Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR PREVENTING BACTERIA PROLIFERATION IN AN
ELECTRIC WATER HEATER
FIELD OF THE INVENTION
(0001) The present invention relates to a method and apparatus for preventing
bacteria proliferation, and particularly but not exclusively, the Legionella
bacteria, in an
electric water heater by the use of heated water within the tank of the water
heater at a
temperature sufficiently high to kill the bacteria.
BACKGROUND OF THE INVENTION
(0002) Although some species of Legionella bacteria can be found in the soil,
most
species live in water that is stagnant and wherein such bacteria survive under
a wide
range of temperatures, typically 20 to 45 degrees C. According to the Centers
for
Disease Control and Prevention, USA, between 8,000 and 18,000 people are
hospitalized with Legionnaires disease each year. It is of great public
concern as its
fatality rate during an outbreak ranges from 5% to 30% in those who contract
the
disease. Actively managing the risk of Legionella in water systems is more
cost
effective than responding to an outbreak. Outbreaks of Legionella pneumophila
can
stem from showers and potable water systems. As water from such sources
aerosolized, individuals can inhale the Legionella containing droplets and the
organism
is aspirated into the lungs.
(0003) The formation and multiplication of such Legionella bacteria is not
only
promoted by the temperature in the customary hot water systems, but also by
the fact
that dead spaces are present in such water distribution systems in which
deposits and
sediment formation can arise, and typically in the bottom zone of water heater
tanks.
Deposits therein can represent a culture medium for bacteria proliferation.
(0004) Most electric water heaters for domestic use have its water tank
constructed
with a dome shaped bottom wall. Such dome-shaped bottom walls form a
surrounding
cavitated zone about the dome-shaped wall where sediments deposit can gather
and
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where water is less agitated. This cavitated zone is spaced from the bottom
heating
element and thus water therein is less hot creating an ideal location for
bacterial
proliferation. Should the bottom element fail, then the water temperature at
the bottom
of the tank will drop. It as also been determined by research that the
Legionella
bacteria does not survive at temperatures above 46 degrees C. When hot water
is not
drawn from a water heater, the water inside the tank becomes stagnant and the
water
temperature stratifies with the cooler temperature being at the bottom region
of the tank.
Water within the cavitated zone below the bottom element of the tank can fall
to about
30 to 40 degrees C which is favourable to bacteria growth. Lowering the bottom
element to place it close to the bottom wall of the tank has not proven to be
a viable
solution.
(0005) Reference is made to US Patents 4,940,024; 5,168,546 and 5,808,277
which
disclose various methods and apparatus to prevent bacteria proliferation in
electric
water heaters. One method teaches adding a heating element in the form of a
belt or
patch on the outside of the tank against the bottom end of the outer sidewall
of the tank
to heat the water at the bottom end of the tank to a temperature preferably
above 55
degrees C. Accordingly, this proposed solution provides an extra heating
element in the
form of a patch heater located in an area which is usually filled with
insulating foam
material and not practical to access should it fail and require replacement or
repair. It is
also costly and consumes more electricity. In US Patent 5,808,277 a third
element is
added into the tank to periodically raise the water temperature at the bottom
of the tank
beyond the pre-set consumption temperature, to a sanitizing temperature to
destroy
bacteria. This is also a costly proposition. US Patent 4,940,024 discloses a
method of
directing the cold water flow of all consumed drinking or domestically used
water
through the lower region of the tank wherein there is no stagnant water and
wherein no
deposits can be formed for bacteria growth. Accordingly, the lower region of
the tank is
continuously flushed with fresh water. This is a costly solution requiring a
new tank
design and cold water conduit network and therefore not a viable solution.
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SUMMARY OF THE INVENTION
(0006) It is a feature of the present invention to provide a method and
apparatus for
the prevention of the Legionella bacteria in an electric water heater which is
different
from the known prior art and which uses heated water within the water tank to
elevate
the temperature of the water in the lower region of the tank sufficiently high
to kill and
prevent proliferation of bacteria.
(0007) Another feature of the present invention is to provide a pump, a water
temperature sensor and a control to pump hot water from the upper region of
the tank to
the bottom end to maintain the temperature in the bottom end of the tank of an
electric
water heater at a temperature sufficient to kill and prevent proliferation of
the Legionella
bacteria while simultaneously preventing temperature stratification in the
lower region of
the tank.
(0008) A further feature of the present invention is to provide a baffle plate
combined
with the bottom electrical resistive heating element of an electric water
heater to prevent
the proliferation of the Legionella bacteria in the bottom end of the tank.
(0009) A still further feature of the present invention is to pump hot water
from the
upper region of the tank of an electrical water heater and to release it in
the immediate
area of the bottom wall of the tank to raise the temperature of the water
above 46
degrees C whereby to kill bacteria.
(0010) According to the above features, from a broad aspect, the present
invention
provides an electric water heater comprising a tank defined by a cylindrical
side wall, a
top wall and a dome-shaped bottom wall. A cold water inlet is disposed for
releasing
cold water under pressure in a lower portion of the tank. Two or more
resistive heating
elements heat water in a respective one of an upper and lower region of the
tank.
Temperature sensing means is provided for sensing water temperature in the
upper and
lower regions of said tank. Control means actuates at least one of the two or
more
resistive heating elements when the water temperature in at least one of the
tank
regions falls below a set point temperature value as sensed by the sensing
means. The
dome-shaped bottom wall has a surrounding cavitated zone thereabout where
water
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temperature is at its lowest and water is less agitated and more prone to
bacteria
proliferation. Convection means is provided to cause heated water from the
tank to be
released in the lower region of the tank in the immediate area of the dome-
shaped
bottom wall and he surrounding bacteria proliferation zone at a temperature
sufficient to
kill and prevent bacteria proliferation.
(0011) According to the broad aspect mentioned above, the convection means is
constituted by a water pump having an inlet conduit extending into the upper
region of
the tank where water is at a high temperature. The water pump has an outlet
conduit
connected thereto which has a discharge end disposed to release hot water from
the
upper region of the tank into the lower region in the immediate area of the
dome-shaped
bottom wall and into a cavitated zone to raise the water temperature therein.
(0012) According to the broad aspect mentioned above, the convection means is
constituted by an inverted shallow cone-shaped baffle plate having an outer
circular peripheral
edge. The baffle plate is retained in the lower region of the tank and houses
a substantial
portion of a lower one of the resistive heating elements extending in the tank
lower region. The
resistive heating element is disposed in a chamber integrally formed along a
top end section of
the cone-shaped baffle plate and in communication with the space under the
baffle plate
immediately above the dome-shaped bottom wall. The chamber has an elongated
slot opening
for releasing hot water from under the baffle plate heated by the resistive
heating element and
causing hot water to flow under the baffle plate against the bottom wall.
(0013) According to a still further broad aspect of the present invention,
there is provided a
method of preventing bacterial growth in a bottom end of a water holding tank
of an electric
water heater in the region of a dome-shaped bottom wall defining a cavitated
zone thereabout.
The method comprises the step of convecting heated water from above the dome-
shaped
bottom wall to the immediate area of the dome-shaped bottom wall and into the
cavitated zone
to maintain the temperature of water in the cavitated zone at a temperature
sufficient to prevent
harmful bacteria growth.
(0014) According to a further broad aspect of the invention, the method
further comprises
before the step of convection, sensing water temperature in the immediate area
of the dome-
shaped bottom wall, and after the step of convection, controlling the duration
of the step of
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convection in relation to the sensed temperature to maintain water temperature
sufficiently high
to prevent bacteria growth.
DESCRIPTION OF THE DRAWINGS
(0015) A preferred embodiment of the present invention will now be described
with reference
to the examples of the preferred embodiment wherein:
(0016) Figure 1 is a simplified section view of an electric water heater
showing some of the
components thereof and illustrating one example of the preferred embodiment
wherein the
convection means is a water pump which displaces hot water from the top region
of the tank to
the lower region thereof closely spaced to the dome-shaped bottom wall of the
tank;
(0017) Figure 2 is a fragmented side view illustrating a modification to the
outlet conduit of the
pump and its relation to the dome-shaped bottom wall;
(0018) Figure 3 is a top view illustrating the modification to the bottom end
section of the
outlet conduit of Figure 2;
(0019) Figure 4 is a fragmented sectional side view illustrating a further
example of the
convection means which is herein constituted by an inverted dome shaped baffle
plate housing
a lowermost one of the resistive heating elements to create a hot water flow
in the immediate
area of the dome-shaped bottom wall of the tank;
(0020) Figure 5 is a fragmented sectional side view, similar to Figure 4, but
providing a
transverse view of the inverted dome shaped baffle plate as shown in Figure 4
and the
lowermost resistive heating element;
(0021) Figure 6 is a top perspective view of an example of the construction of
the inverted
dome shaped baffle plate, and
(0022) Figure 7 is a fragmented section view showing the position of the outer
peripheral
edge of the inverted dome shape baffle plate and its relation to the side wall
of the tank and the
cavitated peripheral zone of the dome-shaped bottom wall of the tank and
further illustrating one
of the spacer clips secured to the peripheral edge of the baffle plate to
center the inverted dome
shape baffle plate with respect to the circular side wall of the tank.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
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(0023) Referring to the drawings and more particularly to Figures 1 to 3,
there is shown
generally at 10 an electric water heater which is comprised of a water holding
steel tank 11
formed by a surrounding side wall 12, a top wall 13 and a dome-shaped bottom
wall 14. A
resistive heating element 15 is mounted to the tank side wall and projects in
an upper region 16
of the tank 12. A bottom resistive heating element 17 is also mounted to the
tank wall 14 and
projects into a lower region 18 of the tank spaced above the bottom wall 14. A
control
thermistor 19 is equipped with a temperature sensor 20 and operates the
resistive heating
element 15 to maintain a set temperature value in the upper region 16 of the
tank. Similarly, a
control thermistor 21 and temperature sensor 22 control the lower resistive
heating element 17
and set at the same temperature as the upper element. Typically, during
ordinary operating
conditions the consumption temperature of the water in the tank 11 is at 60
degrees C.
(0024) As previously described, the Legionella bacteria can survive in
stagnant water at
temperatures of 20 to 45 degrees C but above these temperatures the bacteria
is destroyed.
Because the tank bottom wall 14 is formed as a dome, it defines a surrounding
cavitated zone
23 about the dome adjacent the tank side wall inner surface 24 wherein
sediments in the water
can build up as the water therein is less agitated by the movement of water in
the tank as the
water is heated and rises in the tank and also by the draw of cold water into
the tank through the
dip tube 25 bottom end, space well above the bottom wall, as hot water is
removed from the
upper region 16 through the outlet pipe 26. In order to prevent the
proliferation of the Legionella
bacteria in the bottom end 30 of the tank below the lower resistive heating
element 17 and
particularly in the cavitated zone 23, there is provided a small water pump 27
conveniently
mounted on the top wall 28 of the outer shroud 29 of the tank 11 to pump water
from the upper
region 16 of the tank to the bottom end 30 in close proximity to the bottom
wall. The water
released in the bottom end is typically in the range of between 50 degrees C
to 60 degrees C.
(0025) As shown in Figure 1, the pump 27 has an inlet conduit 31 which extends
into the
upper region 16 of the tank to draw hot water therefrom when the pump is
actuated by a control
device 32 which is connected to a temperature sensor 33 secured against the
tank side wall 11
adjacent the bottom end 30 close to the cavitated zone 23. The pump 27 also
has an outlet
conduit 34 in the form of a dip tube which extends within the tank 11 and has
its discharge end
35 disposed in the bottom end of the tank to release hot water from the upper
region to the
immediate area of the bottom wall and into the cavitated zone 23 to raise the
water temperature
therein above 46 degrees C.
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(0026) It is pointed out that the pump pressure is sufficiently low not to
precipitate sediment
deposits from the cavitated zone into the upper portion of the tank to prevent
mixture thereof
with hot water drawn from the tank as such could cause malfunction of the
mixing valves of
appliances connected to the hot water distribution conduits network. By
positioning the outlet
conduit 34 within the tank in contact with hot water therein there is
substantially no heat loss of
the transfer of hot water from the upper region 16 to the bottom end 30. The
outlet conduit 34 is
also constructed from non-oxidation material, similar to the dip tube 25 and
capable of
withstanding temperatures well above 60 degrees C. The conduits 31 and 34 are
removably
secured to the top wal113 of the tank through suitable leak-proof bushings 31'
and 34',
respectively.
(0027) As shown in Figures 2 and 3, the outlet conduit can also be formed with
a curved
conduit end section 36 and oriented to cause a gentle swirling motion of the
water over the
dome-shaped bottom wall 14.
(0028) With reference now to Figures 4 to 7, there is illustrated another
example of the
preferred embodiment wherein the convection means is constituted by an
inverted shallow cone
shape baffle plate 40, as shown in perspective view in Figure 6. The baffle
plate 40 has an
outer peripheral edge 41 and is of circular contour whereby to be retained in
the lower region or
bottom end 30 of the tank and closely spaced to the inner surface 24 of the
tank sidewall 12. A
resistive heating element chamber 42 is formed in the upper region of the
baffle plate for
housing the lower resistive heating element 17. This chamber 42 is formed as
an open-ended
channel 42' which is in communication with the space 43 under the baffle plate
40 and above
the dome-shaped bottom wall 14.
(0029) The open-ended channel 42' has opposed parallel side walls 44 each of
which has an
inwardly inclined top section 45 to define therebetween and elongated slot
opening 46. The
channel 42' has a rear open end 47 and a closed upwardly slopped front end 48.
The lower
resistive heating element 17 is received in the channel 42' through the open
end 47 and spaced
from the side walls 44. The arms 50 of the resistive heating element 17 may be
disposed in a
vertical plane, as illustrated, or at any angle in the channel 42', depending
on the type of
resistive heating element.
(0030) As illustrated, the diameter of the baffle plate 40 is slightly smaller
than the inner
diameter of the tank side wall 12 whereby to define a space or a
circumferential passage 57
between the outer peripheral edge 41 of the baffle plate 40 and the inner
surface 24 of the tank
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side wall 12. In order to ensure that the baffle plate 40 is centered with the
inner diameter of the
tank inner side wall 24, spacer members 58, as shown in Figures 4 and 5, are
secured spaced
apart about an outer peripheral section of the inverted cone shape baffle
plate 40 and project
beyond the outer circular peripheral edge 41 for frictional engagement with
the inner surface 24
of the tank side wall 12. Because the inner surface 24 of the tank side wall
12 is coated with a
glass lining 59, the spacer members 58 are formed with soft spring arms 60
having a curved
shape for smooth resilient contact with the glass lining to prevent damage to
the glass coating.
They also center the baffle plate 40 with respect to the inner diameter of the
tank. Of course,
the baffle plate is installed and centered before the dome-shaped bottom wall
is welded to the
tank cylindrical side wall 12 and positioned to align the open end 47 of the
channel 42' with the
resistive heating element cavity. The resistive heating element 17 may be
installed in the
channel 42' before installation of the bottom wall. It is also pointed out
that the resistive heating
element 17 can be easily removed and replaced from the channel of the baffle
plate.
(0031) With the baffle plate 40 secured in position, as better shown in
Figures 4 and 5, it can
be seen that the outer peripheral edge 41 of the baffle plate 40 extends
spaced from the inner
surface 24 of the tank side wall 12 and point in the direction of the cavity
zone 23 and spaced
above the top surface 14' of the dome-shaped bottom wall 14 to create the
circumferential water
inlet opening about the baffle plate. As soon as the lower resistive heating
element 17 is
provided power, it quickly heats the water surrounding it in the channel 42
causing the heated
water to rise upwards through the slot opening 46 drawing water form under the
inverted dome
shape baffle plate 40 causing water to be drawn under the baffle plate through
the
circumferential passage 57 thereby drawing heated water from the surrounding
area of the
baffle plate mixed with hot water released through the slot opening back under
the baffle plate
and above the cavitated zone 23 to raise the temperature in that area well
above 46 degrees C
to prevent the proliferation of the Legionella bacteria in this zone which may
contain sediments
and all the area in the bottom end 43 of the tank.
(0032) With further reference to Figures 4 and 5 there is shown a stop bar 51
secured under
tnd across the baffle plate circumferential cone wall 52' and spaced
immediately under the
channel 42. The purpose of the stop bar 51 is to prevent the baffle plate 52
from being
dislodged by moving upwardly beyond its efficient working position, as the
stop bar 51 would
abut a lower arm 50 of the resistive heating element 17. The stop bar 51 is
constructed as a tin
plastic bar oriented vertically to minimize obstruction to water flow moving
upwards from under
the bar. Also, the lower edge 49 of the entrance opening 47 of the channel 42
provides an
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abutment on the resistive heating element 17 should there be an upward
displacement of the
baffle plate 52.
(0033) With respect to both embodiments, the sensor 33 and the control 32
assures the
maintenance of the high water temperature in the bottom end 30 of the tank. If
there is a draw
of water from the tank causing the water temperature in the tank lower end to
drop by the
introduction of cold replacement water through the dip tube 25, the lower
resistive heating
element 17 will be powered for the water in its region to attain its set point
temperature of 60
degrees C and cause heated water to be drawn under the baffle plate. However,
if the water in
the tank is idle for a long period of time and the water temperature in the
bottom end falls below
a value of say 50 degrees C, as sensed by the controller 32, the bottom
element will be
powered by the controller 32 receiving sensed temperature signals from its
connection 33' to the
temperature sensor 33, to raise the temperature in the bottom end 43.
(0034) As shown in Figure 6, it is also conceivable that the outer peripheral
edge 41 of the
baffle plate be positioned in close friction fit with the tank inner surface
24 and water admitted
thereunder through a series of cut-outs 61 formed spaced-apart along the outer
circular
peripheral edge 41 of the inverted cone shape baffle plate 40 to provide water
passage in the
area of the cavitated zone 23 about the dome-shaped bottom wall 14 when the
resistive heating
element 17 is powered.
(0035) It can be summarized that the above described examples of the preferred
embodiment
of the present invention provide a novel method of preventing bacteria growth
in the bottom
region of an electric water heater and particularly, but not exclusively, one
having a dome-
shaped bottom wall 14 where a cavitated zone 23 is formed about the dome-
shaped bottom
wall. Briefly described, the method comprises the steps of sensing water
temperature by means
of a sensor 33 in the immediate area of the dome-shaped bottom wall to assure
that the
temperature of water therein is maintained at a temperature of at least 46
degrees C. The
method also provides convection means to convect heated water from above the
dome-shaped
bottom wall 14 into the area of the cavitated zone 23 and over the dome-shaped
bottom wall 14
to maintain the water temperature above 46 degrees C to prevent harmful
bacteria growth. The
method also comprises controlling the duration of the convection of hot water
above 46 degrees
C in the bottom end of the tank.
(0036) The step of convecting heated water to the bottom end 30 of the tank 11
can be
achieved by the use of a water pump 27 mounted at any convenient location on
the tank to draw
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hot water from the upper region 16 of the tank where water is at a higher
temperature through
conduit means and releasing the hot water above the dome-shaped bottom wall 14
and into the
area or region of the cavitated zone 23 to raise the water temperature therein
above 46 degrees
C and at a water pump pressure sufficiently low to prevent precipitation of
sediment deposits
into the upper region of the tank where water is drawn.
(0037) The step of convecting may also be achieved, as described herein, by
mounting an
inverted shallow cone shape baffle plate under the lower resistive heating
element 17 and
spaced above the dome-shaped bottom wall 14 a predetermined distance therefrom
to direct
heated water generated by the lower resistive heating element to be partially
convected back
into the area of the cavitated zone 24 and over the dome-shaped bottom wall 14
to raise the
temperature in the bottom end 30 of the tank above 46 degrees C. A temperature
sensor 33
and a suitable control 32 assures the operation of the lower resistive heating
element to
maintain such temperature to prevent bacteria growth in the tank bottom end.
(0038) It is within the ambit of the present invention to cover any obvious
modifications of the
preferred embodiment described herein provided such modifications fall within
the scope of the
appended claims
