Torches and burners for flame cultivation and flaming

The invention relates to an LPG combusting burner device for flaming and flame cultivation purposes and an improved hand-held torch that may incorporate such burner device. The burner device includes a primary burner (14) having at least one fuel delivery nozzle or jet (80) arranged within an open combustion chamber (71) defined within a burner skirt (70) such as to direct a stream of fuel towards a flame delivery opening of the skirt (70), and a vaporiser (18) located in heat exchanging proximity to the primary burner (14) and having a pressurization chamber with an inlet for liquid LPG fuel and an outlet for gaseous LPG fuel, the inlet being arranged to be in fluid communication with a source of pressurized LPG and the other being in communication with the nozzle of the primary burner. A metering duct (37) is located in close vicinity or within the vaporiser (18) and arranged to discretely limit the amount of liquid fuel entering the vaporisation chamber via its inlet, and a first pressure reduction duct (52) is provided in the flow path of gaseous fuel between the vaporisation chamber outlet and the nozzle of the primary burner (14) the first pressure reduction duct (52) arranged to impart a discrete pressure drop to gaseous fuel flowing therethrough between the vaporisation chamber (50) and the nozzle of the primary burner (14), the discrete pressure drop being such as to generate a low velocity combustion flame in the primary burner (14) during normal burner operation.

FIELD OF THE INVENTION
 The present invention is concerned with flame cultivation burners and
 flaming torches which find application in agricultural and forrestal land
 management, road surface asphalting and similar tasks which require
 application of heat or a flame onto objects. In particular, the present
 invention is concerned with flame cultivation burners and flaming torches
 which use liquefied petroleum gas (LPG) as a fuel source for the bumers,
 and relates to an improved burner with integral fuel vaporiser and
 improved hand-held flame cultivation and flaming torches.
 BACKGROUND OF THE INVENTION AND PRIOR ART
 Flame treatment or "flame cultivation" is well known in agriculture and
 horticulture to control weeds and pests that adversely affect arable land.
 There is a large body of published patent literature concerned with many
 aspects of this technology, U.S. Pat. No. 2,408,328 setting forth the
 basic principles of flame cultivation, and thus, reference should be made
 to this U.S. patent for a detailed explanation of the technique.
 Flame cultivation and flaming burners can be incorporated in hand-held
 torch devices carried by a person for localised or small scale flame
 cultivation to control weeds and plant pests, particularly in hard to
 access locations, such as drainage trenches, irrigation ditches and the
 like, as well as for igniting vegetation for small and large scale
 clearance of arable land, back-burning operations and the like. Such
 burners also find use in motorised flame cultivators incorporating a bank
 or array of burners supported on a boom carried or drawn by a
 self-propelled agricultural vehicle for large scale surface flame
 cultivation of arable land.
 In the following, the expression torch head and burner will be used
 synonymously to denote a hollow prismatic or tubular skirt or casing that
 houses at least one gas delivery jet arrange to emit a jet of combustible
 fuel (either liquid or gaseous) that is combusted at least partially
 within the casing so as to generate a flame that is discharged under draft
 or pressure from an appropriately shaped burner casing mouth. The specific
 configuration and constructional elements of a flame cultivation and
 flaming torch heads can vary greatly and is influenced by factors such as
 the type of application the device is mainly intended for, eg hand-held
 torches vs torch batteries in an agricultural implement, whether direct
 flaming or hot combustion gas weeding is to be employed, the fuel supply
 source, the need for fuel supply regulators and control devices, whether
 continuous or intermittent high intensity heat is to be applied, the need
 for a pilot flame or igniters, the required heat energy output rate,
 burner type (ie combustion of priorly vaporised or vaporising liquid
 fuel), etc. Accordingly, the large number of patent specifications in this
 field of technology (international patent classes IPC A01 M 15, F23 D 11,
 F23 D 14) concern improvements/inventions that are specific to each burner
 type as well as their specific application.
 Agricultural Flame Cultivators
 In the field of flame cultivation of large tracts of land, U.S. Pat. No.
 3,164,927 (Holloway) discloses a flame cultivator in which a plurality of
 main burners are supported on a traverse rig or boom mounted on the rear
 end of a tractor. Two auxiliary burners arranged to direct a gas jet
 obliquely with respect to the main burner streams (but not intersecting
 therewith) are mounted on opposite sides outside the prismatic casing of
 the main burner. All gas delivery jets are supplied with gaseous fuel
 (butane or propane) from a central supply tank mounted on the tractor via
 a central supply line and branch lines having regulator valves disposed
 therein. The gaseous fuel is usually delivered in equal amounts and with
 equal pressure to the identical fuel delivery jet nozzles, and a mixture
 of gas and air is created in the burners that is ignited for flame
 cultivation purposes. It is readily understandable that gas delivery
 pressure for each jet nozzle can be individually controlled by the
 associated control valve in the gas delivery lines to each burner to suit
 varying operational requirements in relation to energy output and flame
 temperature at each burner. It is also readily apparent that where the
 burner configuration is modified to use gaseous fuels, an additional
 vaporiser will be required, as the fuel is conveniently stored in liquid
 form in a tank, in order to vaporise or convert the liquid fuel into its
 vapour or gaseous form prior to delivery to the burners. A similar
 implement is disclosed in U.S. Pat. No. 3,543,436 (Baxter) and U.S. Pat.
 No. 3,425,407 (Furman et al).
 Hand-operated Flame Cultivation and Flaming Torches
 Hand-operated torches represent another area of application and are mostly
 used to destroy plants, e.g. bush, forest undergrowth and the like, in
 land clearing operations. Such type of torches, referred to as flaming
 torches, will generally be constructed to generate high intensity flames
 capable of setting aflame dead wood and live plants alike, whereas with
 hand-held flame cultivation torches the aim is to destroy unwanted weeds
 and vermin without setting the crop plants on fire, and thus torch designs
 are preferred which generate sufficient heat to destroy the cells of young
 weeds and crop pests without use of high intensity flames that will ignite
 the vegetation. This aspect is of particular importance in arid zones.
 Hand-held torches for agricultural as well as other purposes, e.g. road
 works, roof taring and the like, generally share a number of common
 structural features. They have an elongate handle tube or rod which
 carries at one of its distal ends the torch head which incorporates one
 (or more) burners. In simple designs, the fuel supply line is connected
 directly to an LPG hose that is secured at a regulator at the outlet valve
 of an LPG storage cylinder. More elaborate torches incorporate at the
 torch itself a deadman shutter (valve) to turn on or off fuel supply to
 the burners, as well as other regulators and pressure gages in the fuel
 supply line.
 The size of the LPG cylinder of non hand-held torches would usually be
 determined by the required heat output capacity of the torch burners (as
 represented by fuel consumption). With hand-held torches though,
 manoeuvrability of the torch in the field is paramount. For example, small
 hand-held torches such as those manufactured by Primus and sold under the
 label "Gardener" and which are mainly intended for the hobby gardener, use
 small capacity, disposable or refillable LPG cartridges having a self
 closing connecting valve. Such cartridges can have a capacity of 1 to 3
 litres and are attached directly to a fitting at the end of the torch
 handle. The main limitation as to possible cylinder sizes, however, will
 generally be their weight. The cylinder must be able to be carried by the
 torch operator. This is usually accomplished either using a customised
 back pack or a non-motorised cart. Having regard to restrictions imposed
 by the location where the torch is to be used and the surrounding terrain,
 the back pack arrangement is often used in flame cultivation and
 vegetation torching applications where a larger LPG fuel supply is
 required, e.g. 5 to 15 litres cylinder capacity.
 Smaller hand-held torches incorporate a torch burner arrangement that is
 relatively lightweight. However, heavy duty, high capacity torch burner
 heads can have a substantial weight, eg 2 to 4 Kg. Whilst this may not
 seem much, when mounted at the distal end of a long tubular torch handle
 of 1.5 m or longer, proper balancing of the torch head by an operator is
 often difficult and tiring.
 Torches with Integral Vaporisers
 U.S. Pat. Nos. 3,357,474 and 3,486,497 (both granted to Pivonka) describe
 torches with combined LPG fuel vaporiser and burner arrangements within a
 burner casing or head that can equally be integrated in hand-held torches
 for flame cultivation and igniting vegetation (flaming), as well as large
 scale flame cultivation implements and machinery.
 U.S. Pat. No. 3,357,474 in particular discloses a combined burner-vaporiser
 torch that uses liquid LPG as fuel source for the burner. LPG is a readily
 vaporisable liquid fuel (consisting of liquid propane, butane or mixtures
 thereof, with the possible addition of other hydrocarbons with higher
 ebullition temperatures). Pivonka specifically discloses the need for a
 fuel flow control valve in the liquid fuel line running between the liquid
 fuel storage tank and the vaporiser, the valve being positionable in close
 vicinity at the rear end of the burner/vaporiser torch; only the forward
 end portion of the vaporiser and the forward end of the burner shield
 through which the flames exit the burner will be at a high temperature
 during torch operation. Due to its constructional arrangement, the rear
 end of the burner/vaporiser torch remains at temperatures only slightly
 above ambient temperatures.
 When the flow control valve is fully open, LPG is delivered to the
 vaporiser at a pressure which is essentially dictated by the tank pressure
 of the LPG storage cylinder. The flow rate of liquid LPG into the
 vaporiser can be regulated at the flow control valve. In any event, once
 the liquid LPG exiting the burner nozzles is ignited, because the tubular
 housing of the vaporiser is in proximity of the flame, the vaporiser
 chamber will heat up and vaporisation of liquid LPG in the vaporising
 chamber will take place, thereby lowering the density of the LPG and
 generating high velocity gas streams therein and consequently also in the
 burner nozzle chambers. This results in high pressure expelling of fully
 vaporised fuel through the jet orifices of the burner nozzles once the
 temperature level within the vaporiser is sufficient to ensure full
 vaporisation of liquid LPG within the vaporiser chamber, and spitting or
 flutter in the burner flame is avoided. Pivonka specifically states that
 the vaporiser-burner device is aimed at providing a high velocity burner
 flame. Regulation of the flame, and consequently heat energy output by the
 burners, is carried out solely in the liquid fuel line leading to the
 vaporiser by way of the conventional flow regulator valve. Such type of
 arrangement and regulation has inherent weaknesses.
 Firstly, and this is a well known phenomenon, regulating flow of a
 pressurised liquid fuel, which at ambient temperature is gaseous, by means
 of a flow regulator valve (for example, needle valve) will lead to partial
 vaporisation of the liquid either in the valve housing itself or
 downstream of the valve, with the associated take-up of heat by the
 vaporising resulting in cooling and possible freezing of the fuel lines
 and fitting. If the flow velocity drop is substantial, the valve itself
 will tend to frost-up, the ebullition (boiling) temperature of propane
 fuel being around -42.degree. C. Consequently, liquid fuel supply will be
 irregular, the vaporiser receiving in some instances liquid and in some
 instances partially or fully vaporised LPG, depending upon the ambient
 temperature and pressure induced temperature drop in the fuel supply line
 between regulator valve and vaporiser. It will be further noted that the
 torch of Pivonka specifically ensures that the flow regulator in the
 supply line to the burner-vaporiser torch is not heated by the torch.
 A further drawback is the regulating process during steady state burner
 operation. Regulating the flow of liquid LPG into the vaporiser will only
 regulate LPG gas burning rate but have little effect on flame velocity
 over most of the regulator settings. Flame velocity (and burner nozzle gas
 pressure) regulation would require very fine graduation in a small portion
 of the wide regulation bandwidth of conventional regulators. In any event,
 due to the configuration of the vaporiser and the gaseous fuel delivery
 path to the burner nozzles, notwithstanding liquid LPG flow regulation by
 means of the control valve, the burner will operate in steady state with
 high pressure discharge of gaseous fuel and herewith associated high
 velocity flames which have a long spreading shape. Such high velocity,
 long spreading flames might be appropriate for some flame cultivation
 purposes, eg. to penetrate dense or thick plant growth, but is certainly
 not always necessary nor wanted for flame cultivation or back-burning.
 U.S. Pat. No. 3,177,922 (Pardee) discloses a further flame cultivation
 burner with integrated vaporiser, wherein a flat, box-like burner shield
 or skirt supports a jet nozzle at a rear closed wall thereof. The nozzle
 is mounted such as to direct a vaporised jet of gaseous LPG into the
 inside of the skirt towards the open mouth thereof. The skirt is shaped
 convergent-divergent to provide a venturi effect in the flow path of the
 gaseous fuel-air mixture created within the rear section of the shield,
 combustion air being provided through an air duct opening into the rear
 section of the shield. The vaporiser itself consists of a separate housing
 affixed to the top wall of the skirt, the top wall being common to both
 structures. Liquid LPG is delivered into the vaporising chamber formed
 within the separate housing by means of a tube which at its open end has a
 deflector baffle to enhance fluid distribution into the vaporising
 chamber. A vaporised fuel outlet tube extends from within the vaporising
 chamber through a wall thereof and is in fluid communication through
 appropriate rigid lines and fittings with the jet nozzle. All tubing lines
 extending between the vaporising chamber and the jet nozzle are formed
 such as to ensure that high pressure gaseous LPG generated within the
 vaporising chamber (which is effected by heating the additional housing
 once a flame has been ignited within the burner skirt during normal burner
 operation) passes unrestricted into the jet nozzle from where it is
 expelled at high speed and pressure through appropriately sized orifice
 elements.
 The burner/vaporiser torch of Pardee is essentially limited to work at
 operating pressures (gas pressure at the jet nozzle outlet) dictated by
 the storage pressure of the liquid LPG in its storage tank, as there is no
 means of adjusting the operating pressure and hence flame velocity
 subsequent to vaporisation of the fuel, as pressure drops within the
 vaporiser and burners are comparatively small. Similar problems exist with
 the burner device with integrated fuel vaporiser disclosed in U.S. Pat.
 No. 5,030,086 (Jones)
 WO98/01031 (Boral Gas) discloses a vaporiser for use with a flame
 cultivation device, in which a metering tube is arranged in a hot zone of
 the vaporiser. The metering tube is arranged for limiting the volume of
 liquid LPG flow from the LPG storage tank into the vaporisation chamber of
 the vaporiser. Such vaporiser arrangement disposes with the necessity of
 fluid LPG flow control by means of a regulator with movable parts and
 sealing elements that could be subjected to heat corrosion when arranged
 too close to the hot zone of the vaporiser. Instead, the metering tube
 ensures a constant supply of liquid LPG into the vaporiser, without the
 above mentioned icing problems. The fully vaporised gaseous LPG fuel
 exiting the vaporiser chamber is then regulated in gaseous form using
 conventional regulator valves for controlling distribution of gaseous
 fuels to individual burners of a multi-burner implement.
 Whilst the basic principle of delivering liquid LPG under supply tank
 pressure directly into the hot zone within the vaporiser using a fixed
 metering member with no moving parts and subsequently using conventional
 regulator valves to control delivery pressure (and amount) of gaseous LPG
 fuel to the burners of the torch would seem a viable approach in
 addressing the above mentioned problems associated with the torches of
 Pivonka and Pardee (ie. trying to regulate the operating pressure of the
 burners by way of regulating liquid LPG fuel flow into the vaporiser using
 conventional valves), constructional implementation of such principle
 would result in vaporiser-burner devices that are expensive because of the
 need to incorporate gas pressure control valves in the fuel supply lines
 between vaporiser and burner nozzles, heavy for hand-held implementations
 and lack the necessary compactness typically required for hand-held
 torches. Also, typically, gas temperatures of vaporised LPG fuel generated
 using vaporiser constructions like those of Pivonka, Pardee and Boral Gas,
 will be in the range of 80.degree.-120.degree. C., this requiring use of
 regulator valve seals made of heat resistant materials.
 Taking into consideration at least some of the problems and drawbacks
 associated with the above mentioned prior art burner devices with integral
 fuel vaporisers, the present invention seeks to provide, in at least one
 preferred embodiment thereof, a burner with integral fuel vaporiser which
 is able to deliver a low velocity, ground hogging flame for use in flame
 cultivation that is easy to operate in a safe manner. The burner is to be
 equally useable in hand-held torches as well as apparatus for flame
 cultivating rows of crops.
 It would also be advantageous for at least a preferred embodiment of the
 invention to provide a burner device with integral fuel vaporiser that can
 deliver a high velocity or "booster" flame for use in applications
 requiring increased heat energy output, in particular as required for
 flaming.
 It would also be advantageous for at least a preferred embodiment of the
 invention to address balancing problems in particular encountered with
 hand-held flaming torches that have long torch handles, in a manner which
 will not or only minimally affect the manoeuvrability of the flaming torch
 in operation, and provide a hand-held flaming torch that can be safely
 carried by an operator together with a fuel supply cylinder or canister.
 SUMMARY OF THE INVENTION
 In a first aspect of the present invention there is provided a burner
 device (and similarly a torch head) for use in flaming and flame
 cultivation which includes a primary burner having at least one fuel
 delivery jet nozzle arranged in an open combustion chamber defined within
 a burner skirt such as to direct a stream of fuel towards a flame delivery
 opening (or mouth) of the skirt, and a vaporiser located in heat
 exchanging proximity to the primary burner and having a pressurisation
 chamber with an inlet for liquid fuel, in particular LPG, and an outlet
 for vaporised gaseous fuel, the inlet being connectable to a source of
 pressurised liquid fuel and the outlet being in fluid communication with
 the nozzle of the primary burner. Characteristic of the invention is the
 provision of a metering duct located in close vicinity or within the
 vaporiser and arranged to discretely restrict (as compared to restrict in
 variable or adjustable manner) the mount of liquid fuel entering the
 vaporisation chamber via its inlet, preferably without a substantial
 pressure drop (as compared to a conventional flow regulator that permits
 adjustable flow regulation), and the provision of a first pressure
 reduction duct in the flow path of gaseous fuel between the vaporisation
 chamber outlet and the nozzle of the primary burner, the first pressure
 reduction duct arranged to provide a discrete pressure drop of
 predetermined value for vaporised fuel passing from the vaporisation
 chamber (during normal operation of the torch) to the gaseous fuel
 delivery nozzle of the first burner (more precisely a drop in pressure of
 the gaseous fuel passing through the duct), such as to generate a low
 velocity flame during normal torch operation.
 As used herein, a low velocity burner flame is defined as a combustion
 flame of gaseous LPG delivered through the jet orifice of the burner
 nozzle at between 20 to max 30 PSI gas pressure, whereas a medium to high
 velocity combustion flame is generated by gaseous LPG exiting the nozzle
 orifice at about 35 to 60 PSI or more gas pressure.
 Preferably, the above described burner device can incorporate a booster
 burner arranged to deliver a medium to high velocity combustion flame
 through at least one booster nozzle or jet which is in fluid communication
 with the vaporisation chamber through a second pressure reduction duct
 arranged to reduce the pressure of the gaseous fuel exiting the
 vaporisation chamber by an amount required to generate such medium to high
 velocity flame.
 Whilst the nozzles of the primary and booster burners can be arranged
 within the same burner skirt, it is preferred to arrange the respective
 nozzles within separate burner skirts that are arranged such that flames
 emitting therefrom intersect at a small angle and the wide spreading, low
 velocity flame is superimposed with the narrower, high velocity flame.
 Advantageously, a manually operatable shutter valve is provided to
 selectively permit and cut-off gaseous fuel flow to the booster burner.
 The inventive burner device is intended to be connected directly to the
 outlet valve of a LPG storage tank (either a small cylinder that can be
 carried by a person in a ruck-sack or on a small cart; or a larger storage
 cylinder mounted on an agricultural vehicle such as a tractor, where a
 battery of torches is to be employed similar to the appliance described in
 Pardee, supra). Generally, the liquid LPG fuel delivery pressure of such
 tanks is around 90 to 130 PSI gas pressure, depending on ambient
 conditions. During steady state operation of the burner, where the metered
 liquid LPG entering the vaporiser is fully vaporised in the vaporisation
 chamber, the pressure within the vaporisation chamber will then be only
 slightly smaller than the delivery pressure. The pressure drops
 attributable to the liquid LPG supply conduits and flashing point entry of
 liquid LPG into the pressurisation chamber will be comparatively small (eg
 5 to 10 PSI gas pressure), and it is believed that pressure losses
 otherwise attributable to the metering duct are offset by the about
 280-fold volume increase which the liquid LPG will undergo in attaining
 its fully vaporised, gaseous state in the confined pressurisation chamber,
 thereby generating a pressure increase. The only significant pressure drop
 of fully vaporised gaseous fuel will take place whilst flowing in the
 pressure reduction ducts (ducts with small bore and substantial extension,
 see below) towards the jet nozzle(s).
 In other words, by appropriately sizing the metering duct, the pressure
 reduction ducts and nozzle gas delivery orifices, it is possible to
 manufacture a simple, self-regulating burner or torch head device, where
 pressure increase within the vaporisation chamber (as a result of
 vaporising increasing amounts of liquid LPG entering the chamber via the
 metering duct against insufficient initial back pressure) will eventually
 generate sufficient back pressure within the chamber thereby to regulate
 the amount of liquid fuel entering the chamber and maintain liquid LPG
 intake into the vaporisation chamber about constant (assuming steady state
 supply of enough energy to fully vaporise the properly metered LPG
 quantity within the chamber).
 A torch head construction which incorporates the features of claim 5
 dispenses with the need for any conventional liquid fuel regulators having
 moving parts to adjust flow of the liquid LPG fuel in the supply line to
 the vaporiser, as well as conventional gas valves to adjust pressure of
 gaseous LPG fuel in the flow path between vaporiser outlet and burner
 nozzles. Instead, fluid flow is "controlled" to one setting (as provided
 by the metering member), and gas pressure at the burner nozzle(s) is
 equally set to a non-adjustable level as a function of the pressure and
 temperature level present in the vaporisation chamber and the dimensions
 of the pressure reduction duct.
 The vast majority of torch applications require little or no regulation of
 the flame velocity once the torch has reached steady state operation.
 Accordingly, manual adjustment of LPG flow into the vaporiser is
 superfluous (and non-efficient, see above). For example, hand-held LPG
 burning torches are generally used with conventional LPG storage cylinders
 which provide the fuel and the operating pressure. The outlet or fuel
 delivery pressure at an LPG cylinder will remain about constant (assuming
 constant outside temperature and relatively small fuel flow rates compared
 with cylinder capacity) for as long as there remains a certain level of
 liquid LPG in the cylinder; after full vaporisation of liquid LPG in the
 cylinder, pressure drop will take place over a very short period of time,
 and torch operation will no longer be possible. Accordingly, it is
 possible to use a metering member that does not provide for any manual
 adjustment, as long as such metering member ensures an appropriate fuel
 supply to the vaporiser and consequently to the burner to achieve a
 desired burner operating regime. The additional pressure reduction member,
 which provides a discrete pressure drop between vaporiser chamber and
 burner nozzles is necessary to obtain the required low velocity torch
 flame.
 The metering duct and the pressure reduction duct can be sized such that
 the volume of liquid LPG entering the vaporiser is restricted to a
 specified amount that is vaporisable by a given amount of heat transferred
 thereto by the heater which preferably is the torch burner itself, and
 obtain a desired burner jet nozzle operating pressure that will generate
 the low velocity/spreading flame. Arranging the metering duct in a zone of
 the vaporiser that is heated during torch operation counters the cooling
 effect which flow reduction of liquid LPG in the fuel delivery member may
 cause, thereby ensuring controlled and precise supply of fuel to the
 vaporiser.
 By incorporating the features identified in claim 6, it is possible to
 adapt the fuel delivery member in discrete steps to different types of LPG
 fuels for use with the torch, by using exchangeable parts, without the
 need to exchange the entire fuel delivery member. For example, to achieve
 the same flow rate for different types of LPG, eg pure propane, pure
 butane, gas mixtures, the metering duct will have to have different
 lengths, and/or bore diameters assuming the same fluid delivery pressure
 at the duct inlet. The length diameter variations may be small, though
 perceivable as far as torch operating conditions are concerned. Further,
 the fuel outlet orifice of the fuel delivery member can be incorporated
 into the exchangeable part, thereby simplifying cleaning operations should
 the orifice become clogged.
 The exchangeable nature of the first pressure reduction tube in accordance
 with the features identified in claim 8 enables to incorporate a selected
 one tube of a plurality such tubes in the torch, each tube having
 different lengths and bore diameters to cause a numerically different
 pressure drop between inlet and outlet thereof, and thus provide gaseous
 fuel to the primary burner jet nozzle(s) at a defined (but selected)
 pressure.
 Whilst it is alternatively also possible to incorporate the first pressure
 reduction duct in the main body of the fuel delivery member, thereby
 providing an integral arrangement of metering duct and pressure reduction
 duct within the same part, the number of possible permutations of liquid
 fuel flow rate to gaseous fuel pressure reduction to achieve a desired
 torch operating regime, make this more problematic, mainly ease of
 manufacturing considerations make a multi-part construction of the fuel
 delivery member of the torch desirable.
 A particularly simple torch head design is obtainable in that the fuel
 delivery member is a unitary metallic fitting having an externally
 threaded front hub portion onto which is sealingly fastened the otherwise
 closed tubular vaporiser housing. The closed forward end of the vaporiser
 housing may be T-shaped to reduce overall length of the housing whist
 providing sufficient volume for the vaporisation chamber formed therein.
 Different advantageous features pertaining to the unitary fitting are
 identified in claims 11 to 14.
 Preferred features and shaping of components of the primary burner are
 identified in claims 15 to 18.
 In accordance with a second aspect of the present invention, the torch head
 described above can incorporate additionally the features recited in claim
 19 thereby to provide increased heat output.
 A regulator valve (preferably a shutter valve with on-off regulation only)
 is advantageously disposed between the gaseous fuel outlet of the second
 pressure reduction duct and the booster burner jet nozzle(s) so that the
 torch can be operated with only the primary burner, which provides the
 ground-hogging, wide spreading flame pattern, as well as with the
 additional booster burner which, with its medium to high velocity,
 narrower or more confined flame, will provide additional heat energy when
 required.
 Preferably, the booster burner nozzle(s) is arranged in a separate burner
 skirt to that in which the primary burner nozzle(s) are housed, the
 arrangement being such that the planes in which the fuel streams are
 delivered by the respective jet orifices of the nozzles intersect at an
 oblique or acute angle. Such arrangement will cause the medium to high
 velocity flame to "impinge" on the low velocity flame and "drag" this
 flame with it, thereby creating a flame pattern that is wider than a pure
 high velocity flame and more energetic than the wide spreading flame of
 the primary burner alone.
 The specific ways in which the second pressure reduction duct can be
 embodied are similar to those of the first pressure reduction duct, the
 main difference residing in the necessity of incorporating the shutter
 valve in the fuel flow path to the booster nozzle.
 The heating energy required for vaporisation of fuel in the vaporiser is
 advantageously supplied by one of the burners once the torch has been
 ignited, similar to the torch of Pivonka discussed above. However, with
 the present invention, an arrangement is chosen such that flames exiting
 the mouth of at least one of the burner skirts will radiate heat energy
 only to the tip of the vaporiser housing so as to avoid overheating of the
 vaporisation chamber and its contents.
 In a different , third aspect of the present invention there is provided a
 hand-held flaming and/or flame cultivation torch which includes:
 an elongate handle, a torch head at one end of the handle and a fuel tank
 mounting structure at an opposite end of the torch handle wherein the fuel
 tank mounting structure includes a handle mount fixed at the handle and a
 tank support structure pivoted at the handle mount, whereby in use of the
 torch the weight of the torch head is at least partly counterbalanced with
 the weight of a fuel storage tank secured at the tank support structure
 and the tank is pivotable with respect to the handle such as to remain in
 a substantially vertical or upright position during operation of the
 torch.
 The fuel tank mounting structure with its capability of permitting pivotal
 movement of the fuel tank, eg an LPG cylinder, with respect to the handle
 allows the LPG cylinder to remain in a substantially vertical or upright
 position during operation of the torch which involves directional movement
 and pointing of the torch handle towards a treatment area by an operator.
 This pivotal mounting of the tank at the handle is desirable from a safety
 point of view in that the outlet valve of the cylinder, which is located
 at the top of the cylinder, should remain in fluid communication with the
 vapour phase of the LPG within the cylinder rather than the liquid, to
 ensure proper operation of the torch. This is achieved by keeping the LPG
 cylinder in its upright orientation during operation of the torch. Also,
 the pivoting mechanism enables to set down the torch with the cylinder in
 its upright position on uneven ground.
 In further development of the third aspect of the invention, the hand-held
 torch may the features mentioned in claim 25.
 Preferably, the fuel storage tank mounting structure includes the features
 mentioned in claim 26. This enables fast and simple exchange of the
 cylinder when required.
 There are different ways in which to provide a mounting structure that
 permits cardanic (rotational freedom of movement about 3 mutually
 perpendicular axis), gimbal (rotational freedom of movement about two
 axis) or simple, uniaxial pivotal movement of the LPG tank in relation to
 the torch handle. In one preferred form, a simple gimbal mount is provided
 in that an upper skirt of the cradle is pivotally secured and held between
 a pair of angled support arms which are welded onto a socket hub in which
 the proximal terminal end of the torch handle is received fixed against
 axial movement and, if required, also releasably against rotation.
 During the course of operation of the flaming torch, the centre of balance
 at the handle (the location where the strap harness is preferably secured
 at the handle) will change as the fuel in the cylinder is consumed. To
 accommodate this, it is advantageous to provide at the handle a fixing
 device having the features referred to in claim 29, optional embodiments
 of the fixing device being covered in claims 30 and 31, some other form of
 variable adjustment mechanism known in the art equally being employable.
 To minimise the amount of effort that is required by the operator to point
 the torch head in the direction required whilst being shouldered, the
 second handle grip, which preferably is a loop handle orientated at right
 angles to the torch handle, is advantageously mounted forward of the
 harness strap fixing point (that is towards the torch head) but close
 thereto, and the first handle grip, which is a simple tubular grip member,
 is located immediately rear of the balance point represented by said
 fixing point.
 To improve the safety of the torch operator, a quick release device, such
 as a clasp buckle, can be incorporated either in the shoulder harness
 strap or the harness fixing bracket at the torch handle thereby allowing
 the operator to free him or herself from the torch unit in case of an
 emergency. This improvement addresses concerns that have been expressed in
 relation to known, back-pack shouldered fuel supply tanks.
 The hand-held torch assembly is especially suited for use with larger torch
 heads that provide high energy output. Because of their higher weight, the
 counterbalancing effect of the LPG cylinder will be more appreciated by
 the torch operator.
 Accordingly, the torch head can incorporate a primary burner structure with
 integrated fuel vaporiser as referred to above in the first aspect of the
 invention as well as a booster burner as identified under the second
 aspect of the invention.
 The present invention, in its different aspects, and advantages thereof
 will be more fully appreciated by referring to the following description
 of preferred, non-limiting embodiments of the invention illustrated in the
 accompanying drawings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
 Referring to the drawings, first to FIGS. 1 and 2, there is illustrated two
 variants of a hand-held, flaming torch 1 and 1' which use LPG as fuel
 source and which can be used in numerous applications, such as for flame
 cultivation of terrain that is inaccessible to larger, tractor-carried
 flame cultivator torch batteries, weeding of irrigation channels, burning
 of tree stumps, softening of bitumen surfaces for road works, spot back
 burning operations during forest fires and other applications which
 require localised application of a flame or high heat.
 Torch 1 of FIG. 1 has two main structures, a torch head generally indicated
 at 2 and a fuel tank mounting structure, generally indicated at 3, whereas
 torch 1' of FIG. 2 lacks the fuel tank mounting structure, as the fuel
 supply cylinder for torch 1' is to be carried separately from the torch,
 either using a customised back pack or a non-motorised cart (not shown) as
 is known in the art.
 A straight, aluminium or light weight metal or composite material support
 tube 12 supports at its lower end the torch head 2 which in the
 embodiments illustrated is composed of a primary burner 14, a booster
 burner 16 and a liquid LPG vaporiser 18. These components will be
 described in more detail below.
 Handle or support tube 12 can be comprised of a number of individual tube
 sections for adjusting the length of torch 1, 1' for different
 applications. Handle 12 incorporates near its proximal end a first grip
 member 13 for one hand of the torch operator. A second handle grip in form
 of a loop handle 10 orientated at right angles to the longitudinal axis a
 of handle 12 is fixed thereon in spaced apart relationship from the first
 grip member 13, as seen in FIG. 1, but is not incorporated in FIG. 2.
 Torches 1, 1' incorporate a trigger guard 20 disposed to cover the first
 grip member 13 as well as an actuator lever 22 which operates in known
 manner on a normally-closed deadman (shutter valve) 24 mounted on handle
 12 and which serves to control LPG flow to the torch head 2 as is
 described below.
 Fuel tank mounting structure 3 is supported at the proximal end of handle
 tube 12, the mounting structure being constructed such as to permit
 pivotal movement of an LPG storage cylinder 27 received therein about a
 rotational axis b that is perpendicular to the longitudinal axis a of
 handle tube 12.
 Mounting structure 3 includes a shallow bed cradle 6 made of sheet metal or
 any other suitable, rigid material. It defines a bed shape which conforms
 with the exterior shape of LPG cylinder 27 and has bent terminal flange
 portions 6a for resting the cylinder foot thereon. A number of straps 7,
 which are secured to the cradle in any appropriate manner, serve to strap
 down and secure cylinder 27 on the cradle bed. The upper skirt of cradle 6
 is provided with suitable fastening pins 5 so that it can be pivotally
 secured and held between a pair of angled support arms 4b which are welded
 onto the exterior of a socket hub 4a in which the proximal terminal end of
 tubular handle 12 is received and fixed against axial movement and
 preferably also against rotation, though the latter is not strictly
 necessary.
 Mounting structure 3 is provided with any type of suitable low friction
 bearing elements fastening at pin 5 that permit weight induced, self
 tilting of the LPG tank 27 in relation to the torch handle 12 into its
 upright or vertical position when the torch head 2 is moved and pointed
 towards a treatment area.
 A main liquid fuel supply line 26 in form of a metal braided, flexible LPG
 hose is secured in known manner with one of its fitted ends at the outlet
 valve 25 of the LPG cylinder and is connected with its other end in
 similar fashion to an elbow connector at the deadman shutter 24. Within
 handle tube 12 (see FIG. 2) extends a further fuel supply tube 28 that is
 suitably connected to shutter valve 24. LPG cylinder 27 provides the
 pressurised liquid fuel supply for torch head 2.
 A strap harness 8 is secured on handle 12 at a location intermediate the
 first and second handle grips 10 and 13 such that, in use of the torch,
 the weight of LPG cylinder 27 serves to counterbalance the weight of torch
 head 2 when an operator shoulders the harness. A metallic bracket having a
 tubular portion 9a fixed against axial movement on the handle and having a
 tang 9b with a plurality of eye holes 9c in spaced apart relationship
 serves to secure the strap harness on the tubular handle 12. A connection
 shackle 8a of harness 8 can be selectively inserted and locked in one of
 eye holes 9c. By positioning the shackle 8a in different holes, it is
 possible to adjust the centre of balance of torch 1, when suspended from
 harness 8 shouldered by the torch operator. Accordingly, this simple
 mechanism allows to compensate manually a shift of the centre of balance
 of the torch which is consequential to fuel consumption by the torch, and
 thus due to weight reduction of the LPG cylinder during the course of
 operation of the flaming torch. An alternative mounting arrangement
 includes. a fixing bracket with indexed notches along a horizontal slot
 instead of discrete eye holes as illustrated.
 To minimise the amount of effort that is required by the operator to point
 the torch head in the direction required whilst being shouldered, second
 handle grip 10 is mounted forward of harness strap fixing bracket 9 (that
 is towards torch head 2) but close thereto, and first handle grip 13 is
 located immediately rear of the balance point represented by the fixing
 point. To improve the safety of the torch operator, any suitable quick
 release device can be incorporated either in the shoulder harness strap or
 the harness fixing bracket at the torch handle thereby allowing the
 operator to free him or herself from the torch unit in case of an
 emergency.
 Whilst not shown in the embodiment of FIG. 2, it is understood that torch
 1' can also incorporate a harness 8 and harness attachment structure 9 and
 handle grip 10 as shown in FIG. 1.
 Turning now to FIG. 3 which illustrates in greater detail the combined
 burner--vaporiser arrangement used in torch head 2 of torch 1 and 1'.
 Vaporiser 18 includes a metallic main fitting 30 of unitary
 nature/construction, having a rear cylindrical mounting portion 32, a
 central, radially enlarged cylindrical mounting flange portion 33 and a
 forward, externally threaded, cylindrical hub portion 34. Support tube 12
 is mounted and fixed on the rear mounting portion 32 by any appropriate
 mechanical fastening means, e.g. welding, clamping.
 A central channel 35 extends between axially opposite end faces of main
 fitting 30. As can be seen in FIG. 3, central channel 35 comprises
 portions of varying diameter. Bore 35 ends at both sides in internally
 threaded bores 36, 38, bore 36 disposed to receive in known gas tight
 manner an appropriately threaded connection bush (not shown) secured at
 the forward, flared end portion of liquid fuel supply line 28. Bore 38 at
 the forward hub portion 34 receives a liquid fuel metering tube 39 as
 described herein below. It will be understood that the thread connections
 shown in FIG. 3 are illustrative only of different permanent/non-permanent
 ways of fastening the respective gas/liquid carrying elements to one
 another in pressure and leakage proof manner, different connection types
 being known to those of skill in the art.
 Central bore or channel 35 which has an inner diameter of about 6 mm
 comprises a portion with reduced diameter. This portion serves as a first
 metering duct 37, the inner diameter being about 1 mm and having a length
 of about 40 mm in an actually manufactured device. The metering duct 37 is
 shaped to allow restriction of flow of liquid LPG passing through the
 central bore without any substantial pressure drop. Metering tube 39
 extends forward of main fitting 30 to a length of about 250 mm and is
 mostly comprised of a tubular section with an inner diameter of about 4
 mm, but also incorporates a reduced internal diameter portion (about 1 mm)
 of about 40 mm length at its forward end (indicated at 40) which
 terminates in a flash outlet orifice 42. Accordingly, the combined length
 of both flow restricting portions having the reduced diameter (1 mm)
 totals about 80 mm.
 Vaporiser 18 further includes a tubular vaporiser housing 44 which has an
 enlarged diameter flared rear portion 48 that is internally threaded for
 securing the housing in gas tight manner onto the front hub portion 34 of
 fitting 30. The front portion 46 of housing 34 is shaped as a closed T-tip
 (not illustrated) in order to reduce the axial extension of housing 44
 whilst providing increased volume within housing 44. As will be noted, a
 mostly annular vaporising chamber 50 is defined between the coaxially
 extending housing 44 and metering tube 39, with exception of the housing
 tip region 46 and the radially enlarged coupling zone between housing 44
 and main fitting 30. A first gaseous fuel pressure reduction tube 52 is
 incorporated into vaporiser 18 and located within pressurisation chamber
 50. Tube 52 is secured in any suitable manner at hub portion 34 of main
 fitting 30 (e.g. by way of a leakage-proof threaded connection illustrated
 at 54) so as to be in communication with a first gas delivery channel 56
 formed within the fitting body and extending in the hub and central
 portions 34 and 33. First pressure reduction tube 52 has a length and
 inner diameter such that gaseous fuel entering tube 52 from vaporisation
 chamber 50 at tube inlet 58 will undergo a defined pressure drop before
 reaching channel 56, as will be explained in more detail below. In an
 actually manufactured embodiment, tube 52 has a length, of about 150 mm
 with a bore of 0.8 to 0.9 mm, whilst the diameter of channel 56 is 4 or 6
 mm.
 Also incorporated in vaporiser 18 is a second gaseous fuel pressure
 reduction tube 60 secured in gas-tight manner in a threaded bore 62 in the
 front end of hub portion 34 of fitting 30 so as to be in fluid
 communication with a second gas passage duct 64 which ends in a rearward
 facing surface of central portion 33 of fitting 30 in yet another threaded
 bore 63. The length between the inlet orifice 66 of second pressure
 reduction tube 60 and threaded bore 62 in an actually manufactured torch
 is about 40 mm, whilst the inner diameter is about 1 mm. Accordingly, due
 to its different dimensions as compared to first pressure reduction tube
 52, a different pressure reduction ratio is provided by the second
 pressure reduction tube 60 for gas entering from pressurisation chamber
 50.
 Turning now to primary burner 14, it will be seen that it comprises a
 burner skirt or housing 70 of sheet metal which is open at its rear and
 front end. The cross-sectional shape can be quadrilateral or cylindrical,
 and is tapered from the rear to the front. In case of a cylindrical skirt,
 the front portion is ovoid and reduced in diameter. The front end opening
 72 is covered by a heat/flame resistant gauze or mesh insert 73 which is
 bent concavely into the interior of skirt 70 which defines a burner
 chamber 71. Heat shield plates 74 can be fastened surrounding the front
 end opening 72, shield 74 serving as a deflector for flames existing the
 front end opening 72. In rearward extension of the rear opening 76, which
 provides an ingress passage for combustion air for the burner, is arranged
 a tapered-shaped cover 77 which incorporates a rigid gauze or mesh member
 78 (see FIG. 2) facing handle or support tube 12 and a sheet-metal shield
 member 79. Rigid gauze or mesh member 78 prevents ingress of unwanted
 objects into the burner chamber 71 as the burner is moved close to ground,
 whilst ensuring proper air supply to burner 14. Cover 77 is fastened at
 skirt 70 in any appropriate manner, skirt 70 itself being soldered or
 screwed onto the central flange 33 of main fitting 30, as appropriate.
 Primary burner 14 also includes at least one, but preferably three fuel
 delivery or burner jet nozzles, one of which is shown schematically at 80.
 Each nozzle has an appropriately dimensioned jet orifice outlet (e.g. 0.8
 or 0.6 mm) disposed to direct a jet stream of gaseous fuel towards the
 front end opening 72 of burner skirt 70. Jet nozzles 80 are held in place
 within burner chamber 71 near its rearward end by means of a bent gas
 supply tube 82 which is itself fastened at the flange portion 33 of main
 fitting 30 (at threaded, sealed connection 84) so as to be in fluid
 communication with gas passage duct 56 and thereby receive fuel from the
 vaporisation chamber 50 via pressure reduction tube 52. It should be noted
 here that the inner diameter of gas delivery channel 56 is chosen such as
 to avoid a further, notable pressure drop beyond that which is provided by
 pressure reduction tube 52. Whilst it would also be possible to entirely
 omit pressure reduction tube 52 and instead provide gas passage duct 56 to
 have similar dimensions to those of tube 52 to thereby provide a pressure
 reduction duct that is integrally formed within main fitting 30,
 manufacturing considerations make the presently illustrated design
 preferable. Furthermore, providing the necessary gas pressure reduction
 conduit/duct in a separate element which is removably fastened to main
 fitting 30 allows installation and exchanging of pressure reduction tubes
 that have different dimensions so as to adapt the torch to different fuel
 types and to achieve different operating gas pressures at the burner
 nozzles.
 Booster burner 16 which is arranged on the opposite side of vaporiser 18
 has a similar overall layout to that of primary burner 14 and comprises a
 burner hood or skirt 90 open at its rear end 92 for admitting combustion
 air (a gauze or cage cover like the one of primary burner 14 being
 optional) and having a discharge opening/mouth 94 arranged to direct
 flames emanating therefrom obliquely onto the flames exiting from primary
 burner 14. It will be noted that only a short portion of vaporiser housing
 44, namely the front T-shaped tip 46 extends beyond the plane containing
 both burner openings/mouths 72 and 94. Accordingly, only the terminal end
 of vaporiser housing 44 will be exposed to direct heat in operation of the
 torch, the burners providing the required vaporising energy (heat) for the
 liquid LPG fuel supplied to the vaporiser, as was explained above.
 It will also be noted that the booster jet nozzle 96 (not necessarily only
 one provided) is located more in the central zone of burner chamber 91
 within burner hood 90, but is similarly supported in place by way of a
 rigid, bent fuel supply tube 98 whose terminal end is sealingly secured
 within threaded bore 99 of a gas channel 100 extending in central flange
 portion 33 of main fitting 30. Channel 100 terminates in a rearward facing
 side of portion 33 in another internally threaded bore 101. A stainless
 steel fuel supply tube 102 is appropriately secured at bore 101 and
 extends towards the middle section of torch rod 12 as can be seen in FIG.
 2, where it is coupled to the outlet of a shutter valve 106 secured on
 handle rod 12. Shutter valve 106 incorporates in known manner a biased
 valve body arranged to open and close gas access to fuel supply tube 102
 from a valve inlet, the valve body being operated by a cantilever handle
 108 in known manner. The gas inlet of shutter valve 106 is in fluid
 communication via a further copper fuel supply tube 104 with the threaded
 bore 63 of gas supply channel 64 extending within main fitting 30.
 Accordingly, fluid communication can be established from vaporising
 chamber 50 via second gas reduction tube 60, gas passage or channel 64 and
 supply tube 104 to shutter valve 24, which controls flow of gas, and then
 via supply tube 102, gas passage 100 and supply tube 98 to the burner
 nozzles 96 of booster burner 16.
 As will be noted from the above description, torches 1, 1' incorporate only
 one valve with moveable parts, namely shutter valve 106 in the gaseous
 fuel supply path from vaporiser chamber 50 to booster burner 16, the valve
 to cut/permit liquid fuel flow from the LPG storage cylinder into flexible
 hose 26 (liquid fuel supply line) of the torch being mounted/forming part
 of the LPG cylinder. However, non of these valves serve as conventional
 regulators to adjust pressure or flow rate of the fuel, either in liquid
 or in gaseous form. Of course, shutter valve 106 in the gas supply path of
 the booster burner could be exchanged for a conventional pressure
 regulating valve. It is believed, however, that gas pressure regulation of
 the gaseous fuel delivered to the booster burner 16 during expected
 operational parameters and application of the hand-held torch will be
 superfluous. However, such arrangement may be desirable in other torch
 embodiments, i.e. in burner batteries used in multiburner flame
 cultivators, as the combined burner/vaporiser arrangement of the present
 invention can similarly be incorporated in torch heads (or burner devices)
 used in such flame cultivators.
 The fuel source for the torch is conventional LPG gas in liquid form. The
 size of conventional LPG gas cylinders that can be carried by a torch
 operator on his back or on a wheeled trolley is limited by weight
 considerations and manoeuvrability in rough terrain. Similarly, the amount
 of LPG fuel available for flaming operations will be restricted to that
 which can be safely stored in standard size cylinders under pressure at
 expected ambient temperatures during operation of the torch. Accordingly,
 it is expected that hand-held torch embodiments in accordance with the
 present invention will be operated with gas cylinders with a capacity of
 between 1.5 US Gal (5.7l) and 5 US Gal (19l) under full tank pressures
 (e.g. liquid propane) of between about 90 to about 130 PSI gas pressure
 (6,200 hPa gas pressure to 8,960 hPa gas pressure). In a typical
 application, a 5 kg gas fuel capacity cylinder full of LPG will have a
 cylinder outlet pressure of between 90 and 130 PSIg (6,200 hPa to 8,960
 hPa gas pressure) depending on the exterior ambient temperature.
 This pressure will be substantially maintained in the tank for as long as
 there is liquid LPG in the tank and will only drop over a short period of
 time prior to the tank reaching empty. In the following, an average tank
 pressure of 100 PSIg=6900 hPa gas=7900 hPa absolute pressure will be
 assumed to be present at the storage tank outlet valve. All further
 pressures mentioned will be absolute pressures unless where stated
 otherwise. Assuming a pressure drop of 300 hPa in the fuel supply line
 leading from the tank/cylinder to the vaporiser main fitting 30, liquid
 LPG will be delivered with 7600 hPa pressure at the entrance of the
 metering duct 37 of vaporiser 18. A further pressure loss of about 200-300
 hPa will take place in the metering ducts at 37 and 40 prior to liquid LPG
 exiting into the vaporisation chamber 50 through flash outlet orifice 42.
 With the geometrical values given above for pressure reduction tubes 52,
 60, nozzles 80, 96 and metering duct portions 37, 40, vaporisation chamber
 pressure will be in the vicinity of 7400 hPa during normal, steady state
 burner operation where incoming liquid fuel is fully vaporised into its
 gaseous state (the fuel experiencing hereby a volume increase of about 280
 fold), thereby generating sufficient "back" pressure in chamber 50. The
 required vaporisation heat is provided by the primary burner to the tip
 portion 46 of vaporiser housing 44. It has been found during operation of
 a torch with vaporiser and burners as arranged in FIG. 3, that vaporise
 housing 44, will remain relatively cool (due to the uptake of energy of
 liquid LPG during vaporisation), the temperature of flashing of LPG at the
 outlet orifice 42 being around 20-30.degree. C. Gaseous LPG in other zones
 of the vaporising chamber 50 will have temperatures ranging between
 80.degree. C. to about 100.degree. C. at a pressure of about 7400 hPa. The
 first pressure reduction tube 52 will reduce the pressure of gaseous LPG
 delivered to the primary burner nozzles 80 to about 2900 hPa (equivalent
 to about 25 PSI gas) with burner flame temperature near the transition
 zone gas to flame of about 600.degree. C. Similarly, the second pressure
 reduction tube 60 associated with the booster burner 16 will cause a
 pressure drop of gaseous LPG supplied through the second tube 60 via
 shutter valve 106 to booster nozzle 90 so that. operating pressure at the
 booster nozzle will be about 4000 hPa (about 40 PSI gas), the gas--flame
 transition temperature being about 670.degree. C. Burner gas flow measured
 showed about 42% delivery rate to the primary burner and 58% to the
 booster burner whilst the latter was engaged. The above values are
 approximations only and are valid for torch operation where the booster
 and primary burners are in use after the vaporiser has reached normal
 operating temperatures at which the full amount of incoming, liquid LPG is
 vaporised in the vaporising chamber 50. Different values will apply during
 torch operations in which the booster burner shutter valve 106 cuts
 gaseous fuel supply to the booster burner nozzles 90. However, the
 operating pressure of the primary burner nozzles will still remain in the
 vicinity of 2900 hPa such that a low velocity, wide spreading flame is
 generated in the primary burner 14, widening of the flame being aided by
 the gauze mesh 73 covering the burner outlet 72. On the other hand, the
 comparatively high operating pressure present at the booster burner
 nozzles during booster operation of the torch will provide an additional
 high velocity, and further spreading flame which, due to the oblique
 arrangement of the flame spreading planes of the primary and booster
 burners, will enhance flame penetration. The temperature of the actual
 flame of the primary burner will be about 1080.degree. C. whereas the high
 velocity booster burner flame will reach temperatures of around
 1200.degree. C.
 Torch 1, 1' can be ignited by allowing liquid LPG to flow into the
 vaporiser 18 from where it will exit (in liquid form) through the primary
 burner nozzles 80 where it is subsequently ignited using a conventional
 lighter, as is the case with the torch of Pivonka described above. On the
 other hand, during a short period of time of about 5 to 10 seconds after
 opening the main valve at the LPG cylinder, liquid LPG entering the
 vaporiser chamber will vaporise partially (as it can "pick-up" sufficient
 heat from its surroundings without additional heat transfer into the
 vaporiser), this partially vaporised fuel mixture being able to be ignited
 more readily upon exiting the burner nozzles than liquid LPG.
 Once steady state (normal operation) has been reached, and the vaporiser
 produces gaseous fuel to supply both burners 14, 16, booster burner 16 can
 be fired selectively by means of the handle operated shutter valve 106.
 It should be noted that the flow restriction of liquid fuel into the
 vaporising chamber 50 by way of metering duct 37 and metering tube 39 is
 such that the fuel amount delivered is about equal to the vaporisation
 rate achievable during normal torch operating conditions. As alluded to
 above, because the two pressure reduction tubes 52 and 60 as well as the
 metering tube 39 (which provides part of the overall length of the
 metering flow restricting metering duct) are mounted in removable fashion
 within the vaporisation chamber 50 (the vaporiser housing being readily
 unscrewed from the main fitting), it is possible to exchange these
 components to suit the specific type of fuel employed with the torch in
 order to obtain a desired burner nozzle operating pressure.
 It will be appreciated by those engaged in the field of flame cultivation
 that the torch head 2 illustrated and described with reference to FIG. 3
 can be modified in simple manner for replacing conventional burner
 arrangements used in large scale agricultural flame cultivators such as
 those described in Holloway, Pardee, and Pivonka, supra.