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Design and Development of a Small Scale Millet Dehuller (2018-19)

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 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. The batch type dehuller was feed with either 125 g, 167 g, 208 g and 250 g of millet grains at 9% moisture content and dehulled for either 90 s or 120 s (single pass only). The dehulling efficiency was as high as 99% up to the feed rate of 167 g. At these low feed rates, higher dehulling time did not improve the dehulling efficiency any further. Although, total milling recovery and head grain recovery were a bit low at these low feed rates, any higher feed rate would not be preferable as it reduces the dehulling efficiency. The dehulling time did not have any considerable effect on total milling recovery or head grain recovery at any of the feed rates. So, considering a higher throughput capacity, 90 s dehulling time would be preferable over 120 s.

Keywords: Millet dehuller, Dehuller efficiency, Fabrication of dehuller

Location: FMPE Division, BARI, Gazipur

Start Date:

End Date:

Program Area: Farm Mechanization

Commodity: Huller

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:

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. 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. 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, 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. The chassis of the motor was extended in order to facilitate installation of measurement instruments. Instrumentation and torque data logging: Power from the motor shaft to the dehuller was transmitted via a torque transducer and two pairs of flexible couplings (Fig. 1). The transducer was connected to a laptop computer and torque data (N m) were logged to an Excel sheet as a .csv file. The data were acquired at 10 Hz (10 torque readings per second) during the dehulling period (90 s or 120 s as per the test plan). The motor was connected via an inverter to regulate and measure motor speed. Testing and performance data collection and processing: Kaun grains collected form Plant Breeding Division of BARI was sun dried to a moisture content of 9% for testing. Before test runs, mock tests were conducted and a no load rotor (dehulling cylinder) speed of around 1200 rpm was selected that produced clean rice. It was also found that a dehulling time of 90–120 s (single pass only) were useful to obtain clean ready to cook rice. The mock tests were conducted with the factors and treatment levels three raplication. The dehuller was run idle (empty) for about a minute before each test run to stabilize the dehuller and instruments. Following that a measured quantity of grains (Q_f) was feed in to the feeding chute and dehulling started. Once the desired dehulling time (90 s or 120 s) was over, the dehulled grains were unloaded. During unloading, the grains fell though the cleaning chamber and finally clean rice was deposited to a bowl placed below the delivery chute. The unloading time for each test run was recorded. No load speed of the dehulling cylinder rotor was maintained at 1200 rpm. During loading (dehulling) the motor speed reduced (depending on the load) which was measured using a tachometer and recorded for each test run. Torque data were recorded on a laptop computer for the dehulling period using a data acquisition software. After completion of all the test runs, the collected rice samples were 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. Torque, power, energy and specific energy consumption: An example of the torque signal data logged is presented From the torque data, average torque was calculated. The peak torque was taken as the highest value of torque data recorded during the dehulling period. The average and the peak power requirements were calculated. Dehulling efficiency (η_d): It measures the percentages grains fed remained unhulled (present in the rice) was calculated.

Source of Information: Annual Research Report 2018-2019, BARI, Gazipur, Bangladesh

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

An optimum performance of the machine can be obtained for the combination of 167 g (approximately 170 g) feed rate and 90 s dehulling time. The throughput capacity of the dehuller would then be about 5.8 kg h^-1. However, the results need to be confirmed further for different grain moisture contents, cylinder speeds, etc. The results also need to be shared with farmers and consumers to check whether their requirements are met. It is also recommended that the dehuller be tested for other food grains (e.g., sesame).

Knowledge Management: Report/Proceedings