Fire protection

Contents - Previous - Next

Measures to prevent the outbreak of fire and to limit its effect must be included in the design of buildings. Fire prevention measures include the separation of buildings to prevent fire from spreading and to permit fire-fighting, and a farm or community pond as a source of water for extinguishing fires.

Fire Resistance in Materials and Construction

The ability of a building to resist fire varies widely depending upon the materials of construction and the manner in which they are used. Resistance to fire is graded according to the period of time that an element of construction is able to withstand standardized test conditions of temperature and loading.

Bare metal framework and light timber framing exhibit a low order of fire resistance and both types of construction fail to qualify for a grading of one-hour fire resistance, which in many countries is the lowest grade recognized. In contrast, most masonry walls have good fire-resistance ratings. Timber framing can be improved with the use of fire-retardant treatments or fire-resistant coverings such as gypsum plaster or plasterboard. Steel columns can be protected with plaster or concrete coatings while steel roof trusses are best protected with suspended ceilings of gypsum plaster or plasterboard.

Classification of Fire Hazards

Some types of activities and installations in farm buildings constitute special fire hazards. Wherever practical they should be isolated in a room of fireproof construction or in a separate building away from other buildings. A list of special fire hazards includes:

In addition, lightning, children playing with fire, smoking and lanterns are origins for outbreaks of fire. Thatched roofs are highly combustible and prone to violent fires.

Fire Separation

Fire spreads mainly by wind-borne embers and by radiation. Buildings can be designed to resist these conditions by observing the following suggestions:

Evacuation and Fire Extinguishers

In the event of a fire outbreak, all personnel should be able to evacuate a building within a few minutes and animals within 10 to 15 minutes. Equipment, alleys and doors should be designed to facilitate evacuation. Smoke and panic will delay evacuation during a fire, that evacuation during a fire drill must be much faster.

In animal buildings exit doors leading to a clear passage, preferably a collecting yard, should have a minimum width of 1.5 metres for cattle and 1.0m for small animals so that two animals can pass at the same time. Buildings with a floor area exceeding 200m should have at least two exit doors as widely separated as possible. The travel distance to the closest exit door should not exceed 15 metres in any part of the building.

Fire extinguishers of the correct type be available in all buildings and in particular where there are fire hazardous activities or materials. Water is commonly used for firefighting, but sand or sandy soils are effective for some types of fire. Dry powder or foam type extinguishers are best for petrol, diesel, oil and electrical fires. Regardless of type, fire extinguishers require periodic inspection to ensure their proper operation in an emergency.

Bushfire

The dry season or any period of prolonged drought brings with it a constant fire hazard. Fanned by strong winds and intensified heatwave conditions, a large bushfire is generally uncontrollable.

Firebreaks are an essential feature of rural fire protection and should be completed before the fire season starts. It is desirable to completely surround the homestead with major firebreaks at least 10 metres wide. Breaks can be prepared by ploughing, mowing, grazing, green cropping or, with great caution, by burning, and may include any water-course, road or other normal break which can be extended in width or length.

Shelter belts or even large trees are useful in deflecting wind-borne burning debris. For further protection, all flammable rubbish and long dry grass should be removed from the surroundings of the buildings and any openings such as windows, doors and ventilators covered with insect screens to prevent wind-borne embers from entering the building and starting a fire.

Further reading

Von Blanckenburg, P., Agricultural Extension Systems in Some African and Asian Countries, FAO Economic and Social Development Paper no. 46, Rome, Food and Agriculture Organisation of the United Nations, 1984.

Dillon, J.I., Hardaker, B.J., Farm Management Research for Small Farmer Development, FAO Agricultural Services Bulletin, no. 41, Rome, food and Agriculture Organisation of the United Nations, 1980.

Harwood, R.R., Small Farm Development, Understanding and Improving Farming Systems in the Humid Tropics, Boulder, Westview Press Inc., 1979.

Midwest Plan Service, Farmstead Planning Handbook, Ames, Iowa, Midwest Plan Service, 1974.

Noton, N.H., Farm Buildings, Reading, College of Estate Management, 1982.

Ruthernberg, H., Farming Systems in the Tropics, 3rd ea., Oxford, Oxford University Press, 1980.

Swanson, B.E. (ed), Agricultural Extension, A Reference Manual, 2nd ea., Rome, Food and Agriculture Organization of the United Nations, 1984.


Chapter 9 Crop handling, conditioning and storage

Introduction

Although in many parts of Africa some crops can be produced throughout the year, the major food crops such as cereal grains and tubers, including potatoes, are normally seasonal crops. Consequently the food produced in one harvest period, which may last for only a few weeks, must be stored for gradual consumption until the next harvest and seed must be held for the next season's crop. In addition, in a non-controlled market, the value of any surplus crop tends to rise during this period provided that it is in a marketable condition. Therefore the principal aim in any storage system must be to maintain the crop in prime condition for as long as possible.

Crops grown for food fall into two broad categories, perishable crops and non-perishable crops. This normally refers to the rate at which a crop deteriorates after harvest and thus the length of time it can be stored. While some crops fall clearly into one or another category, others are less well defined. For example cereal grains can be stored for over a year and are considered to be non-perishable, whereas tomatoes are perishable crops and when picked fresh, will deteriorate in days. Tubers such as potatoes, however, may be successfully stored for periods extending to several months.

Although there are methods for preserving many of the perishable crops such as canning, freeze drying etc., but these are normally industrialized processes and not found on farms. It is possible, however, to apply farm-scale methods of preservation to cereals and pulses and the less perishable crops such as potatoes. To do this successfully, it is necessary to know the ways in which a crop can deteriorate and hence the methods for controlling this deterioration. Crops may need conditioning at harvest time to get them into a storable state and they may also require periodic inspection and care during the storage period. Viability of seed must be maintained and susceptibility to damage by fungal and insect pests must be reduced.

The storage and handling methods should minimize losses, but must also be appropriate to other factors such as economies of scale, labour cost and availability, building costs, and machinery cost.

Grain drying

The handling and storage of grains will be discussed in an orderly sequence. First the requirements for safe storage, including the principles involved in both natural and artificial drying, followed by drying methods suitable for the small grower as well as for the larger scale operations of cooperatives and commercial farms.

Finally, various types of storage from family size up to commercial units will be discussed along with management suggestions for preventing damage during the storage period.

Properties of Grains

Cereal grains are edible seeds and as such would eventually be released from the plant when fully mature. Grains can be divided into three groups; cereals (maize, wheat, millet, rice etc.), pulses (beans, peas, cowpeas etc.), and oil seeds (soyabeans, sunflower, linseed, etc.).

Requirements for Safe Storage

Crops left standing unharvested start to show diminishing quantitative and qualitative returns through shatter losses, and attacks by insects, mould, birds and rodents.

It is important therefore that harvesting is completed as soon as possible. In addition, it is necessary to remove dust and contaminants which can include insects, vegetable materials such as bits of straw and chaff, and weed seeds etc. These will fill up pore spaces within the crop and inhibit air movement and add to any possible spoilage problems. The crop must therefore be clean.

One of the most critical physiological factors in successful grain storage is the moisture content of the crop. High moisture content leads to storage problems since it encourages fungal and insect problems, respiration and germination. Moisture content, however, in the growing crop is naturally high and only starts to decrease as the crop reaches maturity and the grains are drying. In a natural state, the seed would have a period of dormancy and then germinate either when re-wetted by rain, or by having a naturally high enough moisture content.

Another major factor influencing spoilage is temperature. Grains are biologically active and respire during storage. One of the products of respiration is heat and reduction of the temperature of the crop can help to reduce the rate of respiration and hence lengthen the storage life by reducing the possibility of germination. Another major temperature effect is on the activity of insect and fungal problems. With a lowering of temperature, the metabolic rate is reduced and consequently the activity causing spoilage.

A damp or warm spot in grain will increase the rate of respiration. In addition to heat, another product of respiration is moisture. The heat and moisture from such a "hot spot" can spread by convection encouraging moulds and bacteria which in turn respire and give off more heat and moisture. It thus becomes a self-generating process. Insect activity also increases with a rise in temperature.

These spoilage mechanisms can also affect the viability of grain required for seed or malting where inability to germinate would render it unmarketable.

The relationship between moisture content and temperature on the storability of crops is shown in Fig. 9.1. It can be seen that the moisture content of grain must be reduced at higher temperatures.

Figure 9.1 Effects in the store at different temperatures and moisture content.

Moisture Content

The moisture content (me) of a crop is normally given on a "wet basis" (wb) and is calculated as follows:

(Weight of moisture / Weight of wet sample) x 100

Occasionally "dry basis" (db) moisture content is given and it is important to know which has been used.

For example, if 100 kg of moist grain is dried and loses 20 kg of water, the mc is:

20 x 100 / 100 = 20% on a wet basis (wb) or

20 x 100 / 80 = 25% on a dry basis (db)

Grain will normally be harvested at a moisture content between 18% and 25% (wb) though it can be substantially higher or lower depending on many factors (such as stage of maturity, season, weather pattern and drying facilities).

Moisture Content Measurement

Moisture can be determined in the laboratory by a number of methods, of which the oven drying method and the distillation method are the most accurate, these are normally used as references for moisture meters used under field conditions.

Laboratory methods requires a representative sample of the grain. Since the mc is unlikely to be uniform throughout a batch of grain, it is essential that:

The oven-drying method is the most straight-forward and requires an accurately weighed sample of grain to be dried for a period of time and then re-weighed. The scales should preferably be electronic unless a large enough sample is used in which case good mechanical scales can be used.

The rapid oven method is one of a number of more rapid laboratory methods that have been designed. Those methods range from simple, inexpensive pieces of equipment to highly sophisticated and expensive instruments.

A typical simple method consists of shining an infrared lamp on a balance pan containing a ground sample of approximately 5g. The sample is exposed to the intense heat of the lamp for a predetermined period and the loss in weight is shown on a scale calibrated in percentage moisture content.

The Salt-jar Method is a simple field method of determining whether maize is dry enough for storage in bags.

A teaspoon full of dry non-iodized salt is placed in a thoroughly dry jar (or bottle) with a tight cover. The salt should not stick to the sides of the jar when it is rolled. Then a cob of maize is shelled, the kernels placed in the jar and the cover put on tightly. The jar is then shaken and rolled gently for 2 to 3 minutes. If the salt does not lump or adhere to the sides of the jar, the moisture is usually below 15%.

Moisture Meters measures one or more electrical properties of the grain which are closely related to moisture content. Although acetylene and hair hygrometer measurement techniques have been used in the past. the most commonly used now are the electrical moisture meters.

Relative Humidity (RH)

Relative humidity as a measure of air moisture is defined in Chapter 7. It is a useful factor relative to grain drying. The relative humidity of ventilating air indicates how much, if any, moisture can be removed from the grain with unheated air and is a basis for deciding on ventilation rates and air temperatures.

Relative Humidity Measurement

Of the devices available for measuring RH, one of the simplest and most accurate is a psychrometer. The temperatures of the wet-bulb and dry-bulb thermometers mounted on the instrument are noted and the values used with a psychrometricchart. In fan systems the psychrometer may simply be held in the air stream to get a reading.

Drying Theory

Equilibrium Moisture Content

All produce has its own characteristic balance (or equilibrium) between the moisture it contains and the water vapour in the air with which it is in contact.

This is known as the equilibrium moisture content (EMC). When food grains containing a certain amount of moisture are exposed to air, moisture moves from the grain to the air or reverse until there is a balance between the moisture in the grain and in the air.

Each food grain has a characteristic equilibrium curve which is obtained by plotting a graph of moisture content against relative humidity and temperature of the air. Curves for some common food grains are given in Figure 9.2. These values must be considered only as a guide since different types and varieties of grain vary in their equilibrium values. The EMC will also vary slightly with the temperature. For most cereals it will drop about 0.5% for every 10C temperature rise at the same %RH of the air.

Figure 9.2 Equilibrium Moisture content for some different crops

Table 9.1 shows Moisture Content Equilibrium Values for a range of produce at 70% relative humidity and 27C, the maximum acceptable level for storage of any sample.

Since sacks are porous and allow air to circulate through the crop readily, it is generally acceptable to allow the grain to be stored at 1 % to 2% mc higher than in bins or containers with non-porous walls.

Table 9.1 Equilibrium Moisture Content Values EMC at 27 C and 70%RH

  EMC
Maize 13.5
Wheat 1 3.5
Sorghum 13.5
Millet 16.0
Paddy 1 5.0
Rice 1 3.0
Cowpeas 1 5.0
Beans 1 5.0
Groundnuts (shelled) 7.0
Copra 7.0

In addition to temperature and moisture content, storage of grains can also be affected by atmosphere. If damp grains is held in a sealed container, respiration of grain and insects, will make use of the available oxygen. As this is depleted, it is replaced with carbon dioxide. This in turn inhibits the activity of the insects and fungal problems and it will decrease to the point that it virtually ceases. Storage in this manner can however cause taints in the grain which render it less acceptable for human consumption.

Storage of seed grain requires conditions which will not only maintain viability at it's peak, but at the same time will avoid all possibility of germination in store. High moisture content and low oxygen storage may decrease viability and therefore should be avoided for seed storage. At the same time, to avoid any danger of germination or fungal and insect problems in store, seed should be dried to 1% to 2% drier than for human consumption. Additionally, it is important to maintain the temperature of the seed as low as possible.

Temperature, Psychrometrics of Drying

Grain to be stored in bins or sacks may have too high a temperature or too high a moisture content or both. If ambient temperatures are low, then air alone may cool the stored grain enough to prevent mould and insect damage while the moisture content is being slowly reduced to a safe level. If the air temperature is too warm (over 10 C), drying may be hastened by heating since by heating the air further, it increases it's capacity to pick up moisture.

Psychrometrics are discussed in Chapter 7, but as an example, Figure 9.3 shows the effect of heating the air, thereby increasing its capacity to pick up moisture.

Example: The ambient air at 25C and 70% RH is heated to 45C and 24% RH. Then upon passing through the grain, it gains enough moisture to again reach 70% RH while the temperature drops to 30.1 C. Then each kg of air will have removed (0.0230.0167) = 0.0063 kg of moisture. Whether the air returns to 70%, RH or some other level, will depend on air velocity through the grain

Figure 9.3 The effect of heating air for drying (from 1500m psychrometric chart).

Loss of Moisture As grain dries, it releases its moisture into the drying air and consequently loses weight.

The weight of grain after drying may be found with the following equation:

W2 = W1- W1 (M1 - M2) / (100 - M2) where:

W1 = Weight of undried grain (kg)
W2 = Weight of dried grain (kg)
M1 = Moisture content of undried grain (%)
M2 = Moisture content of dried grain (%)

For example, if 200 kg of peas at 32% moisture content are dried to 19% moisture content, what is the weight of dried peas?

W2 = 200(32 - 19) / (100 - 19) = 200 - 32.1 = 167.9 kg

When the moisture content of the grain to be dried has been determined, it is possible to check the progress of the drying process by using the following procedure: Before the drying starts, place a weighed sample of the undried grain in a porous sack and bury it in the surface of the bin of grain. At any time during the drying process, the sack may be removed, weighed, and returned to the bin. Then, using the initial weight, the initial moisture content and the newly observed weight in the following equation, the current moisture content at that specific level may be calculated:

M2 = 100 - W1 (100 - M1) / W2

Drying Systems

Selection Systems for drying grains range from thin layer drying in the sun or a simple maize crib to expensive mechanized systems such as continuous flow driers. The choice is governed by a number of factors including:

Rate of harvest: the capacity of the system must be able to keep pace with the rate at which the grain arrives at the store on a daily basis. It is essential that loading and drying does not hold up the harvest.

Total volume to be dried: this may not be the total volume of the crop. If the harvest normally starts as a rainy period finishes, it may be necessary to dry the early part of the harvest, but not the later part.

Table 9.2 Weight of Grain After Drying (% of Original Weight)

Initial

Final mc %

mc (%) 18 17 16 15 14 13 12 11 10
28 87.8 86.7 85.7 84.7 83.7 82.8 81.8 80.9 80.0
27 89.0 88.0 86.9 85.9 84.9 83.9 83.0 82.0 81.1
26 90.2 89.2 88.1 87.1 86.0 85.1 84.1 83.1 82.2
25 91.5 90.4 89.3 88.2 87.2 86.2 85.2 84. 3 83.3
24 92.7 91.6 90.5 89.4 88.4 87.4 86.4 85.4 84.4
23 93.9 92.8 91.7 90.6 89.5 88.5 87.5 86.5 85.6
22 95.1 94.0 92.9 91.8 90.7 89.7 88.6 87.6 86.7
21 96.3 95.2 94.0 92.9 91.9 90.8 89.8 88.8 87.8
20 97.6 96.4 95.2 94.1 93.0 92.0 90.9 89.9 88.9
19 98.8 97.6 96.4 95.3 94.2 93.1 92.0 91.0 90.0
18   98.8 97.6 96.5 95.3 94.3 93.2 92.1 91.1
17     98.8 97.6 96.5 95.4 94.3 93.3 92.2
16       98.8 97.7 96.6 95.5 94.4 93.3

Storage system: In many cases, the storage system and the drying system may be the same structure. For example a ventilated maize crib (see fig. 9.5) used for drying the crop naturally, is likely then to be used to store the crop shelled in bags. Some bin drying systems have a similar dual purpose.

Cost: both capital cost and running cost should be taken into account.

Flexibility: the likelihood of different crops requiring drying should be considered.

Drying Systems fall into two principle groups:

Natural drying using ambient air temperature, and either direct sunlight or natural air movement through the crops.

Artificial drying using fan assistance to move air through the crop with the air either at ambient temperature or artificially heated.

Additionally, drying can be considered in terms of the thickness of the bed of grain being dried, i.e. either shallow layer drying or deep bed drying. Natural drying requires the grain to be in shallow layers whereas certain fans can push air through grain several metres deep.

Click here to continue


Contents - Previous - Next