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Design and Development of a Small Scale Millet Dehuller (2019-20)

M A Matin
Farm Machinery & Postharvest Process Engineering Division, BARI, Joydebpur, Gazipur, Bangladesh

M A Hoque
Farm Machinery & Postharvest Process Engineering Division, BARI, Joydebpur, Gazipur, Bangladesh

M M Billah
Farm Machinery & Postharvest Process Engineering Division, BARI, Joydebpur, Gazipur, Bangladesh

A H M A Faisal
Farm Machinery & Postharvest Process Engineering Division, BARI, Joydebpur, Gazipur, Bangladesh

Abstract/ Executive Summary:

Millets are considered as minor cereals of Bangladesh and commonly grown in marginal land areas (saline coastal areas and chars). Its production contributes to the food security, nutrition and income security of the resource poor farmers in these marginal areas. Dehulling of the millet is traditionally done manually using mortar and pestle. This traditional dehulling is a labour and cost intensive operation and involves human drudgeries. In order to reduce the cost and drudgeries, a low cost millet dehuller was fabricated and tested at the FMPE Division, BARI, Gazipur during 2018-19. It is an impeller type centrifugal dehuller powered by an electric motor (3 Φ, 750 W, 1400 rpm). The batch type dehuller was tested during 2019-20 with millet grains containing varying moisture contents of 12, 14 and 16% (wwb). The feed rate and dehulling time were maintained at 167 g/batch and 90 s, respectively as per recommendation of previous year’s results. Results showed that the moisture content has a significant effect (linear) on the performance of the dehuller. For example, the dehulling efficiency fell slightly by about 1% due to the increase of grain moisture content by 4%. Whilst, the head grain recovery and total milling recovery fell by about 14% and 8%, respectively for the same increase in grain moisture content. The change in moisture content resulted in increased broken grains (11.2–24.0%) that reduced head grain recovery. Analysis of husks data revealed that a considerable percentage (16–22%) of grains (mostly milled rice) were lost with the husks after the cleaning process (by blowing fan). The cleaning loss sharply increased over 14% grain moisture content. Therefore, millets grains should be thoroughly dried to a low moisture content below 12% in order to have high dehulling performance. Review of the blower speed and/or design to reduce the cleaning loss and on-farm and economic performances of the dehuller for millets and other food grains are suggested.

Keywords: Millet dehuller, Merginal land, Fabrication of dehuller, Dehulling performance

Location: FMPE Division, BARI, Gazipur

Start Date: 00-00-2018

End Date: 00-00-2020

Program Area: Farm Mechanization

Commodity: Expeller, Millet

Objectives:

Design and develop a low cost millet dehuller for small scale farmers
Evaluate laboratory performance of the millet dehuller
On-farm evaluate and fine tune the design based on early users’ feedback

Methodology:

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 (Q_f) 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 (W_m). Unhulled grains remaining in the rice samples were separated and weighed (W_u). The rice (pure rice) was then passed through a 0.7 mm sieve to get weights of the head/whole (≥ 0.7 mm) grains (W_w) and broken (<0.7 mm) grains W_b. 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. Dehulling efficiency (η_d): It measures the percentages grains fed remained unhulled (present in the rice). It was calculated using a specific Equation. Total milling recovery (R): It is a measure of percentage of rice obtained from grains fed. Head grain recovery (H): It is a measure of the percentage of the whole grains in the rice. Cleaning loss as unhulled grains (L₁): It is measure of percentage of unhulled grains lost with the husks collected from inside the husk chamber.Cleaning loss as rice (L₂): It is measure of percentage of rice lost with the with the husks collected from inside the husk chamber. Cleaning loss as rice (L): It is a measure of the percentage of rice recovered from the husks collected from inside the husk chamber.

Source of Information: Annual Research Report 2019-2020, BARI, Gazipur, Bangladesh

Article Link (Online Journal):

Funding Source:

1.

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Results indicated that a high dehulling efficiency of about 98.5–99.3% was obtained for grain the moisture contents tested. Thus, a high quality of rice was produced ready to eat. The total milling recovery and head grain recovery started falling down as the grain moisture content was increased. Further, the percentage of broken grains and cleaning loss increased with the grain moisture content. Thus, all the performance parameters fell with the increase of grain moisture indicating the importance of a low moisture content below 12% for the best performance of the dehuller. The cleaning loss was as high as 22% at 16.4% grain moisture content which need to be addressed by reviewing the blower speed and/or design. The dehuller need to be tested on-farm and results be shared with farmers and consumers to check whether their requirements are met. It is also recommended that the dehuller be tested and cost analysis would be done for millet and other food grains (e.g., sesame).

Knowledge Management: Report/Proceedings