Fabrication of the dehuller:
The Plant Breeding Division of BARI has been using a laboratory scale European dehuller to dehull millets. This is a batch type centrifugal dehuller that can produce clean rice ready to cook. In the dehuller a certain amount of grains is fed into the machine (inside the rotating cylinder) and the dehulling starts by impact. Once the dehulling is complete, it is unloaded and next batch of grains is feed into the machine. Following the design of the dehuller, a prototype millet dehuller was fabricated at Mahabub Engineering Works, Jamalpur by reverse engineering during 2018-19. The dehuller has the following major functional components.
Dehulling cylinder: The dehulling cylinder is fabricated of mild steel (MS) sheets, shaft and reinforced rubber sheet and fitted with ball bearings and V-pulley. It consists of three main parts - feeding chute, rotor and a casing.
Feeding chute: The feeding chute is located at the upper part of the cylinder. It has a gate that remains closed due to gravity, but opens up during feeding grains to the cylinder through the chute. The gate become closed (due to gravity) after feeding and remain closed during dehulling. Thus, the gate stops the grains from being coming out of the cylinder during dehulling,
Rotor: A 3-blade impeller type rotor with a concave clearance of 10 mm (as suggested by Kaankuka, 2015) is installed within the casing. At the tip of the blades, reinforced rubber sheet strips are bolted that mainly do the task of dehulling. The middle front part of the concave is corrugated. During rotation of the rotor, the grains repeatedly impact with the impeller and get rubbed by the concave and gradually get dehulled. The shaft of the rotor is mounted on two bearing supports and being operated by an electric motor via V-pulleys (A section) and belts. The casing is made of MS and plastic sheets and seals the cylinder (and the blower). The bottom part of the casing (concave) is split into two halves that can be opened or closed via a spring loaded pedal. This split allows unloading of the dehulled grains to the outlet chute via the cleaning chamber.
Blower: The blower is fabricated of MS sheets, shaft and plastic sheets and fitted with ball bearings and V-belts and pulleys. It is located slightly below the cylinder. The blower rotates, suck air out of the cylinder and creates a vacuum inside the machine. Once the pedal is pressed, the cylinder concave splits and the dehulled grains (mixed with husks) fall out of the cylinder to the outlet chute. During falling, the blower sucks the husks (lighter than grains) of the falling dehulled grains out of the machine. The grains (rice), due to their heavier weights, keep falling down until get deposited on a tray/bowl placed below the delivery chute. Once unloading is complete, the pedal is released to close the cylinder concave splits and start another batch of dehulling.
Prime mover: The dehuller is powered by an electric motor (3 Φ, 750 W, 1400 rpm, 380 V, 50 Hz, Bati Ghar, Nawabpur, Dhaka, Bangladesh) that transmits power to the dehulling cylinder and blower via V belts-pulleys.
Testing and performance data collection and processing:
Kaun grains collected form Plant Breeding Division of BARI that have a moisture content of 12.37%. Using those grains addition samples (medium MC of 14% and high MC of 16%) were prepared by adding calculated amounts of water. Based on the previous years’ experimental results, a no load rotor (dehulling cylinder) speed of around 1200 rpm, feed rate of 167 g batch-1, and a dehulling time of 90 s were selected for the current experiment. The following tests were conducted with the factors and treatment levels stated below (replicated three times) during 2019-20.
The dehuller was run idle (empty) for about a minute before each test run to stabilize the dehuller. Following that the measured quantity (167 g) of unhulled grains (Qf) was feed in to the feeding chute and dehulling started. Once the desired dehulling time was reached, the dehulled grains were unloaded. During unloading, the grains fell though the cleaning chamber, cleaned by a blower, and finally clean rice was deposited to a bowl placed below the rice delivery chute. Some of the husks was deposited in the husk chamber and the rest went out of the machine thorough the husk outlet.
After completion of all the test runs, rice collected at the grain outlet was weighed (Wm). Unhulled grains remaining in the rice samples were separated and weighed (Wu). The rice (pure rice) was then passed through a 0.7 mm sieve to get weights of the head/whole (≥ 0.7 mm) grains (Ww) and broken (<0.7 mm) grains Wb. From the above measurements, the performance parameters of the dehuller were calculated as per Kannkuka (2015) as described below. On the other hand, the husks collected at the husk chamber were hand sorted to find out the weights of the grains (unhulled and milled) lost with the husks.
Cleaning loss as rice (L): It is a measure of the percentage of rice recovered from the husks collected from inside the husk chamber and was calculated as the sum of L1 and L2.