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Патент USA US2412977

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Dec. 24, 1946-.
S, G,v EsKlN
Filed Nov. 19, 1945
22 § 24
1729*. Z.
Samuel 6.51%?!) .
/W/7<%M4/M \
Patented Dec. 24, 1946 >
" 2,412,977
Samuel G. Eskin, Pittsburgh, Pa., assignor to
Robertshaw Thermostat Company, Youngwood,
Pa., a corporation of Pennsylvania
Application November 19, 1943, Serial No. 510,851
2 Claims. (Cl. 201-76)
This invention relates to electric circuit ele
ments having the characteristics of largeresist
ance variation underv temperature changes and,
more particularly, to metallic ?ame sensitive re->
. Several metals and metal alloys have large
positive temperature coe?icients of electrical re
sistivity rendering them valuable in various-cir
cuit applications. These circuit elements may
i be coils of wire, generally termed ballast resis
tors, and in order to utilize them to the fullest
advantage it is sometimes desirable‘to expose
them directly to the heat of a ?ame. In one
application the coil is placed in the ?ame of a
gaseous fuel burner and by its consequent change
in resistance serves as a safety device to shut off
the ?ow of ‘fuel should the ?ame become acciden
tally extinguished.
Of the several alloys which disclose suitable
electrical properties for the purpose described
some‘ will resist oxidation when exposed to the
?ame of burning natural gases. The usefulness
of such materials is limited, however, by the cor
rosion to which they are subject when heated by
manufactured gases containing sulphur. Various
metallic and non-metallic coatings for metals
are known which offer more or less protection
against corrosion from sulphurous atmospheres
but, in the particular services to which this ap
plication relates, it is of the utmost importance.
that the coated material retain the electrical re-.
sistance properties which the uncoated material
It is an object of this invention to protect a
metallic ?ame sensitive resistor from corrosion
without adversely affecting its electrical resist
the ?ame of a gaseous fuel burner 12 connected
with a main fuel pipe I! through which the ?ow
of gaseous fuel is controlled by an electromag
netically operated valve l6. As is customary in
such devices, the electromagnetic valve l6 may
be opened by energization of its coil 23 and closed
by gravity upon deenergization thereof.
Electric current for operating the electromag
netic valve l6 may be supplied from line wires
connected to the primary of a transformer IS.
The valve circuit may be traced from the sec
ondary of the transformer l8 by wire 20, wire
22, coil 23, wire 24, and wire 26 back to the second
ary of the transformer I8. A control switch (not
shown) may be incorporated in the described
circuit to control the energization of the coil 23,
as desired.
In order to cause movement of the
electromagnetic valve IE to closed position when
the burner ?ame is accidentally extinguished, the
?ame sensitive device In is connected in shunt
with the coil 23 by wires 28 and,“ connected
with wires 24 and 22, respectively.
The ?ame sensitive device l0 may consist of ‘
a coil of wire from material having normally a
relatively low resistance and a high positive tem
perature coeflicient of resistance. In this em
bodiment the device I0 is formed from nickel
steel wire containing about I70% nickel-30%
I iron. The electrical resistance characteristics of
30 the ?ame sensitive device in so formed are such
that when heated by the ?ame of the burner l2
sufficient current will ?ow in the coil 23 to main
tain the valve IS in open position, but when
unheated by the burner ?ame, insufficient cur
rent passes through the coil 23 to maintain the
valve IS in open position.
_In the burner control apparatus described, the
ance properties.
Another object of the invention is to permit the
?ame sensitive device It is used to determine
whether the electromagnetic valve I6 is main
coated material to be bent or formed into a coil
or other desired shape and to be repeatedly 40 tained in open position after running conditions
have been established. In order to establish such
heated and cooled without cracking or peeling
conditions it is necessary that the valve l6 be
of the coating from the base material.
initially moved to open position in order that fuel
Another object of the invention is to permit
may ?ow to the burner l2 and the ?ame thereof
formed coils of wire to be coated uniformly.
Other objects and advantages will become ap 45 heat the device ill to bring the'safety features
into operation. Such movement of the valve [5
may .be accomplished by a manually operated
lifting device 32 attached to the solenoid (not
shown) of the electromagnetic valve l6 so that
Fig. 1 is aschematic view of a safety system
embodying the ?ame sensitive device of this in 60 in the starting condition (while the ?ame sensi
parent from the following description taken in
connection with the accompanying drawing,
vention; ‘and
tive device 10 is unheated by the burner ?ame) .
the valve l6 may be manually opened and main
tained in such position until the ?ame sensitive
Referring more particularly to the drawing, the
device 10 becomes suf?ciently heated to render
?ame sensitive device It! is shown as exposed to 55 such manual actuation unnecessary. It will be
Fig. 2 is an enlarged sectional view of the de
apparent, however, that a constant burning pilot
to which the device 90 would be exposed, or other
arrangements known to the art, could be em
ployed without affecting the scope of this inven
As previously indicated, this invention is more.
particularly directed to protecting the ?ame sen
sitive element It from corrosion in the sulphur-.
ous atmosphere of the burner it without sacri?ce.
of the desired electrical resistance properties‘
thereof. Accordingly, the nickel steel wire of
which the device it is formed is provided with a
protective coating which has been found to be
highly resistant to corrosion in suiphurous atmos
The flame sensitive resistor thus formed is pe
culiarly adapted for the purpose described by
reason of the retention of its electrical resistance
properties substantially unchanged, although the
uncoated nickel-steel wire which possessed such
properties is new highly resistant to corrosion
and heat under the severe conditions to which it
is exposed. The invention thus serves the pur
pose of a ?amesensitive device for use in the
burner control apparatus described in connection
with Fig. 1 and will not only remain effective for‘
‘a much greater length of time than an uncoated
coil but will have none of the desired properties
vadversely affected by reason of the protective
pheres as may exist when manufactured gases 15 coating.
are used as the fuel.
The protective coating for the device it) of this
invention is applied by a process known as chrom
izing, whereby the nickel-steel underbody is
While the chromizing process has been outlined
herein and satisfactory cells may be formed by
use of such process, it will be apparent that the
principles of this invention are not limited to
surface-alloyed with chromium by a preferred 20 ?ame sensitive devices produced by following the
method now to be described. The coil of nickel
exact steps set forth. Other methods including,
steel wire vforming the underbody of the flame
variations of the described process may be em.‘
sensitive device 50 is packed in a. mixture con
ployed to produce the desired di?used layer on a‘
taining equal parts of ferrochrome and 60-mesh
flame sensitive electrical resistor without depar-i
alumina and placed in a suitable furnace. No 25 ture from the spirit of the invention as set forth
special surface treatment need be given the coil
in the appended claims.
before the chromizing is started. -The atmos
phere of the furnace is rendered non-oxidizing by
1. A ?ame sensitive electrical resistor compris
the use of a gas such as hydrogen and hydrogen
ing a wire coil consisting of a chromium-free
chloride. Heating of the coil is carried on for 30 nickel-iron alloy of' normally a relatively low
eight hours at 1000° C. which has been found suf
electrical resistance value and a high positive
ficient to form a layer of adequate thickness.
temperature coefficient of resistance value hav
The duration of the treatment may, however, be
ing a chromium-containing layer integrally dif
varied in accordance with the thickness of sur
fused with the surface thereof which is exposed
face layer desired.
to the flame, said layer being adapted to protect
A ?ame sensitive device it so treated has a
said wire from corrosion in sulphurous atmos
surface coating in the form of a di?used layer
pheres resulting from the presence of the flame
integral with the nickel-steel underbody and
and without substantial alteration in the said elec
. the chromium content of the layer appears to
trical resistance value.
decrease from the surface inwardly. The layer
2. A ?ame sensitive electrical resistor compris
generally has a wall thickness from 0.0001"
ing a wire coil consisting of about 70% nickel-.
to 0.0002" and the device so formed shows
30% iron alloy which is chromium-free and has
excellent heat and corrosion resistance under
‘normally a relatively low electrical resistance
severe conditions. The device also has the ad
value and a high positive temperature coemcient
vantage of capability of being bent, wound
of resistance value, said wire having a chromium»
or otherwise formed without cracking or peel=
containing layer integrally di?used with the sur
ing of the coating from the underbody and
face thereof which is exposed to the ?ame, said
will undergo expansion and contraction with
layer being adapted to protect said wire from cor
similar results. It appears, therefore, that while
rosion in sulphurous atmospheres resulting from
the exterior surface exposed to the flame of the
the presence of the ?ame and without substan
burner i2 may consist of pure chromium, the gen
tial alteration in the said electrical resistance
eral character of the coating consists of a chro
mium-nickel-iron alloy intimately bonded with
the nickel-steel wire forming the underbody and.
having the characteristics of all metallic coatings 55
formed by di?usion of metals.
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