This dryer is the simplest dryer. It is usually designed with a storage bin having a perforated floor and a blower to move air through the grain. If the grains just harvested from the field is less than 20 - 22% moisture content, the grain is usually dried with just ambient air, so long as there is enough air flow. The grains must be screened before filling the bin and drying in layers of 0.06m3 to 0.09m3 at a time will decrease drying times. Ideally, if multiple bins are available, each bin can be filled with a layer of grain before adding more to the first bin filled. During a fall with low humidity, ambient drying will have the highest energy efficiency although it will all be electrical energy. During a fall with damp weather, the grain may not be dried down to storage moisture until spring which will greatly reduce the energy efficiency. If the weather is warm and the grain is high in moisture (20 %+) spoilage can be of concern. To reduce the risk of spoilage and long drying times, a small amount of heat can be introduce with a small resistance heater or a solar collector.
Generally the air temperatures are increased up to a maximum of 10°F by these methods while drying times can be reduced significantly especially during damp fall weather where relative humidity exceeds 80%. The basic principle behind the operation of a batch-in-bin dryer is to force relatively large quantities of air through a shallow grain depth so as to obtain a rapid drying speed, thus allowing the producer to accommodate larger harvest rates with other types of in-bin drying.
One reason for the popularity of batch-in-bin drying is the flexibility available when selecting the drying system equipment. Management of a batch-in-bin system is less
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critical than in other in-bin drying units since the operator has an opportunity every 24 hours to correct any mistakes. This also allows for some fine tuning of the drying process. The main disadvantage of a batch-in-bin drying system is that the grain must be handled twice. The drying bin must be leveled and unloaded each day; this requires placing the sweep auger in the drying bin at each unloading (unless power sweeps are used) and frequent moving of any portable handling equipment, unless a second auger was purchased to be used exclusively for unloading the drying bin 2.5.4.1 Recirculating Bin Dryer
These are typically small portable dryers that have a central plenum with a column of grain encircling the plenum. They can be manually or automatically loaded and unloaded. Some are powered from a tractor PTO which allows them to be used almost anywhere. The major disadvantage is they require more supervision and labour per bushel than other types of dryers. A well operated batch dryer has approximately the same energy efficiency as a continuous cross flow dryer with cooling section heat recovery. The recirculating bin dryer incorporates a tapered sweep auger which removes grain from the bottom of the bin as it dries (Figure 2.9).
The sweep auger is usually controlled by temperature or moisture sensors. When the desired condition is reached, the sensors start the sweep auger, which removes a layer of grain as it rotates. After one complete revolution around the bin, the sweep auger stops until the sensor determines that another layer is dry. The process continues repeatedly until the entire grain is completely dried, after which the dried grain is redistributed on top of the grain surface. The dried grain will be partially rewet by the moist air coming through the grain, which reduces drying efficiency.
After all the grain has been dried, the grain is cooled in the bin. The dried and cooled grain is then moved to storage or may be left in the bin. It is common to dry the last bin full of grain using a continuous flow bin dryer as a recirculating bin dryer.
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Figure 2.9. High and Low levels of air recirculation grain dryers.
2.5.4.2 Continuous Flow Bin Dryer
The continuous flow bin dryer is designed to have a tapered sweep auger whose function is to remove grain from the bottom of the bin as it dries, but the grain is moved to a second bin for cooling (Figure 2.10). Up to 2 points of moisture may be removed in the cooling bin if dryeration is used. (Dryeration is a process where hot grain is removed from the dryer with a moisture content 1 or 2 percentage points above that desired for storage.) Usually, increasing the grain depth will reduce the airflow rate, and the drying rate of a continuous flow bin dryer. In a recirculating batch or continuous flow bin dryer, it is the total airflow capacity that determines the drying rate, not the airflow rate. In this type of dryer, the heat recirculation system guides warm exhaust air directly back into the dryer fan inlet. It works best on continuous flow horizontal or tower dryers operated to dry and cool simultaneously.
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Heat Recirculation Performance for a single-stage continuous cross-flow grain dryer, the temperature and relative humidity (RH) of the exhaust air that is warm (at least 20°C-30°C above ambient) with low humidity (40% or less) can be recirculated to reduce energy use. Generally, this is the exhaust air from the lower half of the dryer (including the cooling section).
Fig. 2.10: A heat recirculation system on a two-fan continuous flow dryer operating in the heat/cool mode. (Source: Ontario ministry of Agriculture, Food and Rural Affairs. http://www.omafra.gov.on.ca)
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