USE A PREMIXED PILOT Prembred pllots are recommended beCause the stronger flame anel greater speecl 01 flame propagation contribute to go::idcontact with the ground area ana to sta bili
Trang 1• All Aboul Boilers,· Marine Engineering/Log, February
1974, pp ~53
Bender, Rene J (ed), ·Steam Generation,· Power,
Special Report, June 1964, 48 pages
Burkhardt, Charles H., Domestic and CommerciaJ Oil
Burners, 'Third Edition, New York et ai, McGraw-Hili Book
CompanV 1969,
• Electrode Boilers Make sense.· Enert:t Marketing
cover slory, Electrical World, 8ep1801ber " 1971 pp
70-72
Faust, Frank H and Kaufman, G lheOOore, (ed.),
Handbook of Oil Burning, Oil-Heat lnslilule of America,
Inc., 1951
Griswold, John, Fuels Combustion and FurnactJ$ Arst
EdItion, ThIrd Impression, New York and l.onQ::ln,
McGraw-HIli Book· Company, Inc., 1946
Guide for the Selection, Installation and Operation 01 Oil
Burning Units, Newark, New Jersey, American Boiler
Manufacturers Association, 1971
LeJdcon, Steam Generating Equipment, Third Edition, Artingta'l Virginia, Arrllilrlcan Boiler Manufacturers Ass0cIatiOn 1974
North American Combustion Handbook, Firsl Edilion,
Third Prlnling, Cleveland, Ohio, The North American Manufacturing Co., 1965
• Power From Coal" - Parts I, II, and Ill, Power, Special
Repor1 t;7; the edllorsi, February, March, and April, 1974,
64 pages
Schaphorst, W., "ThouQhts About Fir&-Tube vs
Water-TUbe BoIiEll'S," Power, september, 1972, p 167 SImpson., James H., -Conversion of Boilers to Dual·
Fuel Systems." ASHRAE Journal May 1973, pp 46-54 Steiner, Kalman, Oil Burners, Third Edition, New York,
Fueloil & 011 Heat, 1960
Trinks, W., and Mawhinney, M.H., Industrial Furnactls,
Volume II, Fourth Edition, New York, London, Sydney, John Wiley & Sons, Inc., 1967
Trang 2.~
The purpose of a flame detection system Is 10 delect the presence or absence 01 a safe flame so lhal bJrner 0p
eration may be conllnuedU condil1ons are safe and Inler
fl.4'led if/hey are ~t
FLAME CHARACTERISTICS USED BY FLAME DETECTION SYSTEMS
All flames have certain characteristics In CCM1VT1OI'l in
ch.Jding Ihe following:
ProducUO'1 of heal
Expansion 01 gases
Prod.!cllon' of by-products Emission of Ughl Onfrared 10 ullraviolet)
Ionization of the atmosphere in and around the flame
Flame detection systems have been developed using several of these characteristics with Ihe flame detect
ing ponlon 01 Ihe system emitting a signal or originaJing soma physical action In Iha presence 01 Ihe detected
characteristic
Many flame detection systems designed for use on cb
mastic healing systems use the thermal effect of/he flame
as the method of detection The detecting eiement must
be healed by Ihe flame for operation 01 the system to con
tinue This ,Is true whelher Ihe heat is converted 10 a physi
cal force, as in a bimetal or hydraulic pilot SElnsor, or to an electrical signal as in a Ihermocouple Considerable lime )s required for the SElnsor to heal, and a similar time pericx:1
Is required for It to cool on toss or flame
Larger systems (commerCial and industriaQ require faster flame proving leChnlQJ8S Fast responding systems have been devel~ that use Ih!I Ught emitted by the flame On1rared, Visible, or unraviolet) and the ionization characteristics of the flame
FLAME ROD VS THERMAL SENSORS Flame rod systems dppend on the abllily of the flame to conduct a CUrretlt when a potenltal Is awlied across il (flame lonizalion)
The flame rod must be used wllh a suitable electronic flame safeo;uard control to al11=l1l1'1 the sig'\3l frcm lhe flame rod The flame rod usually is used to detect a gas flame 011 flames are not generally suitable for the awli
calion of a flame rcd because of their higher cperating temperatures
Flame rod defection systems have 4 ifTlXlrtant advan
tages over thermal type pilot sensors:
, o.u !.CKAESPONSE TO FLAME FAILURE-The bi
metal pilOI and the thermocQt.4lJe pilot have a response lime of up to 3 minutes Rarely t:bes this type or pilot r&
spend In less than 1 minute On domesllc Installations, wtlere these flame detection devices are normally em
ploved, low fuel consufT1)tion makes response time less
71
critical Hence the maxil'l"lUTl r8SjX)1"1S8 timings for (his type at equipment have been eslabllshed at 3 miAJtes Thermocouple systems are often used on larger installa·
lions for gas pilot supervislO1 only, as an auxiliary 10 hl\1l
speed rectifying flame rod systems
On larger installations, this slow response to flame fall· ure would be dangerous For exarrple, on a typical larger Installation burning 600 cLblc feet at gas an hour, wring the minute or so It takes the flame sensing device to reccgnlze (hal the flame has been /ost, at least 10 cLblc teet 01
gas can be Introduced Info the conbJslion ct\atrtle( kr
suming that natural gas is used it will take an ad::filional
100 cubic feel of air for Pfoper corrbustlon ThiS amounts
10 a tolal ot , 10 cLblc feet at cornbuslj~ mixture intrawced into the combustion Chamber, 10000Ing for a means
10 be ignited If delayed ignition takes place, lhal voll.lTle of fuel·air mixture COUld caUSEI a SElrious elCplosion For thIS reason, larger jobs need electronic flame safE9J3rd systems that have a response timing of 2 to 4 SElcords
2 PRQVES FLAME AT IGNmON POINT With the bimelal pilot and Ihe thermocouPle pilaf, the pilot is essentially proved at the source
BecauSEI of lhe flexibility of poslllonlng a flame rod, a pilot flame can be proved aJ the point of interseclion with the main flame
3 PROTECTS ITSELF AGAINST FAILuRE OF ITS CQM.!'ONENT PARTS A bimetal pilot or lherrTlOCOl.ple pilot Installed on a large burner Is 5/.bjec1 to the intense heat of the corTt:Iustion c/1aIrt:ler and reflected h&aI frcm radiant brickwork: This heal can cause melal f~, leading 10 Sluggish operation, nuisance shuldJwns, or eYElrt failure or the sansing element In some cases, the bimetal pilot has actually failed In the -on- posllion This, Of courSEl, causes a hazardouS condition by allowing lhe main gas valve to remain open or be opened with no real proof that a pilot flame is present
Wflh eleclronic flame safeg.Jllfd systems, a checking circuit can be built into the syslem If abnOrmal cordlJons occur In the flame delee/or circuit-SUCh as c:pen circuits,
shOrt circuits or leakage resistance to grOU'"d-they simulate absence not presence, at flame and cause the system to fail safe
4 LONG WFE Of CQNSISTE!'lT OPERATION-As pointed out abOve, the intense heat encot.rIlered on larger installations causes metal fatig.Je in thermocouple pilots and bimetal pilots
With electronic flame safE9J3rd syslen'lS, hoW8\'8r, a flame rod or photocell is the flame sensing device TIle
flame rod normally has a temperature raUng In excess
or 2,000 degrees, so it can withsland the hi\1l 1Iame
lempel'alures
71-97558-1
Trang 3FLAME CONDUCTIVITY VS FLAME
RECTIFICATION SYSTEMS
There are 2 basic principles In flame rod delecllon sys
tems-flame conductivity and flame rectification Conduc
tivity systm are, for the mOSI part, no longer used
Eilher lype of system depends on the ab.lllty at the
flame 10 conduct current when a voltage Is applied across
The ac VOltage ~Iied 10 the electrodes looks like this:
In a 60 Hzsyslem, II changes its direction /polarity} 120
times a second At one instant, one of the electrodes is
positive, and 1/120 01 a second later it is negaliva /J.s the
voltage Changes polarity, the flame current (ion flow) will
Change direction
For a CondJClivity system, the areas 01 the 2 electrodes
(called flame and ground electrooes) are equal and the
flame current between them is the same In both directions
This is the principle 01 a conduclivity system, When an ac
voltage is applied across the flame electrode and the
ground electrode, alternating currenl proportional to the
awtled voltage flows through the flame
Because the flame current in a conductivity system Is
ac, this system cannot differentiate between a leakage
current and an actual flame current It i!$ possible for the
system to falsely indicate the presence 01 a flame (with
possibly dangerous results) if the flame electrode is
shQrted to grcx.nd through a leakage circuit with about the
same resistance as the impedance of a flame A carton
depOSit on the base of the flame electrode could form a
very effective leakage path and cause a false flame indica
lion (A direct short of low impedance would, 01 course,
make the system Inoperative.)
lhe flame rectification system also uses 2 electrooes,
bul with 1 irfllortant difference-lhe grOUnd eleclrode is
2 eleclrlXies in the flame Heat frem the flame causes
!!lQIawJ~ be~weE!ln the electrOCies 10 colli.de wllh each
other so forcibly as to knock; some electrons out of the ai
oms, proC:liClng ions ThiSts called flame Ionization Posi
tively Charged·,1ons flow to the negatively charged
electrocJe; negatively charged electrons flow to the posi
tively charged eleclrode
always designed to be much larger than the flame elec· trode (flame r~ FOI" effective operation, the area of the ground electrode must be atleasl4 limes thai 01 the flame rod Usually, the ground electrode will be the burner head Because of the difference In electrode size, more current flows In one direction than In the other When the flame rocl is positive, more current flows
Only the ionized path through a flame and the different sized electrodes can provide the rectified current required for the operation of the electronic network in a rectification system Should a high resislance leakage to ground occur
in the flame circuit, it senc:ls an ac signal Into the network, and lhe system shuts down safely The rectification system does re'Cognize the difference between a high resis· tance leakage to ground and the presence of a flame
Trang 4The 'requirements thai must be satisfied when applying
a recUfying flame rod are:
1 Stable flame - The name 10 be proved must be sta
ble-continuously in contact with Ihe flame grounding
area We are primarily concerned with pUOI flames
since 1beS8 are the most common and the most difficult
awllcalions
3 Flame rod prq)9rly located in the flame
4 A clrcuillo carry the flame signal 10 the amplifier in
the primary control
These 4 requirements will be covered In detail in the
sections thai follow
REQUIREMENT l-STABLE (PILOT) FLAME
The pilot to use if the pilot is 10 be proved by a flame rod
should have the following characteristics
1 11 shoold be a premixed pilot
2 tt should be a strong pilot
3 II should be poSitioned where it will smoothly ignite
the main burner
USE A PREMIXED PILOT
Prembred pllots are recommended beCause the
stronger flame anel greater speecl 01 flame propagation
contribute to go::idcontact with the ground area ana to sta
bility with respeclto the flame electrode
Raw gas pilots are generally difficult to Slfl9rvise elec
tronically because of the difficulty in securing a slable
flame ground In ad:l'ilion, lheflame o1a raw gas pilot may
fluctuate excessively and changeposilion in response to
minor draft variations, cOrTJlllcaling the prd:llem 01 flame
Fig 1 shows the 2 types of pllots Note, with the raw gas
pilol, how the Insulaling layer of raw gas keeps the flame
from contacting the groond area until near the end With
the raw gas pilot reaching oul for the air necessary for
combusllon, even this small contact is uncertain Com
pare this with the flame-l<>groond-area contact in Ihe
premiXed plio!
USE A STRONG PILOT
Make the pilot flame strong enough to be reasonably
stable under the most adverse conditions of draft and
modulation If the pilot flame leavElS the flame electrode for
a period longer than the flame relay liming, nuisance shut
downs are sure to result Increase the gas pressure to the
pilot, or enlarge the pilot orifice_if necessary to provide a
stronger flame Increasing the gas pressure tends to harden and lengthen the pilot flame, increasing ils stability under adverse conditions This stability is especially necessary when the main burner fires with hl~ pressure gas
Adjust the air mixer 10 reduce traces 01 yellow in the pilot to
a minimum
POSITION THE PILOT WHERE IT WILL SMOOTHLY IGNITE THE MAIN BURNER
Follow the burner manufacturer's recommendations
on pilotlocauon, if available; if not, Iry to find the location most favorable to the smooth ignition 01 main flame For example, with a mullJple-head or multiple-Jet burner,the ideal location isat U1epolnt where gas first emerges from bJrner ports, aM a point near the LpSheam ed;J:I of the burner (with respect to the direction of the drafl) The pilot can normally be mountec;l yertically between bu'ner heads as shawn in Fig 2 wilh the pilol flarne playing upward across the junction of the gas slreams coming from
al I6ast 2 heads
Fig 3 shows a satisfaclory pilot installation on an inspirator or venturi type burner The pilot is firing in the general direction of the main flame The pilot is strong enough
to intersectlhe main flame envelCf.lE!, aM the flame rod is Iocatea where it can prove both the pilol and main flames simultaneouSly Fig 3 is a QOOd example o1a pilot that is in
a p::lSition 10 smoothly igdte the main burner
Fig 4, however, shows an exaggerated pcx:lr installation Here the pilot has been located so that no part of II comes near intersecting the main burner envelCf.lE!, and
the Rame rectifier is positioned where a lazy pilol flame ~jr
ing at low gas pressure) can still coolacl it Obvioosly this installation can only result In delayed ignition and ro Ql starts
On radiant type burners, t~ pilot is often ITlOl.nted alongside the burner or fires throogh one of the b.Jrner c:penings In the radiant burner block as shown in FlO 5
AG 1- FLAME CONTACT WITH GROUND
AREA-RAW GAS PILOT VERSUS PREMIXED PILOT
Trang 5Here the pilot fires In the dlrec:lion of draft and provides a
flame which readily Inl9rwcls with t!"le main burner flame
Fig! 6, 7, 8, and9 smw recomlTlElOdad pilolloc:ations
on other burner types
PosmON THE PILOT $0 rr FIRES IN THE GENERAL
DIRECTION OF THE DRAFT
Fig 315 an 8l1:C9l1enl BlI:arrple at a piJOllhai is localed 10
1Ire In the direction at t!"le drall Obviously, drall etfecl
would not pull th8 piiOI away from lhe main 11am& In fact,
prevailing draft would anecl both pilOl and main flames in
FIG 2-TYPICAL MOUNTING OF FLAME
RECTIFIER' P·ILOT ON -MUlTIPLE HEAD
GAS BURNER
11=j):::1!yP'LOl lkd=j
FIG 3-RECOMMENDED PILOT INSTALLATiON
ON AN INSPIRATOR OR VENTURI TYPE
BURNER
the same manner Any drilllrlQ of one flame would be accompanjedbya almUaralfling 0' l!"le other The result will
be smooth reliable Ignition
Never instalilhe plio! burner so thallhe pilot flame can st1i1l Joaposiflon whete il will not positively IglU8lhe main burner, but can still make conlact wilh the flame electrode For ell:atJl)le, i1 the pliol burner Is installed horizontally or inclined the flame electrode must NOT fie along the lop of (he pilot burner assembly where a weak or ~Iazy' pilot
PILOl
FIG 4-INSPIRATOR OR VENTURI TYPE
BURNER WITH EXAGGERATED POORLY POSITIONED PILOT AND FLAME RECTIFIER
TDP VIEW
"" J
' - - - FIG 5-TYPICAL MOUNTING OF FLAME
RECTIFIER PILOT ON RADIANT INSHOT TYPE BURNER
Trang 6flame, inadequate to liQht the main burner, can curl up
around lhe shank 01 lhe 1lame eleclrode
PROTECT THE PILOT FROM THE EXTREME HEAT
OF THE COMBUSTION CHAMBER AND RAOIAnON
00 not locale the pilot burner nozzle where the main
11ame will impinge on il under any f1rinQ conditions tf p0s
sible, keep the pilot oorner below or behind the main burn
er ,(as in F(Q 3) so Ihat the burner frame and refractory help
aiDE VIEW
BV~~ER
""
FIG 6-POSSIBLE MOUNTING OF PILOT AND
FLAME ELECTROOE ON RING TYPE
BURNER
FLAIoIC
RECTIFlU
TOP VIEW
FIG 7-POSSIBLE MOUNTING OF PILOT AND
FLAME ELECTRODE ON TUNNEL
BURNER,
75
10 shield the pilot burner Locating lhe pilot In the secondary air stream will also provide a cooling effect KEEP THE PILOT VENTURI ACCESSIBLE AND AWAY FROM HIGH TEMP1::'AATURE AREAS
The pilot venturi /TILlS! be accessible 10 make air adjustments 10r the proper amount of premi:Jclng Preferably, the venturi ml:lCer should be localed outside the combustion chamber and usually outside of any wind boll; area Fig 2 shows the venturi located where il is readily accessible Hi.;;tl-temperature locations should be avoided because Changes in air temperature al the pilol mi:lCer
characteristics
POSlllVE COMBUSTION CHAMBER PRESSURES Positive combustion charrtler pressures are caused t¥ lhe rapid expanSion of lhe fuel mixture in the oorner One result is pilot instability, causing erratic flama prO\ling In severe cases the pressure may be emugn to snuff oullhe pliol
It may be necessary to relocate the pHot venturi where the pressure will be equaliZed between it and lhe pilot nozzle; e.g., below lhe oorner bed on an ~hol burner
SIDE VIEW
FIG 8-TYPICAL MOUNTING OF PILOT
ASSEMBLY ON MULTIPLE HEAD INSHOT BURNER
PILcr
TOP VIEW
FIG 9- TYPICAL MOlJNTING OF PILOT
ASSEMBLY ON SINGLE- PORT INSHOT BURN.ER
71-9755&-1
Trang 7REQUIREMENT 2-ADEQUATE GROUND
AREA
,,,.
•
Ensure that the ground area In contact with the flame
exceeds the area of the flame rod normally In COTllact with
the flame by a raUoOf alleast 4 10 1 This ratio Is sufficienlly
large 10 prevern grOUnd area problems regardless Of the
pas/lion of lh" rod in the flame
WHAT IS GROUND AREA?
Ground area is any malerial In contact wllh (he flame
that will carry the flame currenl10 ground Grourd area Is
dasignecHnto the buTner ItseH In ad::Iition, melal burner or
COrnbusl.ion chamber pal1&, refractory, and other maleri·
als In contact with the flame all act as pan 01 the ground
area (Refractory does conliJcl once It Is healed· this Is
why the flame rod must not be In contact with refractory.)
A typical method of pro iding ground area tor a pilO!
flame is shown in Fig 10
Ground area is provided on the C7005 Rectifier Pilot by
"bomb fin" grOUrd plales that are part of the burner noz
·l.~( ~lECTIItlDE -.x"
FIG 10-C7005 FlAME RECTIFIER PILOT
zle This type of pilot will rarely present ground ralio ptcblems
It is not usually necessary to provide special grounding a.sserrtlIles for main burner flames-the flame COntact with the walls of the cont:ustion cham/::le( and wllh burner parts is generally sufficiant 10 provide' an acceptable ground-lo-flame-rod ratio
ADDING GROUND AREA TO EXISTING BURNERS PILOT BURNERS
If the existing pilot bJrner does not proviae sufficient ground area for the flame, it may be replaCed with a rectifier pilot like the C7005 which does have sufficienl ground
II may also be filted wilh a special ground assembly, if available; or grOUnd area may be added to the pilot USing one ot the methods Sl'lJwn below
•
FLAT PLATE MULTIPLE ROD
TIle flat plale asserrbly is similar to the type at ground area used on the C7005 It may be construc.led simply by welding together pieces of high-temperature steel The assembly is then welded 10 the burner head
TIle multiple rod assembly is constructed b\' welding plecesofflame rod loa melal strap The slrap is then fitled around lhe burner head and welded In position TIle rods may also be welded directly to the burrier head, or the head may be tapped and the rods screwed Into the tapped holes
MAIN BURNERS
If gtwnd area musl be added 10 a main burner flame, add rods or pipe grOUnded 10 the boiler or furnace wall and projecting Inlo the flame al all times
RODS VS FLAT PLATES FOR GROUND AREAS
A curious phenornenal will be noted when using flal metal surfaces as a grounding medium in lhe flame envelope Allhc>lJl;t1 the metal may bEl complelely Immersed In the flame, effective ground area In contacl with the flame wlllex:lsi only around lhe edgeS of the tlal metal surface A thin layer of unburned gas tends 10 insulate the center portions of (he flal surface from the burning fuel and IonIzed
gases adjacent 10 lhe ftame This condition Is present, but
to 8 much lesser degree, when rod materials are used for flame grounding
Trang 8Rod materials also offer a much more flexible means of
accompliShIng grounding, as they may be mounted In any
number and In any pattern necessary to Insure contact
wilh the flame under all conditions of varyIng drafts The
qusstlon 01 flash-over does not enler the picture with rod
grounding either, as II may with f1afplate ftame groundlng
Protel;tion from flame snuffing In strong drafts Is perhapS
even grealer with rod grOLrlding than II Is with flat plate
groundino The flame will liang behinc:l a rod, whereas lhe
plate lends tpblock the air flow, Ihus wiping the flame from
Ihe surface ,
EFFECT OF SOOT OR SCALE BUILDUP
Soot or scale buildup on a flame rod can rasull in reduc
tion of the grOUnd-te>-flame-rod-area rallo to IBS5lhan the 4
to 1 rEQJired for prc:per rectiflcallon and'flame currenr Un- ,,',.
der -no flarne-, condltions, sool and' scale are nonconduc
tors of eleClricity However when flame is awlied, they
actually become condJclors The problem Is In the addi·
tlonal area acX1ed 10 the flame red by fhis bui~ Ad:fI
tlonal surface area provided by Ihe hills and valleys of lhe
buildup material can easily cb.bIe or triple the flame rod
area Too much buildup can decrease the flame currene
below the value required to hold in Ihe flame relay, and
burner shuldown will occur This points out Ihe necessity
of inspecting and Cleaning the flame red periedically
The best inc:licator or adequate ground area is a flame
signal reading of prq:>er size and steadiness The flame
signal is measured with a dc microammeter (Honeywell
W136A or equivalent) connected in series with the F lead
Most HoneY'N8l1 conlrols have a meter jaCk which aute>
malically places Ihe meter in serles with the flame rod
Refer to the inseructions paCked with the flame safe
guard control for Ihe exact flame slglal measurement
procedure
WlIh most Honeywell conlrols, the flame siliT'i!l should
be at least 2 microart'f.lElres and s1eadj The reading 00
'\ained on self-Checking systems will be much higher-re
ter to the controllnsfrucfions If the flame signal reading is
not correct, the ground area shOuld be the firsl ilem
checked If In doubt, ~ more ground area 19lTIpOfarily
and recheck the flame slglal tt a satlsfacfory reading is
obtained, adj permanent ground area
,
The location of the flame ro:::l In the flame en'l9lc:pe
must meel the following requirements
, The location must provide the req.Jlred type of flame
4 The locatron should prevent changes In the flame ro:::l posillon or al leasl eliminate the PJSSlblllty of any change causing a dangerous situation
TYFES OF FLAME SUPERVISION The flame rod may provide any Of 3 possible types of flame supervision:
Pilot anc:l maIn flame slmultaneoosly
Pilot flame only
Main flame only
BOTH PILOT AND MAIN FLAME SUPERVISED
If at all possible, the flame rod should be Iocaled af the intersection of the pilot and main flames This type 01 awljcation proves lhalthe plloe Is adecp.Ja.le for main flame ignition anc:l proves lhe main flame COO!lI1l.OJSIy dJring the run cycle
Figs 3, 6, 7, and 8 show flame rods awlied fa super
vise the pitof flame at the poinl of intersection with Ihe main flame
PILOT FLAME ONLY SUPERVISED
On some installations it may be Impossible to prO'fflthe pilot and' main flames SimUltaneously because of vari· ations in the main flame envelq:::.e al different tiring rates Where II is not possible to prove lxlth pilot anc:l main flames, proving the pilot only is the nexl best q:ltion Fig 11 shows a mechanical fan type burner where the gas pressure rotates Ihe fan blades 10 provide torced draft for the burner [n this burner, the flame position varies with the firing rate, making illmpossible to prOl/e lxlth pilo! anc:l main flames Alsa, b&cause Of the design of the cermus
tion chamber, It Is Il'fl)OSSible fa check the main flame and
pilot flame without sub~ !!.ng the pilot asserriJlv to the Intanse combustion chamber temperature
Proving the pilol only is also ckJne on con1:linatlon IOJ2S" oil burners, particularly on the horizontal (otary type wllh the gas ring added
When proving a pilot closer 10 Its source than at the ignl tion point for the main flame, we can sHI! prove the presence of an adequate pilol flame by supervising the pilot
gas prBS5ure
Fig 12 shows a raverse acting gas pressure switch
used to intern pt the circuli to the main gas valve whenever the pilot pressure drops below the point at which rell· able ignllion would occur
MAIN FLAME ONLY SUPERVISED When only the main flame is to be supervised, the flame ro:::l must be located where It will remain in the flame eovelq:::.e during atl variations In firing rates and draft adjusfments On larger burners, special conslc:leratlon mUSl bEl given to main flame welq:::.e changeS that occur 0'l9f
Trang 9considerable distances wllh variations In firing rates cer
tain ring and gun burners produce regular fluctuations in
flame loCation, even during steady firing AWlicalions or
this kind shouldJ)e carefully checl<ed under all possible
draft cerdtions and LKlder all firing rates before the instal
latioo is considered complete
ENSURE THAT THE FLAME ROO REMAINS IN CON·
TACT WITH'THE FLAME UNDER ALL CONDITIONS
,
aiDE VIEW
FIG 11- MECHANICAL FAN-TYPE BURNER IN
WHICH PILOT FLAME ONLY IS
AVOID LOCATION THAT ALLOWS FLAME ROO TO
BE IN PILOT FLAME WHEN PILOT CANDLES Avoid locating lhe rod Immediately aoove the pilot flame (see Figs '3 and 14) If the plio! pressure Is rEdJC8d for any reason, the pilot flame would obViously decrease in its int9l"\!'lJty, and a candling effect would be noliceable If the flame rod Is localeo: immediately above the pilot, this candling could allow the flame rod to sense the pilot even lhough the pilot flame is in no posilior'lto smoothly or sa/ely ignjte the main burner Bringing the roo In from the side or from undemealh the pilol flame avoids the POSSibility that lhe flame roo will cootac! the pilot flame under a low pressure condition where the pilot is incapable of prO\lidlng safe ignition
LOCATE THE FLAME ROD TO PREVENT DANGEROUS CHANGES IN POSITION Keep the flame roo short, avoiding bends i1 possible Fig 15 is an e.:ceilent example 01 an application that meets this r9CIuirement Nole how the flame roo is brought
PILOT 8U~N,~
.RONG PO~TIIl<
Of ~O~
FIG 13-IMPROPERLY POSITIONED FLAME
ROD MAY PROVE AN INADEQUATE PILOT FLAME
NOTE· INE fLANE H!CTIlOOE
INOI.JLO 6E LOCA TED ON
EIT~!~ ~IO[O~ 8ELOW
T~[ PI~OT
WRONG
FIG 14-IMPROPER POSITION OF FLAME
ELECTRODE ON C700S FLAME RECTIFIER PILOT
Trang 10do n vertically Irom the Iq') of the burner assembly This
llame rod will not sag and dTOCP away rrom the location
where il can properly supervise the pilol and main flame
Had the flaiile fOCI been broughl In from behind, it would
have required a longer flame rod with a bend The longer,
bent flame ro:! could drocp out 01 p::sition and no longer
provide proper supervision of the pilot
Other argu~ls in favo~.at a ShOrt flame rod without
bends are: (1) a shorterro:!lessens the chance of excess
flame rod area distortinQ the rCJd.area-lo-grouroarea ratio
upon which uame rectiticatio:n depends, and (2) straight
flame rodS are easier 10 replace than rlXiS that musl be
bent to reach the flame-proving point
LOCATE THE FLAME ROD 6aOW OA BESIDE THE
FLAME IF II CANNOT BE APPUED VEATICALLY
It the flame, rod cannot be positioned SO lhat it comes
down from directly above the flame as in Fig 15 locate it
below or bes~ the piiOI flame FIg.13 Shows how a flame
rod may plCve an Inadequate pilot if it is lX'SifiOMd along
the top o1lhe pilot burner In ad1ition, the ro:! must not be
located where II can drocp Into a position where it mighl be
able to prove an unsafe pilot If the rod is lOCated beside or
below the pilot, it will, if affecled by excessive heat condi·
lions, drcq::l away rrom the pilot flame and cause a sate
shutdown
UNUSUAL APPLICATIONS
RUNNEA PILOT INSTALLATION
Fig 16 shows a burner with mullip/e firing ports iglited
by a runner type pilotlhat is proven at the extreme encI at
-o
o
o
FIG 15-INSTALLAnON WIlli SHORT,
STRAIGHT FLAME ROO
the runner This application Is used on many cast Iron sec
lional bollars, and is ideal for the flame rectification sys tam, The sna.ll, constantly burning pilot may be proved by
a thefmOcQ1 1)le IypQ assambly, The constanlly burning pl
lat ignites the runner pilot and that, In turn, fires each oftha burner ports The rUMer pilol is checked at the1ar end of the runner,
A runner piiOI or this type should have adeq.ele grounding area (the small flame al each runner port Is grounded), Ar'rof recbJClion in pressure would make ilself lell flrst at the far end of the runner Therefore, in lhe event
ofpilot r9liJclJon the flame rod would no longer be able 10 prove Ihe pilot at lhe end afthe runner ar'd would not allow the main burner valve to open
MULTIPLE PILOT INSTALlATION
If the runner typepilot is nol salisfactory lor thepartieuIar installation, end it It is necessary to have more than 1 pilot assen'tlly within a burner for safe i~llIon !hen sach pilol should be SlCle/Vised by its own flame ro:! an::! flame safeguard control With the RA890 Primary Controls, this individual supervision can be accompliShed wil.toJl aUlfjliary equipment
Fig 17 ShOws one RA890 and one flame rod asserrbly
for sach pilot being- proven The Interconneclion is SirTlJfe because of tne fleXibility of the RA890 The control circuit can be isolaled from the electronic network After the first pilot haS been prOViEln, terminal 5 of the first relay provides the power to the control circuit at the relay checking !he second pilot
It the second pilot is also proven, power Is made available to operate the main burner valve With this arrangemenl, it makes no difference wheth9r Ihe p~ots are intermittent, Igli!ad for eaCh burner operallon, or c0n
stantly burning In any case, they would haY8 to be pfO'f91 separately or the main burner vaNe could nol be energized If either pilot fails, the bUrner Is IrTlITtIilCiaIely sh.rt
down
-
Trang 11
-REQUIREMENT 4-PROPER FLAME CIRCUIT
•
"
We have disCussed lhe ·relatiOnshlp 01 the flame, flame
rod, and grCll.S1Cf-area When the flame rod and the ground
area arelmmel'SEld In the flame, and a vollage Is appIJEld, a
current flows In order to use this current flow, we must
prO"lIda a circuli In which the current now Is noll,Jl'"lCUly af
fected ~ cspac:itance, Interference from OIher vollage
sources, or leakage resistances to ground The spectfic
Slaps thpt are rBCJ.lirfld 10 provide this type of circuli are
coverEld in lhe following paragraphs
PROTECT AGAINST LEAKAGE RESISTANCES AT
THE FLAME ROD INSULATOR
The flame rod insulator Is normally a nonconcilctor of
eleclrlClty However, when lhe te~rature at the insula
tor exceeds 500° F, the resislance of the ceramlc material
decreases enough so that II will conduct current This
leakage current may be enough to decrease the flame cur·
rent bEllow the value requirEld to hold In the flame relay,
and burner shutOOwn will occur Dirt, soot, and moisture
on the insulalOf can also cause leakage current gr~f
enou~ to cause shuloown
For proper qJ9ration of the rectification system, 1\ is
necessary to mainlain alleast a 20 megohm insulating ra
siSiance in lheftame roo circuil If the insulating resistance
drq?S below 20 megohms, a prc:portionate drop In flame
reclification current occurs, evenlually reachinQ a low
point where InsuffiCIent output shuts down the system
Fig 18 Sh::lws BIl instalialiOfl with flame and ignition elec·
trode leads properly protecIEld
The temperature at the flame rod insulator must be
ccnsiderEld when selecting a flame roo location, and Ihe
insulalor must be cleanEld periodically
FIG 17-MULTIPLE PILOT INSTALLATION
USE PROPER WIRE AND WIRING TECHNIQUES FQR THE FLAME LEAD
No 14 wire lralEld for 90 C or hlg,er) Is recommended for the flame lead Actual wire size Is not critical: No 141s
easy to handle and to PJII throug, conclJlt The Increased resistance lhal occurs wllh decreased wire size Is not Sltt ntficant (For example, 200 feet of No 30 wire-were II practical to use such small wire - nas a resistance of only
20 ohms Cc:rTl)ar8 Ihis 10 the internalt~nceof 1500 ClhrTlS In the meier we connect In series with Ihe flame rod when reecllng the flame current.)
The type of Insulation Leed on the flame lead Is IrrporIant, because It must protect against leakage resistance 10 grCU1C1 When a suitable wire Is used, leakage will not occur from the wire Itself Taped TEFLON (Honeywell Part
No R1298020) is recommendecl for high lemjJElrature installations
The maximum length oflhe flame lead is IImllecl by the electrlc:al capaCitance between the F lead and ground (condJll); In olherwords, bytheamouni of currenllhat can
flow from the lead 10 ground tf the capacitance is perml!· ted (by excesslve lead length) 10 exceed 0.02 microfareds, the ttame SI\7ltt1 will be maskEld and relay operation wlll become uncertain In practice, 1he flame lead can be Lp to
150 feet long, provided proper wire Is used and an acceptable flame sl\7ltt1 Is measurEld althe primary conlrol The flame lead may be run in conduit with other line vollage wiring without being affectEld by slray electromatt netic current pickup lhe flame lead should not be run in the same conduit with hi~ voltage Ignition transformer wiring
PROTECT AGAINST THE POSSIBILITY OF IGNITION INTERFERENCE
IQl1ition inlerference is a false signal superimposed.on the flame signal II is caused by ignilion current feeding through the flame 10 the flame rod From the flame rod it feeds Ihroug, the F lead 10 the primary control and back to ground When the iQl1llion current is small it will 00 no damage to the relay, but it will either increase or decrease the flame current Whelher the currenl increasElS (positive
FIG l8-INSTALLATION WITH FLAME AND
IGNITION ELECTRODE LEADS PROTECTED FROM EXTREME HEAT
Trang 12lnlerlerence) or decreases (negallve inlerlerencel d&
pends on the phase relationship of the ac vollage in the F
lead and In the Ignition electrode Subtractive differential
may ~.sutricienl to cause the relay to dr(fl out If the inler
ference /s severe enoug., the relay ilself will be damaged
When ignition Interference damageS a relay, the relav
must be replaced
rgnillon Interlerence Is most easily detected ~ rGading
[he flame currenl with U'" ignilion l::olh on and oft A differ
ence greater than 1/2 microampere indicates the presence at ignition lnterlerence; rearrange the name eleclrode ignition electrode, and ground Check for cor· rect spacing of the ignilionel~troda.1he ad::Iillon atmore groUnd area In the form of a flat plate between the flame rod and the Ignition electrode 10 give a beller grOLl"ld for the ignition and 10 recl.Jce the current to lhe fleme rod may
be sufflclenl to cure the prcblem
Trang 13The flame rod is held In position by a flame rod holder or
rectifier pilot 8SSeirDty The method used depenc:E on the
size type, and physical requIrement 01 the burner
FlaIM roo holders provide a mount tor lhe flame roo,
insulation 10 prOlect against leakage resistance 10 ground
al the mounling point, and terminals for connecting the
flame roo 10 the el9clronlc n~rk in lhe primary control
Holders are provided wlltdJl flame rods to allow seleclion
of the prcper type and length of rod for the particular
awJication
In standard holders, rods are held in place by chuck
and setscrew arrangement In miniature -spark plug"
type assemblies, the rods are screwed inlo threaded
bases ',_
Rectifier pilots are cCllTbinaUon flame rod holders aOO'
pilol burnaIS The flame electrode Is sU!=p1ied as part attha
reclifier and is permanWllly held in position for proper pilot
provIng Honeywell rectifier pilots may beprovided with 19
nllion electrodes for automatic Ignition systems If desired
TYPES OF FLAME RODS
Whenever the flame rod extends into the main flame
envelope, the temperatures encountered may have some
effect on the type of rod selected
Most Honeywell flame reds are made of a stainless
sleel alloy called Kanthal A·1 which has a maximum oper
ating temperatLJl'e rating of 2462 F 11350 C) Two rcx:ls are
made of a similar material called Jellif Allay ~ Kn
with a maximum temperature fating of 2200 F ['200 C) These
raUngs are ac!lq.Jate for atmospheric burners and for most
low pressure partial premix burners However, as the de
gree 01 premixIng Increases, the flame lends 10 shorten,
becomes colorless, and burns wilh hiQher velocJUes and at
higher temperatures
On power burners where extremes In temperatures are encountered, e'tUfl Kantha! rOdS may deform and delerioreJa In many of these cases It Is necessary 10 use a ceramic rod Ttls ceramic malarial recommended is Globar, which ha6 a maxImum temperature rating of approximately 2600 F [1425 C)
NOTE: It lhe rod materials mentioned above are used
at higher temperatures than those Irdicated, rod deterioratlon may require a planned replacement program Un
~rted horizontally molKlted electrodes may sag al somewhaJ \oW8t temperatures, varying with rod lenglh and diameter (Standard diameters of electrodes available are 3116 inch for Kanthal, and 3IB inch 10r Globar.) Ability oftl1e rod to maintain i\s position when mounted horizonlally shOuld be determined by testing If the temperature is within 400 degrees of the rated maximum
FLAME ROD LENGTH Honeywell Kanthal A-l rcx:ls are available in several lengths from 3-1/2 10 48 inches These rcx:ls are adaplable
10 the standard flame rod holders or to the miniature
~spark plug" type assemblies Jellrl Alloy "K- rcx:ls are available in 20 and 26 inch lengthS; they are for use with the C7008A miniature assembly only G\Qbar rcx:ls are available only in 12 inch lenglhs The Glebar rod is 1TIOlJnted in a holder that can be filled with a chuck assembly to adapt the holder to the larger diameter of the ceramic rod Chucks are available for the C7004B and tor the C7011A
The flame rod used should be as short as possible for the flame to be proved Most flame rod holders may be fitled wllh a threaded pipe to add ~rt to the long flame rod rltesung indicates that the rod musl be supported
Trang 14C7005A B RECTIFIER PILOT ASSEMBLY Pliol burner and flame delector electrode for Industrial or commercial burners-Ignition electrode on C70Q5B - Kanlhal rcxl Included
Q179A, B RECTIFIER PILOT ASSEMBLY
Pilol burner and flame detector electrode lot commercial or industrial burners-Ignition
eleclrcde on 0179A-Slainless staal electrode(s)-Selsction of pilol flame patterns
at79C, 0 MINIATURE RECTIFIER PILOT ASSEMBLY Gas pilot burner, flame detector electrode, and ignilion electrode for commercial or indJs· trial burnars-Q179D has a thermocouple adapter In place oflhe ignition elecfrode-KanthaI electrodes mounted in ceramic insulators-Stainless S!eellargel to stabilize Il'1e flame
and provide correct flame ground area-Selection 01 mounting brackel:s and target configurations
I ~ Small holder mounts on 1/2 NPT male or female pipe fitting-Kanthal A·, rod {order
REFER TO THE HONEYWEll FLAME SAFEGUARD CATALOG FOR DETAILS OF EACH MODEL
Trang 15CHECKLIST FOR FLAME ROD APPLICATIONS
o Flame rocl remains in gcxx:l contact with flame under all normal firing conditions
o Flame rod iS~ as short as possible -preferably applied from below or beside the flame proved
o Ground area is alleasl 4 limes the area of (he rod in the flame
o Flame lead has Insulation raledfor 90 C or higher-usually No 14 wire for normal installations, Honeywell specification R1298020 101 hig, temperature installations
o Flame lead Is as short as possible
o Temperature at the flame rod insulator cbes not exceed 500°F
o ,A,pplicalion protected against ignilion inlerterence
0 Mounting pasHian permits service
o FLAME CURRENT CHECK INDICATES STEADY FLAME SIGNAL OF AT LEAST 2 MICROAMPERES (8 microamperes for saM' checking systems using the R4Q75,R4138, or R4181 controls)
Trang 16tion whose wavelengfh 1alls wilhin the range of falls within Ihe Infrared banct, ils waves are too long 10 seen
waves 100 shOrt to be seen sors are divided Into 3 groups,
Fig 1 shows radiation inlensity at gas
flames, and hot refractory with respect to
also shows the response ranges at ultraviolet
RECTIFYING PHOTOCELL FLAME DETECTORS
In a rectifying flame detection system, alternating volt· negatively charged electrons are attracted to the pos.iage is applied to the flame (FJ and ground (G) terminals of livety charged anode, and at this moment current will naw
Ihe primary contrOl, but the operalion at the electronic net· The magnitude at the current depends on the lnlensi!¥ of work depends on direct currenl It is the jcb of the flame the light reaching the cathode-Which in turn determines
•
I ',' detector to permit the flow of current, and 10 convert alter· the amount at electrons which are emilled by the actIve
nating current to direct current, when II senses the pres coaling of the cathode A fraction of a secord rater, when enC'El of a flame the allernating CUrrent flow-isrev-ersed, the anode is nega
tive and the cathode p:::ISilive The anode, which is not
PHOTOCEll OPERATION coaled with active malerlal, cannot give off electrons; tal
The rectifying photocell is a high vacuum type photo sequenlly, ciJring this part at the'Cycle, no current wilt !low
FIG 2- CONSTRUCTION OF THE RECTJFYING
PHOTOCELL WHEN ANODE IS POSITIVE ANO CATHOOE NEGATIVE, CURRENT FLOWS
In this way, the photocell acts as a true rectifier, changing
ac to de when flame it; presenl, and permitting no current
at all to flow when lhe flame Is not present
The rectifying photocell has one very Important advantage O'>'er Ihe cadmium sulfide and Infrared (lead sulfide) type detectors, which are nonrectifying, in thai it is protected against a false flame signal ciJe to Ihe presence at a hir/1 resistance shOrt If a high resistance short Is p1acad across Ihe photocell, It would provide an alternate current path and only allernaling current would flaw in the circuit Consequenlly, the relay would drop oul and the bumef would shuf down because dc only must be present lOoper· ate/he electronic network of the relay
The flame rectification clrcuil will distinguish between The cathode of the reclifler tube is coated with caesium
the presence ot an oil flame and a high resistance short to oxide or some other active material which has lhe property
grOUnd This characteristic of flame rectification systems
of emilling electrons whenever light strikes il Thus, in the
makes the use of ~cial shielded cables unnecessary presence at an oit flame, light striking the cathode causes
No 14 wire (rated for 90 C or higher) Is recommend9d for"electrons to be emitted
Consider what happens when alternating vollage is ap normal temperature installations 0100se insulation plied to lhe cell AI the instanlthe anode ~rale) at the ph0 which allows no ¥preclable leakage resistance from wire tocell is p:::ISitively charged with respect to the cathode, the 10 wire or from wire to ground
Trang 17RECTIFYING PHOTOCELL APPLICATION
PhotOC8lIs are generally applied to commercial and in
d.tstnal 011 burners, where the aO::led safety of shorted
lead protection [os r9qJlred Photocells are nol used 10 d&
teet gas flames bec:au6e a well·adjusted gas flame emits
Insufficient visib6e light There are 4 basic ree:,Jiremenls for
a g::od phoCoceIl awllcalioo
1 Photocell must have a gqod view of the flame
2 PholocelLmuSt be prot.ected from the light emilledtlt
hol1'9fractory
3 Terrpera'ure al the cell must be under 155 F
4 Proper wire must be used for the flame lead
Each of these 4 basic requiremenls will be covered
briefly In the following sections
1 PHOTOClU MUST HAVE A GOOD VIE:W OF
T1iE FLAME
The reclifying J:holocell muSI be located where it wlll
continuously siltll a stable portion of the flame being de
tected PhOtocells may view lhe burner name through a
hole In the cornbusllon cha~r wall, Irom inside the blast
h tle of a gun type burner, or from some other location
within the burner itself which is determined by lhe oorner
manufacturer 10 provide adEq.Jate sighting Cell housings
are desiQ'l9Cllo adapt the plug-in type pholoceilio any of
these awlications
2 PHOTOCELL MUST BE PROTECTED FROM THE
LIGHT EMITTED BY HOT REFRACTORY
Glowing refractory surtaces emil ligh! in the visible
range If the photocell is Sighted so It resp:lllds to Ihis light,
the flame relay will be held in atlhe end of the firing cycle,
and the burner cannot be restarted Lmtil the flame relay
drops 00 care should be taken in a.w1ying the photocell
to aimit at aportion of the refractory which will remain rela
ti""'ly cool An Olitice or filter may also be used with most
cell mounts to restrict the viewing field of the cell Of to cut
cbwn the lotal li\11t reaChing the celi faCe so that hot re
fractory will not be detected
3 TEMPERATURE AT THE PHOTOCELL MUST
REMA.lN UNDER 165 F
R9Clification lype phOIOCeJlS must be protected from
arrbient lerrperatures above 165 F The coating on the
cathode of the cell will break cb'M'l al higher temperalUres
A photocell damaged by excessive heat can be recog
nized by the cracked bluish appearance of the face of the
cathode
4 PROPER WIRE MUST BE USED FOR THE
flAME LEAD
Use of Ihe wrong wire lor the flarT'lB delector lead can
cause signall0S5 due fa moisture in the insulation, actual
insulation damage due 10 high lemperatures, or capaci·
tance affects between the flame (ead and ground For rec
tifying photocell awlicalions where the f1ama lead is nol
Slbiecfed to terrperatures in excess of 125 F, No 14 wire
(rated for 90 C or higher) is recommended for lhe flame
sig1a1 leadwir8!i For high temperature installations, spe
cial wire (sUCh as Honeywell Specification No A1298020,
rated J.4) 10 400 F contir.u::tJS ckJty, or 9CJ,JIvalent) must be used for the flame signal leads
MOUNTING THE PHOTOCEll ON THE BURNER
On gun type oil burners, the photocell is oRen mounted
in the blast tube itself behind Ihe all nozzle and Ignllion eleclrcx:le In this position, the csl1 is cooled by the airstream in the blast tube Different mounts are used for standard and shell-head type burners
If excessive heat is reaching the Cell, a filler may be adjed 10 cut down the heat on lhe cell face A filter may also be adjed to restrict the visible light of a hot refractory which strikes Ihe ceU face If lhe photocell signal Is weak, a magnifying lens may be actled 10 gel a beller view of lhe Ilame
Burner mounted photocells will generally have been lested by the burner manulactuter for proper line at sIght and proleclion trom excessive heal If a burner mounted photocell falls or is damaged, il is best to replace II with a similar cell mounted in the same location as the old one
FIG 3- BLAST TUBE MOUNTING OF PHOTOCELL
Trang 18FIG,S - ADDING A FILTER OR MAGNIFYING
LENS TO A C7013A PHOTOCELL MOUNT
MOUNTING THE PHOTOCELL ON A SIGHTING
PIPE
The photocell, in an appr~riate holder, may be
mounted on a sighting pipe 10 view the all 'flame from a lo
cation remote from the burner itself A typical applit.alion
on a horizontal rotary burner is Shown
Cl'lIlIlJl PHQTOCfLL
This type 01 mounllng Is used, of course wher1 the
burner is not a type easl1y adapted 10 an internal celiloca
lion Ills also used, however, wMre it is dimcult toproteci
the cell from high temperatures atlheburner, or when ills
necessary to select a particular locallon to avoid hot re
fraclory sensing The sightrng pipe musl be localed so that
the flame is sighted under all firing conditions The cell
should generally be as close to the flame as possible
consistent with lemperalure restrictions
The viewing angle of the photocell plane retall'ffl to the
burner plane should be as small as possible This will di
minish as much as possible lhe effect of changes in the
flame size on the slghllng ability of the cell In general, it Is
best to aim the cell al a !XIlnl abOut 10 10 25 Inches ahead
of the nozz~ or spinner cLP
For steam or air atomizing burners, 1 or 2 additional detectors ShOllld be connected in parallel to Ql'lSure sl~lIng
of the flame at all firing rates
If possible, the slghtlng pipe should be slanted d0wn
ward to prevent soot or all vapor trom accumulating In the pipe Viewing wind::>Ws, lenses, and filters must be clean
at all times IIlhe sighting pipe slants t.pward, cleaning will have to be done more IrequenUy If the pipe muSI slanl up
ward, forced ventilation would be beneficial
Two types of cell holders are available for sl~Ung pipe mounting-C7003 and C7010
The C7003 cell holder has a finned conslructlcn which protects lhe ceU1rom excessive heat EIther type of holder permits removal of the cell for Cleaning or replacemenl, and haS provision for adc:Iing orifices Q( fillers
a lappifIQ provided in the cell holder body may be usecllo provide forced venlllatron Forced ventilation may also be used 10 keep lhe sighting pipe face free of all base vapor which ml~t block Ihe ceU's view of Ihe flame
LfflllT LENGT OF fllQUNTI/OG PII'E TO
1 IIiCIiES_2 IN PIPE
FQII C1OO3 I_I/~ ,,,
for hoi refraclory hold-in, opera,te Ihe burner until refraclory reaches Ifs maximum f~rafure Shut down the burner and lime lhe flame relay dropoullflhe flame relay holds in for 5 10 10 seconds or longer after actual disappearance of the flame, a slide with a smaller ortflce 8hould
be Inserted In fronl of the cell When a slide Is changed, flame current should be checked wllh a microarml8ler to
Trang 19
be certain that a flame CtJrrent of at least 2 mlcroarrps Is
ctltainad Recheck for hot refractory hold-In at the end of
the run cycle If hot refractory hold-In cannot bEl prevenled
thrClU(j1ll'!e use of orifice slIdes, the j:hotocell will have fa
be slg.led at a cooler refractory background
Orifice slidBs mi~ be used in one other situation With
the photocell pr~ installed, flame current should be at least 2 microamps Readings as hlg as 4to 7 mlcroan-ps might be obtaIned If the pholocell Is reasonably close to lhe flame (2~ feaI) FOl maximum j:holocellllfe a lens or smaller orifice shcL1ld bEiused 10 limll the flame current 10 5
microamperes
OPERATIDN OF INFRARED DETECTORS
Infrared'08I9Clors; unlike rectifying j:holocells, may be
used 'If!lh either oil 01 gas flames Since more than 90 per
cent of the flame's lolal radiation is infrared, these delee
lors receive aJl1)Ie radialloo of QUite hi~ intensity, and so
will C(l9rale w~-very weak flames as well as with very hot
ones -i',
The Infrared dBleclQ( can responc:110 infrared rays emit·
ted by a hoi refractory, even when the refractory has vis
Ibly Ceasad to rjow Infrared radiation from a hot refractory
is steady, Whereas radiallon from a flame has a flickering
characteristic The Infrared d:!lection system responds
only 10 a tliCkering infrared radiation; j( can rejecl a steady
511;11al trom hoi refractory The refractory's slaarjy sIgnal
can be made 10 fluctuate If It Is reflected, benl, or blocked
by smoke or fuel miSi within the combustion chamber
Care must be taken when applying an infrared system to
ensu:e Its response to 1lame only
CELL CONSTRUCTION
The sensitive I\'at~rial uSl1ld,in -the infrared detector is
lead sulfide The electrical resistance o'f lead sulfide drops
when ex~to infrared radiation If a voltage is applied
across lhe lead SUlfide irI Ihe cell, currenl will flow when
the cell is exposed 10 infrared radiation
The construction at the cell may be altered to vary Us
sensitivity The C7015A Infrared (Lead Sulfide) Flame D&
lector providBs a sel~!ion of 4 plug-in cells
FIG 8- LEAD SULFIOE DETECTOR
The mounting collar faslens the ce/i holder 10 a 314 inch
sighting pipe The bushing also Includes a viewing wind::Jw
which proteclS the cell A replacement bushing is avail
able which haS a magnifying lens In place of the viewing
window to concentrate available radiation on lhe cell face
The d9!ectOl' typically mounts on a 3/4 Inch black iron
pipe
If refractory mUSl be removed, II should bEl t~_r~
sllghlly away from the ends of the sighting pipe 10avoid
blockIng the cell's view The siglling pipe should nol bEl
extended all the way thrClU(j1lhe refractory to avoid canductlng excessive heal back 10 lhe cell
ACC8SSOlies-heat block, seal-ofl adapter, pipe nipple, and orifice-may be added if roquired, as shown In
1 The cell musl have a goo:l view of the flame
2 The cell must be protected from excessive hoI refractory radiation
3 The cell must bEl protecled from temperatures In 8)(cassof 125 F
4 Correc! wiring procedures must bEl usoo for lhe flame detector leads
Each of these eppIicalion requirements will bEl discussed In the following sections
1 THE CELL MUST HAVE A GOOD VIEW OF THE FLAME
The infrared sensor musf continually sl~l a stable p:>r· lion of lhe flame being dBtected The sensor is commonly appliedlo detect bolh gas pilot andmain flame or gas pilot and main oil flame In this case, Ihe &ansor musl be carefully aimed at lhe inlersecUon of pllOI and main flames,
A lead sutflde cell, like other j:hotocellS, views an area
as ~to a point It is unable 10 pinpolnf pilot flame location as easJJy as a flame rod tt the delector is to prove the pilot flame only al the ignilion p:>lnt the area viewed by the cell must bEl only thai area Where!he pilot can successfully lightlhe main flame The Viewing area must not be SO largelhat weak and wavering pilots can 9f1ergize lhe cell and cause lhe flame relay to pull in