WW-07356 Reviewed 1998
Harold A. Cloud, extension agricultural engineer
R. Vance Morey, associate professor
Reviewed by Bill Wilcke
Copyright © 1997 Regents of the University of Minnesota. All rights reserved.
Converting High-Speed Dryers
The typical high-speed drying process rapidly reduces corn moisture content using heated air and then cools the corn rapidly in the dryer before transfer to storage (figure 1). Dryeration (figure 2), a process developed at Purdue University in the late 1960s, modifies this high-speed drying process by transferring the corn hot to a cooling bin, eliminating in-dryer cooling. The hot corn is allowed to "steep" or "temper" in the cooling bin at least 4 to 6 hours before being cooled slowly. After cooling, the corn is transferred to storage. Dryeration provides three advantages over high-speed drying with in-dryer cooling: increased dryer capacity, reduced fuel consumption, and better corn quality (less stress cracking, less fine material, higher test weight).
In-storage cooling (figure 3) is another alternative to in-dryer cooling. Hot corn is transferred directly to the storage bin rather than to a special cooling bin as in dryeration. In-storage cooling eliminates the extra handling step that is sometimes a disadvantage in dryeration. Although dryeration has advantages over in-storage cooling related to energy savings, dryer capacity, and corn quality, elimination of the extra handling step required in dryeration often makes in-storage cooling a feasible alternative. Both dryeration and in-storage cooling are discussed in detail in this publication.
Figure 2 shows the steps involved in the dryeration process. The corn is delivered hot from a high-speed dryer to a dryeration bin where it is allowed to "temper" or "steep" without airflow for at least 4 to 6 hours before being slowly cooled. The capacity of the high-speed dryer is increased because the dryer is not used for cooling. In a batch dryer the cooling time is eliminated. In a continuous-flow dryer a burner can be added to the cooling section, converting the entire dryer column to full heat.
Immediately after high-speed drying the inner portions of the kernels are wetter than the outer portions. During the tempering process the moisture equalizes throughout the kernel. During the slow-cooling process following the tempering period two to three percentage points of moisture are removed. By contrast, little water is removed during rapid in-dryer cooling. Since the heat contained in the corn is more effectively utilized for removing water during cooling with dryeration, less fuel is required and more capacity is realized for the high-speed dryer.
The kernel stresses developed in final stages of high-speed drying and rapid cooling also are minimized by dryeration. This improvement in kernel quality produces corn that is less susceptible to stress cracking and damage during subsequent handling operations.
When a high-speed dryer is used for dryeration, the drying air temperature may be increased. This is possible because the corn is discharged at a higher moisture content and is in the dryer for less time. Increasing the drying air temperature increases the dryer capacity and improves the fuel efficiency of the high-speed dryer. When drying air temperature is increased, the corn quality (test weight and stress cracking) should be carefully monitored to insure that satisfactory quality is maintained.
Typically, a 20 to 40 percent savings in energy and a 50 to 75 percent increase in dryer capacity result from dryeration. The actual energy savings and capacity increases depend primarily on the initial corn moisture content. Weather conditions and corn temperature also affect energy use and capacity.
After cooling has been completed, the corn is transferred from the dryeration bin to aerated storage. During the tempering period condensation can build up in the corn next to the bin wall, which can lead to spoilage. As a result, corn that has gone through the tempering period in the dryeration bin should not be left there for storage. If the dryeration bin is to be used for storage of the last corn dried, the recommendations for in-storage cooling should be followed.
A separate cooling bin (or bins) is necessary for the dryeration process. The physical arrangement of the dryer and the cooling bins should facilitate ease of switching bins and transferring cooled corn to storage as shown in figure 4. The best arrangement is to provide two cooling bins, each holding at least 24 hours of drying capacity. Hot corn is delivered to a bin throughout the day and allowed to steep with the cooling fans off. After the first hot corn delivered to the bin has steeped at least 4 to 6 hours, the cooling fan is turned on while additional hot corn is delivered to the bin. Typically, the cooling fans are started in the evening if hot corn is first delivered to the bin in the morning. On the second morning, the dryer discharge is switched to the second dryeration bin. During the second day, cooling is completed in the first dryeration bin and the corn is transferred to storage. The dryeration process is alternated from bin to bin on each succeeding day.
With larger-capacity cooling bins, more than one day's drying can safely be accumulated before transfer to storage. If the maximum benefit is to be obtained from dryeration, the operation of the cooling fans should be adjusted to provide at least a 4- to 6-hour tempering period for all hot corn.
If only one cooling bin is used for dryeration it should be large enough to hold several days drying capacity. Hot corn is delivered to the bin, allowed to temper and then cooled on a daily basis. Each day more hot corn is added to the bin. Again, to provide the maximum benefit from dryeration, the operation of the cooling fan should be adjusted to provide at least a 4- to 6-hour tempering period for all hot corn. This always requires some initial checking and experimentation on the specific cooling rate of the fan-bin system. Operation of the cooling fan can be controlled by a 24-hour timer or a 1- to 2-hour percentage timer. A percentage timer control allowsthe operator to adjust the fan operating time to any percentage of the total period.
When only one cooling bin is used, there are several methods for finishing the cooling and transferring the corn.
Complete unloading of a flat-bottom bin is not necessary if the corn remaining in the bin stays cool, dry, and in good condition. However, the cooling bin should be emptied completely before it is filled the last time for storage.
The success of the dryeration system depends on good design and equipmentselection including:
The cooling bins should be sized to hold at least the maximum 24-hour capacity of the high-speed dryer. Remember that when used in dryeration, high-speed dryer capacity will be increased drastically (50 to 75 percent). Also, if the bins are not unloaded each time, the volume of corn remaining after gravity unloading must be deducted from the bin capacity. The bushels of corn remaining in the bin after gravity unloading can be estimated by multiplying the cube of the bin diameter by 0.11. Thus, a flat-bottom, 24-foot diameter bin would have about 1,520 bu corn remaining in the hopper after gravity unloading (0.11 x 24 3 or 0.11 x 24 x 24 x 24).
Long-range needs must be considered in setting up a dryeration system. A cooling bin set up today to handle the capacity of a 4-row combine may be required to handle a 6- or 8-row combine in the future. Extra capacity in the cooling bins is not wasted since they also can be used for storage.
Grain bins can be equipped with a wide range of airflows (cfm/bu) depending on what is to be accomplished. Storage bins equipped with aeration have airflow rates in the range of one-fifth to one-twentieth cfm/bu. This is enough air to change the temperature of the stored grain as the seasons change and to maintain uniform temperatures throughout the storage bin. Drying bins equipped with full perforated floors and large fans have airflow rates in the one to one and one-half cfm/bu range. This is enough air to dry grain as a result of the drying ability of the natural air.
In many cases, aerated storage bins or drying bins can function as cooling bins for dryeration or in storage cooling. It depends on the airflow rate in the bin and the rate at which hot corn is delivered to it from the high-speed dryer.
In a dryeration bin, the air is moved upward so the first corn delivered to the bin is the first corn cooled. The tempering period is provided by delaying the start of the cooling fan at least 4 hours. The cooling fan is then started and the dryer continues to deliver hot corn to the bin. Cooling and drying continue at the same time. The hot corn delivered to the top of the bin should not be cooled before it has been tempered for at least 4 to 6 hours.
When the bin is full or drying is stopped it should take at least 4 hours to finish cooling the corn. Based on this operating procedure, the fan should deliver enough air to keep up with the dryer. If the cooling fan is perfectly matched to the dryer capacity, and the fan is started 6 hours after the start of the drying, (cooling and drying continue) cooling will be completed 6 hours after completion of drying.
A minimum airflow rate of 12 cfm for each one bushel per hour (bu/hr) of dryer capacity provides cooling capacity to keep up with the dryer. The cooling fan should be selected to deliver this airflow when the cooling bin is full. It is unrealistic to try to perfectly match the cooling rate with the drying rate. At the shallower depths, cooling proceeds faster than at the deeper depths because of the higher airflow. Various adjustments, based on experience, can be made to the fan starting and operating times to make sure that all hot corn goes through at least a 4-hour tempering period before it is cooled.
The airflow required is based on dryer capacity rather than bin size. A minimum airflow rate of 4,800 cfm would be required for a dryer delivering 400 bu of hot corn per hour to the cooling bin (12 cfm/bu/hr x 400 bu/hr). In 24 hours, this dryer would fill a 9,600 bu cooling bin (400 bu/hr x 24 hours). The cooling fan delivers 4,800 cfm when the bin is full, resulting in an airflow rate of one-half cfm/bu based on the full bin capacity. On the other hand, if this dryer is delivering to a 19,200 bu bin, it will take 48 hours to fill it and the same 4,800 cfm keeps up with the dryer. In this case the fan delivers one-quarter of a cfm/bu based on the full bin.
Although a full perforated drying floor is preferable for cooling bins, proper cooling can be accomplished with partial floors or duct systems. For a partial floor and/or duct system, adequate perforated surface areas and duct sizes are important. Air velocities must be kept reasonably low to prevent excessive static pressure build-ups and consequent reduction in fan performance.
The air velocities entering the grain through the perforated area should not exceed 25 to 30 feet per minute. To keep the velocities at this level, one square foot of perforated area must be provided for each 25 to 30 cfm delivered by the fan. In a duct or partial floor system, the velocities of the air should not exceed 1,500 to 2,000 feet per minute any place it moves through an opening, transition, or duct. This requires a free opening of one square foot for each 1,500 to 2,000 cfm moving through the duct or transition. Any restrictions (flanges, braces, floor supports, etc.) will increase the velocity and create an additional static pressure build-up.
The additional grain handling required with dryeration can sometimes lead to bottlenecks. There are three major grain transfer operations in a dryeration system: unloading wet corn, moving hot corn from the dryer to the dryeration bin(s), and moving cooled corn from the dryeration bin(s) to storage. This is one more transfer operation than is normally required with a conventional operation. Sometimes the extra transfer step can be handled by intermittent operation of the three steps with the existing transfer equipment. This may mean shutting down one or more transfer operations while the transfer of cooled corn from the cooling bin to storage is underway. In this case high transfer rates are required to avoid long periods of downtime for the other operation. In other situations extra materials handling equipment, such as a second leg, may be added, allowing for continuous transfer at slower rates. The extra transfer operation may be a particular concern where only one cooling bin is used since it is important to rapidly empty the cooling bin so the drying operation can continue. Each installation is different, but it is important to consider grain transfer in developing a dryeration system. It is also important to note that the capacity of some materials handling equipment such as augers decreases when moving wetter, hotter corn. Such corn may not flow well through downspouts at angles less than 45° F. This may be a potential bottleneck in moving hot corn from the dryer to the cooling bin.
In-storage cooling (figure 3) is similar to dryeration with one exceptiondelayed cooling to allow for tempering or steeping is not recommended. With in-storage cooling the corn is delivered hot to a storage bin equipped with adequate airflow, cooled in the bin and left in storage in the same bin. If hot corn is delivered to the bin and cooling is delayed as in dryeration, condensation may build up on the sidewalls and in the adjoining corn. If this condensation build-up occurs and the corn is left in storage, the wetter corn around the sidewalls may spoil. The seriousness of the problem depends on outside temperature. Cold weather causes more condensation, resulting in more spoilage when the bin walls warm up.
For in-storage cooling, the cooling fans must move enough air to keep up with the high-speed dryer. This means that the airflow of the cooling fan or fans is dictated by the maximum dryer capacity instead of the bin capacity. Dryer capacity will increase by 20 to 40 percent when in-storage cooling is practiced. The necessary airflow is basically the same as that recommended for dryeration12 cfm/bu of maximum hourly high-speed dryer capacity.
For existing bins equipped with aeration systems, the amount of hot corn that can be delivered to the bin in one hour is dictated by the airflow of the aeration fan. A typical storage bin equipped with aeration will have an airflow of about one-tenth cfm/bu based on a full bin. Thus, a 12,000 bu aerated storage bin should have an airflow of at least 1,200 cfm when the bin is full. As a result, hot corn could be delivered to the bin at a rate of 100 bu/hr (12 cfm/bu/hr x 100 bu/hr = 1,200 cfm). Over a full 24-hour period this amounts to 2,400 bu and the bin can be filled over a period of 5 to 6 days. If the bin is filled faster than this, additional airflow is necessary for cooling to keep up with the high-speed dryer. When storage bins equipped with aeration duct systems are used for in-storage cooling it is a good practice to deliver some cool corn to the bin first to help prevent possible problem areas between the ducts. Experimentation with a probe thermometer can provide the information for successful operation.
For in-storage cooling, the fan should be started as soon as hot corn is delivered to the bin and it should continue to run as long as there is warm corn in the top of the bin. Once the bin is cooled to outside temperature, the fan can be operated for aeration to lower the bin temperature below 35° F in the late fall and early winter. Procedures for fan management for aeration are discussed in the publication Management of Stored Grain with Aeration, FO-1327.
The general recommendation for grain storage aeration systems has been to move the air downward through the bin and exhaust it through the fan at the bottom. For dryeration and in-storage cooling, however, it is recommended that the air be forced upward through the bin and exhausted at the top. In most cases, the cooling fans will be operating while the high-speed dryer continues to deliver hot corn to the bin. If upward airflow is used, the corn in the lower part of the bin is not subjected to the heat and moisture removed from the last corn delivered in the upper portions of the bin. With upward airflow, there will be heavy condensation on the roof and upper portions of the bin walls (particularly in cold weather). In dryeration, when the cooled corn is transferred out of the bin, the small quantity of wet corn around the bin walls will be blended with the rest of the corn in the bin and will cause no storage problems. Sidewall condensation will not cause problems for in-storage cooling if the fans move enough air to keep up with the high-speed dryer and are started as soon as hot corn is delivered to the bin.
Proper management requires accurate checks on moisture content of the corn before and after it is cooled, and on temperatures of hot corn discharged from the dryer and cooled corn in the bin. A bin probe thermometer is useful to monitor cooling progress. Temperature cables are highly recommended for monitoring temperatures in bins of 20,000 to 25,000 bu or larger. An indoor- outdoor thermometer with one bulb located in the air exhaust from the top of the bin is helpful in checking cooling progress. However, a low reading on this thermometer does not necessarily mean the bin is cool. The corn in the top center of the bin will be the last to cool and should be checked with a bin probe thermometer. The fan should never be stopped as long as there is warm corn in the bin.
CAUTION should be exercised when inspecting bins during cooling of hot corn. The hot, moist air coming off the grain can be hazardous to breathe, and can potentially cause excessive dehydration leading to heat stress problems. No one should climb into the bin under these conditions. Care should be exercised even when looking through the door into the bin since metal surfaces may be slippery and eyeglasses may fog.
To gain experience in the management of a dryeration or in-storage cooling system, check corn temperature immediately after high-speed drying and in the bin during and after cooling. The best way to check corn temperatures after high-speed drying is to place a sample in a thermos bottle, insert a thermometer and read it as soon as it stabilizes. Leave the hot grain in the thermos until immediately before the next sample is taken. This keeps the thermos warm for the next sample.
Electrical moisture testers in common use do not yield reliable moisture contents on hot corn immediately after being discharged from the high-speed dryer. Some experimentation is necessary to obtain reasonably reliable results with these moisture meters. One method is to rapidly cool a sample of hot corn by forcing air through the sample with a small fan as illustrated in figure 5. A moisture correction is necessary because some moisture is removed during rapid cooling and because electrical testers may not give accurate readings on freshly dried cooled corn. The correction can be determined by using well-mixed duplicate samplesone rapidly cooled and checked, the other being sealed in a moisture-tight container, allowed to cool for 24 hours and then checked. The duplicate sample can also be checked by oven drying or the Brown-Duvel oil distillation test.
Checking the moisture content of the hot corn from the dryer is necessary in managing a dryeration or in-storage cooling system. However, it is also necessary to sample the corn in the bin after cooling to insure that the desired moisture content is reached.
Batch dryers, both automatic and manual are converted to dryeration by simply omitting the cooling cycle and unloading immediately after drying. In some staged batch dryers the proportion of heating and cooling time in the final stage can be adjusted. In these dryers the final stage is set for heat all of the time.
Continuous-flow dryers are generally more difficult to convert to dryeration. Probably the best way to convert most continuous-flow dryers is to add a burner unit to the cooling section. This will allow the dryer to be used for normal operation with in-dryer cooling by merely inactivating the burner in the cooling section. Some continuous-flow dryers use only one fan unit to deliver air to both the heating and cooling section. In these dryers a burner can be added to the air stream going to the cooling section. Sometimes removal of the divider between the heating and cooling sections and rearrangement of the diverter to the cooling section provides a satisfactory conversion for dryeration.
In some 2-fan continuous-flow dryers, particularly those with high airflows (in cfm/bu), satisfactory conversion may be achieved by eliminating the cooling fan and removing the divider between the heating and cooling sections. In low-airflow dryers this may reduce the cfm/bu too much. Also, since the airflow of the drying fan will be increased, the burner capacity may not be adequate.
In all cases conversion of continuous-flow dryers to dryeration should be done only by the manufacturer or after consultation with the manufacturer. The manufacturer should be able to provide recommendations on dryer performance and operation safety following conversion to dryeration.
With both dryeration and in-storage cooling, hot corn must be handled by augers, legs and downspouts out of the high-speed dryer. Condensation can occur in this equipment, particularly in colder weather. Condensation in the trunk of a bucket elevator may run down into the boot and freeze. Equipment should continually be checked for serious condensation accumulations. Periodic cleaning of elevator boots, hoppers, and other equipment will help minimize these problems.
When hot corn is delivered to a bin through a downspout, warm moist air may move up the downspout and condense. This condensation may run back down into the bin or, if condensation occurs in the elevator head, it may run into other bins through their downspouts. A manual- or gravity-operated flapper valve at the end of the downspout near the bin to prevent air moving up the spout will help minimize the problem.
Downspout condensation problems can be eliminated by installing exhaust fans in the roof of the bin that will move 25 to 50 percent more air (at one-eighth to one-fourth inch of water static pressure) than the cooling fan. The exhaust fans, operating at a slight negative pressure (one-eighth to one-fourth inch of water) will keep the top of the bin under a slight suction thereby preventing warm moist air from moving up the downspouts.
Another solution could be to pressurize the downspouts using a small fan to force air down the spout into the bin.
|WARNING: Flowing Grain is Dangerous
Never enter a grain bin or other grain storage area while the grain is flowing. Flowing grain will exert forces against the body great enough to pull the average size person under the grain in only a few seconds leading to death by suffocation.
Development of these publications was partially supported by the Minnesota Energy Agency under an Energy Policy and Conservation Act (P.L. 94-163) grant.
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