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 wasting money.

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

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 in spoilage.

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 %

 

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 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 content.

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 80^oF 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 130^oF (except 105^oF 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 filling.

Safe wheat storage period is affected by its moisture content and temperature. Figure 5 shows the effects of wheat temperature and moisture content on the duration of storage. It is clear that the higher the wheat storage temperature and/or the higher its moisture content the shorter the safe storage duration. The following are few tips that may help maintain the wheat quality during storage:

• Remove the previously stored wheat before placing newly harvested wheat into storage bins. Sweep the bin wall and floor as well as under the aeration ducts to get rid of grain kernels that may contain insect larvae and mold spores. Apply an approved insecticide both inside and outside the bin to delay insect population development before placing wheat in the bin.
  
  
• Apply aeration to cool down wheat, which was dried with heated air. Aeration will control grain temperature even if it starts heating during storage, but this may only be a short-term solution to avoid further damage to grain quality. If aeration cannot control hot spots, move wheat to another bin to break up these hot spots.
  
  
• Explore stored wheat conditions once a week during warm weather to protect it from deterioration caused by molds or insects. Consider adding temperature measurement cables to monitor conditions during storage and an automated controller for aeration fans to start cooling stored wheat below 60°F as soon as possible in late summer.
  
  
• Feel the top 6 to 12 inches of wheat to monitor temperatures and insect and mold activity. Insert plastic insect traps below the grain surface to monitor insect activity and check them during weekly inspections to control damaging populations. Make sure to secure these traps to a fixed member of the bin.

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.

Table 7. Heating value of fuel as well as their corresponding 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:

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)

Example 1:

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) = $2.25/h

Example 2:

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

Working with moving grain required special precautions and training. Therefore, check the following safety tips with coworkers and everyone who supposes to work around grain bins.

• Never enter a grain bin or gravity unload vehicle when grain is flowing. Several accidental deaths occur every year during handling and unloading grain. Therefore, lock out the control circuit on automatic unloading equipment before entering or cleaning a bin or repairing conveyors. Flag the switch on manual equipment so someone else does not start it. Do not enter a bin unless you know the nature of previous grain removal, especially if any crusting is evident.
  
  
• Avoid walking on any surface crust. Crusted or bridged grain can collapse and could bury workers. Do not depend on a second person-on the bin roof, on the ground, or at some remote point-to start or stop equipment on your shouted instructions. If a grain bin is peaked close to the roof, be extremely cautious. Crawling between roof and peak can cave grain and block the exit.
  
  
• Request help from coworkers when entering a bin. When entering a questionable bin or storage, have two outside and one inside workers. Attach a safety rope to the man in the bin with the two men outside capable of lifting him out without entering the bin. One man outside cannot do this and cannot go for help while giving first aid. That person may fall or over-exert in the panic and haste of getting off the bin or running to the control point.
  
  
• Be careful when working around flowing grain. Flowing grain can trap and suffocate a worker in seconds. Equipment noise can block out shouts for action or assistance. With modest flow rates of a 6" auger, a worker is helpless only 2-4 seconds after stepping into the cone of flowing grain. This worker will be totally buried in grain within 20 seconds at a grain flow rate of 1,000 bu/h.
  
  
• Be wary and alert while working with out of condition grain. Grain that has gone out of condition-there may be molds, blocked flow, cavities, cave-ins, or crusting. Always wear a respirator capable of filtering fine dust to work in obviously dusty-moldy grain. Never work in such conditions, even with protection, without a second person on safety standby.
  
  
• Run aeration fans for several minutes prior to entering a grain bin to ensure oxygen is present. Stored grain can consume oxygen from the air within the bin and suffocate a person entering the bin.