FAQs on drying

list collapse Since solar energy is free, would it not be great to have a solar rice dryer?

While the idea of using the free energy from the sun for drying is very exciting, most solar dryers for rice have failed. One exception is the Solar Bubble Dryer (SBD), which is a recent development and was commercialized as Version 1 in September 2014. It is very promising with respect to replacing sun drying and eliminating the losses cause by spreading the grains on the ground and exposing them to the elements.

Solar energy can be used for two purposes:

  1. To generate heat in order to increase the temperature of the drying air for reduction of the relative humidity of the air to enable/speed up the drying process.

  2. To generate electrical energy to move the drying air through the grain bulk or over the grains and to transport the evaporated water away from the grains.

The SBD uses the overflow drying principle that requires only a small blower with little power requirement and used relatively low drying air temperatures. The drying tunnel itself, which also contains the grains, serves as the solar collector that generates heat and the SBD therefore does not require much additional area for solar panels for generating heat.

Attempts to use solar collectors for heated air drying have mostly failed though due to the relatively low solar energy from the sun per square meter and the high air flow rates required in heated air dryers. Heated air dryers also need strong blowers to force the drying air through the grain bulk.

Using solar collectors to generate heat for rice drying in heated air dryers is not economically feasible and there are technical constraints.

An example, flat bed dryer with 1t capacity

1. Heating the drying air

  • Assuming an average solar radiation of 500W/m³ and 70% collector efficiency, one square meter of solar panel can generate 1,260 kJ/h or roughly 15,000 kJ/day.
  • The desired drying rate in heated air dryers for optimum dryer use, grain quality and economic operation is 1% moisture content per hour. To achieve this air with a temperature of 43°C needs to be moved through the grains. In the tropics average ambient air temperatures are somewhere between 2535 degrees so on average a temperature rise of 10 K is needed, which would require around 50 kJ/h energy for heating the air. For one ton batch capacity this would require more than 40 m2 collector area for the solar collector.

2. Generating electricity for the blower

  • For each ton dryer capacity this requires 0.71 kW motor power for the blower to force the drying air through the grain bulk at around 0.2m/s air velocity. Powering the motor with photovoltaic is not economically feasible.

Advantages of solar energy

  • Solar energy is freely available during the day and is environmentally friendly.
  • A solar dryer can be completely independent from other energy sources and can be used anywhere.


  • Most heat for drying is needed when it rains or at night when solar radiation is low.
  • High investment cost and space requirement for solar collectors for heated air drying, the solar collector area needed is around 10 times the area of the drying bin.
  • Temperature control is a major problem.


The Solar Bubble Dryer is a promising concept for using solar energy for drying since it uses overflow drying and low temperatures.

For heated air drying solar collectors are not economically feasible since the specific investment cost (cost per ton installed capacity) and the space requirement remains major constraints to using solar power for heating the drying air.

Contact postharvest@irri.org

list collapse How to prevent fly ash in the paddy?

There is a lot of fly ash or black sooth in my paddy which reduces its market value. How can I reduce it?

Most paddy dryers are direct fired, which means they don't use a heat exchanger and the flue gas of the furnace is mixed with ambient air and then blown into the dryer. If the furnace - fan combination is not designed properly the fan can suck ash from the furnace and blow it into the air distribution system of the dryer. If the air velocity is high enough the ash or sooth even gets carried into the grain bulk.

Most furnaces have a fly ash separation device, e.g. by using a set of baffles that forces the air to turn sharply and separates the ash trough centrifugal force. Other furnaces use a circular air flow like in a cyclone for ash separation. If you find too much fly ash in the drying air check the following:

  1. Is the ash separation device designed according to the specifications?
  2. Has fly ash accumulated in the ash separation device? If so, clean it.
  3. Is the air velocity according to the specification? Too high air flow can suck excess ash from the furnace.

 Contact postharvest@irri.org

list collapse The drying time of my flat bed dryer is too long, what could be the reason?

A properly designed and well maintained flat bed dryer should have a drying rate of around 11.5% moisture reduction per hour. 1%/h drying rate means that if paddy is harvested at 22% moisture content and the final moisture content after drying is 14% the moisture has be removed by 8% and it would take around 8 hours for drying. If the drying time is longer than the expected drying rate would suggest there are three possible reasons:

  1. The paddy is very wet
  2. The dryer performed well when newly installed but only recently started having longer drying time
  3. The dryer always had a long drying time

1. The paddy is very wet

If the paddy was very wet because of harvesting after rain or early harvest before physiological maturity obviously the drying also takes longer since the drying rate is fixed by the design of the flat bed dryer.

2. The dryer performed well when newly installed but only recently started having longer drying time 

Some operating parameters must have changed. Check for the following:

  1. Check the air temperature. It should be around 43ºC. Adjust if necessary.
  2. Check the airflow rate, a sheet of paper should float on top of the drying bin at each location. If not:
  3. Check the fan rotor speed, sometimes operators reduce the speed to extend the engine life or to save fuel
  4. Check for slippage of the fan belt or any other
  5. Check the grain layer depth, was the dryer overloaded?
  6. How dirty was the grain? A lot of dirt and dust between the grains can hinder airflow.
  7. Check whether fly ash has accumulation the plenum chamber or in the air distribution system

3. The dryer always had a long drying time

If the dryer has never achieved a drying rate of 1%/h there is something wrong with the design. Possible causes are:

  • Most likely the fan does not perform according the specifications. Often fans are not properly designed and deliver only a fraction of the required air flow. Fan design is hardcore engineering and any tinkering will lead to failure.
  • The air ducts might be too small limiting air flow.
  • The plenum chamber below the drying bin might be too small.
  • The furnace might not provide sufficient heat; check the drying air temperature inside the plenum chamber.
  • The drying bin might be too big for the blower. Often manufacturers and users use a bigger drying bin in order to increase capacity but maintain the blower designed for a smaller bin. While the bin holding capacity increases the drying capacity expressed in t dried per hour might actually decrease. 
Example: In South Sumatra users doubled the drying bin capacity from 3.3 to 8 t but tripled the drying time from 8h to 24h. In reality they reduced the dryer capacity from 0.41t/h to 0.33t/h.
  • Check all other design parameters.