Types of Seed Dryers for Heated Air Drying
The heated air drying can be further broken in the following categories.
1. Layer- in- bin dryer
2. Batch- in – bin dryer
3. Batch dryer
4. Continuous dryer
1. Layer- in – bin dryer:
In this method, the bin is filled to a specific depth depending upon seed moisture, the drying unit and bin sizes. This depth is dried down to a state moisture content for storage, before the next layer is added. The drying air is usually controlled by a humidistat set at 55 percent to prevent over drying of the lower layers. Although this method of drying is fairly slow, the seed is uniform dried between the top and bottom of the bin.
The compounds that make a bin perform as a dryer are fan, heater, operating and safety controls , transition , duct, foundation, ring , false floor , seed leveller, sweep unloaded and unload auger. The layer-bin- dryers range from 21 to 40 feet diameter and 5 to 20 horse power. It is the most efficient but slowest drying method. After the initial layer, fills must be separated by three to four days while each new layer dries.
2. Batch- in – dryer:
In this method of drying, the quantity of seed at a given moisture content can be dried with a given drying unit. The high moisture seed is placed in a drying bin, dried and cooled and removed to a storage bin.
The drying equipment used as similar to that of layer drying; the main difference being in higher capacity heaters and fan, grain levellers, sweep unloaders and unload auger equipment.
Seed depths are typically 2.5 to 4 t; the deeper , the seed , depth , the lower the air flow and the slower the drying.
3. Batch Dryers:
Batch dryers are bins with an inner air chamber surrounded by two parallel perforated steel walls to contain a designed thickness of seed. The fan heater unit is connected to one end, or side of the plenum as heated air for drying and natural air cooling can be forced through the seed. Batch dryers, generally rectangular or cylindrical, are usually described by the volume in bushes that a dryer will hold per fill e.g. 300 bushel dryer. Fan power ranges from 3 to 30-40 horse power. The number of batches per day may be eight to ten for small dryers but only two or three for large units. Dryer capacities are rated on average moisture removed of ten points.
All batch dryers depend on fast handing. Wet holding capacity is needed to avoid seed receiving, dry seed transfer, and dryer operating cycles.
4. Continuous Dryers:
Continuous flow dryers move seed through heating and cooling sections of the column continuously. Seed flow is controlled by volume metering devices at the lower side of the seed column. Heated air is forced through the upper 2/3 to 3/4 of the seed column at 100 to 150 CFM/bu. Continuous dryer sizes are usually given in bushel/hour capacity at 10 points moisture removal. Power needs are from 7 ½ horse power to 60 horse power. Some dryers use multiple sequence start fan motors to keep starting loads low.
Wet holding is necessary for both one and two leg system. With one leg, seed from the dryer is accumulated in a dry holding bin for periodic manual transfer. With two legs a low capacity elevator leg moves dry speed to storage continuously.