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Mobile maize dryer development at farm and cooperative/collector levels
by Kriang-Krai Mekvanich
INTRODUCTION
Maize has been one of the major cash crops of Thailand. About 65-70 per cent of the 4-5 million tons annual production has been exported to other Asian countries. The remaining produce was utilised by the domestic feed industry. Maize production is increasing due to the utilisation of hybrid seed and high market demand from the feed industry.
Harvesting of maize is usually done in two periods: July to October and December to January. Wet maize harvesting and storage practiced commonly in the first period has posed serious problems especially due to the high incidence. of aflatoxin. There is therefore an urgent need for improvements in both on-farm handling practices, and cooperative/collector drying. Traditionally farmers shell their maize at the time of selling to cooperative/collector. It is also a critical time for drying shelled maize to avoid hazards associated with microorganism development. Recommendations have been issued to dry freshly shelled maize to a safe moisture content, within a 24-48 hour period.
Considering numbers of advantage of local drying at farm and cooperative/collector levels such as
From these advantages various types of mobile dryers have been tested/developed. In this paper two types of mobile dryer will be presented: the DAK mobile heater and the US mobile dryer.
THEORY OF DRYING
Drying has two basic stages: (1) diffusion of internal moisture to the surface of the kernel, and (2) removal of external moisture by air flowing around the kernel. Vapour pressure is increased internally within the kernel which causes moisture to diffuse through the micropores of the seed coat. The grain temperature largely establishes the rate of diffusion, and therefore must be controlled in order not exceed a maximum rate which would result in damage. Removal of the exterior moisture for a given air flow is also dependent upon the air temperature. These two stages must be balanced to produce high quality uniformly dried grain.
In addition to varieted characteristics of the grain, the drying rate is also allected by atmospheric conditions. Hard and fast rules cannot be set due to these variables. It will be necessary to dry several batches to determine the exact dryer settings in a specific area.
Grain matures at 30% to 35% moisture. While some grain may be harvested easily at 30%, others do not harvest well above 20%. Therefore, grain should be harvested as soon as possible after maturity, as long as grain damage is at a minimum and gleaning is thorough.
To properly store grain, the grain moisture content must be compatible with the length of time the grain will be in storage, and with the grains intended end use. This moisture content will vary due to locality.
| Grain type | 1 year storage (% moisture) |
| Corn | 13% |
| Wheat | 13-14 % |
| Barley | 13% |
| Rice | 12% |
| Grain Sorghum | 12% |
| Soybeans | 11 % |
| Edible Beans | 14-16% |
For long-term storage up to 5 years, or for grain stored for seed, moisture levels should be 2% lower than shown in the above Table.
The Storage and Processing Research Centre, Agricultural Engineering Division, Department of Agriculture. Thailand, with support from the ASEANEEC R1 Project, has designed and developed a mobile heater for drying maize on the cob in the farmer's storage.
Two models were tested at the collectors and farmers level. Both prototypes are mobile heat source units consisting of a corn cob furnace, a heat exchanger, and a blower mounted on a mobile frame.
The first prototype has a capacity of 5,100 megajoules or 2 tonnes capacity to dry maize from 25% to 16%. The corn cob furnace is constructed from bricks and cement and has a 1.22x1.52 main fire grate placed at 45° angle with a horizontal plane, and a 15x61 cm small auxiliary grate also at 45° but, in the opposite direction of the main fire grate (see Figure 1). The back wall of the furnace is connected to the fire tube of the heat exchanger. The furnace is designed to burn 300 kg corn cob per hour.
The heat exchanger is a parallel flow baffle type with 36 fire tubes of 7.62 cm diameter enclosed in a 122 cm diameter outer shell. The heat exchanger is 2.4 m long with a 30 cm diameter smoke stack of 10 m height. The flame and smoke produced by the furnace passes through the fire tubes of the heat exchanger and the smoke stack on the opposite side of the furnace. Cold air enters the heat exchanger at the opening toward the furnace end, and hot air leaves the heat exchanger at the far end of the furnace by means of a blower driven by a 7.5 Hp electric motor.
The blower which draws hot air from the heat exchanger and forces heated air through the grain mass is a forward curve centrifugal type rotor. The rotor is 53.5 cm in diameter capable of delivering 140 cubic meters per minute (c.m.m.) of 33 mm static pressure art 980 rpm.
The second prototype (see figure 2) is a smaller scale model. The furnace (0.7 cubic meters) has a 65x85 cm main fire grate and 65x15 cm auxiliary grate designed to burn 60 kg corn cob per hour. The heat exchanger has 20 fire tubes of 8.89 cm diameter. The blower is driven by an 8 Hp Diesel engine.
RESULTS:
Results of development work and field testing of the two prototype mobile heater units have shown the following:
US MOBILE DRYER
The dryers described in this paper are radial heat flow, recirculating batch type mobile dryers. The three imported dryers have been trialed at Petchaboon and Nakom Sawan provinces, the main maize growing area under USAID "Appropriate Technology Transfer Project". One 12 tonne unit was manufactured by GT Inc., U.S.A., utilising diesel oil as the burner fuel and two 2 tonne units were made by Morridge Company., U.S.A, utilising LPG as burner fuel. The dryers utilise either tractor Power Take off (PTO) or an electric motor as the prime mover. The structure can be divided into 6 main components (see figure 3);
Battery voltage supply which must be in the range of 11-18 volts.
Tractor PTO speed in the ranged 440-540 rpm.
Seed/grain type selection such as maize seed, maize grain, rice seed, rice grain etc.
Maximum hot air temperature.
Maximum grain temperature.
Difference between the operating hot air temp and the setting.
Difference between the operating grain temp and the setting.
Specification of Mobile GT dryer (GT 580)
There are four steps in operating the dryer from loading to unloading. Maize grain is loaded either from the grain sack or dump truck through loading hopper, loading auger and vertical auger to holding bin. After completion of loading, the loading auger clutch is disengaged. The grain will be circulated within the bin by vertical auger (see figure 4)
Once the bin almost filled with the grain, the ring burner can be started. Drying components can be set and controlled manually in Morridge dryer, or by microprocessor in the GT dryer. Hot air is forced by fan to the hot air chamber, then though the grain wall to bin outside. Drying is continued until the grain moisture content reaches 15.5% (see Figure 5).
In general, the maximum grain temperature is set at 45.5° for seed and 60° for grain.
After the completion of drying, the burner is shut off. However, the fan is still operating to cool the grain down until the temperature difference of the grain to the ambient temperature is less than 5°C to avoid condensation in storage At this stage, the moisture content of the grain is decreased 1-2%, with the grain outloaded of 14-14.5% mc (see Figure 6).
By pulling the unloading head to the unloading position, all the dried grain is conveyed out of the dryer. (see Figure 7).
From actual operations under humid tropic conditions (ambient temp 30-37° and ambient relative humidity 65-85%) from August to November in the years 1987 and 1988, it was found to dry maize from average 21-22% to 15%,
