Note: Descriptions are shown in the official language in which they were submitted.
CA 02280613 1999-08-17
WATER HEATER WITH HEAT SENSITIVE AIR INLET
Field of Invention
The present invention relates to air inlets for water heaters, particularly to
improvements to gas fired water heaters adapted to render them safer for use.
Background of Invention
The most commonly used gas-fired water heater is the storage type, generally
comprising an assembly of a water tank, a main burner to provide heat to the
tank, a pilot
burner to initiate the main burner on demand, an air inlet adjacent the burner
near the
base of the jacket, an exhaust flue and a jacket to cover these components.
Another type
of gas-fired water heater is the instantaneous type which has a water flow
path through
a heat exchanger heated, again, by a main burner initiated from a pilot burner
flame.
For convenience, the following description is in terms of storage type water
heaters
but the invention is not limited to this type. Thus, reference to "water
container," "water
containment and flow means," "means for storing or containing water" and
similar such
terms includes water tanks, reservoirs, bladders, bags and the like in gas-
fired water
heaters of the storage type and water flow paths such as pipes, tubes,
conduits, heat
exchangers and the like in gas-fired water heaters of the instantaneous type.
A particular difficulty with many locations for water heaters is that the
locations
are also used for storage of other equipment such as lawn mowers, trimmers,
snow
blowers and the like. It is a common procedure for such machinery to be
refueled in
such locations.
There have been a number of reported instances of spilled gasoline and
associated
extraneous fumes being accidently ignited. There are many available ignition
sources,
such as refrigerators, running engines, electric motors, electric and gas
dryers, electric
CA 02280613 1999-08-17
light switches and the like. However, gas water heaters have sometimes been
suspected
because they often have a pilot flame.
Vapors from spilled or escaping flammable liquid or gaseous substances in a
space
in which an ignition source is present provides for ignition potential.
"Extraneous
fumes," "extraneous fumes species," "fumes" or "extraneous gases" are
sometimes
hereinafter used to encompass gases, vapors or fumes generated by a wide
variety of
liquid volatile or semi-volatile substances such as gasoline, kerosene,
turpentine, alcohols,
insect repellent, weed killer, solvents and the like as well as non-liquid
substances such
as propane, methane, butane and the like.
Many inter-related factors influence whether a particular fuel spillage leads
to
ignition. These factors include, among other things, the quantity, nature and
physical
properties of the particular type of spilled fuel. Also influential is whether
air currents
in the room, either natural or artificially created, are sufficient to
accelerate the spread
of fumes, both laterally and in height, from the spillage point to an ignition
point yet not
so strong as to ventilate such fumes harmlessly, that is, such that air to
fuel ratio ranges
are capable of enabling ignition are not reached given all the surrounding
circumstances.
One surrounding circumstance is the relative density of the fumes. When a
spilled
liquid fuel spreads on a floor, normal evaporation occurs and fumes from the
liquid form
a mixture with the surrounding air that may, at some time and at some
locations, be
within the range that will ignite. For example, the range for common gasoline
vapor is
between about 2 % and 8 % gasoline with air, for butane between 1 % and 10 % .
Such
mixtures form and spread by a combination of processes including natural
diffusion,
forced convection due to air current drafts and by gravitationally affected
upward
displacement of molecules of one less dense gas or vapor by those of another
more dense.
Most common fuels stored in households are, as used, either gases with
densities
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relatively close to that of air (eg. propane and butane) or liquids which form
fumes
having a density close to that of air, (eg. gasoline, which may contain butane
and pentane
among other components is very typical of such a liquid fuel).
In reconstructions of accidental ignition situations, and when gas water
heaters are
sometimes suspected and which involved spilled fuels typically used around
households,
it is reported that the spillage is sometimes at floor level and, it is
reasoned, that it
spreads outwardly from the spill at first close to floor level. Without
appreciable forced
mixing, the air/fuel mixture would tend to be at its most flammable levels
close to floor
level for a longer period before it would slowly diffuse towards the ceiling
of the room
space. The principal reason for this observation is that the density of fumes
typically
involved is not greatly dissimilar to that of air. Combined with the tendency
of ignitable
concentrations of the fumes being at or near floor level is the fact that many
gas
appliances often have their source of ignition at or near that level.
The invention aims to substantially raise the probability of successful
confinement
of ignition of spilled flammable substances from typical spillage situations
to the inside
of the combustion chamber.
Summar~of the Invention
The invention relates to a water heater including a water container and a
combustion chamber adjacent the container. The combustion chamber has at least
one
inlet to admit air and extraneous fumes into the combustion chamber. The inlet
is formed
from a heat sensitive material and has a plurality of ports. The inlet is
capable of
permitting air and extraneous fumes to enter the combustion chamber and
prevents
ignition of extraneous fumes outside of the combustion chamber. The water
heater also
includes a burner associated with the combustion chamber and arranged to
combust fuel
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to heat water in the container.
Brief Description of the Drawings
Fig. 1 is a schematic partial cross-sectional view of a gas-fueled water
heater
having a single air inlet according to aspects of the invention.
Fig. 2 is a cross-sectional view of a water heater taken through the line II-
II in
Fig. 1.
Fig. 3 is a schematic plan view depicting a portion of the base of a
combustion
chamber of a water heater including an air inlet.
Fig. 4 is an enlarged schematic plan view of an air inlet shown in Fig. 2 with
the
burner and fuel supply apparatus removed for ease of understanding.
Fig. 5 is a cross-sectional view taken through the line A-A of Fig. 4.
Fig. 6 shows a top plan view of a preferred air inlet of the invention.
Fig. 7 illustrates a plan view of a single port taken from the air inlet shown
in Fig.
6.
Fig. 8 is a detailed plan view of the spacing of part of the arrangement of
ports
on the inlet plate of Fig. 6.
Fig. 9 shows two adjacent ports, taken from an air inlet of the type shown in
Fig.
6, the left hand port depicting a state prior to exposure to heat caused by
combusted
vapors and the right hand port depicting a state subsequent to exposure to
heat caused by
combusted vapors.
Fig. 10 is a top plan view of a main burner, pilot burner, thermocouple and
air
inlet arrangement in a combustion chamber of an especially preferred
embodiment of the
invention.
Fig. 11 is a side view of the structure illustrated in Fig. 10 rotated by
90°.
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Fig. 12 is an exploded view of the main burner, pilot burner and thermocouple
arrangement shown in Fig. 10.
Fig. 13 is a side view of the structure illustrated in Fig. 12 rotated by
90°.
Detailed Description of the Invention
Conventional water heaters typically have their sources) of ignition at or
near
floor level. In the course of attempting to develop appliance combustion
chambers
capable of confining flame inside appliances, it has been discovered that a
type of air inlet
constructed by forming holes in a sheet of heat sensitive material in a
particular way has
particular advantages in damage resistance when located at the bottom of a
heavy
appliance such as a water heater which generally stands on a floor. It has
further been
discovered that providing holes having well defined and in a controlled
geometry assist
reliability of the air intake and flame confining functions in a wide variety
of
circumstances .
A thin heat sensitive plate having many ports of closely specified size
formed, cut,
punched, perforated, etched, punctured and/or deformed through it at a
specific spacing
provides an excellent balance of performance, reliability and ease of accurate
manufacture. In addition, the plate provides damage resistance prior to sale
and delivery
of a fuel burning appliance such as a water heater having such an air intake
and during
any subsequent installation of the appliance in a user's premises.
In experiments conducted with a number of metallic air inlets it was observed
that
some variants were more effective than others in flame confinement function.
Certain
ones enabled a flame to burn in close contact with the inside surface of the
air inlet plate,
thereby leading to substantial temperature rise of the plate on its outside
surface, by heat
conduction. In some instances, this was observed to involve turbulent
combustion
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oscillations which further heated the inlet plate.
It was found that an excessive rising temperature of the perforated plate in
contact
with the flame could possibly transfer heat by conduction through the
relatively thin metal
plate to the extent that it could reach a sufficiently high temperature (of
the order of
1250°F or 675°C) such that a failure might possibly occur under
some conditions caused
by hot surface ignition of the spilled fumes on the outside of the combustion
chamber.
During experimentation, which was designed to create potential ignition
conditions
not likely to occur under normal operating conditions and, with a video camera
filming
the inside of the combustion chamber, it was discovered that a potential mode
of failure
occurred in some instances to involve heating particularly the periphery of
the inlet plate
at a faster rate than that in the center. Associated with this observation has
been the
phenomenon of the periphery of the inlet plate tending to closely retain the
flames formed
on the combustion chamber side of the air inlet plate, whereas towards the
center,
regardless of whether the air inlet plate is rectangular or circular in shape,
there was
evidently more of a tendency for flames to lift off the surface, further into
the combustion
chamber. Where the flames are closely retained the inlet plate becomes visibly
hotter,
which indicates excess temperature.
The invention addresses ways of meeting such extreme conditions. The invention
also address ways of avoiding detonation wave type ignition that we discovered
propagates from the inside to the outside of the combustion chamber through
the inlet
plate under certain circumstances, by minimizing the amount of flammable fumes
which
may enter the combustion chamber before initial ignition inside the combustion
chamber
occurs; and, also, by avoiding prolonged combustion incidents.
It will be appreciated that the following description is intended to refer to
the
specific embodiments of the invention selected for illustration in the
drawings and is not
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intended to limit or define the invention, other than in the appended claims.
Turning now to the drawings in general and Figs. 1 and 2 in particular, there
is
illustrated a storage type gas water heater 62 including jacket 64 which
surrounds a water
tank 66 and a main burner 74 in an enclosed chamber 75. Water tank 66 is
preferably
capable of holding heated water at mains pressure and is insulated preferably
by foam
insulation 68. Alternative insulation may include fiberglass or other types of
fibrous
insulation and the like. Fiberglass insulation surrounds chamber 75 at the
lowermost
portion of water tank 66. It is possible that heat resistant foam insulation
can be used if
desired. A foam dam 67 separates foam insulation 68 and the fiberglass
insulation.
Located underneath water tank 66 is a pilot burner 73 and main burner 74 which
preferably use natural gas as their fuel or other gases such as LPG, for
example. Other
suitable fuels may be substituted. Burners 73 and 74 combust gas admixed with
air and
the hot products of combustion resulting rise up through flue 70, possibly
with heated air.
Water tank 66 is lined with a glass coating for corrosion resistance. The
thickness of the
coating on the exterior surface of water tank 66 is about one half of the
thickness of the
interior facing surface to prevent "fish scaling" . Also, the lower portion of
flue 70 is
coated to prevent scaling that could fall into chamber 75 and possibly
partially block off
air inlet plate 90.
The fuel gas is supplied to both burners (73, 74) through a gas valve 69. Flue
70
in this instance, contains a series of baffles 72 to better transfer heat
generated by main
burner 74 to water within tank 66. Near pilot burner 73 is a flame detecting
thermocouple 80 which is a known safety measure to ensure that in the absence
of a
flame at pilot burner 73 the gas control valve 69 shuts off the gas supply.
The water
temperature sensor 67, preferably located inside the tank 66, co-operates also
with the gas
control valve 69 to supply gas to the main burner 74 on demand.
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The products of combustion pass upwardly and out the top of jacket 64 via flue
outlet 76 after heat has been transferred from the products of combustion.
Flue outlet 76
discharges conventionally into a draft diverter 77 which in turn connects to
an exhaust
duct 78 leading outdoors.
Water heater 62 is mounted preferably on legs 84 to raise the base 86 of the
combustion chamber 75 off the floor. In base 86 is an aperture 87 which is
closed gas
tightly by air inlet plate 90 which admits air for the combustion of the fuel
gas combusted
through the main burner 74 and pilot burner 73, regardless of the relative
proportions of
primary and secondary combustion air used by each burner. Air inlet plate 90
is
preferably made from a thin perforated sheet of heat sensitive material such
as plastic.
Where base 86 meets the vertical combustion chamber walls 79, adjoining
surfaces
can be either one piece or alternatively sealed to prevent ingress of air or
flammable
extraneous fumes. Gas, water, electrical, control or other connections,
fittings or
plumbing, wherever they pass through combustion chamber wall 79, are sealed.
The
combustion chamber 75 is air/gas tight except for means to supply combustion
air and to
exhaust combustion products through flue 70.
Pilot flame establishment can be achieved by a piezoelectric igniter. A pilot
flame
observation window can be provided which is sealed. Cold water is introduced
at a low
level of the tank 66 and withdrawn from a high level in any manner as already
well
known.
During normal operation, water heater 62 operates in substantially the same
fashion
as conventional water heaters except that air for combustion enters through
air inlet plate
90. However, if spilled fuel or other flammable fluid is in the vicinity of
water heater
62, then some extraneous fumes from the spilled substance may be drawn through
plate
90 by virtue of the natural draft characteristic of such water heaters. Air
inlet 90 allows
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the combustible extraneous fumes and air to enter, but confines potential
ignition and
combustion inside the combustion chamber 75.
The spilled substance is burned within combustion chamber 75 and exhausted
through flue 70 via outlet 76 and duct 78. Because flame is confined by the
air inlet plate
90 within the combustion chamber, flammable substances) external to water
heater 62
will not be ignited.
The air inlet has mounted on or adjacent its upward facing surface a thermally
sensitive fuse in series in an electrical circuit with pilot flame proving
thermocouple 80
and a solenoid coil in gas valve 69.
With reference to Fig. 1, the size of air inlet plate 90 is dependent upon the
air
consumption requirement for proper combustion to meet mandated specifications
to ensure
low pollution burning of the gas fuel. Merely by way of general indication,
the air inlet
plate of Fig. 1 should be conveniently about 40 square inches of perforated
area when
fitted to a water heater having about 34;000 Btu/hr (approximate) energy
consumption
rating to meet ANSI requirements for overload combustion.
Fig. 3 shows schematically an air inlet to a sealed combustion chamber
comprising
an aperture 87 in the lower wall 86 of the combustion chamber and a heat
sensitive
material or plastic air inlet plate 90 having a perforated area 100 and an
unperforated
border or flange 101.
Holes in the perforated area 100 of plate 90 can be circular or other shape
although
slotted holes have certain advantages as will be explained, the following
description
referring to slots.
Figs. 4-5 show a preferred arrangement of air inlet 90 with respect to lower
wall
86 of the combustion chamber.
It is intended that air inlet 90 be substantially sealed against lower wall 86
to
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prevent air and/or extraneous fumes to pass between facing surfaces of inlet
90 and lower
wall 86. Inlet 90 has an outer flange 101 that extends beyond the edge of the
opening
in lower wall 86. Flange 101 may be attached to a corresponding portion of
lower wall
86 by several methods such as forming, press-fitting or fasteners. Other means
of
securing or fixing air inlet 90 to lower wall 86 are possible, such as heat
resistant
adhesives and the like.
Air inlet 90 also most preferably has a raised portion 204 that extends above
the
upper surface of lower wall 86. This assists in ensuring that condensation
generated in
flue tube 70 does not lie or congregate on air inlet 90 so as to occlude the
openings/slots
therein.
Fig. 6 shows an air inlet 90 as will be described to admit air to combustion
chamber 75. Air inlet 90 is most preferably a plastic plate having many small
slots 104
passing through it. The air inlet should have a thickness of at least about
0.18 inches or
more. Depending on the plastic and its mechanical properties, the thickness
can be
adjusted. Portions of air inlet 90 away from ports 104 need not be formed of
the heat
sensitive material since such portions need not deform in response to elevated
temperatures.
Fig. 6 is a plan view of an air inlet plate having a series of ports in the
shape of
slots 104 aligned in rows. All such slots 104 have their longitudinal axes
parallel except
for the edge slots 107 at right angles to those of the ports 104 in the
remaining perforated
area 105. The ports are arranged in a rectangular pattern formed by the
aligned rows.
As mentioned above, the plate is most preferably at least about 0.18 inches
thick. This
provides air inlet 90 with adequate damage resistance and, in all other
aspects, operates
effectively. The total cross-sectional area of the slots 104 is selected on
the basis of the
flow rate of air required to pass through the air inlet 90 during normal and
overload
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combustion.
The slots 104 are provided to allow sufficient combustion air through the air
inlet
90 and there is no exact restriction on the total number of slots 104 or total
area of the
air inlet, both of which are determined by the capacity of a chosen gas (or
fuel) burner
to generate heat by combustion of a suitable quantity of gas with the required
quantity of
air to ensure complete combustion in the combustion chamber and the size and
spacing
of the slots 104. The air for combustion passes through the slots and not
through any
larger inlet air passage or passages to the combustion chamber. No such larger
inlet is
provided.
The water heater of the invention thus includes a water container and a
combustion
chamber adjacent to the container. The combustion chamber has at least one
heat
sensitive inlet to admit air and extraneous fume species into the combustion
chamber.
The inlet has a plurality of ports, each port having a limiting dimension
sufficient to
confine ignition and combustion of the extraneous fumes within the combustion
chamber.
The water heater also includes a burner associated with the combustion chamber
and
arranged to combust fuel to heat water in the container.
Fig. 7 shows a single slot 104 having a length L, width W and curved ends. To
confine any incident of the above-mentioned accidental ignition inside the
combustion
chamber 75, the slots 104 should be formed having at least about twice the
length L as
the width W and are preferably at least about twelve times as long. Length to
width
(L/W) ratios outside these limits are also effective. Slots are more effective
in controlling
accidental deflagration or detonation ignition than circular holes, although
beneficial effect
can be observed with L/W ratios in slots as low as about 3. Above L/W ratios
of about
15 there can be a disadvantage in that in an air inlet 90 of thin flexible
plastic possible
distortion of one or more slots 104 may be possible as would tend to allow
opening at the
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center of the slots creating a loss of dimensional control of the width W.
However, if
temperature and distortion can be controlled then longer slots can be useful;
reinforcement of a thin inlet plate by some form of stiffening, such as cross-
breaking, can
assist adoption of greater L/W ratios. L/W ratios greater than about 15 are
otherwise
useful to maximize air flow rates. A particularly preferred length L is about
6 mm and
a particularly preferred W is about 0.5 mm.
To perform their ignition confinement function, it is important that the slots
104
perform in respect of any species of extraneous flammable fumes which may
reasonably
be expected to be involved in a possible spillage external to the combustion
chamber 75
of which the air inlet of the invention forms an integral part or an
appendage.
Fig. 8 shows slot and inter-port spacing dimensions adopted in the embodiment
depicted in Fig. 6. The dimensions of the ports are preferably the same as in
Fig. 7 and
have a length L of 6 mm and a width W of 0.5 mm. The ends of each slot are
semicircular but more squarely ended slots are suitable. The chosen
manufacturing
process can influence the actual plan view shape of the slot. Blanking such
large numbers
of holes can be difficult as regards maintaining such small punches if the
comer radii are
not well rounded. The photochemical machining process of manufacture of air
inlets 90
with slots 104 is also more adapted to maintaining round cornered slots.
The interport spacing illustrated in Fig. 8 performs the required confinement
function in the previously described situation. The dimensions are preferably
as follows:
A about 2.0 mm and B about 2.0 mm.
Fig. 9 shows a port 104 in two states, one state shown on the left hand being
a
port prior to exposure to heat caused by combusted vapors or fumes and the
right hand
drawing showing a port depicting a state subsequent to exposure to heat caused
by
combusted vapors or fumes. The change in size and shape of port 104 as shown
in Fig.
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9 is brought about as a result of ignition of extraneous fumes having passed
through air
inlet 90 and ignited on the surface of air inlet 90 facing combustion chamber
75. The
presence of flames at or near the surface of air inlet 90 causes its
temperature to increase,
thereby causing the heat sensitive material forming the air inlet to increase
and at a
particular point begin to soften and approach and/or reach its melting
temperature at
which point the walls or edges of port 104 begin to change in shape and the
port shrinks
as the material of the plate flows and fills into the port.
The result of the decrease in the total open space of the air inlet is the
decrease in
entry of air and extraneous fumes into the combustion chamber, thereby
reducing
combustion and, given sufficient time, choking off combustion all together.
Of course, there are a multiplicity of ports 104 in air inlet 90. Some of
ports 104
may be caused to close off completely while leaving others slightly open, but
not
sufficiently to permit continued combustion within the combustion chamber.
Materials suitable for forming the heat sensitive or plastic air inlet 90
should most
preferably possess crystalline characteristics such that the heat sensitive
material or plastic
will flow or partially flow when heated. In addition, the material should
possess heat
deflection temperatures in excess of about 400°F and melting points in
excess of about
500°F. Amaco polymers AMODEL, a glass fiber-reinforced grade of
polyphthalamide
(PPA) resin or Phillips 66 RYTON, a glass fiber-filled polyphenylene sulfide
(PPS)
compound are especially preferred examples of suitable materials for plastic
air inlets.
Of course, other materials having the appropriate heat sensitivity,
machinability, strength
and durability may be utilized.
Referring to Figs. 10-13, they collectively show fuel supply line 210 and
pilot fuel
line 470 extending outwardly from a plate 250. Plate 250 is removably sealable
to skirt
600 that forms the side wall of combustion chamber 75. Plate 250 is held into
position
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by a pair of screws 620 or by any other suitable means. Pilot fuel line 470
and fuel
supply line 210 pass through plate 250 in a substantially fixed and sealed
condition.
Sheath 520 also extends through plate 250 in a substantially fixed and sealed
condition
as does igniter line 640. Igniter line 640 connects on one end to an igniter
button 220
and a piezo igniter 660 on its other end. Igniter button 220 can be obtained
from
Channel Products, for example. Each of pilot fuel supply line 470, fuel supply
line 210
and sheath 520 are removably connectable to gas control valve 69 by
compression nuts.
Each of the compression nuts are threaded and threadingly engage control valve
69.
Sheath 520, preferably made of copper, contains wires (not shown) from
thermocouple 80 to ensure that, in the absence of a flame at pilot burner 73,
gas control
valve 69 shuts off the gas supply. Thermocouple 80 may be selected from those
known
in the art. Robertshaw Model No. TS 750U is preferred.
The pilot burner to air inlet relationship is quite important in stand-by or
pilot only
mode of operation. The hood of pilot burner 73 should be located over ports
104. This
creates conditions for smooth ignition of flammable vapors as they flow
through the ports.
A pilot located away from the ports can result in at least two undesirable
conditions:
rough ignition of vapors and delayed ignition of vapors which could result in
a small
deflagration within combustion chamber 75. This deflagration could possibly
produce a
pressure wave which could push flames through ports 104 and ignite any vapors
remaining outside the water heater.
The location of thermocouple 80 is important. Quick shutdown of gas valve 69
is desirable for several reasons. Disablement of gas valve 69 results in pilot
burner 73
outage and subsequent main burner 74 shutdown. Therefore, main burner 74
cannot be
ignited, which may result in the development of undesirable pressure waves
within
combustion chamber 15 while flammable vapors are being consumed on the air
inlet
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plate. Flammable vapor spills may result in vapor concentrations that migrate
in and out
of the flammable range. Vapors adjacent air inlet 90 may ignite and be
consumed for a
short period of time before ports 104 have an opportunity to reduce in size or
close off
extraneous fumes in order to self extinguish. Disablement of gas valve 69
(i.e. pilot
burner 73 and main burner 74 shutdown) removes the water heater as a source of
ignition
in the event that vapors should again reach a flammable concentration level.
It is to be understood that the invention disclosed and defined herein extends
to all
alternative combinations of two or more of the individual features mentioned
or evident
from the text or drawings. All of these different combinations constitute
various
alternative aspects of the invention.
The foregoing describes embodiments of the present invention and
modifications,
obvious to those skilled in the art can be made to them, without departing
from the scope
of the present invention.
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