Natural Drying

Contents - Previous - Next

The traditional methods used by farmers for drying grain rely on natural air movement to reduce moisture content to a safe level for storage. In additional they may utilize the extra drying capacity gained by exposing the produce to the sun. With good ventilation through the store the grain can be harvested just after it is ripe (about 30% MC for maize) but most methods allow some of the drying to take place naturally while the crop is still standing in the field.

Natural drying may be divided in three principle approaches:

Field Drying

The method of leaving the crop standing in the field for drying is popular in areas where maturity of the crop coincides with the beginning of a dry season. However, a crop left unharvested is exposed to attack by insects, birds, rodents, wild animals, strong wind and occasional rain showers which can damage and reduce the crop considerably. These factors are particularly important with the new improved high yielding crop varieties that are often more susceptible to damages from the environment than the traditional varieties. For instance a hybrid maizecob has less leave cover than the old maize and therefore more open to attack by insects and birds.

Field drying of the crop will also delay the clearing of the field. This should be taken into account in areas where the field should be prepared for a second rainy season or where the humidity is high enough at the end of the growing season to allow for an additional crop of for instance beans.

Shallow Layer Natural Drying

The harvested crop is spread on hard surface ground, on roofs or purposely built platforms or trays. Exposed to the sun the crop will dry fairly quick depending on the humidity of the ambient air. The produce should be stirred frequently to ensure even drying. The disadvantage of the method is that the crop has to be brought in or covered every evening or before rain. The labour may be reduced considerably by placing the crop on a plastic or tarpaulin sheet for easy handling or on a platform/tray covered by for instance transparent plastic as shown in figure 9.4

Figure 9.4 Tray Drier for natural drying of crops in shallow layers. The tray has meshwire bottom and a pitched roof of transparent plastic.

Ventilated structures for natural drying

The very small producers may suspend bundles of the crop from trees or poles so they are freely exposed to the air. With larger quantities the harvested crop may be heaped on platforms or racks and topped by a layer of straw for rain protection. The method is commonly used for sheaves of paddy and cereals as well as for cob-maize and groundnut plants. Since the drying is depending on the free flow of air through the crop, the heap should be made as open as possible.

The next step is to have a more permanent ventilated structure in which the crop may be heaped for drying well protected from rain.

For maize the tradition in most part of Africa is to leave the crop in the field until the moisture content has falled to about 18% and then continue the drying of the maize on the cob with or without husk (sheath) in a granary which most commonly has the shape of a circular woven basket placed on a platform 1-3 feet above the ground. The predrying in the field in normally necessary because the basket is too tightly woven or too wide to allow for sufficient ventilation.

This "two-step" drying worked fairly well with the traditional farming systems where the farmer used maize with good sheathcover and could break new farmland regularly. The high increase in population experienced in many countries has resulted in scarcity of good land which forces the farmer to use the same land for the same crop year after year. In most cases this will lead to an accumulation of pests (e.g. insects). This together with the higher susceptibility to insect attack by most improved high yielding crop varieties (see the Field Drying section) shows that the crop should be harvested as early as possible, just after maturity, and moved away from the field for quick drying and safe storage. For maize the circular traditional granary may still be used with some modifications. The basket has to be more loosely woven or the wall can be made slatted with at least 40% air space, and with a diameter varying up to l50cm depending on the humidity of the air. The restriction for the width makes it more economical to build the drying structure rectangular as soon as the production exceeds the yield of 5-9 bags. The rectangular structure in Figure 9.5. with slatted walls and floor is called a Ventilated Maize Crib. Although it can be used with small modifications for any crop that need to be kept ventilated, it is mostly used for drying of maize on the cob without the husk.

Figure 9.5 Ventilated maize crib for drying maize on cobs. The structure has slatted walls and should be placed with the long wall facing the prevailing wind.

The crib can be constructed in many different ways but what is important for the drying effect is the width and that the long wall is facing the prevailing winds. The width may vary from 60cm in very humid areas to 180cm in areas with semi-arid climated. Except for this extreems a width of 100-150cm as recommended in East Africa will suit many maize growing areas. The walls should not limit the airflow through the maize, this requires at least 40% openings. In areas with rodents the floor should be lifted 90cm above the ground and the legs fitted with ratguards. If the width does not exceed what is recommended for the area it is possible in a ventilated crib to dry maize with an initial moisture content of 30% without getting problems with mould, but if it takes too long time (more than 10- 15 days) to get the moisture content below 18% mould may develop regardless of where the maize is, in the field or in a store.

The drying rate is depending on the relative humidity of the air and the air velocity. When the moisture content of the produce has reached the equilibrium with the humidity of the ambient air the drying will stop. Maize will dry down to about 13.5% m.c. if the mean relative humidity of the air is 70% (Figure 9.2).

Table 9.3 Capacities of Crib at different Lengths (section length 150cm)

Crib width 150cm. No. of sections Volume in m No. of bags wet maize on cobs No. of bags of 90 kg dry shelled maize
1 2.7 26 9.3
2 6.0 58 20.7
3 9.4 91 32.5
4 12.8 124 44.8

Artificial Drying

If the air humidity is too high to allow grain to be dried adequately by natural means and storage methods do not facilitate further drying, it is necessary to dry the produce by using forced air or heat or the two in combination. Various local methods using available materials have been developed. In some areas storage is restricted to the amount which can be dried on a heat supply similar to that available from a kitchen fire. Thus panicles of paddy and maize stored on horizontal grids are kept dry by heat from a fire which is lit occasionally underneath the grid and the heap of panicles is turned at regular intervals to prevent the development of mould. There are raised granaries beneath which fires are lit to complete drying. The produce receives a characteristic odour and flavour when exposed directly to smoke from the fire as well as to the hot dry air. This problem is overcome by using driers designed with a hot-air chamber or heat-exchange unit and smoke stack or chimney. See Figure 9.6.

Figure 9.6 An Oil Barrel Drier.

The fire is lit at the mouth of the oil-barrel tube, hot air and smoke is exhausted via the chimney. The heated barrels in turn heat the surrounding air which rises through the crop.

When heat is used to dry grain there must be some provision for aeration as well. Either very thin layers or frequent stirring is advisable as natural convection currents seldom move enough air.

The forms of artificial drying may be characterized by the depth or thickness of grain being dried. There are:

Large Scale System driers can be derived into the following categories:

They may also be either high temperature or low temperature systems.

Air Volume Requirements

Whatever the system, artificial drying depends on forced air ventilation with, or without, added heat. Knowing the amount of moisture to be removed, together with the moisture carrying capacity of the air under the existing conditions, it is possible to estimate the weight of dry air required to complete a given drying operation. The humid volume of air is found on a psychrometric chart and from that the total volume for drying can be determined. Drying will take place as long as the RH of the drying air is below the equilibrium of the produce.

For example, the air described in Figure 9.3 contains 0.0167 kg moisture/ kg dry air at 25C and 70% RH. The holding capacity of this air is 0.0186 kg moisture/ kg dry air when fully saturated, and the specific volume is 1.04 m 1 kg dry air.

Table 9.2 shows that e.g. one tonne of grain dried from 22 to 16% m.c. will yield 71 kg of water (1.000 - 0.929) x 1000 kg = 71 kg).

Weight of air required = 0 0186 - 0.0167

Air volume is then 37368 kg x 1.04-= 39963 m kg

If the same air is heated to 45C the RH will drop to 23.6% and the holding capacity when fully saturated will increase to 0.025 kg moisture/ kg dry air.

The specific volume is now 1.11 m/ kg dry air (Figure 9.3)

Weight of air required =71 / (0 025 - 0.0167)

Air volume is then 8554 kg x 1.11 = 9495m or 1583m/tonne and % moisture reduction.

From this result the total volume of air and rate of flow is calculated in order to complete the drying operation in the necessary time.

Experience shows that the air volume needs to be increased somewhat depending on air velocity and grain depth. Air leaving a drier using high air velocity and shallow grain layer, is seldom fully saturated with moisture. Certain minimum airflow rates are necessary to prevent the formation of mould during drying. These rates are given in Table 9.4. It should be noted that these figures are for loose grain through which air can be blown.

Table 9.4 Minimum Required Airflow Rates for Wheat and Shelled Maize

  Grain Moisture percent, w.b. Airflow m/s/m
Wheat 20 0.06
18 0.04
16 0.02
Maize 25 0.10
20 0.06
18 0.04
16 0.02

Deep Layer Driers

These consist of beds, bins, silos or rectangular warehouses equipped with ducting or false floor through which air is distributed and blown through the grain. The depth of the grain layer may be from 30cm and up to 350cm.

In deep layer driers unheated or slightly heated air (less than 6 C) is forced through the grain by a mechanical fan. The grain dries first at the point where the air enters, a drying front passes through the mass in the direction of air movement, and the grain at the air discharge location dries last. Most of the drying occurs just below the drying front in a layer called the drying zone which develops and then moves through the bulk (Figure 9.7). The depth and rate of progress of the drying zone depends largely on the dampness of the grain and the air speed. A low ventilation rate results in a shallow slow moving zone whilst a higher rate produces a deeper zone which progresses more quickly.

Figure 9.7 Deep Layer Drying.

The grain furthest from the air source will remain wet, and may even become wetter (due to condensation), until the drying zone begins to move out of the crop.

For successful results, the drying zone must reach the surface before the grain in this area deteriorates. It is therefore normal practice to limit the depth of grain so that the drying front reaches the top in time.

Although increasing airflow increases the drying rate, it will be noted in Table 9.5 that the static pressure due to the resistance of the grain to the flow of air, rises at a very rapid rate. In general therefore, it is common practice to limit the airspeed through the crop to 0.10 - 0.15 m/s to avoid the necessity of excessive fan capacity.

Table 9.5 Typical Resistante to Airflow (Pa) per meter of Crop Depth

Crop Air speed through crop m/s
0.05 0.10 0.15 0.20 0.25
Wheat 140 330 570 850 1070
Maize 70 180 320 500 720
Peas 50 110 190 290 410

Note: Values for the other small grain cereals, such as rice, are similar to wheat and values for very fine seeds such as herbage seeds may exceed 2500Pa for 0.10 m/s airspeed.

A floor drying system in a godown or warehouse type of building is shown in Figure 9.8.

Figure 9.8 Floor drying system.

The crop is piled over the lateral ducts which are fed with air from a main duct. The main duct is often large enough for a man to walk inside in order to close off laterals where the grain is already dry.

The lateral ducts can be installed above or below floor level. The above-ground laterals are cheaper but will have to be removed during unloading the store. Below-ground laterals are left in place and can be driven over.

Fan capacities

When planning for deep layer driers it is important to have the fan performance in mind. Figure 9.9 shows typical fan performance curves for modern high pressure propeller fans.

Figure 9.9 Fan performance curves for some modern, high pressure, propeller fans.

Example

A village cooperative is planning a deep layer drier. Find a suitable size of the drier and choose a suitable fan. The following data is given:

Quantity of grain: 10 tonnes of maize/batch
Time available for drying 60 hours (6 days)
Initial moisture content (MC) in maize 21%
MC reduction for sack storage 6%
Incoming air at 25C and 50% RH
Assumed exhaust air at 85% RH and 19.5C
Air volume required to remove 1 kg Water:
From the 1500m Psychrometric chart it is
found that the given air can remove, 0.0143 - 0.0118 = 0.0025 kg H 20/ kg dry air.
The volume of incoming air is 1.03m/ kg dry air.
Required air volume to remove 1 kg of water.
1.03 / 0.0025 = 412m/ kg H2O

Moisture to be removed from maize

W 1 - W 2 = W1(M1 - M2) / (100 - M2) = 706 kg H20

Total air volume required 412 x 706 = 290824m
Total air flow/hour 290824 / 60 = 4847 m/ h
Minimum Air Velocity required 0.07 m/ s (from Table 9.4)

Try different heights of the layer considering the airflow resistance.

  Height of layer (m)
1 1.5 2
Floor area required (density of maize 720 kg / m) 13.9 9.3 6.9m
Airflow 4 x 7/13.9= 349 521 702m /hm
Air velocity 0.10 0.14 0.20m s
Airflow resistance 180 480 1000Pa

From Figure 9.9 it can be seen that the 2.2kW fan can easily manage a 1.5m layer, that is; 4850m/h at 480Pa. Under the same conditions with wheat instead of maize the airflow resistance would be 330, 860 respectively 1700Pa and the layer should therefore be reduced to 1m or a centrifugal fan with higher performance to pressure would be more suitable.

The calculations assumes ideal conditions and the real moisture reduction may be decreased because of other climatic conditions or moisture content in the grain. The fan performance should therefore be a bit higher than calculated.

In the example the grain depth was given as 150cm. However, this sort of drying and storage unit may have a capacity of 300-400cm. To avoid the problem of spoilage in upper layers, it is normal practice to dry in batches of 150cm before adding more grain. The additional grain will then start drying from the point.

Commercially available bins for drying and storage are normally made of corrugated steel. Round bins have no theoretical limit to the diameter. However, for practical purposes, a diameter of 7 to 8m is likely to be the maximum. The minimum diameter is dictated by the ability to roll the sheet to a tight radius and is likely to be approximately 2-3m.

Rectangular bin sizes are limited by the ability of a straight length of wall to resist thrust. The practical limit is about 3m and these bins may well be built "nested" together (Figure 9.10). It is possible to omit the cross wall and replace it with tie-rods.

Figure 9.10 Nested bins with rods replacing cross-walls.

Another type of in-bin drier is a radially ventilated bin, in which there is a vertical perforated duct up the centre of a circular bin. The bin wall is of perforated steel or of timber staves alternating with perforated steel strips. The distance between the duct and the bin wall is 1 m to 2m, depending on bin size. The air path through the grain is thus limited to the radius of the bin. The air velocity will also decrease gradually towards the outer wall. They are normally used as a batch drier with the grain then transferred to a store for either bulk or bag storage. When drying wet grain the height in the bin should be decreased in order to increase the air velocity and eliminate too high pressure on grain in the bottom of the bin.

Shallow layer driers

Batch Driers

These are shallow layer dryers, often in the form of a tray with a perforated base. The dimensions may be 1 to 2m wide and 2 to 4m long with the grain bed being 150 to 300mm deep. The drier can also be built vertical with channels for both inlet and outlet air going through the grain, see Figure 9. 11. Warmed air is blown into the plenum chamber beneath and then up through the grain. This type of dryer is suited to a smaller operation than continuous-flow driers. They may be either mechanically or manually loaded and unloaded.

Figure 9.1 1 Section showing the principle of a vertical shallow layer batch drier.

Continuous-flow Driers

The grain passes through these driers in a continuous flow at a controlled rate. The grain is kept in a thin sheet, approximately 100 to 150mm deep and hot air is blown through the crop. In this system, the air temperature can be substantially higher than in bulk driers. The rate of throughput can be controlled and hence the length of time exposed to the hot air. This is adjusted according to the amount of moisture to be removed. The latter part of the path through the drier is an ambient air section to cool the grain. Continuous-flow driers are high in cost and are applicable only in highly mechanized situations.

Grain Cooling

Failure to cool grain that has been dried with heat may cause an increase in mc great enough to seriously shorten its storage life.

It can been seen on a psychrometric chart that for a given air mc (absolute humidity), a drop in air temperature causes an increase in RH. It follows that if hot grain is allowed to cool naturally the RH of the air in the bin will rise and, if the saturation temperature is reached or passed, condensation can cause the grain mc to rise again. To prevent this possibility, after drying, the grain should be cooled until ambient temperature is reached.

Figure 9.12 The principles of a Continuous-flow drier.

The methods adopted to cool grain are dependent on the drying system.

Sun-dried grain can reach high temperatures if in the direct sunlight. If it is to be stored in any container through which air cannot freely pass, it should at least be left shaded for an hour or more before storing.

Air can circulate around sack stacks to some extent and therefore can cool naturally. Even so, it is preferable to ventilate to cool the stacks.

Fan ventilated batch driers of all types, including sack driers, should have the fan left running with no added heat until the crop is at ambient temperature before discharging the crop from the drier. This is most easily determined by comparing the temperatures of the incoming and exhaust air and waiting until there is essentially no difference.

Cooling Buffer Storage

Low volume ventilation (LVV) or aeration may be employed to cool grain that has been put in storage. Although it can be used in conjunction with other driers as a cooling system, the main objective of LVV is to cool the grain positively at harvest time and thus prevent infestations of insects and mites and the development of mould. The deterioration of viability is slowed and the migration of moisture from warm spots to cooler ones in the grain mass is avoided.

It must be stressed the LVV is not a drying system. Consequently if grain is too wet at the start (over 18%) it will be unlikely to store well, and for human consumption it would be preferable to start with a mc lower than 18%.

Principles

Ambient air passed through the grain at the rate of 6 m/ h to 8 m/h for each tonne in the storage has proved adequate in practice. Depending on the static pressures involved, this range of ventilation rates would require from 190 to 560W per tonne.

Drop in temperature occurs in three ways: a Removal of respirational heat by airstream b Contact cooling of the grain by colder air c Evaporative cooling when the RH of the cooling air is below the EMC level of the grain.

Air flow may be upwards or downwards and investigations have shown little real difference in overall effect.

Once the grain is cooled and the ventilation stopped, it is advisable to turn the fan on every 2 to 3 weeks to check for storage problems. A musty odor will indicate a moisture and temperature problem.

Equipment

Fans to be used for grain cooling can either be centrifugal or single-stage axial fans. Motors ranging from 370 to 746W cover the vast majority of fan sized used. They are usually small enough to be picked up by hand and runs on 13 amp switched outlets. The volume of air delivered varies with the climate but should at least be 10m/h and tonne.

Ducts similar to those in on-floor storage are satisfactory.

Management

With the aim of cooling the grain, only air that is cooler than the grain under treatment should be used.

The preferred method of cooling grain is to blow when ambient air is 3C cooler than the warmest grain. This entails knowing the temperature of the grain in the bin or heap. A spear thermometer or a thermistor will be needed, the quicker reaction of the latter greatly speeds up the chore of taking grain temperatures at several points. In a bin the hottest spot will be in the centre some 1.2 to 1.8m below the surface with upwards ventilation or about 1.2m above the duct with downwards airflow. In a natural heap the hottest places are the apex, the shoulders or at the foot of the side wall (Figure 9.13).

Sack Drying

Grain in sacks can be dried in a stack or the sacks may be laid one or two layers on a platform drier as shown in Figure 9.14.

A platform drier consists of a plenum chamber with an open top of wire nesh, bamboo or other means of supporting 2 to 3 layers of sacks. Using an airflow rate of 0.1 m/ s per m of platform area, air heated to about 14C above ambient should reduce the cm about 0.5%/hr, though a temperature increase of 6C to 10C may be more usual.

In the stack system, a perforated plenum tunnel is used to form the base of the stack and to distribute the air uniformly. See Figure 9.15. The initial moisture content determines how large the stack can be: 8 sacks high for an initial mc of 25% and 12 to 13 high for an initial mc of 18%. A fan is used to blow air through the stack. This is normally diesel powered.

Figure 9.13 Warmest areas in grain.

Figure 9.14 Platform drier with concrete panels on brick piers.

With both platform driers and sack-stack drying, there are some points which need to be borne in mind. Firstly, any gaps between sacks should be filled with empty bags or straw to minimize air leakage. Secondly, as pointed out earlier grain should be cooled before being left for storage.

Drying Problems

Overdrying grain with excessive temperature can set up stresses in the individual kernels leading to cracking and loss of viability. Another effect of overdrying is that all moisture lost below the safe storage mc is a loss in the value of the crop if not considered when the grain is sold. For example, 10tonnes of grain at 15% mc weights 340 kg less at 12% mc.

Air Short-circuiting means that the air will always take the path of least resistance which, with grain, is usually the shortest route possible through a batch. Figure 9.16 illustrates this principle and emphasizes the need to level the grain and provide a uniform depth in any forced air system.

Figure 9.15 Sack drying with a diesel powered fan.

Figure 9.16 Air taking shortest path.

Dirty Crops such as grain with a large proportion of chaff, fine seeds and dirt becomes more difficult to dry as the resistance to airflow increases due to spaces between grains being blocked. Table 9.5 while dealing with clean grain, shows the great effect that small particle size has on the resistance to air flow. It is important therefore to have grain as clean and uncontaminated as possible.

Cleaning techniques range from the traditional winnowing of crops by throwing them into the air, to sophisticated modern high throughput equipment. The two techniques used on small farms are winnowing and sieving.

Sieving is usually a two stage operation. The first sieve is just coarse enough to let the grain through while rejecing all larger particles. The second sieve is just fine enough to hold the grain being cleaned, but it passes weed seeds and particles that are smaller than the grain.

Grain may sometimes have a preliminary cleaning before storage to remove the majority of contaminants, and then a second more thorough cleaning before sale. This would apply in particular to seed grain.

Instruments

The measuring of temperature, relative humidity, static pressure and air flow are discussed in Chapter 7. However, the special situations found in making these measurements relative to grain drying and storage will be discussed in more detail.

Thermometers

Although mercury-in-glass thermometers are fragile and rather slow acting, they are probably the most dependable means of measuring temperature. They may be protected by mounting in a groove in a wooden or metal probe so that temperatures deep in piles or bins may be checked. Care should be taken to allow several minutes for the temperature to stabilize.

Thermistors and thermocouples are convenient for remote measurements but they are more costly and subject to misadjustment.

Air Flow Meters

Airflow meters similar to the one shown in Figure 9.17 are available to measure the vertical air speed through grain being dried in bulk. The conical clear plastic tube contains an aluminium disc which can slide on a wire mounted along the axis of the tube. A metal cone at the base of the plastic tube supports the instrument on the grain and collects the emerging air. The plastic tube is graduated in m/ s and the air speed is read at the point where the disc is 'floating' on the air passing up the tube. In order to obtain consistent and accurate readings the disc should move freely on the wire.

For very simple and rough airflow assessment in a fan ventilated bin, a square of light material such as a handkerchief, about 300mm square, laid on the surface of the grain should be lifted if enough air is passing through the crop.

Figure 9.17 Grain airflow meter.

Manometers

The quantity of air delivered by a fan is related to the static pressure against which the air is being delivered. By measuring static pressure and referring to the relevant fan performance data, an approximate guide to the quantity of air being delivered can be obtained. Figure 7.8 shows a simple manometer.

For all types, it is important that the sensing head (static tube) be mounted in a position in the main air duct where the mean static pressure can be monitored. In practice, a position near the top of the main air duct and a distance of at least twice the fan diameter from the fan is normally satisfactory. The lower the airflow at the sensing location, the truer the static pressure reading will be.


Contents - Previous - Next