FAQs on drying
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:
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
2. Generating electricity for the blower
Advantages of solar energy
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.
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:
A properly designed and well maintained flat bed dryer should have a drying rate of around 1—1.5% 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
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:
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: