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For drying a ton of rice the re-designed fixed bed batch dryer from 22% to 13% moisture content at the ideal drying temperature of 43 degrees Celsius with a calm breeze of 1 m/s at 60% relative humidity, the drying time for the 1x3 meter dryer is 7 hours.
For drying a ton of rice the re-designed fixed bed batch dryer from 22% to 13% moisture content at the ideal drying temperature of 43 degrees Celsius with a calm breeze of 1 m/s at 60% relative humidity, the drying time for the 1x3 meter dryer is 7 hours.
==Construction Instructions==
[[Image:Dryer1.JPG|thumb|center| Step 1]]
Step 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
[[Image:Dryer2.JPG|thumb|center| Step 2]]
Step 2:
* Cut a hole in only one side of the dryer
* The hole should be made in part E
[[Image:Dryer3.JPG|thumb|center| Step 3]]
Step 3:
* Place fan C inside the dryer close the hole from step 2
[[Image:Dryer4.JPG|thumb|center| Step 4]]
Step 4:
* Attach mesh layer to the top of the frame
* Use nails spaced 10cm apart
[[Image:Dryer5.JPG|thumb|center| Step 5]]
Step 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
[[Image:Dryer6.JPG|thumb|center|Step 6]]
Step 6:
*Attach aluminum pipe B between the hole made in step 2 and the husk oven A
*Attach smokescreen D inside the aluminum pipe above the husk oven A
[[Image:Dryer7.JPG|thumb|center|Final Dryer Design]]
[[Image:Dryer8.JPG|thumb|center|Final Dryer Design Top view]]
The final product should look like the pictures below.


== See also ==
== See also ==

Revision as of 05:05, 16 April 2010

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Summary

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

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

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)
  • Advantages: Simple, inexpensive, fast drying
  • Disadvantages: Reduction in milling yield (due to re-wetting of dry grains), risk killing seeds, labor intensive

Re-circulating Batch

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

Continuous Flow Dryer

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

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

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.

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. 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 off 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. **Link to photo**

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. **Link to photo**

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.

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 goo 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 circulate batch dryer.

Larger scale fixed bed batch dryers have 4-10 ton capacity 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. 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. 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 43oC). 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.

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.

In designing this apparatus, the evaporation rate was calculated to estimate drying time for different designs. The rice was assumed to have a constant rate of evaporation. However, due to the falling-rate period, this assumption causes the calculated drying time to be slightly lower than is should be to reach the desired moisture content.

Evaporation rate of water in a large pool is given by:

and

where

  • g is the amount of evaporated water in kg/h
  • k is the evaporation coefficient in kg/m2h
  • v is the wind velocity in m/s
  • A is the area is m2
  • x is the saturation humidity at the surface temperature (43 degrees Celsius for optimum drying) in kg water/kg air
  • h is the relative humidity of the ambient air

The desired amount of water to be removed from the rice is given by:

where

  • d is the desired amount of water to be remove from the rice in kg
  • m is the initial mass of the rice
  • MCo is the initial moisture content (usually between 20% and 25%)
  • MC is the desired moisture content (usually between 9% and 14%)

The total drying time is given by:

where

  • t is the total drying time in hours

For drying a ton of rice the re-designed fixed bed batch dryer from 22% to 13% moisture content at the ideal drying temperature of 43 degrees Celsius with a calm breeze of 1 m/s at 60% relative humidity, the drying time for the 1x3 meter dryer is 7 hours.

Construction Instructions

Step 1

Step 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
Step 2


Step 2:

  • Cut a hole in only one side of the dryer
  • The hole should be made in part E
Step 3


Step 3:

  • Place fan C inside the dryer close the hole from step 2
Step 4


Step 4:

  • Attach mesh layer to the top of the frame
  • Use nails spaced 10cm apart
Step 5


Step 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
Step 6


Step 6:

  • Attach aluminum pipe B between the hole made in step 2 and the husk oven A
  • Attach smokescreen D inside the aluminum pipe above the husk oven A
Final Dryer Design
Final Dryer Design Top view


The final product should look like the pictures below.

See also

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