step in drying grain is
determining the desired, or target,
grain moisture content level. Under
drying grain reduces safe storage
time, increases the potential for qual
ity losses and increases the likelihood
of high moisture price dockages
upon sale. Over drying grain reduces
income due to increased drying costs.
In addition, since grain is usually sold
on a weight basis, one of the expenses
involved in drying grain is the “cost”
of the weight loss that occurs during
the drying process. This weight loss
by drying is referred to as “shrink”
and is expressed as a percentage of
the original quantity before it is dried.
Shrinkage should be considered to
accurately determine the total cost of
mechanical drying. Shrinkage tables
provide bushel weights for various

University of Arkansas "On-Farm Grain Drying Methods" — page 2

Read page text

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
moisture content levels of grains (see https://www. 
uaex.uada.edu/publications/pdf/FSA1078.pdf). When 
choosing the desired target moisture content, safe 
storage time, grain shrinkage and buyer’s require
ments should be considered.
      Grain conditioning by drying and cooling to target 
ranges should begin immediately after harvest. If 
possible, avoid leaving grain in carts and buggies for 
more than a few hours or overnight. As indicated 
earlier, grain temperature and moisture content dic
tate how quickly grain quality and market value are 
reduced. Drying and cooling freshly harvested grain 
will delay spoilage and must begin within 24 hours 
and preferably within 12 hours after the harvest. 
Equilibrium Moisture Content
      The moisture in grain creates vapor pressure. 
In a like manner, the moisture in the air around the 
grain also creates vapor pressure. Moisture moves 
from areas of high vapor pressure to areas of low 
vapor pressure. This moisture movement continues 
until the vapor pressures in the grain and air are 
equal. The point at which vapor pressure in grain 
and air are equal is called the Equilibrium Moisture 
Content (EMC). The EMC is dependent on three fac
tors: air temperature, air relative humidity around 
the grain and grain type. EMC values for grain 
decreases as air humidity decreases or air temperature 
increases (see https://www.uaex.uada.edu/publications 
/pdf/FSA1074.pdf). Thus, grain drying will occur 
as long as the EMC is less than the current grain 
moisture content. If the EMC is greater than current 
grain moisture content, drying will not occur. Instead, 
additional water will be added to the grain bin. Water 
will increase the potential for mold and needs to be 
removed as soon as possible.
KEY CONCEPTS
l
Moisture moves from high to low vapor
pressure areas.
l
Grain drying occurs when the vapor pressure
in the grain is greater than the vapor
pressure of the air surrounding the grain.
      If the current air conditions will not result in 
grain drying, the easiest way to adjust EMC is by 
heating the air. Heating air lowers the air relative 
humidity and thus lowers the EMC and decreases 
drying times. As a result, after heating air the new 
relative humidity must be measured or calculated 
before determining the new EMC. 
Temperature and Humidity
      As indicated earlier, air temperature and humid
ity determine EMC level and thus the drying capacity 
of the air around the grain. Since ambient air temper
atures and humidities fluctuate over time, the EMC 
and drying potential of air also fluctuates. Therefore, 
in drying systems that use ambient air (with or with
out low levels of supplemental heat), air temperature 
and humidity should be monitored and used to deter
mine when the drying system should be operated. If 
mismanaged, drying opportunities could be missed or 
moisture could be added back to the grain environ
ment increasing storage risks and wasting the energy 
to run fans again to redry.
      The ability to heat drying air increases the oppor
tunities to dry grain and provides more control over 
the grain drying and storage process. If the EMC of 
ambient air will result in grain drying, adding heat 
will reduce drying time and lower the final grain 
moisture content. The reduced drying time is usually 
desirable. However, if mismanaged, there is an 
increased risk of over drying grain. There is also 
energy cost to run fans and heaters. If the EMC of 
ambient air will not result in grain drying, adding 
heat can provide drying that otherwise would not 
take place. As a result, the decisions of what type of 
grain drying/storage system to install and when to 
run fans and/or heaters become a process of balancing 
risks and economic inputs.
      For manually controlled systems, the temperature 
for determining EMC should be an average tempera
ture over the drying period. The relative humidity 
should be the average expected during the drying 
period. However, several companies make automated 
grain drying controls which measure grain moisture, 
air temperature and air humidity. These automated 
controls can take much of the “guesswork” out of 
grain drying. Temperature can be read with a ther
mometer in the plenum or on the farm. Ideally, 
temperature and relative humidity should be mea
sured on farm, but local weather information has 
been used in the decisionmaking process with 
acceptable results.
      Evaporating moisture from grain requires energy 
in the form of heat. In general, it takes 1,100 BTUs 
of heat to vaporize one pound of water at 100% effi
ciency. Heat energy can be supplied by the natural 
heat content of air or by supplemental heating. The 
amount of moisture that air can absorb and transport 
as it moves through the grain column is dependent 
primarily on EMC along with some influences from 
air velocity, the distance the air travels and grain 
moisture content. As air moves through the grain 
column, it absorbs moisture and thereby loses some 
or all of its drying capabilities.

University of Arkansas "On-Farm Grain Drying Methods" — page 3

Read page text

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
.... 
V, 
0 u .... 
C 
QJ 
E .... 
V, 
QJ 
> 
C 
Combination 
and Dryeration 
High Temp 
Bin Drying 
Natural Air/Low 
Temp Drying 
Field 
Drying 
> 
Control Over Drying Process > 
Grain Drying Options
      Grain drying strategies can be divided into the 
following approaches: field drying, natural air drying, 
low temp drying, high temperature drying, and com
bination or dryeration. Allowing the grain to dry in 
the field is the most widely used method. In many 
cases, partial field drying is often used in conjunction 
with postharvest drying to reach target storage 
moisture content. Natural air/low temp grain drying 
is best described as filling or partially filling bins 
with freshly harvested grain, then running fans to 
force air through the bins until the desired moisture 
content is achieved. High temperature drying is 
either conducted in the bin or within a pass dryer. 
Air is heated to high temperatures and forced 
through the grain until the grain dries. Combination 
and dryeration is done by partially drying grain with 
high temperature dryers, and then the remainder 
of the drying process is done with low temperature 
air and fans. Each method has its advantages and 
disadvantages. In general, more drying process con
trol reduces potential risk. However, an increase in 
control is usually associated with increased invest
ment costs and energy costs (Figure 1). 
FIGURE 1. General relationships among manage
ment control, initial investment cost and operational
energy costs for various grain drying approaches. 
Field Drying
      Grain drying begins in the field after the grain 
is fully mature. A layer of tissue is formed between 
the seed and the plant which blocks additional mois
ture and nutrient inputs from the plant. At this 
point the maximum grain quality and yield are set. 
Once the grain matures and the layer of tissue is 
formed between the seed and the plant, the sun and 
air can remove moisture and dry grain at a rate of 
½ to 1 percent per day. Once moisture reaches near 
storage goal level, drying slows. Drying using this 
method is very common. Most producers fielddry 
grain to a certain moisture content and then harvest 
and dry further or market the grain at harvest. The 
disadvantage with this method is the reduced control 
of the drying process and potential exposure to 
weather and pests which causes damage. In addition, 
over drying grain usually increases shatter and 
losses during harvest. Field drying should be used to 
manage grain moisture at the time of harvest. The 
time of year grain reaches maturity and the weather 
conditions can have a major impact on how quickly 
the grain will dry to a moisture content acceptable 
for storage or sale. 
Natural Air Drying
      Natural air drying is the most common onfarm 
drying method in Arkansas. It refers to the process 
in which grain bins are filled or partially filled with 
grain and then natural air is moved through the 
grain with fans (Figure 2). This is typically done in 
bins equipped with a perforated floor, drying fan, 
grain spreader, sweep auger and unloading auger. 
Stirring devices may also be added. However, they 
are not economical for natural air drying systems in 
most grainproducing areas because over drying is 
usually not a significant problem. Loading the drying 
bin may be accomplished by either a portable auger 
or a bucket elevator. The loading rate should be 
sufficient to empty a large trailer truck in no more 
than 2 hours, thus a minimum of approximately 
500 bu/h capacity.
      As dry (lower vapor pressure) air passes wet 
(higher vapor pressure) grain, moisture moves from 
the grain into the air. The addition of water to the air 
reduces its ability to dry the grain it passes through 
next. This process continues as the air moves through 
the column of grain until the air no longer dries the 
grain, or the air exits the grain. As the fans continue 
to run, a drying front moves from where the air 
enters the grain to where it exits the grain. Behind 
the drying front, the grain is at EMC. Ahead of the 
drying front, the grain is above EMC. The vapor 
pressure and flow rate of the air entering the grain 
determine the formation of this drying front and how 
quickly it moves through the grain. The air flow rate 
depends on fan properties as well as the type and 
depth of the grain. As grain depth increases, air flow 
rates decrease. Therefore, increasing grain depth 
slows the drying front and increases the amount of 
time it takes for all the grain to reach EMC, and the 
potential for grain quality losses.
      A common mistake with natural air drying is 
to add too much grain to the bin at once. This will 
increase drying times and delay the grain drying 
process which increases the likelihood of grain qual
ity losses. Therefore, it is commonly recommend to

University of Arkansas "On-Farm Grain Drying Methods" — page 4

Read page text

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
t 
t 
t 
Moist Air 
t 
t 
t 
Wet Grain Layer 
Drying Zone 
Dry Grain Layer 
t 
t 
t 
Dry Air 
t 
t 
t 
11----,1'1¥ 
+
+-
~-T"""l'-T"""l'""-1::: 
only add 45 feet of grain to a bin at a time. Then 
avoid adding more grain until the layer is dry. 
Depending on the system setup, several bins can be 
loaded alternatively, or the dry grain can be moved to 
another bin.
      Successful grain drying with natural air is usu
ally the most energyefficient method of drying. It is 
also the slowest method and has the greatest poten
tial for grain spoilage. Consequently, natural drying 
requires the highest level of management if spoilage 
and/or aflatoxin problems are to be prevented. Much 
of the risk is because there is little “reserve” capacity 
for speeding up the drying process in that the inlet 
air conditions vary with the weather. Typically, the 
bin is filled only once each harvest season. If spoilage 
begins, the midcourse corrections are limited to 
either (1) drying immediately using another drying 
method or (2) selling before the grain degrades 
because of unacceptable damage levels. 
Low Temperature Drying
      Low temperature drying refers to the process in 
which grain bins are filled or partially filled with 
grain and then air with little (<10°F) heat added is 
moved through the grain with fans (Figure 2). This 
is typically done in bins with a perforated floor or 
ducts. Usually, electricity is the thermal energy 
source, hence the term “electric drying” is sometimes 
used instead of “low temperature” drying. However, 
LP gas and solar energy may also be used as thermal 
sources.
      The low temperature drying method is assumed 
to always have potential for drying grain within the 
accepted moisture contents associated with longterm 
storage. This is contrasted with natural airdrying 
where outside air conditions may not allow further 
drying. When air is heated, its temperature and 
volume increase, but its moisture level remains 
FIGURE 2. Grain bin utilizing natural air/low 
temperature drying. 
constant. This results in a lowering of the relative 
humidity of the air and allows for a possible net 
transfer of moisture from the grain to the air. Mois
ture transfer continues until the grain and air come 
into equilibrium. In grain drying, the drying fan con
tinuously supplies air as moisture is transferred from 
the grain and removed from the bin. With low tem
perature drying, sufficient heat is added so that 
drying can continue until normally acceptable final 
moisture contents are reached.
      In this method, a perforated floor is required. 
A grain spreader, underfloor unloading auger and 
sweep auger usually are included. A stirring device 
may also be added. Filling the bin may be accom
plished by either a portable auger or a bucket 
elevator.
      In low temperature drying, grain is dried and 
stored in the same bin, thus minimizing handling and 
labor costs. Generally the comparative total cost for 
drying decreases as less energy is used to heat the 
drying air even though more energy is required to 
operate the drying fans. Thus, successful low temper
ature drying is relatively economical in terms of 
energy costs when compared to higher temperature 
methods. Some of this advantage is lost when electric
ity is used as the thermal energy source because it is 
usually more expensive than LP gas on per unit of 
energy basis.
      Successful low temperature drying is defined as 
drying the grain to a desired moisture content with
out excessive economic losses through either energy 
costs or grain spoilage. Potential for drying increases 
when adding heat to increase the drying air tempera
ture. Unfortunately, the rate of grain spoilage also 
increases with higher temperature. Thus, low temper
ature drying is generally restricted to conditions 
where grain moisture is relatively low, nearly 20% 
for corn. Low temperature drying has little reserve 
capacity; that is the system dries grain at a low 
steady rate that cannot be altered greatly. The 
dependability of the system is reduced further if 
solar energy is the heat source.
      Perhaps the greatest risk associated with low 
temperature drying is the yeartoyear variability of 
weather. The same drying strategy may not be used 
every year. Care should be taken to avoid aflatoxin 
contamination. 
High Temperature Drying
      High temperature drying is done either in the bin 
or in a dryer. There are four approaches to high tem
perature drying: inbin batch drying, recirculating bin 
drying, continuous flow bin drying and pass drying. 
Inbin batch drying is similar to natural air/low tem
perature drying except that air temperatures are

University of Arkansas "On-Farm Grain Drying Methods" — page 5

Read page text

EMc Downloads/PDF Viewer

EMc Downloads/PDF Viewer

University of Arkansas "On-Farm Grain Drying Methods"

GEAPS Exchange EMC Presentation