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Drying Mechanism for Rice

This article features instructions on how to build a fixed batch heated air drying bin from local materials. For further information on rice farming and appropriate practices, see Rice.

Methods of Drying

Field Drying

• Rice is left to dry in fields
• Very low relative cost
• Advantages: Loosens panicles for ease in threshing
• Disadvantages: Very poor quality rice, cannot dry in rain (eg. wet season) or at night^[1]

Sun Drying

• Rice dries on pavement or drying mats by heat from the sun
• Very low relative cost
• Advantages: Better quality than field drying, relatively inexpensive
• Disadvantages: Poor milling quality, labor intensive, cannot dry in rain (eg. wet season) or at night^[1]

Heated-Air Drying

Fixed-Bed Dryer

• Heated air flows through batch
• Low relative cost ($150 for circular batch – also can be considered in-store dryer due to long drying time, see below – and $1500 for flat bed)^[2][3][4]
• Advantages: Simple, inexpensive, fast drying
• Disadvantages: Reduction in milling yield (due to re-wetting of dry grains), risk killing seeds, labor intensive^[1]

Re-circulating Batch

• Rice moves through sections that alternately dry and temper it
• Medium relative cost ($10,000)^[3]
• Advantages: Good quality yield, large capacity, low labor for mixing, lower moisture gradient
• Disadvantages: Requires servicing and skilled laborers, moving components^[1]

Continuous Flow Dryer

• Rice and air flow in two distinct directions
• High relative cost (large scale productions)
• Advantages: Large capacity
• Disadvantages: Complicated mechanisms^[1]

In Store Drying

• Low temperature deep bed drying
• High relative cost (large scale productions)
• Advantages: Energy efficient, large bins, excellent grain quality, dries in storage
• Disadvantages: Very long drying time, need bulk handling system, risk spoiling with faulty power supply^[1]

Additional Notes

Research done into enhancing sun drying with solar collectors has shown that the sun cannot be intensified enough to increase drying time enough to justify the added cost. Also, just like sun drying, solar collectors cannot be used to dry rice at night or in the rain.^[1]

Economics of Drying

There are several apparent economic benefits to using mechanical dryers to dry rice. These include an increased market value of the higher quality product, more secure income from reduced weather risks, and the ability to process more grain in a given amount of time. In addition, most markets sell rice by weight or by volume. Drying reduces both the weight and volume of the paddy.^[1]Generally, in order to justify the cost of drying rice mechanically, the increased profit due to higher rice quality must outweigh the cost of installing and running the machine as well as the loss in weight or volume of the paddy. Taking into account other economic considerations including maintenance, depreciation, and interest, it is very difficult to break even over the lifespan of a mechanical dryer in small-scale operations.

Flat bed fixed batch dryers (heated air drying fixed batch flat bed dryers) are the most commercially successful mechanical dryers and are most often used in small to medium sized rice mills. Although they may not offer the best technical solution to drying rice, they seem to balance the cost and profit requirements well. The Vietnamese Flat-Bed Dryer (over 6000 installations at a cost of $1500) has a capacity of four to ten tons and can dry the rice in six to eight hours. The IRRI Flat-Bed Dryer ($1000) has a capacity of one ton and can dry the rice in six to eight hours. According the IRRI's website, the dryer has been installed thousands of times since the 1970's however admits that the design is very outdated.^[3][1]

The circular batch dryers (low temperature deep bed dryers) have experienced modest success. Examples include the IRRI seed dryers and the Vietnamese Low Cost SRR and SRT dryers. Although relatively inexpensive ($100-$150), they have small batch sizes (maximum one ton) and take one to two days to dry the rice.^[3][1]

As a general rule, a dryer should reduce moisture content at a rate of one percent per hour to be considered efficient. For example, if you wish to reduce the moisture content of your paddy from 22% to 13% (for a storage time of eight to 12 months), your dryer should be able to dry the rice in nine hours.^[1]

Design

The goal of this design was to create a dryer that could maintain rice quality at a low capital and operating cost yielding a reasonably high dry time while remaining accessible to small-scale processing. The fixed batch dryer produces good rice quality and has a fast drying time, however it has a high capital and operating cost. These costs are primarily associated with the type of material used, fuel, and complexity of the machine (requiring skilled laborers to repair). Because of the number of units sold of the fixed bed dryers compared to the circular batch dryers, the fixed bed dryer is obviously preferred. Therefore, this design sought to optimize the current fixed bed dryer by incorporating successful features of the circular batch dryer.

Larger scale fixed bed batch dryers have a 4-10 ton capacity and are fueled by large furnaces, while the smaller scale circular batch dryers are fueled by small coal, wood, or rice husk fires. Fixed bed batch dryers, due to their large capacity, are often made of metal while the circular batch dryers are often made from less expensive local materials such as bricks and bamboo. Both utilize a metal wire mesh to keep rice in place while allowing warm airflow through batch. They also both use inexpensive axial fans to propel air through the system.^[5][6][7][4]The redesigned fixed bed dryer is made from less expensive local materials, similar to the circular batch dryer. It is also fueled by burning biomass in the form of rice husks.^[8]This decreases operating costs and, along with the appropriate heat transfer system, produces a higher temperature than the circular batch dryers.

Circular batch dryers dry the rice at a small temperature differential while fixed bed dryers dry rice at a higher temperature (optimal at 43^oC). This allows circular dryers to have a much smaller moisture gradient. A high moisture gradient in the batch allows for re-wetting of rice with lower moisture content, which reduces rice quality. To reduce the moisture gradient in the fixed bed dryer, the rice is not to be at a depth greater than 70 cm.^[9][10]

Evaporation of Water

Rice drying is characterized by an uneven evaporation of moisture. In the preheating period, most of the heat supplied to the rice is used to heat the rice up, so very little evaporation occurs. Next, in the constant-rate period, the rice is fully heating and water is evaporating from the surface of the grain. Next, in the falling-rate period, the moisture from the surface of the grain is nearly all evaporated and moisture from the interior of the grain must make its way to the surface before evaporating. This drastically slows the rate of evaporation and causes the rice grains to further heat up.

在设计该装置时，计算蒸发速率以估计不同设计的干燥时间。大米被认为具有恒定的蒸发速率。然而，由于下降速率周期，该假设导致计算的干燥时间略低于达到所需水分含量的应有时间。

大型水池中水的蒸发速率由下式给出：^[11]

[math]\displaystyle{ \text{g=kAx（1-h）}\，\！ }[/数学]

和

[math]\displaystyle{ \text{k=（25+19v）}\，\！ }[/数学]

哪里

• g是蒸发的水量，以千克/小时为单位
• k 是蒸发系数，单位为千克/平方米h
• v 是风速，单位为 m/s
• A是面积为m2
• x 是表面温度（43 摄氏度，实现最佳干燥）在 kg 水/kg 空气中的饱和湿度
• h 是环境空气的相对湿度

从大米中除去所需的水量由下式给出：

[math]\displaystyle{ \text{d=m（MCo-MC）}\，\！ }[/数学]

哪里

• d是从大米中除去的所需水量，以千克为单位
• m是大米的初始质量
• MCo是初始水分含量（通常在20%和25%之间）
• MC是所需的水分含量（通常在9%至14%之间）

总干燥时间由下式给出：

[math]\displaystyle{ t=\frac{dMCo}{g} }[/math]

哪里

• t 是总干燥时间（以小时为单位）

为了干燥一吨大米，重新设计的固定床间歇式干燥机在43摄氏度的理想干燥温度下从22%到13%的水分含量，在60%相对湿度下以1米/秒的平静微风，1x3米干燥机的干燥时间为7小时。

热损失分析

下图显示了通过长度为L且内径为r1和外径r2的管道的热损失的方程和图表，该管道具有传导传热系数为k，其中包含温度为Tinf1和对流传热系数为h1的热流体，周围是温度为Tinf2和对流传热系数为h2的冷流体。Ts1 和 Ts2 分别指管道在内径和外径上的表面温度。^[注12]

不锈钢 AISI 304 的传导传热系数 （k） 等于 14.9W/mK。钢比不锈钢便宜，因此将用于设计，但是将假设它们的k值大致相等。在气体的自由对流中，对流系数（h）通常在2到25 W/m2K之间。在气体的强制对流中，对流系数通常在25至250 W/m2K之间。^[12]对于此应用，h1将被假定为50 W / m2K，因为风扇引起的空气通过管道的轻微强迫，h2将被假定为5 W / m2K，因为管道将暴露在平静的风中。一根钢管在环境空气温度为30摄氏度，含有温度为525摄氏度的加热空气（冷生物质火灾假设），内径和外径分别为20厘米和25厘米，外管表面在88摄氏度的温度下将遇到1.7千瓦/米的热损失。

为了在43摄氏度下冷却一吨大米，必须用0.7-1.0 W的风扇将14 W热量的空气推入稻田。^[1]如果没有关于生物质火灾产生的热量（以瓦特为单位）的信息，很难确定干燥温度是否会达到所需的水平。然而，廉价的圆形干燥箱目前使用类似的加热系统来产生略高于环境空气温度的干燥温度。此外，管道还有一个额外的90度弯曲，有可能减慢空气，允许更多的热量通过管道损失。考虑到这些因素，系统可能不会产生高于所需水平的温度。在高于43摄氏度的温度下干燥大米会对大米的质量产生不利影响。^[注1]

最终设计

Please note: THIS DESIGN HAS NOT BEEN TESTED

Please refer to the IRRI Example Dryers site to view pictures of the described dryers. The first picture is of the IRRI STR/SRR dryer and the second is of the Vietnamese flat-bed dryer. This new design combines the higher grain quality and fast drying time of the flat-bed dryer and the inexpensive materials and heat source of the circular batch dryers. Frequent mixing of the shallow bed of drying paddy will help reduce the high moisture gradient usually associated to flat bed dryers.

• Advantages: Mixing allows rice to reach a more constant moisture (lowering the moisture gradient) which improves the milling quality, mixing also promotes airflow which increases the velocity over the grains and prevents the heat chamber from becoming too hot (which drastically reduces rice quality), inexpensive and fast-drying
• Disadvantages: No substitute to two-stage dryer, temperatures may peak above the ideal 43 degrees Celsius if mixing is not frequent enough, labor intensive, still slightly more expensive than circular dryer, filter may miss some smoke or other combustion by-products creating smoky rice, power supply required (unlike field and sun drying)
• Notes on Use: Fill top drying bin with 50cm of rice. This reaches the dryer's capacity of one ton. Supply fuel basin with burning rice husk to keep a constant heat source.
• Future work: The fan pulls air through system but makes flow difficult to analyze. A CFD and heat transfer simulation would test assumptions and design validity.

Cost

• Fuel basin, pipe, fan, and cloth filter - $100 (estimated)^[13]
• Lumber - $25 ^[14]
• Wire mesh - $70 ^[15]
• Bricks - $20 (estimated)

Total estimated cost - $215

Required Materials

• Fire basin (approximately 50 cm diameter and 50 cm high) supported by bricks (A)
• Steel piping between 20 cm and 30 cm diameter with 50 cm opening (B)
• Axial flow fan (0.7-1W) (C)
• Smoke filter (same diameter as aluminum piping) (D)
• Plywood (1/2 inch thick)- 2 x 1 m x 0.3 m (E)
• Plywood (1/2 inch thick)- 2 x 1 m x 0.7 m (F)
• Plywood (1/2 inch thick)- 2 x 3 m x 0.3 m (G)
• Plywood (1/2 inch thick)- 2 x 3 m x 0.7 m (H)
• Plywood (1/2 inch thick)- 8 x 0.3 m x 0.1 m cut at 45 degree angles at ends for bracing (I)
• Woven wire meshing or perforated metal (openings no larger than 0.05 inches) – 1 m x 3 m (J)
• Extra bricks to support wire meshing (K)

Required Tools

• Power supply for fan
• Nails
• Hammer
• Hand saw
• Measuring tape or meter stick
• Jigsaw (optional)

Construction Instructions

1

• Attach parts G and E together using the braces I
• Use 2 nails in each brace (16 nails total for this step)
• Hammer each nail through part G or E into part I

2

• Cut a hole in only one side of the dryer
• If a jigsaw is available, cut a circle with a close tolerance to the aluminum piping.
• If no jigsaw is available, cut a square from the bottom of the frame with a hand saw.
• The hole should be made in part E

3

• Place fan C inside the dryer close to the hole from step 2

4

• Attach mesh layer to the top of the frame
• Use nails spaced 10cm apart

5

• Repeat step 1 using parts F, H and the remaining I
• Use 2 nails in each brace as before
• Place the second frame on top of the wire mesh as shown

6

• Attach aluminum pipe B between the hole made in step 2 and the husk oven A
• Be sure to allow a small gap so that the husk oven may be removed and refilled with fuel
• Attach smokescreen D inside the aluminum pipe above the husk oven A

7

Final Dryer Design

Final Dryer Design Top view

Final Product

The final product should look like the pictures below.

References

See also

• Rice