A radiant panel igniter

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Research Paper (National Research Council of Canada. Division of Building

Research); no. DBR-RP-150, 1962-01-01

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A radiant panel igniter

Ser

TH1

N21r2

no. 150

c . 2 EZDG

NATIONAL

RESEARCH

COUNCIL

CANADA

DlVlSlON O F BUILDING RESEARCH

A RADIANT' P A N E L IGNITER

R E P R I N T E D F R O M

lNSTRUMENT PRACTICE, NOVEMBER 1961, P. 1398

-

1399

R E S E A R C H P A P E R N O . 150

O F T H E

DlVlSlON O F BUILDING RESEARCH

OTTAWA

JANUARY 1962

T h i s publication i s being d i s t r i b u t e d by the Division

of Building R e s e a r c h of the National R e s e a r c h Council. It

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O r d e r o r a cheque m a d e pay-

able a t p a r in Ottawa, to the R e c e i v e r ~ e n e r a l

of Canada,

c r e d i t National R e s e a r c h Council) to the National R e s e a r c h

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G o v e r n m e n t Specifications B o a r d .

A

Radiant Panel

0

N E used by the Fire Section of the Divisioil of of the pieces of apparatus that is frequently Building Research is the one-foot-square, gas-fired radiant pailel illustrated in Fig. 1. I t is the source of radiation with which, firstly, the Section's field and laboratory radiometers arc calibrated and, secondly, measureinents are made of the intensities that will ignite various materials. It also constitutes the essential component of the small-scale spread of flame test apparatus specified in British Standard No. 476: 1953.

I t requires both a gas and an air supply and to ignite it mailually is an operation in ~vhich some considerable care must be exercised. The igniting arrangements described in this note make the operation of the panel easier and safer without substantial espenditure of time and money.

Circuit Arrangement

The circuit diagram is shown in Fig. 2. For con- venience it may be considered divided into three portions, the igniting circuit (top right), the pilot flame-sensing circuit (bottom right), and the solenoid valve switching circuit (to the left of thc Aame-sensing circuit).

For norinal operation switches

S,

ancl

S,

arc left closed and S, is turned to " automatic ". When thc

main s\vitch S, is closed, power is applied to the pi!ot gas solenoid valve and to the igniting circuit. The

"Fire Research Section, Division of 13uilcli11g liesearch, National liesearcll Council, Ottawa, Canada.

Igniter

lattcr consists of an automobile ignition coil, the circuit of which is broltcn about four times a second by relay A. The output of tlic igiiition coil ignites tllc ~ i l o t gas to give a pilot flame a t tllc bottom right-hancl corner of the radiant pailel (Fig. 1). When the pilot flame is ignited the transfer of ions across the ionisatioll gap triggers the cold cathode triode and hence operates relay B. The source potential for the triggering electrode of the cold cathode triode is d.c.; that for the anode is a x . with a superposed d.c. equal to the peak value of the a.c. This arrangement was the most convenient that could bc devised with a view to ensuring that the cold cathode triode would extinguish in the event of flame failure.

Relay 73 is a sensitive device and its oilly function is t o operate rclay C. The latter disconnects the igiiiting circuit and operates the solenoids turning on t h e main gas and air.

To check thc functioning of the ~vhole arrangement a push-button circuit brealter is included in the supply to the pilot gas solenoid valve. When this supply is cut off the cold cathodc triode extinguishes, hence relay C drops off, the main ga5 and air valves arc closed and the igniter circuit is rc-energised. Switches S, and S, and thc " on " position of switch S, are incol-porated

to allow by-passing of the pilot flame-failure c~rcuit and operation of the furnace without a pilot. No such requirement is foreseen i11 the immediate future and in fact precautions have been talten to prevent casual

operation of S, which could switch on the main gas prior to the presence of a pilot flame.

Fig. 2. Circtlit of radiant panel igniter. Ifz the solenoid valve szuitching circuit signal l a m p s are sltozen

in

jarallel with the solenoid valves, and also to indicate zultett the pilot is lit and when the m a i n s i ~ p p l y is szeitclzed on.

Reliability of the Equipment

The reliability of equipment of the type described above almost invariably hinges on the saie operation of the flame-failure circuit. In this respcct thc arrange- rncnt described is quitc good. I t has becn designed on a fail-safe basis and in the event of thc failurc of a power supply or ol a relay the main gas valve will not open. The only harmful failure involving thc cold cathode triode \vould be a short from anode to cathodc and the liltelihood of this is remote.

An occurrence that must bc guarded against is a short between the two wires constituting the ionisation gap. This is probably the worst iault that could possibly develop ior it could allow thc switching on of the main gas supply without thc presence of an igniting source. Two stcps liavc been taken with a view to eliminating tliis. Firstly, the conilections to thc gap are made with earthcd screened lcads so that in thc event of breakdown ol tlie iilsulatioil the cold cathodc triode cannot firc. I t was found that there is a limita- tion to the length of the screencd lead which can be connected to thc triggering clectrode, corresponding to an increase in the associated capacity to a value of about 0.002 PI!'. I t appears that the ion current in the triggering circuit, with the potential source described, is not sufficient to ensure that the triggering gap remains continuously fired. The circuit thereforc behaves as a resistance capacity relaxation oscillator. Fortunately such an oscillatory discharge is as effective as a continuous discharge if the frequency is high compared with the 60 c/s imposcd on the anodc of the triodc. If a substantial length of screened lead had been required it would have been liecessary to eliminate tlie effect altogether by using a highcr potential source.

This could be conveniently derived by adding another stage to the rectifier circuit so that it constitutes a Cockroft-Walton generator.

The second measure helping to obviate a short betwcen the ionisation gap wires is to have a physical arrangement which rnaltcs it unlikely that they will touch. Thc nlost convenient way of ensuring this is usually to bring the two wires in from opposite sides of the flamc. Close to the gap the wires pass through a 2-inch thickness of concrcte which could develop fairly high tcmperatures. Some care was necessary here since it was found that high temperatures sufficiently reduced the resistivity of most insulating materials to trigger thc cold cathodc triode. Thus it was evcn found that ceramic beads, whcn heated, did not provide adequate insulation between the two ionisation gap leads. The nlinimunl valuc of resistance which will trigger the triode is of the order

lo7

to lo8 ohms. The problem was overcolnc by lcaving a clearancc around the wires and cnsuring that thc bulk of thc concrete was con- nected to a n earth potcntial point so that in the event of contact between the wires and the concrete the cold cathode triode would fail to fire.

Conclusion

The arrallgelnents described greatly facilitate the ignition of a radiant panel and provide several safety features at low cost.

Acknowledgements

This is a contribution from the Division of Building Research, National Research Council, Canada, and is published with the approval of the Director of the Division.