GB2137331A - Method and apparatus for heating or cooling explosive or flammable material - Google Patents

Abstract

An explosive or flammable test piece is placed in a test chamber (1) which is provided with a heat exchanger (2) which forms part of a continuous sealed conduit (6, 7) through which a heat exchange gas is circulated. The use of a heat exchange gas allows for faster heating responses by the heat exchanger and reduced corrosion and leakage problems compared with the use of conventional heat exchange liquids such as glycol or water. <IMAGE>

Description

SPECIFICATION Method and apparatus for heating or cooling explosive or flammable material This invention relates to a method and apparatus for heating or cooling explosive orflammable material with reduced risk of explosion or ignition.

It is often necessary to heat explosive or flammable substances or equipment or apparatus containing such substances in a test chamber enclosure to establish their physical characteristics orto condition them priorto some other sort of test. In conventional systems, there is always the risk that the substance maycomeintocontactwith a heating element, which being very hot may cause the substance to explode or ignite.

Techniques to minimise this risk have involved temperature control ofthe heater surface temperature orthe use of an intermediate heating liquid such as glycol.

Heater surfacetemperature control relies on the hot spots on the heater being correctly identified and sensors being attached to the correct points. Also it relies on the hot spots remaining constant with time and on the airflow pattern overthe heater remaining fairlyconstanttoo. In addition, heatersurfacetemperature control entails regular re-calibration of several controllers.

Glycol heating relies on passing heated glycol through a heatexchangerfitted insidethetest chamber. However,the use of glycol while avoiding development of hot spots on the heater has several attendant disadvantages. Thus, glycol is highly corrosiveand over a period oftimethe heat exchange fluid conduits, their connections and the heat exchange fluid pump develop leaks. The chemical process which causes the corrosion the accelerated by heating and any heating and cooling which the test apparatus is subjected to places further stress on pipework already weakened by the glycol.Furthermore, glycol is an electrolyte, eating away at the pipework in one part of the apparatus, causing potential leak points, and at the sametime building up deposits in another part, blocking the system, creating poor thermal transfer and causing excessive back pressure to further stress potential leak points. Finally, afluid leak, should it occur, is very difficultto repair and may cause immeasurable damage to the test piece which may have to be written off.

Even when the indirect heating fluid is water, the risk of leaks and subsequent damage to both test chamber and test piece remains. If the water is not drained from the system adequately priorto a sub-zero temperature test, the risk of damage is self-evident.

We therefore sought a method of heating or cooling explosive orflammable test pieces using a test chamber provided with a heat exchanger in which the problem of exchange medium leakage was reduced and preferably in which a broader operating temperature range cou Id be achieved than is possible with glycol or water.

We considered using a heat pump to extract heat from the surrounding ambient and transfer itto the test chamber via a heat exchanger. The principle of operation would be very similarto that of the domestic refrigerator with the refrigerant being compressed into a liquid and giving up its latent heat of evaporation to the test chamber. The liquid would be pumped round and allowed to expand into a gas, so absorbing its latent heat of evaporation once more from the surrounding ambient. However, liquids suitable for use in such a system may also be corrosive, reactive or toxic. Fu rthermore, the operating temperature range for such a system is relatively narrow, boiling points of available refrigerants being generally about 45 to 50"C.

We then surprisingly found thatthe disadvantages ofthe conventional systems could be overcome and a broaderoperatingtemperature range could be achieved bythe use of a gas as the heat exchange -fluid. This was surprising as it had previously been thought that gases would have insufficient thermal capacity to act satisfactorily as the heat exchange fluid in heat exchangersfor apparatusfor heating or cooling explosive or flammable materials. However, therelativelylowerthermalcapacityofgaseous rather than liquid heat exchange fluids has in fact resulted in a positive advantage in that the heating response is faster i.e. thetemperature ofthe heat exchanger can be raised or lowered more rapidly.

In one aspect, our present invention thus provides a method of heating or cooling explosive or flammable material which method comprises placing said material, optionallywithin a piece of apparatus or equipment, in a test chamber provided with heat exchange means whereby heattransfer may occur between the contents of said chamber and a heat exchange fluid in said heat exchange means, wherein said fluid is a gas.

The heat exchange means used in the method of the invention preferably forms part of a sealed continuous conduitthrough which the gaseous heat exchange fluid is cycled repeatedly, for example by a pump or a fan. The conduitwill be provided upstream of the heat exchange means with means for adjusting the temperature of the gas to the desired level as well as means for cycling the gas through the conduit.

In a further aspect, our invention provides explosive orflammable material heating or cooling apparatus comprising a test chamber provided with a heat exchange means, said heat exchange means forming part of a sealed continuous conduit which is provided with meansforcycling gastherethrough and means for adjusting the temperature of said gas.

In a preferred embodiment, the test chamber ofthe apparatus is provided with means for generating an airflow, i.e. a flow of the fluid atmosphere within the test chamber, overthetest material placed therein.

The test chamber airflow is unable to mix with the separate heat exchange gas flow due to the heat exchanger system being totally sealed. The heat exchange gas may be heated by conventional heaters and then blown by a fan through the heat exchange means where it heats the test chamber air before being drawn backtothe heaters. The hot gas flow is therefore completely sealed and recycling.

The temperature ofthe heat-exchange gas is preferably monitored using temperature sensing means and adjusted using heating or cooling means located upstream ofthe heat exchange means to ensure that the temperature ofthe gas at the heat exchange means remains at a level thatthetest piece can tolerate safely. This can be achieved by the provision to the apparatus of control means capable of arresting the operation of the means for cycling the heat exchange gas to the conduit ifthetemperature sensing means shows the gas temperatu re to have risen above a preset value, eitherthe desired test temperature or a safety limit value, or ifthe pressure within the conduit deviates from within a preset range.

Alternatively, the temperature sensing monitor can serve to control cooling means located in the conduit intermediately ofthetemperature sensing means and the heat exchange means orto activate diverting means to divertthe flow of gas to the heat exchange meansthrough a cooling means if the gas temperature is too high.

The heat exchange gas is preferably one which does not corrode the conduit or any other components of the apparatus it comes into contact with, e.g. fans, heaters, temperature and pressure sensing means etc.

Furthermore, the gas is preferably one which is inert with respect to the test material so that should a leak develop causing the gas to come into contact with the test material there will be no reaction between the two. Suitable gases include nitrogen,the noble gases, i.e. helium etc, and mixtures thereof although for reasons of economy and simplicity airwill generally be acceptableasthe heat exchange gas.

To minimise the danger of dust or particles of the test piece entering the heat exchange conduit should a leak develop, it is preferred thatthe gas within the conduit should be maintained ata pressure higher than that ofthe test chamber.

Thus the method and apparatus of the invention result in the following advantages: intrinsic safety because the heat exchanger is the hottest part ofthe test chamber and can never reach a highertemperaturethanthe heatexchangegas heating it. The gas can be limited to the maximum safe temperature by a temperature controller, preferably backed up bya safety control system; the completely sealed independent recirculating flow of gas to transfer heat from the heaters to the test chamber; virtually any range of temperatures can be achieved in the same test chambers since the limitations of boiling pointorfreezing point of a liquid heat exchange medium do not apply; Simplicity in terms of setting up, calibration and maintenance;; reduction in breakdowns and increase in apparatus working life resulting from the avoidance ofthe need to use a corrosive fluid such as glycol; reduction of problems of spillage and handling such as would result from the use of a heat exchange liquid, thus making the method and apparatus ofthe present invention more suitable for applications which involve transportation; the avoidance ofthe needfortopping up since the heat exchange gas conduit is completely sealed; waster heating responses than are achievable with a heat exchange liquid; by the use of a sealed heat exchange fluid conduit, contaminationofthe heatexchangefluid bythe surrounding air is avoided - in vented systems using oils as the heat exchange fluid such contamination can cause the effectiveness ofthe heat exchange system to deteriorate; and wherethe heat exchange gas conduit is sealed water condensation within the conduit (as a result of operating in a moisture laden atmosphere) is avoided thus avoiding corrosion or blockage ofthe conduit.

There are numerous applicationsthroughoutthe test industryforthe invention both in research and development establishments and also in prooftesting and temperature conditioning applications. Examples include the research into physical properties of explosives at differenttemperatu res, the temperature testing of devices containing flammable propellants and the temperature testing of devices using flammable liquids as coolants.

A preferred embodiment of the apparatus and method of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic view of an apparatus constructed in accordance with the present invention; and Figure 2 shows a graph illustrating the heating response of an apparatus constructed according to the presentinvention.

Referring to Figure 1, a test piece (not shown) is placed inside test chamber 1 in which is located heat exchanger2. For optimum results,the test chamber should have aforced airflow so asto minimise temperature variationsthroughoutthe chamber and across the test piece, and also to maximisethe rate of heattransferfrom the heat exchanger.

Forced airflow however is not essential to the working ofthe apparatus since heat will be transferred albeit moreslowly by conduction and naturalconvec- tion. Whereforced airflowis used howeverthemotor forthe fan or other means used to achieve the forced airflow should be dust-tightto prevent dust creeping down the fan shaft and into the armature.

Heat exchanger 2 is the means by which heat is transferred to the test chamber. The heat exchanger is a totally sealed unit in that hot gas passing through it is unable to mix with the air in the test chamber.

For maximum efficiency of heattransfer, the heat exchanger 2 is preferably a finned coil with forced chamber airflow passing through it. Othertypes of heat exchanger, for example radiators, etc can however be used with orwithoutforced chamber airflow.

Since heat exchanger2 is the only source of heat in the chamber, limiting its surface temperature below the danger level of the test piece provides an inherently safe method of heating the test chamber.

Intestchambers using forced airflow, it is importantto note that in well-insulated chambers the operation of airflowforcing fans can drivethe chamber temperature up to about 45 to 500C over a period of time. Where this approaches the safety temperature ofthe test piece concerned, forced airflowshould not generally be used.

The gaseous heat exchange medium 8 is caused to circulate around a continuous sealed conduit 6,7 by a fan (not shown) located in heater box 3. Heater box 3, in which there is also located a heater for heating the circulating gas, and heat exchanger2 form part of the sealed conduit.

Thus, gas is heated in the heater box 3, driven down inlet duct 6 in the direction ofthe arrows shown in Figure 1 to passthrough heatexchanger2fromwhich it emerges into and is returned through return duct7 to heater box 3.

Heater box 3 is itself a totally sealed unit in thatthe gas flow passing through it is unable to mix with the outside air. The box is preferably lagged to minimise heat loss. The heater in the box heats the gas to the requiredtemperaturewhilstthefan blows itthrough to the heat exchanger and then sucks it back into the heater box in a sealed recirculating system.

The heater(s) should normally be placed upstream ofthe fan to ensure good mixing ofthe gas.

Alternatively however, where the inlet duct 6 is of sufficient length to provide good mixing, or where a mixing chamber is provided in the conduit between heater and heat exchanger 2, the heater(s) may be placed downstream ofthe fan thus prolonging the working life of the fan.

Inlet duct 6 and return duct 7 are totally sealed againstthe outside environment and preferably are lagged to minimise any heat loss. The efficiency ofthe apparatus can be optimised by keeping the inlet and return ducts as short as possible.

The temperature of gas 8 in inlet duct6 is monitored with sensor9 which is suitably a fast response device which feeds a signal proportionaltothemeasured temperatureto controller 10 which comparesthe measured temperature with a preset value and sends a control signal to heater control 4to cause the power inputto the heaterto be adjusted as necessaryto raise or lowerthe temperature ofthe gas in heater box 3.

The hot gas entering the heat exchanger is thus controlled so as notto exceed a pre-settemperature.

Since the heat exchanger cannot be hotterthanthe gas entering it and since the heat exchanger, as the test chamber's heat source, has the hottest surface temperature in the test chamber, the test chambers heater safety requirements are met simply by setting the correcttemperature on the controller 10. In practice, there is an additional safety margin in that thermal losses between the ducting and the heat exchanger inlet, together with inefficiencies in the heat exchanger itself, may result in the actual surface temperature of the heat exchanger being less than the value set on controller 10.

Controller 10 may be an on/offtype, on/offwith energy regulators or overdamped three term. Prop ortional only control with orwithout manual reset is not generally suitable sincevariations in ambient and chambertemperature could affectthefinal settling temperature. Similarly, Proportional and Integral control is generally unsuitable because ofthe large overshoots which may be caused, and underdamped or critically damped three term controllers are also generally unsuitable for the same reasons.

Thetemperature control system of heater, heater control 4, sensor 9 and controller 10 is backed up by an independent safety cutout system which operates if the heat exchange gas becomes too hot.

Thus a second temperature sensor 11, the safety sensor, is located in inlet duct 6. This should be a fast response sensor independent of sensor 9 sending a signal proportional to the measured gas temperature to cutout control 12. With the signal from safety sensor 11, cutout control 12 constantly monitors the gas temperature in inlet duct 6. The cutout control 12 is setto a temperature just a few degrees above the temperature set on controller 10. Should the measured temperature exceed the value set on cutout control 12, it will send a signal to heater control 4 and fan control 5to shutdown the power to heater(s) and fan. The system will remain shutdown until thefault condition has been cleared.

The cutout control 12 acts as a trigger and therefore is preferably an on/off type device so that it can respond quickly and unambiguously to any over temperature signal.

It may be considered a sound policy to set the cutout control 12 to the same value as the maximum permissible chambersurfacetemperature or even sufficiently below that temperature to prevent a malfunction from causing an overshoot of the heat exchange gastemperature from reaching the safety limit This would mean having to lowerthe temperaturesetting on controller 10 and consequentlythe temperature ofthe heat exchange gas itself so that it does not become hot enough to trip the safety cutout under normal operation. Whilst this policy provides added safety protection it does reduce the maximum airtemperature that can be achieved inside the test chamber in direct proportion to the reduction in the control temperature.

The switch gearforthe heater control 4 referred to above may suitablytake the form of relays or contactors or thyristors, ortriacs, or zero switching solid state relays etc. The heater power may be regulated on an on/off, time proportioning, or percent output proportioning method. However, regardless of which type of switch gear is used for heater control 4, there should preferably be an additional isolation contactor in series with the main powerfeed to the heater(s) so that the heater(s) can be disconnected automatically should the switch gear of the heater control 4jam in the "on" position.

Where the gas in the conduit 6,7 is pressurised to above atmospheric pressure for additional protection in the case of leaks, a pressure sensor 13 is preferably provided which in the event of the gas pressure moving outside a preset range will send a signal to fan control 5, and preferably also to heater control 4, to shut-off powerto the fan and the heaters.

There are two important points regarding the pressure sensor 13. Firstly, it should preferably be situated inthatpartoftheconduitwhich is neither pressurised nor de-pressurised by the action of the fan in heaterbox3. This isto preventany leaks being masked bythe high pressurefrom thefan orto prevent nuisance tripping because of the low pressure upstream ofthefan. The best location is usuallyatthe furthest distance in terms of ducting from the heater box. The second point relates to the pressures generated in the conduit by the heating and cooling of the heat exchange gas.Extra pressure generated by heating generally presents no problems since the pressure sensor can be set to trip on falling pressure at just above atmospheric and any leaks would thus quickly bring the pressure down to the trip level. Low pressures generated by the test chamber being cooled can cause nuisance tripping if the heat exchange gas is not pressurised to a high enough pressure initially.

As a further precaution, pressure sensor 13 is preferably attached to an alarm 14which reacts to a signal from pressure sensor 13 and acts to sound a warning signal and to shutdown the test chamber and both the fan and the heater(s) in heater box 3 by sending an appropriate signal to fan control 5 and heater control 4. Both the heat exchange gas system and the test chamber will preferably remain shutdown until manually reset once the fault condition has been cleared.

Itshouldbenotedthatwhiletheoperation of an apparatus according to the invention in which the heat exchange gas is heated priorto entering the heat exchanger has been described, the apparatus and method ofthe invention are equally applicable to the performance of cold tests where the heat exchange gas entering the heater is coolerthan ambient.

The integrity of the heat exchange gasflow system against leaks is most important. If there are any leaks, they are most likely to occur in the joints in the ducting. However, the worst case is if the heat exchangershould leak and draw potentially explosive chamber air in through the leak along the return duct and onto the heater(s) in the heater box.

This problem may be overcome by positioning the heat exchanger closerto the heater box outlet than its inlet. The principle behind this feature is thatthefan in the heater box causes a pressure differential around the sealed gas conduit system. The pressure in the ducting nearthe heater box outlet is higherthan atmosphericwhilstthat in the ducting nearthe heater box inlet is lowerthan atmospheric. As the higher pressure gas travels around the conduit, its pressure reduces dropping to atmospheric approximately half way round and to even lower levels by the time the gas reenters the heater box.Placing the heat exchanger nearthe heater box outlet means that any leaks which might occur in the heat exchanger are in the higher than atmospheric pressure part ofthe conduit and the positive pressure at th at point prevents any chamber airfrom being drawn in.

Sound manufacture and regular maintenance should further guard against leaks, but additional safety can be provided using either or both ofthe following: (a) non-combustible heat exchange gas: should any explosive mixture be drawn into the gas flow through a leak, then the risk of explosion as it is drawn onto the heater(s) is minimised by the inability of the gas to support combustion. Typical gases could be nitrogen orthe noble gases. Such a technique could safeguard the system till the leak is found during routine maintenance.

(b) Pressurised gasflow: should there by any leak, the initial flow of gas out ofthe pressurised system prevents the ingress of any explosive dust. A pressure sensor 13 can be provided in the conduit and checked regularly to ascertain ifthere are any leaks. If pressure measurements are madewhilsttheapparatus is operating, allowances should be madeforthe pressure change generated bythefan and also the pressure change caused by heating the gas or (for low temperature tests) cooling it.

In Figure 2,the heater response of an apparatus constructed according to the invention is shown.

Curves A, B and C are respectively the measured temperatures ofthe heat exchange gas in the inlet and return ducts 6,7 and the temperature ofthe airwithin test chamber 1 plotted as a function of time. The test chamber concerned measured 24" x 24" x 24" (61 cm x 61 cmx 61 cm) and in the heater boxtwo 550W heaters were used. The heat exchange gas was air.

Claims (13)

1. A method of heating or cooling explosive or flammable material which method comprises placing said material in atestchamber provided with heat exchange means whereby heattransfer may occur between the contents of said chamber and a heat exchange fluid in said heat exchange means,wherein said fluid is a gas.

2. A method as claimed in claim 1 wherein said gas is pumped through a sealed continuous conduit comprising said heat exchange means and upstream therefrom means for adjusting the temperature of said gas.

3. A method as claimed in eitherofclaims 1 and 2 wherein said gas is a gas substantially unreactive with said explosive orflammable material.

4. A method as claimed in either ofclaims 1 and 2 wherein said gas comprises air, nitrogen, a noble gas -or a mixture oftwo or more thereof.

5. A method as claimed in any one of claims 1 to 4 wherein said gas in said heat exchange means is maintained at a pressure in excess ofthatwithin said chamber.

6. A method as claimed in any one of claims 1 to 5 wherein the temperature of said gas is monitored and adjusted using heating and/or cooling means to ensurethatthetemperature of said gas in said heat exchange means remains atatemperature said material can safely tolerate.

7. A method as claimed in any one of claims 1 to 6 substantially as hereinbefore defined with particular referenceto the accompanying drawings.

8. Explosive orflammable material heating or cooling apparatus comprising a test chamber provided with heat exchange means, said heat exchange means forming part of a sealed continuous conduit which is provided with meansforcycling gas therethrough and means for adjusting the temperature of said gas.

9. Apparatus as claimed in claim 8 wherein said test chamber is provided with means for generating an airflow overtest material placed therein.

10. Apparatus as claimed in either of claims 8 and 9 wherein said means for cycling comprises a fan located within said conduit and provided with drive means.

11. Apparatus as claimed in any one of claims 8 to 10 further comprising control means capable of arresting operation of said means for cycling if the temperature of said'gas upstream of said heat exchange means rises above a preset value or if the pressure of said gas deviates from within a preset range.

12. Apparatus as claimed in any one of claims 8 to 11 wherein said heat exchange means is a finned coiled section of said conduit means.

13. Apparatus as claimed in any one of claims 8 to 12 substantially as hereinbefore described with particular reference to the accompanying drawings.