 I am Ken Hellevang, Agricultural Engineering Specialist with the NDSU Extension Service and a professor in the Agricultural and Biosystems Engineering Department at North Dakota State University. The focus of this presentation is energy efficient grain drying. During the presentation I will indicate if there are variations between the types of grain. Recommended long term moisture contents permit storing grain at warmer temperatures without grain deterioration due to mold growth. Mold growth starts at about 65 to 70% relative humidity. The maximum recommended moisture content is the grain moisture content in equilibrium with air at about 70 degrees and 60% relative humidity. The recommended long term storage moisture content for corn is about 13% for oil sunflower is about 8% and for soybeans is about 11%. Many recommendations for flax indicate a moisture content of 7% but based on equilibrium moisture content a moisture content of 8% should be adequate. The amount of drying in the field depends on parameters such as corn maturity, hybrid, moisture content, air temperature and relative humidity, solar radiation and wind speed. A predictor of the drying rate is potential evapotranspiration or PET which is based on parameters similar to those that affect drying. Values for PET are available on sites such as the North Dakota Agricultural Weather Network. About 1 inch of potential evapotranspiration results in about 4 percentage points of corn field drying. Field drying is extremely slow during cold winter months and the corn will dry only to about 20 or 21% moisture content based on the equilibrium moisture content for average monthly air temperature and relative humidity conditions. Corn that remained in the field during the winter of 2008 and 2009 for example dried from 25% to 30% initially in November to 17 to 20% when harvested in February and early March. Natural air drying is very energy efficient if the system is properly designed and managed. It is not efficient or effective on very wet grain or at November to March temperatures. The moisture holding capacity of air is related to its temperature therefore natural air drying becomes inefficient as outdoor temperatures average below about 40 degrees. The power required to move air flow through the grain increases rapidly as the air flow rate increases. So enough air flow to dry the grain before it deteriorates is necessary but the system becomes inefficient at higher air flow rates. Typically the maximum corn moisture content that can be efficiently dried is about 21% with a minimum air flow rate of 1 CFM per bushel. Natural air drying corn will work very well in October drying the corn to about 13.5% moisture within about 42 days using an air flow rate of 1 CFM or cubic feet per minute per bushel based on North Dakota average climatic conditions. The fan heats the air about 3 degrees assuming a static pressure of about 4 inches water gauge. Starting to dry on October 15 rather than October 1 increases the final corn moisture content to 15.8% and lengthens the drying time from 42 days to 65 days using an air flow rate of 1 CFM per bushel. Use local climatic conditions in making drying decisions. Much of the corn in North Dakota is not harvested until the end of October or even later. Because the air temperature is 20 degrees colder in November the moisture holding capacity of the air is about one half of what it was in October. Therefore the drying time is approximately twice as long. It will take about 70 days to dry corn during November instead of the 42 days in October using an air flow rate of 1 CFM per bushel. Of course there are only 30 days in November so only about one half of the corn is dried during the month. Also the relative humidity is higher so the corn that is dried will only dry to about 16% moisture. Warming the air by 5 degrees in November 2 degrees in addition to the 3 degrees from the fan reduces the drying corn moisture to about 14.6% moisture in the drying time to about 65 days. Warming the air by 10 degrees reduces the corn moisture to about 12.5% so the corn is dried to a moisture content lower than desired. Also the drying time is still about 51 days with an air flow rate of 1 CFM per bushel. Warming the air by 10 degrees reduces the drying time from 70 days to 51 days but also reduces the corn moisture content from 16% to 12.5%. The corn is dried to a lower moisture content than desired and the drying time is still nearly 2 months. This is a very expensive method of drying the corn. Natural air drying becomes inefficient at temperatures below 35 to 40 degrees. At temperatures below about 35 degrees it is better to cool the corn to about 20 degrees, store the corn over winter and dry it in the spring starting when temperatures average about 40 degrees. Natural air and low temperature corn drying works well in the spring. Corn will dry to about 15% moisture during April and 13% during May based on average North Dakota conditions. Actual final moisture content will be lower due to the fan heating the air. Start drying when the average outdoor temperature is about 40 degrees. Drying will take 40 to 50 days. The maximum recommended moisture content for natural air and low temperature corn drying is 21 to 22%. Higher moisture corn is expected to deteriorate before it is dried so it should be dried in a high temperature dryer. The type of fan used affects the quantity of air moved through the corn. Shown in the table are the air flow quantity, air flow rate and operating static pressure that various types of 10 horsepower fans will move through a 21 foot diameter bin with a corn depth of 20 feet. The axial flow propeller fan delivers an air flow rate of 1.07 CFM per bushel. The inline centrifugal fan delivers an air flow rate of 0.98 CFM per bushel. The low speed centrifugal fan delivers the most air flow of the four fan types. 1.41 CFM per bushel. It is generally the fan type that will provide the most air flow per horsepower for typical corn drying depths. The high speed centrifugal fan delivers the least amount of air flow, 0.99 CFM per bushel. When two types of grain will be dried in the same bin, a fan will need to be selected that provides adequate air flow for drying both types of grain. Generally the fan that delivers the most air flow through wheat and corn is the low speed centrifugal fan. A fan that delivers adequate air flow through wheat cannot be assumed to move an adequate amount of air through corn. Frequently the inline centrifugal and high speed centrifugal fans may have been adequate for wheat but will not be adequate for corn. Some people will estimate the fan size needed using a rule of thumb of 1 horsepower per thousand bushels of corn. This is only true for an air flow rate of 1 CFM per bushel and a corn depth of about 19 feet. It is best to use a fan selection program or tables to select the fan horsepower needed. Generally an air flow rate of 1 CFM per bushel is selected which will dry corn at moisture contents up to about 21% before deterioration occurs. The speed of drying is related to the air flow rate. Drying is too slow at low air flow rates to dry the grain before deterioration occurs. However to double air flow rate requires increasing fan horsepower by a factor of 4 to 5. Therefore high air flow rates are not practical because the fan horsepower required becomes excessive. Many people desire to dry corn using a natural air and low temperature drying system in a tall bin. This too is not feasible. For example a 180 horsepower fan would be required to provide an air flow rate of 1 CFM per bushel for a 42 foot diameter bin of corn that is 36 feet deep. Not only is the fan horsepower required not practical but the operating static pressure would be about 17 inches. Which exceeds the capability of common grain drying fans. The maximum depth of wet corn that can realistically be dried is about 22 feet. For wheat the recommended maximum depth is about 18 to 20 feet. High temperature in bin drying can be an energy efficient method of corn drying. Some type of stirring is required to prevent over drying the corn at the bottom of the bin. The moisture content variation will be less than about 1 percentage point using stirring. The maximum drying rate will be obtained with a corn depth of about 6 feet. Greater depths reduce the air flow moved by the fan and therefore the drying rate. Drying a full bin permits larger batches so this is frequently done even though the drying rate is less. Use the highest drying temperature that will not damage the grain to obtain the most energy efficient drying. Typical drying temperatures are up to about 160 degrees for corn. Another version of high temperature in bin batch dryer is shown on this slide. The heated air coming from cooling the grain mixes with the heated drying air. This heat reclaim improves the energy efficiency of the dryer. There is a moisture variation across the batch of grain. The grain is removed when the desired average moisture content is obtained. The energy consumption should be similar to a cross flow dryer with energy efficient features. A continuous flow bin dryer is shown on this slide. This system removes the dried corn from the bottom of the bin and transfers it to an adjacent bin for cooling. Again use the maximum air temperature and limit grain depth to maximize dryer capacity and efficiency. Drying air comes in contact with the driest corn so the drying air temperature must be limited to prevent grain damage. A traditional cross flow dryer requires about 2,500 BTUs to remove a pound of water from corn. Frequently dryers with energy efficient features are compared to the traditional cross flow dryer when comparing the energy efficiency of the dryers. There is considerable variation in grain temperature and moisture content across the drying column of a traditional cross flow dryer. The energy efficiency of a high temperature dryer is improved by using the highest allowable drying temperature that will not damage corn quality. Most cross flow dryers will have an air flow rate of about 70 to 90 CFM per bushel. The energy required to remove a pound of water is about 4,000 BTUs at 150 degrees and only about 2,800 BTUs at 200 degrees with an air flow rate of 75 CFM per bushel. A mixed flow dryer typically has an air flow rate of 40 to 45 CFM per bushel. Even though this graph is for a cross flow dryer, the energy efficiency benefit of using a lower air flow rate can be observed. The lower air flow rate will typically be associated with a slower drying rate. Use the maximum drying temperature that will not damage the grain to reduce the energy requirement to remove the moisture and increase the drying capacity. The simplest cross flow dryer is a batch dryer. The grain is dried with heated air going through the grain until it is dry followed by cooling with unheated air going through the grain. As was shown in the diagram of a cross flow dryer, there is a variation in grain temperature with grain near the inside wall approaching the plenum air temperature. Because of this, the drying temperature must be limited to minimize grain damage which results in a lower energy efficiency. There is a potential for a loss in grain quality to the corn near the inside of the drying column due to excessive grain temperatures. Continuous flow dryers with pressure heat and pressure cooling use heated air for drying in the upper portion of the dryer and unheated air in the lower portion for cooling. The same concerns of the grain temperature near the inside wall approaching the plenum air temperature exist in the continuous flow dryer as it did in the batch dryer. Because of this, the drying temperature must be limited to minimize grain damage which results in a lower energy efficiency. There is a potential for a loss in grain quality to the corn near the inside of the drying column due to excessive grain temperatures. Some cross flow continuous flow dryers have the option of adding heat for drying in all of the dryer. After drying, the corn is transferred to a cooling bin that has a perforated floor and adequate air flow for rapid cooling. Some of the moisture is removed during cooling in the bin. Condensation may be a problem when the warm air contacts a cool bin roof and wall. Since the entire dryer is used for drying, this type of dryer will provide the best drying capacity for the least dryer purchase cost. The cost of cooling fans and bins must be considered in the total system cost. Since some moisture is removed during cooling in the bin, corn can be removed from the dryer at a moisture content slightly above the final desired moisture content. By removing some of the moisture during cooling in the bin, there is an improvement in dryer energy efficiency. Also, in bin cooling, typically will reduce the amount of stress cracks in the corn, improving the corn quality in comparison to a cross flow dryer with cooling in the dryer. Another option for a full heat dryer is to use a lower temperature in the lower portion of the dryer. This improves the grain quality, but there is a loss in energy efficiency due to the lower temperature in the bottom portion of the dryer. However, the top portion can be operated at a higher temperature, which may compensate for the loss in the lower portion. Since this is a variation of a full heat dryer, the items discussed for a full heat dryer apply to the two heat dryer as well. Some type of heat reclaim on the cooling portion of the dryer captures the heat removed from the corn while cooling it, and that captured heat is used to preheat the air for the drying portion of the dryer. Heat reclaim will improve the energy efficiency of the dryer by at least 20% in comparison to a heat and cool dryer. The drying capacity is less than a full heat dryer, but since the corn is cooled in the dryer, only a normal aeration system is required in the storage bins. In addition, less management is required in comparison to in-bin cooling. Suction or vacuum cooling is available in both tower dryers and horizontal high temperature dryers. The expected increase in drying energy efficiency is about 20%. Dryers that reclaim air coming from the final portion of the drying column in addition to suction cooling have the best energy efficiency of the cross flow dryers. There will be a small reduction in drying capacity due to the small amount of moisture in the reclaimed drying air. Since the air is coming through nearly dry corn, the reduction in drying capacity is small. The grain exits the dryer within 20 degrees of outdoor temperature, so normal aeration in storage is adequate to remove the remaining heat. This table from one manufacturer shows their estimate of drying cost for the various types of dryers. It shows the expected reduced cost or improved energy efficiency for the full heat, vacuum cool, and vacuum cool with heat reclaim. There is little difference between the full heat within bin cooling and the dryer with vacuum cooling. The most energy efficient dryer incorporates the heat reclaim and the vacuum cooling. There are a variety of features that improve dryer energy efficiency. Using a higher temperature on the wettest grain in the upper portion of the dryer improves energy efficiency since energy efficiency increases with drying temperature. The higher temperature does not affect grain quality due to evaporative cooling. Some dryers use a grain turner or inverter to move the grain from the inside of the drying column to the outside of the column. A grain turner or inverter reduces over drying, moisture variation, and excessive kernel temperatures. It also permits using a higher dryer temperature which improves energy efficiency. Another approach to reducing excessive grain temperatures and creating a more uniform grain moisture content is removing the grain more rapidly near the inside of the grain column. This dryer has two metering rolls with the inside removing grain more rapidly than the outside metering roll. This too permits using a higher drying temperature without damaging the grain. Still another approach is using what one company calls a moisture equalizer. Again grain on the inside of the drying column moves more rapidly than the outside. This also permits using a higher drying temperature which improves energy efficiency and dryer capacity without damaging grain quality. Company tests showed that corn test weight was higher in their dryer with the moisture equalizer than without. In a recirculating batch dryer the grain is mixed during drying creating a more uniform grain temperature and moisture content. The continual handling of the grain increases the potential for mechanical damage. The recirculating batch dryer generally has a wider grain column than a cross flow dryer which gives more time for the air to absorb moisture from the grain. Due to the longer grain retention time in the dryer the typical drying temperature is lower to maintain grain quality. The lower plenum temperature results in a lower energy efficiency. A mixed flow dryer uses a lower air flow rate per bushel than a cross flow dryer which increases energy efficiency. A common air flow rate is 40 to 45 CFM per bushel or cubic feet per minute per bushel. The mixed flow dryer is about 20% more energy efficient than the basic cross flow dryer. The energy efficiency of the mixed flow dryer is similar to that of a cross flow dryer that incorporates energy efficiency features. The moving grain design minimizes exposure to the hot plenum air which reduces the potential for grain damage. Dry when it is warmer if possible since it takes more energy to dry at colder temperatures. The energy cost for high temperature corn drying is about 22 cents per bushel at 40 degrees and increases to about 25 cents per bushel at 20 degrees. The energy cost is based on a $1.80 per gallon propane and a normal dryer efficiency. A dryer with air recirculation is more energy efficient and will reduce the drying cost. Air recirculation captures the heat from the cooling portion of the dryer. The table shows the amount of energy required to evaporate a pound of water from grain. At 40 degrees it takes 1,070 BTUs per pound to evaporate liquid water. It will take more energy to evaporate water from the grain. The minimum amount of energy required to evaporate water from corn is about 1,200 BTUs per pound of water. A realistic minimum for a grain dryer is probably about 1,500 BTUs per pound. The actual energy efficiency depends on many factors. An energy efficiency of 1,800 to 2,000 BTUs per pound is a reasonable expectation for an energy efficient dryer, but it will frequently take more energy than that to dry the grain. Combination drying is an energy efficient drying system that utilizes the high temperature dryer to remove some of the moisture and uses the energy efficient natural air or low temperature drying to remove the final moisture. It requires more energy to dry corn from 17% to 15% moisture than from 25% to 23%. Combination drying uses the more energy efficient natural air drying to remove the final moisture, which is more energy intensive drying. Combination drying greatly increases the capacity of the high temperature dryer since only some of the moisture is removed and the corn is not cooled in the dryer. Combination drying requires the bin is equipped with adequate airflow for natural air drying and that the climate permits completing drying by the desired time. Dryeration is energy efficient because the most difficult moisture to remove when the grain is nearly dry is removed by the heat stored in the kernel during the cooling rather than in the dryer. Dryeration reduces energy consumption by about 20 to 25%, increases the drying rate of the dryer at least 50%, and reduces the potential for stress cracks in the corn kernel. Dryeration is a process where grain is removed from a high temperature dryer at temperatures near 130 degree kernel temperatures without cooling. With a moisture content about 2 to 2.5 percentage points above the desired storage moisture content. The hot grain is placed in a dryeration bin where it is allowed to temper without cooling for at least 4 to 6 hours. Then the grain is cooled in the dryeration bin and about 2.5 percentage points of moisture are removed by the airflow before it is moved to final storage. The expected moisture removal is about one quarter of a percentage point for each 10 degrees that the corn is cooled. Condensation will be extensive during the tempering and cooling period so the grain must be removed to a different bin for storage to prevent grain spoilage. Wet grain mixes with dry grain during the transfer to storage. Within storage, cooling grain is dried to within one to one and a half percentage point of the desired final grain moisture content in the high temperature dryer. Then it is transferred to the storage bin and immediately cooled. Condensation will cause storage problems unless the corn is cooled at nearly the same rate as the bin is being filled. An airflow rate of at least 12 CFM or cubic feet per minute per bushel times the per hour fill rate will cool the corn at that fill rate. For example, if hot corn is being added to the bin at a rate of 500 bushels per hour, an airflow rate of 12 times 500 equals 6000 CFM of airflow will cool the corn at that fill rate. This should be considered a minimum in storage cooling airflow rate. Farmers have found condensation to be a problem even with adequate airflow when outdoor temperatures are less than about 50 degrees. So it is recommended to cool the corn to about 90 or 100 degrees in the high temperature dryer to reduce the condensation potential when outdoor temperatures are cool. A little moisture removal will occur during cooling, but it will be much less than that which is obtained by using dry aeration. More information is available on the NDSU grain drying and storage website. The website can be found by doing an internet search for NDSU grain drying and storage.