Contents - Previous - Next
To conduct fumigations of commodities on a nonemergency basis, it is advisable to install a specially designed fumigation chamber. For many commodities the infestation may be controlled by fumigation at atmospheric pressure. Vacuum fumigation is recommended for the treatment of certain densely packed or absorbent materials. It is also used when a rapid turnover of certain goods is required. This technique is discussed in Chapter 9.
The purpose of a fumigation chamber is to allow fumigations to be carried out efficiently, safely and economically. The basic elements for design and construction should be incorporated in all chambers with variations made to suit individual needs. An effective fumigation chamber must be:
There is room for very wide variation in the design of atmospheric fumigation chambers. The fumigant to be used, the siting and size of the chamber, the type of goods and methods of handling, the availability of building materials, as well as the expenditure that can be allowed for increased efficiency and ease of operation, must all be taken into consideration. If chambers are used constantly or are loaded with heavy materials, a sturdy construction is necessary (Figure 22). However, light, portable chambers made from plywood or similar material may be appropriate for some types of treatment. A simple portable plywood chamber that was designed for travel from one forestry nursery to another is illustrated in Figure 23.
A portable gas-tight tent (shown in Figure 26) was devised by Brown and Heseltine (1964) and a full description of its construction and operation was given by these authors. It proved to be very useful for the fumigation of small commodity lots and for nursery stock.
While the safest arrangement is to have the fumigation chamber located outside the main buildings, there may be circumstances in which it is safe to have it inside. If the chamber is properly constructed and equipped, it may be placed in a well-ventilated part of a building not regularly used by employees. When a chamber is located where people are constantly working it may be separated from the working space by an additional wall with a ventilated space between.
The cost of loading and unloading the chamber largely determines the cost of fumigation. Therefore, by choosing 8 site which will permit the handling of goods in an efficient and economical manner, these costs can be reduced to a minimum. Some suggestions for locating a chamber in a warehouse are given in Figure 24.
After the removal of treated materials from a chamber, fumigants continue to diffuse. Therefore, the commodity must be kept on an open platform or in a well-ventilated room for 24 hours after treatment.
One important way to protect employees from exposure to fumigants is by means of a well designed exhaust system. This system should be equipped with one or more fans or blowers powerful enough to draw a continuous draught of air through the door into the outside atmosphere. A movement of air equivalent to one complete air change every one to three minutes in the empty chamber is recommended. If the chamber is located near dwelling houses, the exhaust pipe should terminate well above the roof of the building so that fumes may not enter occupied rooms during the aeration process. In some countries the spent fumigant is absorbed by passing the exhaust gases through absorbing solutions and degrading to harmless end products (Mori, 1980). For chambers up to 56 m³ in capacity (2 000 ft³ ), an exhaust stack reaching 4 m (13 ft) above the roof or any any nearby structure should be adequate. When fumigants are thoroughly mixed with air, they dissipate rapidly on discharge into the open air from the fumigation chamber.
If the chamber is to be permanently fixed inside a building, it may incorporate a part of the floor, two existing walls and even the ceiling. A generalized plan indicating some of the essential features of an atmospheric fumigation chamber is shown in Figure 25. Suggestions for constructing a chamber from various materials, together with details of accessory equipment, are given below.
The chamber size should be such that it can normally be loaded to its full capacity. There is, however, no difficulty in fumigating a partly filled chamber. While the dimensions of a fumigation chamber can only be decided by the owners, as a general guide a chamber should be approximately twice as long as it is wide with a height of 2-3 m. Even distribution of fumigant is more easily obtained in such a chamber than in a square one or one more than 3 m high.
FIGURE 24. - Possible positions for fumigation chamber in relation to storage facilities (UK, 1973)
FIGURE 25 - Generalized plan of an atmospheric fumigation chamber.(J.E. King)
The maximum economic size is probably of the order of 100 m3. If a greater capacity is required, two or more smaller chambers often prove more economical and easy to handle. One chamber can be unloaded and reloaded while the second chamber is in use. The dimensions of the chamber are as important as the overall capacity, particularly where mechanical handling machinery is used. for instance, the size of pallets, if used, will dictate the proportions of the length and width. The height will depend on whether goods are loaded manually or with a fork lift truck (UK, 1973).
The most satisfactory type of chamber, and one which is likely to give the minimum of trouble from leakage, is one with a concrete floor, walls of brick or poured dense concrete or other similar solid building materials and a flat roof of reinforced concrete. Chambers made of timber framing covered with sheets of material are likely to produce initial difficulties in sealing and continuing difficulties due to deterioration or damage. When made of light materials, chambers are prone to leakage and may need an impermeable flexible film lining on the inner surface or regular renewal of the painted finish.
Paints should be applied, preferably by spraying, to give a continuous, impervious surface. A relatively durable finish can be obtained with an alkali-resisting primer followed by two coats of black oil paint and two coats of white. The use of the two colours facilitates inspection of damage after the chamber has been used (UK, 1973).
Walls. The outside walls of the chamber may be constructed wholly or in part of concrete, concrete blocks, sheet iron, plywood or tongue and groove boards. All these materials may also form part of the inner lining, but careful sealing of all joints and seams is essential.
Rough concrete and brick sorb fumigants and, if used, must be covered with hard finishing cement, with two or three layers of asphalt paint applied over its surface.
Plywood sheets, held by a framework of 5 x 10 cm ( 2 x 4 in) timbers, may be used for the interior walls of chambers.
All seams between sheets and at junctions of the floor with walls and ceiling must be carefully sealed with materials such asphalt cement or similar caulking compounds. With plywood it is advisable to seal all the inside surfaces with a primer, which is then covered with a suitable resin-base varnish. This protects the wood from moisture and reduces loss of fumigant through sorption by the wood.
If a plywood-lined chamber is to be subjected to rough usage with heavy bays or cartons, it is suggested that a sheet-metal lining be added. All joints and nail holes in the sheet metal should be carefully soldered.
When tongue and groove boards are used, they must be in two layers, placed diagonally, with a layer of roofing paper between. Insulation may be pieced between the layers if required. All joints must be sealed with great care, using a suitable cement. The inside surface should then be covered with a plastic film lining or with a primer and varnished as recommended above for plywood.
If a chamber detached from a building is to be used in the winter, it is recommended that the outer walls be of cement or brick. The inner wall may be made of plywood, which is separated from the outer wall by insulating material and a vapour barrier. Careful attention must be paid to the proper sealing of the plywood to prevent an accumulation of fumigant in the interspace.
Sheet or corrugated iron has been used successfully in warmer climates for the walls of chambers. When metal is used, great care must be taken to ensure proper sealing. All seams and joints are liberally filled with mastic end the edges of the overlapping sheets covered with mastic tape. In chambers of this type, the ends, top end bottom of the corrugated iron well rest firmly in neoprene gaskets. It is necessary to have more support members than in a standard sheet iron building, in order to reduce expansion end contraction (Barnes and Reilly, 1956).
Ceiling. Lighter gauges of the materials recommended for the walls may be used for the ceiling. With plywood, a thickness of 1 cm (0.4 in) is usually sufficient. A detached permanent chamber must have a roof above the ceiling to provide all-year-round protection from the weather.
Floor. When the chamber is in constant use, and especially for treating bagged goods, a concrete floor is best. This should be reinforced to bear the greatest expected load. The concrete must be hard-finished to provide a gasproof, nonabsorptive surface. Tongue and groove lumber or plywood, satisfactorily sealed as described above, may be used if built to withstand the loads. Plywood flooring of 1.2 cm (0.5 in) is often suitable for plants and nursery stock if it is well finished in the way described above.
Doors. The chamber can be provided with one or two doors, depending on its size and function. For larger chambers the use of two doors allows the loading of untreated material in one end of the chamber with unloading after treatment from the opposite end (Figure 24a and b). If a chamber is located inside a building, it may be built through a dividing wall with loading and unloading doors on opposite sides of the wall. The segregation of untreated and treated stock in this manner reduces the possibility of crossinfestation.
The door should be as light as possible, it must give a gas-tight fit and it should be of good quality to withstand constant use. A stiff steel or timber frame clad with sheet metal on the inner surface may be satisfactory. The size of the door will be governed by the requirements for loading and unloading, but the smaller the door the easier it is to ensure a gas-tight fit.
Lift doors, which are raised above the chamber during loading and unloading, are very practical when a chamber is inside a building, because they are out of the way when open and do not interfere with the movement of the goods. Lift doors are operated by means of a counterweight and, if this works properly, opening and closing are easy.
Doors sliding on rails are better for installation on the outside of a building, where they can be rolled out of the way along an outside wall. An ordinary hinged door may be installed in a chamber if the materials to be treated can be moved in and out easily. Generally, the doors may be made of the same wood or metal materials, suitably finished, as recommended for the walls.
Door gasked. Proper closure is important because fumigants, such as methyl bromide, escape readily through improperly sealed doors. The best seal is provided by a continuous strip of rubber or neoprene that is soft enough to give a good seal but resilient enough to recover well after continued pressure. Natural rubber may deteriorate after some time if used with methyl bromide, but is readily renewed. Overhead doors or hinged doors should fit neatly into flanges. The best seal is obtained with three strips of rubber, two on one surface, with a gap between that is just wide enough to receive the third gasket, which is on the opposite face. Sliding doors will carry only one strip of gasket on one surface, and this should therefore be 2.5 to 5 cm (1 to 2 in) wide. With all types of doors, particular care must be taken to make a gas-tight join between gasket strips, especially at the corners; this is best effected by liberal use of the adhesive material.
Door clamps. Door clamps are relied on to give a tight seal, and experience has shown that they should be placed not more than 30 cm (1 ft) apart all the way round the edge of the door.
For overhead and hinged-type doors, refrigerator door fasteners are recommended. Small, hinged doors not greater than 1.8 m (20 ft2 ) in area may be closed with ordinary sash fasteners 20 cm (8 in) apart. Sliding doors are forced against door frame gaskets by screw fasteners (Barnes and Reilly, 1956).
If the floor of the chamber is above the outside floor or ground level, the clamps may be extended all along the bottom. A sloping ramp is then required to load and unload the chamber. This may be drawn back when the door is closed. If the chamber floor is at outside level, it is not possible to clamp the bottom of the door unless special provision is made. A suggested arrangement is a shallow trench in the cement floor, which should be wide enough to allow sideways movements of the door and fasteners.
Circulation and venting. Proper circulation and postfumigation venting of the fumigant/air mixture are essential in atmospheric chambers. Efficient circulation ensures that the fumigant is rapidly and evenly dispersed throughout the chamber so that no part of the load is overdosed or underdosed, while forced ventilation removes the fumigant so that the chamber can be safely unloaded after the treatment. Circulation and forced ventilation are essential in large fumigation chambers and advisable even for small ones. A variety of fan systems can be used to achieve adequate circulation or ventilation and their size and capacity are governed by the volume of the chamber and the flow rate required. More details on types of fan systems are given by the U.K. Ministry of Agriculture, Fisheries and Food (UK,1973).
It is suggested that with most treatments of commodities the rate of air flow should give approximately one complete change of air every one to three minutes, based on the volume of the empty chamber. In selecting fans or blowers, due consideration must be given to friction in the exhaust pipe and other ducts in the system. Suppliers of equipment will provide the necessary specifications of the equipment required for a particular installation.
When nonflammable fumigants such as methyl bromide or ethylene dibromide are used, no protection from sparking is needed. With HCN, and other fumigants with wide flammability limits, the motors of fans should be totally enclosed or, preferably, placed outside the chamber, with the blade shaft inserted in a gas-tight connexion in the wall.
The system should be designed so that the same fan will be used for circulation and venting. One method is to place the exhaust door, or port, near the fan so that when it is opened the fumigant/air mixture will be blown out into the open air. Another way is to use a blower with a large inlet and outlet. A diameter of 18 cm (7 in) is suitable. The outlet is fitted to a pipe of the same size. By means of a suitable blast gate or valve operated from outside, the air can be recirculated throughout the chamber by means of ducts or blown out through the exhaust stack into the open air.
If the ducts are used to draw the fumigant/air mixture from the bottom to the top of the chamber, distribution will be greatly improved from the beginning of the treatment. Ducts are best made from galvanized iron; the diameter required to give the suggested air flow of one complete change of air every one to three minutes will depend on the size of the chamber. A suggested arrangment is shown in Figure 25.
Exhaust door or port. The exhaust port may be a small door in the wall of the chamber; it should be at the opposite end of the chamber from the door so that after treatment, during venting and unloading, fresh air is drawn through the chamber from the open door. This exhaust port may slide open or move on hinges as a trapdoor. Experience has shown that the trapdoor may become a serious source of leakage. Great care must be taken to ensure that this door fits tightly in suitable gaskets and is firmly latched so that gas leakage is kept to a minimum. It is advisable to have the port in an accessible place so that it can be constantly checked.
Heating and liqhtinq systems. A heating system is required if the chamber is used in cold weather. Sorption of fumigant on foodstuffs increases as the temperature decreases and, therefore, at lower temperatures larger amounts of fumigant are required and postfumigation ventilation is slower. As a general rule the temperature of the chamber and its load should be 15°C or above. If steam or hot water is available, it may be conducted through pipes set on opposite walls. A standard heating unit with blower may be used, which will also serve for fumigant circulation. However, such a blower should not be used for postfumigation venting unless the heat is turned off. Only nonflammable fumigants should be used in contact with steam pipes.
Electric or radiant heaters may be used if they provide adequate heat at short notice and if the elements are totally enclosed so that glowing wires may not come in contact with the fumigant. It is important to protect the heating units from mechanical damage and to protect the goods being fumigated from scorching or damage by the heater.
Lighting inside the chamber is often necessary since windows are usually omitted from fumigation chambers. The lights should be arranged so that the loading does not obscure them and they must be adequately protected against damage.
Application equipment. Gaseous-type fumigants are introduced from outside the chamber through tubing. Copper tubing is preferred for permanent installations, although a type of plastic tubing, which is not affected by the fumigant, may be used. The size and type of tubing will vary according to the fumigant used. Methods of application of important fumigants have already been given under separate headings in Chapter 6.
It is recommended that gaseous-type fumigants, such as HCN and methyl bromide propelled into the chamber under pressure, be discharged into a shallow galvanized iron evaporating pan, above 10 cm (4 in) wide and 5 cm (2 in) deep and two thirds the length of the chamber, which is suspended from the ceiling well above the load. This will speed evaporation and distribution, especially if the circulation fan blows downward above the surface of the liquid.
Liquid-type fumigants, such as ethylene dibromide, may be poured from a measuring cylinder into a small evaporating pan near the door. When nonflammable fumigants are used, they may be evaporated after the door is closed by warming the pan with a small, totally enclosed heater or heat lamp.
Accessories. There are several items of equipment that contribute to safety or help to achieve good results in the use of a fumigation chamber. All chambers that are not under constant surveillance during actual fumigation should be padlocked from outside. Also, a warning notice should be hung, or tacked, on the door while treatment is in progress. A small window or marine port (obtained from a marine hardware store) permits a view of the inside of the chamber so that thermometers or other instruments can be read from the outside.
One or more thermometers are essential for obtaining accurate readings of temperature in the free air space and in different parts of the load. The thermometers may be entirely inside the chamber or they may have gauges or dials on the outside with cables leading to the sensitive bulbs placed, as required, inside the chamber.
Sampling tubes for fumigant analysis, the number to be determined by the requirement to obtain samples of the gas from all representative parts of the space under fumigation, are useful accessories, which can aid in achieving efficient treatments. If they are installed at the time the chamber is built, they are available for use any time. The most practical arrangement is to have several copper connexions of about ().5 cm (0.25 in) outside diameter set at intervals at convenient points along one or more walls of the chamber. These are carefully sealed at both ends with removable caps or plugs. For sampling, the caps are removed and plastic or copper tubes leading to the gas analysis equipment are fitted to the outer ends of the desired sampling tubes.
When a chamber is inside a building, it is advisable to have a red electric light bulb over the door. This is turned on during the treatment to indicate that gas is inside the chamber. With fixed chambers, it is desirable to have an offset control panel on which as many as possible of the recording instruments, electric switches, valve handles and other controls are situated. Such a unit simplifies operations and improves the appearance of the installation.
Pressure Leakage Test
Atmospheric chambers must retain the fumigant during the exposure period without appreciable loss through leakage to the surrounding atmosphere. The gas tightness of a chamber can be checked by a simple test where a positive air pressure is created and maintained in the chamber for a set length of time. An opening should be provided in the chamber to use a blower or other means to introduce air for creating the positive pressure. This pressure can be measured with an open-arm manometer filled with kerosene, which records the difference in kerosene levels in the two arms of the manometer. The time lapse for the chamber pressure to recede from 50 to 5 mm should be 22 seconds or longer. Inability to develop or maintain adequate pressure indicates considerable leakage and the chamber should be checked for leaks at seams, gaskets and other points. Repairs should be continued until the time for the pressure to recede is more than 22 seconds (for testing yes tightness of large structures see Chapter 11).
A smoke bomb or other device may be used in an effort to determine the areas of leakage (USDA, 1976).
Loading. The manner of loading the chamber will depend on the type of commodity and the method of handling it . A space of at least 30 cm (12 in) should be allowed between the top of the load and the ceiling. Goods can be stacked close to the side walls, provided they are not too near the heating units, but they should be kept clear of the end walls and at least 50 cm from the circulating fan. Bagged or packaged goods should not be placed directly on the floor. If these goods are not placed on platforms or pallets, a wooden floor rack with ample space between the slats should be provided. When methyl bromide is used, no extra space need be left between individual packages and bags because this fumigant penetrates well.
Temperature. The next step is to take the temperature of the material to be fumigated. Probe type "meat" thermometers have a pointed end and are suitable for placing in many materials. They are slow to respond, however, and sufficient time must be allowed for a correct reading. It is advisable to take temperatures in at least four well-separated points in the load. The temperature in the free air of the chamber should also be accurately determined.
For the purpose of selecting the appropriate dosage of fumigant, it is best to take the lowest temperature recorded, either in the commodity or the free space. As a general rule the temperature of the chamber and its load should be 15°C or above. Under certain circumstances, the fumigator should take into consideration the temperature at which the material was kept for several days before the treatment, because this factor might influence the response of the insects during the fumigation. This point was discussed fully in Chapter 2.
Computinq the dosage. The dosages for various fumigants and commodities are given in the Schedules. In some instances the concentration x time (c x t) products have been worked out and are given also; their use is dependent upon an accurate determination of concentrations present in the chamber throughout the treatment. Table 14 gives the ml per 100 ft³ (2.83 m3), equivalent to certain dosages in terms of mg/l for a number of fumigants. For practical purposes, one mg/l is equivalent to one oz/ 1 000 ft³ 3.
Closure. When it is time to start the fumigation, the door is firmly clamped. (With liquids, discs or tablets this may be done after the fumigant is dispensed.) With most of the fumigants commonly used in chambers, the amounts introduced as dosage do not usually exert a significant positive pressure, especially when a full load sorbs some of the gas, or when there is a small amount of leakage. Therefore, all exhaust vents or ports must be tightly closed before the fumigant is introduced. If it is found in practice that considerable positive pressure is produced by the gas, one of the exhaust vents may be left slightly open while the fumigant is being introduced. If the fumigants that are used occupy a comparatively large part of the air space, 8 vent must be left open during application. Examples of such fumigants are the mixtures of ethylene oxide with carbon dioxide, and of ethylene dichloride with carbon tetrachloride.
Chambers used in the open may be equipped with small air pressure relief valves or exhaust tubes, which can be closed or capped as soon as application is finished.
APPLICATION OF FUMIGANT
The general manner in which each of the more important fumigants may be applied has already been described in Chapter 6.
Fumigants, such as HCN and methyl bromide, are discharged from cylinders or other containers placed outside the chamber. When HCN is dispensed into an atmospheric chamber, it is necessary to apply additional pressure to the cylinder. This may be done with a tyre pump, in accordance with detailed instructions supplied by the manufacturer. When cylinders are used, they are placed on a platform scale and accurately weighed. The required dosage is then deducted from the total weight and the scale set at the lower point. The discharge valve on the cylinder is opened and closed quickly when the weight bar is again in balance.
Fumigants which are liquids at room temperatures are poured into a shallow tray or trough inside the chamber or are poured onto burlap sacks or similar material from which they quickly evaporate. After the chamber door is closed, the circulating fans are started and the draught across the liquid or the sacked material hastens evaporation. As stated before, some fumigants with high boiling points, such as EDB, should be gently heated with a hot plate to hasten volatilization.
TABLE 14 - DOSAGE TABLE FOR FUMIGANTS USED IN SMALLER CHAMBERS
Quantities of liquid in millilitres per 100 cubic feet at 20°C equivalent to dosages in pounds per 1 000 cubic feet (to be used for measuring smaller quantities of liquids before evaporation in small chambers).
lb/l 000 ft³
Millilitres of liquid/100 ft³
|Acrylonitrile 34% + carbon tetrachloride 66%||2.1||8.5||17.0||25.6||34.1||68.2||102.3||136.4|
|Ethylene dichloride 75% + carbon tetrachloride 25%||2.1||8.4||16.9||25.3||33.8||67.5||101.3||135.1|
|Ethylene oxide at 7°C||3.2||12.7||25.5||38.2||51.0||102.0||153.0||204.0|
|Methyl bromide* at 0°C||1.6||6.5||13.0||19.5||26.1||52.2||78.3||104.4|
100ft³= 2.83m3- 11b = 16oz - loz/1 000 ft³=g/m³= mg/litre
1 fluid oz (Br) = 28.4 ml
1 fluid oz (U.S.) = 29.6 ml
1 ml = 0.035 fluid oz (Br)
1 ml = 0.034 fluid oz (U.S.)
* Methyl bromide is often dispensed as a liquid held under pressure in a graduated measuring glass, as a "280-ml applicator"
If HCN discs are used in a chamber, they are scattered by the operator. In small chambers of up to 28 m³ (1 000 ft³) capacity, the discs can be scattered from the partly opened door. For this type of application, the fumigator must always wear a respirator.
After the fumigant is applied in the closed chamber, the circulating fans are operated for 15 to 30 minutes. With many commodity treatments, this initial circulation will suffice. When continuous or intermittent circulation is required, as for some fruit and plants, this is mentioned specifically in the fumigation schedules, which follow Chapter 15.
The period of exposure to yaseous-type fumigants begins when the discharge of the fumigant is completed. With liquids and discs, the exposure should be timed from the moment the door is firmly closed.
The exposure periods for the various kinds of treatment at atmospheric pressure are given in the fumigation schedules. Fruit, vegetables, plants, bulbs and nursery stock are exposed usually for 1.5 to 4 hours; seeds and plant products, for 16 to 24 hours. For particularly sorptive commodities that may substantially deplete fumigant concentration, some allowance for this depletion may be necessary (see Thompson, 1970).
Venting and aeration
At the end of the treatment, venting should be commenced by opening the exhaust port or valve and starting the fan. The chamber door should be opened slightly to allow fresh air to flow in. At least 10 or 15 minutes should elapse before the door is fully opened. The time of this interval will depend on a number of factors, but the door should not be fully opened until the operator is assured by appropriate chemical tests, instrumental tests, or from long experience, that it is safe to enter the chamber to begin unloading. First aid kits and gas masks should be available and in good condition at all times.
MOBILE AND PORTABLE CHAMBERS
Fumigation chambers which can be readily moved from one place to another provide a flexible means of disinfestation. Such chambers, like the one illustrated in Figure 26, are often similar in construction and operation to stationary chambers, but of the lightest possible weight. The portable chambers shown in Figures 23 and 26 have already been discussed. Two other types, making use of a convenient water seal for closure, are described below. These may only be used for fumigants such as methyl bromide and ethylene dibromide which are only slightly soluble in water. The actual surface of water exposed to the fumigant is small and does not affect the gas concentration significantly.
Portable Drum Fumigator
A small fumigation chamber (Figure 27), about 200 1 (7 ft³) in volume, can be easily made from a clean steel or iron drum of the same size (50 gallons), as described by Johnson (1940). The best closure is a water seal arrangement with a lid fitting into a collar, or trough, built round the outside of the drum.
The head of the drum is cut out with a cold chisel and the rough edges smoothed. The collar is made of 20- to 24- gauge galvanized sheet metal and is tightly soldered or welded to the drum. The top of the collar should be about 2.5 cm (1 in) below the rim of the drum, so that if water is spilled it does not fall into the drum. A tap or plug near the bottom of the collar permits drainage.
The lid or cover is made of the same material as the collar. It consists of a disc and a rim, which fit neatly into the collar as illustrated in the figure. Disc and rim are carefully soldered or welded to provide a gas tight seam. One or two handles on the lid facilitate lifting.
A small fan or blower is necessary for the proper distribution of the fumigant. It may be operated by electricity from an ordinary lighting circuit or a battery, or manually by a hand-crank on the outside of the chamber with a chain drive connected to the blower axle. The manual arrangement is practical, as no reliance is placed on electric power. However, a special connexion must be fitted where the crank axle passes through the chamber wall. Heat may be provided by ordinary light bulbs, manually or thermostatically controlled. Fans and heating bulbs should be placed below the rack on which the load rests. This rack may be made of rat wire of 12 mm (0.5 in) mesh, or similar material.
Where continuous circulation is required in a small chamber, a suitable small fan, with a high impedance motor, can be connected in series with electric light bulbs located on the outside of the chamber to serve as a rheostat. In this manner, the fan may be operated continuously during the fumigation periods with no increase in temperature because of the heat developed by the motor (Richardson and Roth, 1966).
Equipment such as dispensing tubes, plungers and thermometers may be inserted through gas tight fittings, for example rubber stoppers, in holes in the lid or sides. The water seal in the collar is satisfactory for fumigants such as methyl bromide or ethylene dibromide, which are only slightly soluble in water.
Methyl bromide may be dispensed from the 280 ml applicator already described, or from 20 ml ampoules which are placed in clips in a tray inside the lid before it is closed; the ampoules are thus broken by the plunger.
It should be pointed out that if an ampoule containing 20 ml of methyl bromide is released inside a drum of 200 l (approximately 50-gall) capacity, the resulting concentration will be about 170 mg/l (10.5 lb/1 000 ft³ or 170 g/m³). With this concentration, injury to growing plants may occur even if the exposure period is shortened to give an appropriate concentration x time (c x t) product. For plants, therefore, it is better to use the type of applicator illustrated in Figure 27 so that a small dosage may be applied.
FIGURE 27 - Portable drum
fumigator for use with fumigants slightly soluble in water.
F. 1-lb can CH3 Br
F. 28U-cc applicator
G. Evaporating pan
H. Water jacket
I. Drainage tap
K. Metal grille
L. Circulating fan
M. Heating lamps
If ethylene dibromide is used, the required amount is poured as a liquid into the evaporating pan. This can be done through a stopper after the lid is closed. Ethylene dibromide evaporates slowly and it should be heated with a small heating element without exposed wires, placed below the evaporating pan, in order that a full concentration of gas may be reached at the beginning of the treatment.
When the lid is placed in the water collar, it is necessary to release some of the air trapped inside the drum in order to allow the rim to be well below the water level and to prevent positive pressure being developed inside the drum. The air can be allowed to escape through one of the stoppers in the lid.
The drum should be painted on the inside with a primer and then given a resinbased varnish or other finish appropriate to the fumigant principally used.
Mobile Water-Sealed Fumigator
The fumigator illustrated in Figure 28 is particularly suitable for use in large nurseries or similar establishments, where there is a fairly continuous flow of material requiring treatment.
The figure shows clearly the essential features of the fumigator. It has a water-seal to make a gas tight system and is designated primarily for use with methyl bromide for the treatment of nursery stock or rolls of turf which may harbour Japanese beetle larvae or other soil pests. With minor alterations to the design, other water-insoluble fumigants could be used. It is safe to use methyl bromide gas under an aluminium cover, but the liquid should never be left in contact with the metal for any length of time because of the possibility of fire.
The air pressure release valve prevents the top section of the fumigator from being too buoyant and keeps it well seated in the water seal. Also, positive pressure created by the introduced fumigant may be released through this valve.
Mobile fumigation chambers that have facilities for applying and circulating the fumigant and employ gaskets to give gas tight seals are produced commercially. Models with capacities of 10 and 20 m are available.
Plain Steel Drums with Clamp-Type Lids
Where steel drums, equipped with removable, clamp-type covers and rubber gaskets, are available, they can be used as fumigation chambers without modification, except possibly for the holes required for the insertion of tubing and wire. The rubber gasket provides an adequate seal, but it should be checked occasionally for tightness.
FIGURE 28 - A water-seal
fumigator with mobile base. Cover section fits inside trough when
ready for fumigation
(Plant Pest Control Division, U.S. Department of Agriculture)
A. Cable attached to overhead pulley system to raise and lower cover section
B. Pressure relief valve
C. Internal electric fan
D. Connexion for fumigant applicator
E. Place for load
F. Water drainage point
G. IU-cm trough filled with water for sealing cover section
Contents - Previous - Next