GB2213695A

GB2213695A - Apparatus for soil sterilisation

Info

Publication number: GB2213695A
Application number: GB8830040A
Authority: GB
Inventors: Edward Mcgrath
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-01-15
Filing date: 1988-12-22
Publication date: 1989-08-23
Also published as: GB8800885D0; GB8830040D0

Abstract

Soil sterilisation apparatus for the introduction of a gaseous sterilant into a growing medium such as soil or the like, comprising an elongate duct (13) having means at one end for connection to a source of sterilant gas and a plurality of transversely extending branch delivery ducts (19) extending laterally from the elongate duct in a comb-like structure, each branch duct (19) having a plurality of openings (20) in the side wall and a shroud (27) surrounding the branch duct (19) at least in the region of the openings (20) and held spaced from the branch duct (19) by an internal perforated spacer (26) adjacent an outlet end (33) having an associated tapering end stop member (18) and, at the other end, an imperforate annular support plate (29). <IMAGE>

Description

APPARATUS FOR SOIL STERILISATION The present invention relates generally to apparatus for soil sterilisation and to methods of effecting such sterilisation.

In environments where cultivation is undertaken on a regular basis, for example within greenhouses and the like, the population of pests and fungi can increase dramatically from the regular presence of favourable circumstances. Unfortunately, it is necessary to maintain these growth-promoting conditions for the plant crop and techniques for inhibiting or restricting the growth of unwanted pests or fungi have to be performed in order to cleanse the soil or other growing medium to make it possible for an adequate crop of the desired plant species to be obtained. One technique for achieving this end is the use of high temperature usually applied by transmitting steam through the soil.

The principles of steam sterilisation are widely known.

The steam serves solely as a heat-transmitting medium and is injected into the soil in order to raise the temperature thereof. Upon introduction into the soil the steam condenses, giving up its heat to the soil and thereby progressively infiltrating the soil with a heat front" which proceeds to propagate through the soil in the manner of a way front from the point of injection.

It is known that in order completely to destroy not only the harmful pests and diseases within the soil, but also to inactivate virus particles it is necessary to maintain the soil at a temperature of at least 180 degrees Fahrenheit for a period of not less than 15 minutes.

However, since the introduction of steam into the soil is a commercial process involving an expenditure of resources both in heating the water to form steam, delivering the steam to the soil and supervising the process to ensure than an adequate temperature is achieved, and since the cost of this process must be offset against the proceeds obtained from cultivation of the soil subsequently, it is essential that efficient operation of the sterilisation processes be undertaken in order to ensure that the cost of sterilising the soil does not exceed the value which can be obtained from such sterilised soil. For this reason it is not possible simply to supply an excess of steam as rapidly as possible since the steam condensation rate must be maintained at a level sufficient to ensure that the majority of the heat in the steam is given up to the soil before the steam passes out.The quantity of steam being produced must therefore be calculated from the quantity of water in the boiler and the rate at which it is evaporated, and the area of soil treated thereby can be calculated with reference to the number of outlet points in the steam distribution system. Such calculations can be upset, however, if one or more of the delivery outlets become clogged or at least seriously blocked by soil since the rate at which the steam passes out from that particular delivery point will be very much less than the mean and correspondingly other outlets will deliver more than the calculated value. This results in the dual disadvantage of a waste of steam from those outlets delivering more than the calculated value and an inadequate sterilising effect in the soil regions immediately surrounding the blocked outlet.Not only does the sterilising process cost more, in such circumstances, but also the beneficial effect may be lost at least partly, and in some cases entirely, by the subsequent growth of pests or fungi from the region improperly sterilised during the process.

Conventional systems for effecting sterilisation include the so-called "grid" steaming system in which forked grids are buried in trenches at approximately two thirds of'the depth of soil intended to be sterilised. Another known system is the so-called Hoddesdon pipe, which is a single pipe as opposed to a forked grid. The function is similar to the grid, and the pipes are buried to the same depth as a grid would be. Pipes may be something in the region of two metres long and the spacing is normally not more than 25t greater than the depth to which they are buried. Hoddesdon pipes have been found to be useful when the whole floor is steamed in a greenhouse having relatively few obstructions, such as wide-span greenhouses where large mechanised cultivations can be used to prepare the soil for steaming.Both grids and Hoddesdon pipes are left in place, however, when not in use and consequently constitute a fixed investment for the greenhouse. By contrast, an alternative known system is the so-called "comb" system which consists of a main pipe of rigid material having a plurality of parallel spikes or hollow tubes spaced from one another and closed at the free end joined to the main pipe in a structure resembling a comb. Spaced from the free end of the spike the tube is restricted with a "waste" and through the waste are formed a number of holes to allow the steam to escape into the soil when the comb is inserted down through the surface. The horizontal tube is fitted with wooden handles at each end allowing it to be manipulated and pressed into the soil.With such apparatus the soil within an area such as that defined by a greenhouse can be sterilised progressively by moving the comb, or a series of such combs, through the soil introducing the spikes vertically at regular intervals and supplying steam for a known period of time at each location. Such an arrangement has the advantage of being less expensive since it does not require a fixed capital investment in association with any given area of ground, but the spikes have the disadvantage that the holes in the wasted portions can become clogged during insertion or extraction and, as before, clogging of the delivery outlets results in uneven or imperfect sterilization.

The present invention seeks to provide apparatus by which soil sterilisation by the introduction of a sterilising gas such as steam can be achieved without the risk of clogging to the delivery apertures and without the necessity for extensive expenditure on permanent delivery ducts.

According to one aspect of the present invention, therefore, there is provided soil sterilisation apparatus for the introduction of a gaseous sterilant into a growing medium such as soil or the like, comprising an elongate duct having means at one end thereof for connection;to a source of sterilant gas and a plurality of'transversely extending branch delivery ducts projecting laterally from the elongate duct in a comb-like structure, in which each branch duct has a plurality of openings in the side wall thereof and a shroud or baffle surrounding at least that part of the branch duct wall pierced by the openings and spaced therefrom to allow the passage of sterilant gas.

The shroud or baffle then protects the outlets from the branch delivery ducts from the ingress of soil and, as will be described in more detail below, the outlet from the shroud can be formed in such a way that the entry of soil or clogging thereby is extremely unlikely.

In a preferred embodiment of the invention the said branch delivery ducts are closed at their free ends remote from the elongate duct, and the closure of the ends of the branch delivery ducts are preferably achieved by a stop end member having a tapered, conical shape the wider end of which is not less than the diameter of the branch delivery duct itself.

Preferably the stop end member has a conically tapering surface converging to a pointed tip end coaxial with the branch delivery duct. Embodiments may be envisaged, however, iz which the conically tapering surface converges to a rounded or frusto-conical tip end rather than to a pointed tip end since such may nevertheless be sufficiently intrusive to allow the branch delivery duct to be pressed into soft soil. It is possible to envisage circumstances in which the conically tapering surface of the stop end member may advantageously be non-coaxial with the branch delivery duct as well, for example being offset to one side or the other.

The conical surface of the stop end member preferably diverges at its wider end to a diameter greater than that of the shroud surrounding the branch delivery duct.

The end of the shroud remote from the elongate duct may be spaced from the stop end member, and a portion of the branch delivery duct projects therefrom to support the stop end member.

The end of the shroud spaced from the elongate duct may be held substantially coaxial with the branch delivery duct by an apertured plate located inwardly of the open end of the shroud. Again, strict coaxiality is not essential and an asymmetric or non-coaxial configuration may be adopted and is intended to fall within the scope of the invention defined herein.

The stop end member may include an internal reinforcing spigot extending within the branch delivery duct at least as far as the said apertured plate, and such spigot may provide substantially the whole of the support for the conically tapered stop end member. The axial distance separating the end of the shroud remote from the elongate duct from the branch delivery duct stop end member is preferably less than the diameter of the shroud and may also be less than the diameter of the branch delivery duct itself.

The end of the shroud nearest the elongate delivery duct may be held concentrically, coaxially or otherwise with respect to the branch delivery duct by an imperforate plate sealingly secured thereto and to the branch delivery duct.

The present invention also comprehends apparatus for sterilising soil or other growing or cultivation medium by permeating it with a sterilising gas, including at least one soil penetrating gas delivery member having a smooth, cylindrical outer surface, a tapering tip at the free end to facilitate introduction of the member into the soil or other cultivation medium, means for introducing a sterilising gas into the other end of the delivery member, and gas outlet means in the form of an at least partial circumferential outlet opening extending around the periphery of the member for the escape of gas introduced into the inlet end thereof, the said outlet opening communicating with an interior annular axial passage within the member having a perforated baffle therein spaced axially from the peripheral outlet opening in a direction towards the said inlet end of the member.

Apparatus formed in accordance with this aspect of the invention may incorporate a plurality of such members in a comb-like delivery system.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of a structure formed in accordance with the present invention; and Figure 2 is a sectional view of one of the soil-penetrating elements of the apparatus of Figure 1.

Referring now to the drawings the apparatus shown comprises the steam delivery component of a system having means (not shown) for boiling water to generate steam, and for delivering the steam in a substantially dry state into a greenhouse or to another area to be treated. The delivery tube from the boiler to the distribution outlet may be a pipe of between 11/4 inch and 2 inch diameter.

The larger diameter pipes are preferable for longer distances in order to avoid the pressure drop which occurs in smaller diameter pipes. For example, in a 300 foot long pipe the pressure drop at 11/4 inches from, typically, 100 lbs per square inch at the boiler to zero at the outlet, with a steam flow in the region of 500 ibs per hour. At the same length in a larger pipe, for example 11/2 inches, the pressure drop over the same length would only be 24 Ibs.

Because of the high temperature in the pipes leading to the distribution outlets it is usually advisable for these to be insulated if economic considerations justify it.

It is also necessary to be able to establish accurately the temperature of the soil or other growing medium being sterilised, and this may be achieved using a surface-mounted soil temperature sensor. It is unnecessary to probe into the surface since it can be assumed that the "heat front" discussed above progressing from the distribution points below the surface will arrive at the surface only after the soil or growing medium between the delivery outlet and the surface have been heated to the same temperature. A rise of temperature at the surface to the required level will therefore indicate that the underlying soil has also been raised to the required temperature.

As shown particularly in Figure 1, a steam supply pipe 11 delivers heated steam to a distribution "comb" comprising a rigid tubular main pipe 13 having a closed end 14 and a suitably curved inlet end 15 with a coupling 16 schematically indicated in Figure 1, for connection to the steam supply pipe 11 in any suitable way. This may be by flexible connectors or other pipe coupling means.

Projecting from the tubular main pipe 13 are a plurality of parallel steam delivery outlet pipes 17 in communication with the tubular main pipe 13 at one end and provided with a conically tapering solid tip 18 at their free ends. The detailed structure of each steam delivery outlet pipe 17 is illustrated in more detail in Figure 2.

Referring now to Figure 2 the steam delivery outlet pipe 17 shown is identical with all the others of the array illustrated in Figure 1 and comprises a straight cylindrical perforated pipe section 19 having a plurality of apertures 20 extending over an intermediate portion thereof. At its upper end (in relation to the orientation illustrated in Figure 2) the straight cylindrical perforated pipe section 19 has an unperforated portion 21 the open end of which is welded to an opening in the tubular main pipe 13 so that the interior of the two pipes communicate. At its other end the straight cylindrical perforated pipe section 19 has a distal end unperforated pipe portion 22 to which the solid conically tapering tip 18 is secured.For this purpose the tip 18 has a cylindrical spigot 23 extending coaxially with the conical tip 18 and having an external diameter equal to the internal diameter of the distal end unperforated pipe portion 22. The spigot 23 has a nose end boss 24 of smaller diameter defining an annular shoulder 25 which abuts against a perforated disc 26.

The distal end unperforated pipe portion 22 is welded to the solid conically tapering tip 18 and the interfitting engagement of the cylindrical spigot 23 and the nose end boss 24 with the annular shoulder 25 pressed against the perforated disc 26 forms a very secure and solid fixing for the conically tapering solid tip 18.

Coaxially surrounding the perforated portion of the straight cylindrical perforated pipe 19 is a cylindrical shroud 27 which is made of such a diameter that there is a narrow annular space 28 between the perforated pipe section 19 and the shroud 27. At its proximal end (proximal, that is, to the tubular main pipe 13) the shroud 27 is secured to an annular proximal end plate 29 secured sealingly to the unperforated proximal pipe portion 21. The connection between the annular proximal end plate 29 and the cylindrical shroud 27 is also sealed so that the annular space 28 between the cylindrical shroud 27 and the straight cylindrical perforated pipe section 19 is sealed against the escape of gas.

At its distal end the shroud 27 is secured as by welding to the perforated disc 26 so that the shroud 27 is securely held at each end coaxial with the straight cylindrical perforated pipe section 19. The shroud 27 has a distal end axial projecting skirt 30 extendingbeyond the perforated disc 26 towards the conical solid tip 18, but having a rim 31 spaced by a short distance from the wide end 32 of the conically tapering solid tip to leave an annular peripheral slot-like opening 33 extending around the steam delivery outlet pipe 17.

The diameter of the wide end 32 of the conically tapering solid tip 18 is greater than the diameter of the cylindrical shroud 27 so that when the steam delivery outlet pipe is thrust into the ground the solid tip 18 parts the soil and the shroud 17 enters the aperture thus formed without crumbs of soil entering the annular peripheral slot opening 33. When in place the steam delivery outlet pipe conveys steam from the tubular main pipe, arriving from the steam supply pipe 11 in the manner described above, which steam passes through the interior of the perforated straight cylindrical pipe section 19 passing out through the apertures 20 into the annular space 28. The steam cannot escape upwardly at the proximal end of the shroud 27 because of the sealing closure of the annular proximal end plate 29.However, the perforated disc 26 allows the steam to pass through into the annular region within the distal end axially projecting skirt 30 from where it can pass out through the annular peripheral slot opening 33 into the soil and expand through the interstices of the soil crumb structure heating the soil in a region around the annular peripheral slot opening 33.

Because of its particular configuration the blocking or clogging of the annular peripheral slot opening 33 by the soil into which the steam delivery outlet pipe 17 is inserted is avoided so that a given rate of delivery of steam from the steam delivery outlet pipe 17 can be guaranteed. In this way all of the steam delivery outlet pipes 17 of the comb-like structure illustrated in Figure 1 will deliyer steam at the same rate so that a complete and thorough sterilisation of the soil can be achieved at a predictable rate of delivery so that the length of time for which the steaming operation is performed can be calculated accurately. As mentioned above, this calculation is important for ensuring thorough sterilisation of the soil without wasting heat by applying steam for any longer time than is absolutely necessary with the steam being supplied at the correct rate. The optimum rate of steam supply to the soil requires a loose crumb structure enabling the steam to permeate slowly giving up its heat to the soil as it rises to the surface. If the steam supply rate is too great the steam will be forced to the surface before losing all its heat and any residual heat in the steam will be wasted to the atmosphere without heating the soil.

Claims

1. Soil sterilisation apparatus for the introduction of a gaseous sterilant into a growing medium such as soil or the like, comprising an elongate duct having means at one end thereof for connection to a source of sterilant gas and a plurality of transversely extending branch delivery ducts extending laterally from the elongate duct in a comb-like structure, in which each branch duct has a plurality of openings in the side wall thereof, and a shroud or baffle surrounding at least that part of the branch duct wall pierced by the openings and spaced radially therefrom to allow the passage of sterilant gas.

2. Soil sterilisation apparatus as claimed in Claim 1, in which the said branch delivery ducts are closed at their free ends remote from the elongate duct.

3. Soil sterilisation apparatus as claimed in Claim 2, in which each branch delivery duct is closed at the free end by a tapering stop end member the wider end of which is not less than the diameter of the branch delivery dut.

4. Soil sterilisation apparatus as claimed in Claim 3, in which the stop end member has a tapering surfaces converging to a pointed tip end substantially coaxial with the branch delivery duct.

5. Soil sterilisation apparatus as claimed in Claim 3 or Claim 4, in which the tapering conical surface of the stop end member diverges to a diameter greater than that of the shroud surrounding the branch delivery duct.

6. Soil sterilisation apparatus as claimed in any of Claims 3 to 5, in which the end of the shroud remote from the elongate duct is spaced from the stop end member, and a portion of the branch delivery duct projects therefrom to support the stop end member.

7. Soil sterilisation apparatus as claimed in any of Claims 3 to 6, in which the end of the shroud spaced from the elongate duct is held substantially coaxial with the branch delivery duct by an apertured plate located inwardly of the open end of the shroud.

8. Soil sterilisation apparatus as claimed in Claim 7, in which the stop end member includes an internal reinforcing spigot extending within the branch delivery duct at least as far as the said apertured plate.

9. Soil sterilisation apparatus as claimed in any of Claims 2 to 8, in which the distance from the end of the shroud to the wider end of the tapering branch delivery duct stop end member is less than the diameter of the shroud.

10. Soil sterilisation apparatus as claimed in any of Claims 2 to 9, in which the end of the shroud nearest to the elongate delivery duct is held concentrically with respect to the branch delivery duct by an imperforate annular plate sealingly secured thereto and to the branch delivery duct.

11. Soil sterilisation apparatus for the introduction of a gaseous sterilant into a growing medium, substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

12. A soil sterilisation system comprising a source of a gaseous sterilant, a sterilant distribution system, and soil sterilisation apparatus as claimed in any of Claims 1 to 11.

13; Apparatus for sterilising soil with a sterilising gas, or other growing or cultivation medium by permeating it, including at least one soil penetrating gas delivery member having a smooth cylindrical outer surface, a tapering tip member at the free end to facilitate introduction of the member into the soil or other cultivation medium, means for introducing a sterilising gas into the other end of the delivery member, and gas outlet means in the form of an at least partial circumferential outlet opeing extending around the periphery of the member for the escape of gas introduced into the inlet end thereof, the said outlet opening communicating with an interior annular axial passage within the member having a perforated baffle therein spaced axially from the peripheral outlet opening in a direction towards the said inlet end of the member.