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The goal of rice drying is to reduce its moisture content to meet the recommended
levels for safe, long-term storage. When placed in bins, rice should be dried quickly
to a moisture level of about 12% to minimize any quality deterioration. Rice drying
can be accomplished in bins by blowing large volumes of dry air through the grain.
This website will explore the challenges of rice drying and storage and provide the
required spread sheets that might help producers maintain their rice quality and reduce
the drying and storage costs.
Rice quality is the major factor affecting its market value. Immediately following
harvest, rice quality is typically at its peak. A primary measure of rice quality
is head rice yield which is greatly influenced by drying. The final quality of rice,
ready to market, is sensitive to all post-harvest processes, such as drying, handling,
storage, and milling. Rice is graded using several classifications designed to characterize
rice quality. Accordingly on-farm rice drying and storage has the potential to increase
harvest efficiency, reduce harvesting delays, and provide more control over grain
quality, all of which contribute to overall market/delivery time flexibility.
Air is typically being used as a tool for rice drying by forcing large quantities
of unheated or heated air though the grain bulk. Air temperature and relative humidity
(RH) are the two key characteristics that determine the drying rate and the grain
final moisture content. For a given air temperature and relative humidity, rice will
only loose moisture until a certain moisture and eventually achieve a state of equilibrium
with the environment. This property is called "Equilibrium Moisture Content” (EMC).
Thus, temperature and relative humidity properties of the drying air determine rice
moisture level. The EMC may be determined by measuring air temperature and relative
humidity. A sling psychrometer is one of the tools used for measuring relative humidity,
and is relatively inexpensive. The following table present the EMC of rice in equilibrium
with air at various temperatures and relative humidity levels. To illustrate the use
of the EMC table, assume that air at 70% relative humidity and 60oF temperature is
being forced through rice kernels in the bulk, rice will not dry below 15.1%. If air
under the same RH (70%) is heated, the EMC will be reduced.
Accordingly, rice may be dried without adding any heat to the drying air if the EMC
is low enough as compared to rice moisture content. Careful monitoring of the EMC
and management of drying times to optimize low values will provide the most economical
drying. In many cases, particularly at night, the addition of heat is needed to condition
the air to the correct its EMC.
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.
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 mentioned earlier, air is the medium used to carry moisture away from the rice
during drying and conditioning. The air is typically forced into the bottom section
of a bin under a perforated floor supporting the rough rice using one or more fans.
This open area is called the plenum. The fans are mounted to the plenum using a transition
that allows even distribution of air flowing through the bed of rice. Even air distribution
is critical to allow complete drying of the rough rice and avoiding “hot spots” where
sections of rice do not dry properly resulting in spoilage. Rice 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 rough rice 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 rough rice 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 rice to reach a sufficiently
low moisture content to prevent spoilage. Properly designed dryers optimize the balance
between airflow rate and air temperature to dry rice to the proper moisture content
to maximize quality while minimizing overall costs.
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 rice 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 rice. The axial flow fans, however, could
provide adequate airflow for aeration of already dried rice, which required much less
airflow than when drying. Axial flow fans are also typically creates much more noise
during operation than centrifugal fans, which should be considered when locating drying
facilities near residences. The second type of grain drying fans is the centrifugal
fan. 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 rough rice, 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.
The actual amount of air needed to dry rough rice depends on its initial moisture
content. The following table shows the minimum recommended airflow rate to dry rough
rice at various levels of initial moisture contents. Fans sized specifically for corn
or soybeans will not move enough air for rice placed in bins at the same grain bed
depths due to the higher static pressure developed by rice.
Recommended Minimum Airflow Rates for Drying Rough Rice
Initial Moisture Content
(cfm per bushel)
11% to 13%
13% to 15%
15% to 18%
18% to 20%
Table 3. Rice weights of one market standard bushel at a moisture content of 13.0
When rice 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. This
is because rice buyers 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
rice 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 rice in this case, so it is very important to avoid
over drying of the rice to prevent lowering its the value.
The following example will provide an illustration of applying the shrink factor on
rice. Suppose a farmer harvests a portion of a field and dries the rice to the market
standard moisture content of 13.0% and the dried wheat weighs 100,000 pounds. This
means that the weight of the water in the wheat is 0.130 x 100,000 = 13,000 pounds.
The weight of the dry matter in the wheat is 100,000 – 13,000 = 87,000 pounds. At
45 lbs per bushel at standard base moisture content of 13.0%, the farmer will get
credit for 100,000 pounds/45 pounds per bushel = 2,222 bushels of rice.
Alternatively, suppose this same portion of the field is harvested and the rice is
dried to 16% moisture content. The farmer would still harvest 87,000 lb of dry matter,
but rice would now contain more water because of the higher moisture content. At 16%
moisture content, the total weight of the water in this rice is now 16,571 pounds.
The total weight of rice is 87,000 + 16,571= 103,571 pounds. As mentioned earlier,
the elevator will not be willing to buy excess water. The elevator applies the shrink
factor to adjust the quantity. From the previous table, the market standard bushel
weight at 16% moisture content is 46.61 pounds per bushel. The total bushels calculated
accounting for the greater moisture content is now 103,571 pounds divided by 46.61
pounds per bushel equals 2,222 bushels, which is the same as if the moisture content
were 13.0%. The total price the farmer would receive remains the same. However, the
farmer would also need to pay drying costs to the elevator to remove the extra moisture.
In-bin rice 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 photo below).
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. Rough rice 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 rice if some adjustments
are made to compensate for rice high resistance to air flow. The simplest adjustment
is to reduce rice 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 5 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 rice. 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 1.77 cfm/bu. The following
table can be used to select rice depth for known moisture content and fan size. If
high moisture rice 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 rice. 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 rice
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 rice. Operate drying
fans only during low humidity hours to finish drying to around 13.0%. This management
scheme will minimize the amount of rice over drying in the bottom of the bin. It should
be mentioned that excess heat could cause severe over drying. Table 7 shows the approximate
degrees of heat needed to dry wheat to 12% moisture content.
Number of wheat bushels in grain bins
Maximum safe drying depth for wheat with typical bin and fan combinations.
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 rice. These units typically have very high airflow rates, and they do not require
supplemental heat for daytime drying when harvesting rice 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.
Listed below are recommended steps provided by researchers and grain processing experts
that can be adapted for on-farm rice drying. Following these steps can greatly increase
the chance of having one of the best rice-drying seasons for your farm, which in turn
could lead to an increase in total farm gross profit.
Tips for Rice Harvesting:
Working with either moving and/or stationary grain requires special precautions and
training. Therefore, it is necessary to keep in mind the following tips while working
with rough rice as well as other grain: