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The goal of wheat drying is to reduce grain moisture content to meet the recommended
levels for safe, long-term storage. When placed in storage, wheat should be dried
quickly to a moisture level of about 12 percent to minimize any quality deterioration.
Wheat drying can be accomplished in bins by blowing large volumes of dry air through
the grain. This website will explore the challenges of wheat drying and storage.
Currently, there is a limited amount of wheat harvested at higher moisture and dried
on-farm in Arkansas. Majority of the harvested wheat is transported directly to grain
terminals. However, there has been a dramatic increase over the last few years in
on-farm storage and drying systems that have been primarily used for other crops such
as corn. These systems could be used for wheat. As a result, Arkansas wheat growers
often debate the subject of whether or not to dry and store wheat on the farm.
One argument against on-farm wheat drying is that the cost may be higher than commercial
drying costs, particularly when the cost of electricity is high. Another argument
against on-farm wheat drying is that it must be done at one of the busiest times of
the farming season. The time required to effectively monitor and manage the drying
process can add to the already significant pressure on growers.
However, arguments for on-farm wheat drying include creating a higher quality finished
product. Growers realize that when wheat grain is re-wetted in the field several times
while awaiting field drying to dockage levels, the quality is compromised. Therefore,
implementing on-farm drying of the freshly harvested wheat will produce higher quality
grain. Growers who dry and store their wheat also gain more flexibility to manage
their operations and timing of when to sell their wheat.
Additionally, early wheat harvest would allow earlier planting of double crop soybeans,
which generally means greater yields on double crop. Overall, on-farm drying and storage
of wheat is becoming an appealing practice especially when the producer can use the
drying and handling equipment in rotation with other grains such as rice. This chapter
will explore the basics of on-farm wheat drying and storage.
In order to dry wheat, high flow rate of air (heated or unheated) is typically forced
though the grain bed. The air is used as a medium to carry away moisture from the
grain. The air temperature and relative humidity (RH) determine how much moisture
the air can hold. Air flow and rate (or velocity) determines the drying time required
and the final grain moisture content.
Air at a specific temperature and relative humidity continually passing through the
wheat will cause the wheat to dry to a specific moisture content since the wheat moisture
will eventually achieve a state of equilibrium with the air. This property is called
"Equilibrium Moisture Content” (EMC). Thus, temperature and relative humidity properties
of the drying air determine the wheat's final moisture level. Table 1 indicates the
EMC values of soft red winter wheat in equilibrium with air at various temperatures
and relative humidity levels. Arkansas wheat producers grow about 99.5% soft red winter
wheat.To illustrate the use of the EMC tables, assume that air at a relative humidity of
70% and temperature of 60 oF is being forced through a bed of wheat. The soft red winter wheat will not dry below
moisture contents of 13.9%. If the air at 60 oF and 70% RH is passed through a heater and heated to 80 °F, not only will the temperature
increase, but the RH will decrease to about 35% and the EMC decreases from 13.9% to
less than 9%. Under heated conditions, the air can hold more water and will cause
the wheat to dry to lower moisture content. This is the key to achieving more reduction
in moisture, but caution must be taken not to over dry the wheat causing damage and
To determine the value of EMC in your area now, click the link Determination of the current temperature and relative humidity based on your zip code.
Table 1. Equilibrium Moisture Content of Soft Red Winter Wheat
Relative Humidity (%)
To learn more about Equilibrium Moisture Content, see the fact sheet titled: "Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts"
To determine the values of EMC for various grains, download the Excel sheet by clicking
the link: "Equilibrium Moisture Content"
As previously mentioned, air carries moisture away from the wheat during drying and
conditioning. The air is typically forced into the bottom section of a bin under a
perforated floor supporting the wheat using one or more fans. This open area is called
the plenum. The fans are mounted to the plenum using a transition that allows efficient
pressurization of the plenum that allows even distribution of air flowing through
the bed of wheat. Even air distribution is critical to allow complete drying of the
wheat and avoiding “hot spots” where sections of wheat do not dry properly resulting
Grain producers should select the manufactured fan that best fits their drying needs.
Over sizing, the fan leads to unnecessary energy consumption in the form of electricity
from the fan motor and gas or electricity from the air heater. The greater the airflow
rate the more energy required to heat the air to a specific temperature. Also, higher
airflow rates through the wheat result in higher pressure requirements for the fan
resulting in higher purchase costs for the fan and less efficient operation. On the
other hand, under sizing the fan size will cause too little airflow resulting in drying
being too slow.
Airflow rate through the wheat and air temperature directly control the drying rate.
The higher the airflow rate and temperature accelerate the drying rate and increase
the cost. Proper dryer operation allows the wheat to reach a sufficiently low moisture
content to prevent spoilage faster than the rate spoilage occurs. Properly designed
dryers optimize the balance between airflow rate and air temperature to dry the wheat
to the proper moisture content to maximize quality while minimizing overall costs.
Types of Fans
Grain drying fans are classified as either axial-flow or centrifugal flow. Each type
of classification could be used to optimize the airflow rate and minimize the energy
consumption for maintaining grain quality. In both types, air is forced into the bin
by the fan.
Axial-flow fans move air parallel to the axis or impeller shaft. This type of fan
is suitable for grains that create low static pressure, less than 4 inches of water
which generally not the case of wheat unless the bed depth is shallow. The axial flow
fans, however, could provide adequate airflow for aeration of already dried wheat,
which required much less airflow than when drying. Axial flow fans also typically
create much more noise during operation than centrifugal fans, which should be considered
when locating drying facilities near residences.
In the centrifugal fans, air enters one end of the impeller parallel to the shaft
and exits perpendicular to the shaft. Centrifugal fans used for grain drying and storage
generally have backward-curved blades. They are usually the most efficient type of
fans when static pressure is greater than 4 inches of water and are typically capable
of generating much greater pressure than axial fans. Because of high resistance to
air flow generated by wheat, centrifugal fans are the ideal fans to use for drying
operations, which generally require moving air flow rates of 1 to 4 cfm/bu. Centrifugal
fans also operate with less noise than axial fans.
Recommended Minimum Airflow Rates for Drying Wheat
The actual amount of air needed to dry wheat depends on its initial moisture content.
Table 2 shows the minimum recommended airflow rate to dry wheat at various levels
of initial moisture contents. Fans sized specifically for corn or soybeans will not
move enough air for wheat placed in bins at the same grain bed depths due to the higher
static pressure developed by wheat.
Table 2. Minimum Recommended Airflow Rates
To select and maintain your fan, please see the fact sheet titled "Selection, Performance and Maintenance of Grain Bin Fans"
Wheat kernels contain dry matter and water. The dry matter translates to the actual
value of the wheat. A base moisture content of 13.5% is typically used to price wheat.
Variations of the wheat moisture content change the weight of the water in the wheat
and its weight per standard bushel as shown in the following table.
Table 3. Wheat weights of one market standard bushel at a moisture content of 13.5
Moisture Content (%)
When wheat is delivered to the elevator at moisture content above its base standard,
buyers apply a “shrink factor” to adjust the quantity for the excess moisture so they
will not pay the price of excess water. Applying the shrink factor approximates the
equivalent number of bushels that would be in the load if wheat were dried to the
base moisture content. Conversely, some farmers often deliver wheat to the elevator
at moisture levels below the base. There will be no compensation factor increasing
the price of the wheat in this case, so it is very important to avoid over drying
of the wheat to prevent lowering its the value. The following example will provide
an illustration of applying the shrink factor on wheat.
Ex. Suppose a farmer harvests a portion of a field and dries the wheat to the market
standard moisture content of 13.5% and the dried wheat weighs 100,000 pounds. This
means that the weight of the water in the wheat is 0.135 x 100,000 = 13,500 pounds.
The weight of the dry matter in the wheat is 100,000 – 13,500 = 86,500 pounds. At
60 lbs per bushel at standard base moisture content of 13.5%, the farmer will get
credit for 100,000 pounds/60 pounds per bushel = 1,667 bushels of wheat. At a price
of $5 per bushel, the farmer sells the wheat for $5/bu * 1,667 bu = $8,333.
Alternatively, suppose this same portion of the field is harvested and the wheat is
dried to 16% moisture content. The farmer would still harvest 86,500 lb of dry matter,
but wheat would now contain more water because of the higher moisture content. At
16% moisture content, the total weight of the wheat is now 102,976 pounds and contains
102,976 – 86,500 = 16,476 pounds of water. As mentioned earlier, the elevator will
not be willing to buy excess water. The elevator applies the shrink factor to adjust
the quantity. From Table 4, the market standard bushel weight at 16% moisture content
is 61.79 pounds per bushel. The total bushels calculated accounting for the greater
moisture content is now 102,976 pounds divided by 61.79 pounds per bushel equals 1,667
bushels, which is the same as if the moisture content were 13.5%. The total price
the farmer would receive remains the same at $8,333. However, the farmer would also
need to pay drying costs to the elevator to remove the extra moisture.
If the farmer harvested the same portion of the field and over dried the wheat to
a moisture content of 10%, then the total weight of the wheat is 96,111 pounds including
dry matter and moisture. The over dried wheat is not adjusted for moisture, so the
standard bushel weight of 60 pounds per bushel is used. A total amount of wheat is
calculated to be 96,111 pounds divided by 60 pounds per bushel = 1,602 bushels. The
farmer receives a price of 1,602 bushels x $5/bushel = $8,010 or about 4% less than
if the wheat was at 13.5% moisture content. Costs are also added for the extra electricity
and gas consumed removing the extra moisture from the grain during over drying.
In-bin wheat drying processes can utilize either natural air (unheated) or low temperature
air (slightly heated usually less than 10 °F) to dry grain in bins (see figure 1).
The air is forced up through the grain with fans until the grain moisture content
is sufficiently reduced. This is typically done in bins with a raised perforated floor
to ensure even airflow, but can also be done using air ducts laid on the concrete
bin floor prior to adding grain. Wheat is among the grains that offer high resistance
to airflow requiring a significant pressure output from the fans, which is represented
as static pressure in inches of water. In-bin drying methods used for other crops
such as corn and soybeans can be adapted to work properly for wheat if some adjustments
are made to compensate for wheat’s high resistance to air flow. The simplest adjustment
is to reduce wheat depth in the bin to half that normally used for corn or soybeans.
In addition, the installation of appropriately sized centrifugal fans may help deliver
higher airflow rates under higher static pressures.
Bin capacity measured in bushels of grain shown in Table 4 increases by increasing
the bin diameter and/or the grain depth. For example, a grain bin with 28 ft diameter
filled to a level height of 16 ft height can hold up to 7,882 bushels of wheat. Increasing
the grain depth increases the static pressure that the fan has to overcome to provide
the same cfm/bu. For example, if air is delivered to a grain bin that holds 9,048
bushels creating static pressure of 8.00 inches of water, the airflow rate will not
exceed 16,000 cfm (1.77 cfm/bu) using 2-15 hp fans or 24,000 cfm (2.65 cfm/bu) using
3-15 hp fans. It should be mentioned that for a 30-ft or 32-ft bin, wheat depths greater
than 20 feet would generally reduce airflow rates to less than 1 cfm/bu even with
the use of 3-15 fans. Accordingly, Table 5 can be used to select wheat depth for known
moisture content and fan size.
If high moisture wheat is to be dried and stored in the same bin, extra care is advised.
If the initial moisture content is 17% or more, use heat to dry the top layer to less
than 17% before adding more wheat. Generally, moderate airflow (2-5 cfm/bu) along
with a temperature-rise less than 20°F are required. Stirring devices, re-circulators,
or automatic unloading augers can be used to increase drying rate. After drying the
wheat to 17%, use unheated air to dry it to about 15%. During this period, run the
fan continuously to provide uniform drying and moisture distribution within the wheat.
Operate drying fans only during low humidity hours to finish drying to around 13%.
This management scheme will minimize the amount of wheat overdrying in the bottom
of the bin. It should be mentioned that excess heat could cause severe overdrying.
Table 6 shows the approximate degrees of heat needed to dry wheat to 12% moisture
Figure 1. Grain bin.
Table 4. Number of wheat bushels in grain bins
Table 5. Maximum safe drying depth for wheat with typical bin and fan combinations.
Table 6. Approximate degrees of heat needed to dry wheat to 12% moisture content.
Batch and Continuous Flow Drying
High temperature batch or continuous flow dryers are usually used to dry large capacities
of wheat. These units typically have very high airflow rates, and they do not require
supplemental heat for daytime drying when harvesting wheat at 18-20% moisture range.
If heat is used, the drying air temperature can be limited by cycling the burner on
and off or by changing the gas burner orifices.
The following are some tips that may help wheat producers achieve better grain quality
while minimizing the drying cost:
Harvest wheat at 20% or less moisture content. Wheat requires extra care for harvesting
above 20% moisture, so this is the practical upper limit for drying. The more typical
harvest moisture content is around 14 to 16%. Adjust the combine to minimize the amount
of trash collected with the grain in order to reduce the pressure loss of air passing
through it and increasing airflow rate.
Load grain into clean bins immediately after harvest. Bins should be cleaned and sanitized
prior to harvest to minimize insect problems. Move wheat from the field to grain bins
as soon as possible. The amount of time before spoilage begins depends on grain moisture
content and air temperature. A safe rule of thumb is to hold freshly harvested wheat
in carts or trucks no longer than 12 hours. Warm air temperatures greater than 80oF and higher grain moisture levels are the most critical factors for decreasing the
time required for the grain to spoil.
Check the moisture content of each load of grain as it is placed in the drying bin.
There can be some variation in moisture content, but you need to know the average
moisture content of the bin to determine the minimum necessary air flow needed and
the allowable depth of grain in the bin.
Open air exits and start the fan as soon as the grain depth is about 1 foot deep on
the perforated floor. Be sure to use spreading devices or some other means to keep
the grain leveled as the bin is being filled. If the grain is allowed to cone, there
will be an increase of small particles in the center of the cone/ or central portion
of the bin resulting in the air not being able to reach this grain because of increased
resistance to flow. This makes it very hard to dry and control moisture uniformly
in the grain bin and may cause spoilage.
Add wheat to drying bin in shallow layers until the moisture content decreases. High
moisture wheat (18 to 20%) can be added in 4 feet layers on top of dry grain if the
fan can provide at least 3 to 4 cfm/bu through the total depth in the bin. As an example,
6 feet of 20% moisture wheat can be dried to 14.5%, then 4 more feet of 18 to 20%
moisture wheat can be placed on top of that. The fan must work against the static
pressure developed by 10 feet of grain to provide at least 4 cfm/bu for the 4 feet
of wet grain.
Level wheat inside each drying bin continuously – never allow coning to occur. Some
manual work may be required to maintain a level surface on the top when the maximum
depth is reached. This will ensure uniform airflow through all the grain, assuming
it has been placed in the bin with a good spreader.
Use stirring devices when drying wheat if possible. If stirring devices are used,
the temperature can be set as high as 130oF (except 105oF maximum for seed wheat). Stir augers will blend the wheat sufficiently to prevent
it from drastically over-drying near the bottom of the bin.
Monitor the moisture content of wheat daily. Wheat must be cooled to avoid nighttime
condensation on the inner walls. If the heat has been on long enough for the complete
mass of wheat to be warmed and the weather is clear and dry with humidity below 60%,
turn the heat off when the moisture content of the grain drops to within 1% of the
target moisture content. Continue running the fans, and the residual heat in the grain
will finish the drying process.
Aerate with natural air once the grain is below 13% moisture content. Wheat should
be cooled as much as possible with early summer conditions. Cooling air should be
checked for humidity, being careful to aerate when humidity is below about 60% or
better yet when the EMC content is at or below the target moisture level. Avoid aeration
with high humidity air since it will add moisture back to the grain.
Probe the bin periodically to check for insect infestation and grain temperature increase.
Wheat temperature increase usually means moisture migration. Aerate whenever this
is detected. If the problem is in the center of the bin and aeration is not effective,
move the grain to another bin to solve this problem. Problems in the center of the
bin usually indicate that a lot of fines and/or trash accumulated in this area during
It is very important to maintain the on-farm wheat drying cost at a minimum in order
to maximize profits (return on investment). Producers interested in drying their wheat
need to determine the total pounds of water they will remove from one bushel of grain.
The number of BTUs to extract 1 pound of water will vary from 1,100 to 1,400 BTU/pound
depending on how easily moisture is given up by the kernel. A good estimate is to
use an average of 1,200 BTU/pound of water removed to calculate the energy needed
to remove 1 pound of moisture. Table 8 summarizes the BTU/unit of fuels as well as
the burning efficiencies.
Wheat drying costs may be estimated using the following equation(s):
Fan motor cost:
Fan motor cost ($/h) = fan HP x 0.7475 (kW/HP) x electricity cost ($/kW.h)
Fuel cost ($/bu) = [BTU/lb water x (lb water removed/bu) x fuel cost ($/unit of fuel) x 100] /
[(BTU/unit of fuel x burning efficiency %]
Wheat drying cost ($/bu) = fan cost ($/h) x drying time (h/bu) + fuel cost ($/bu)
Assume that a producer has a 30 HP fan with electricity cost at $0.10 kW-h, no demand
charges are applied; determine the cost per hour of operation for this fan?
Fan motor cost ($/h) = fan HP (30) x 0.7475 (kW/HP) x electricity cost ($/kW-h) (0.10)
Determine the drying cost per bushel of wheat to dry from 19.0% moisture wheat down
to 13.5% moisture using LP at a cost of $2.40/gallon.
Look at Table 3 above for 19.0% wheat; it is determined that there is 4.07 pounds
of water per bushel above the value for 13.5% wheat (64.07-60.00). The following is
an estimate assuming:
Fuel cost ($/bu) = [1200 x 4.07 x 2.40 x 100] /[92,000 x 80%] = $0.16/bu