Patent Publication Number: US-10775051-B2

Title: Bacteria preventive water holding tank construction for electric water heaters

Description:
FIELD OF THE INVENTION 
     The present invention relates to water holding tanks of electric water heaters and more specifically to the construction of the bottom end section of the tank and an improvement thereof to prevent bacteria proliferation and preferably the  Legionella  bacterial. 
     BACKGROUND OF THE INVENTION 
     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 70 to 115 degrees F., according to some studies. The Centers for Disease Control and Prevention, USA, has reported that 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. 
     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. 
     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 gather and where water is less agitated and most often stagnant. This cavitated zone becomes progressively narrow as the inner surface of the dome-shaped bottom wall merges towards the bottom end section of the tank circumferential side wall forming a narrow gap in which sediments accumulate and pile up to form a bed of sediments. This bed of sediments is spaced the furthest from the bottom heating element and thus water therein is less hot creating an ideal temperature location for bacterial proliferation. Should the bottom element fail, then the water temperature at the bottom of the tank will drop. In a study reported in 2011 and entitled “Sporadic Legionnaires disease: the role of domestic electric hot-water tanks”, by S. F. Dufresne, et al, and published by the Cambridge University Press 2011, water samplings were extracted from the drains at the bottom of several domestic water heaters and analysed. This study revealed a few positive samples at water temperature of 133 degrees Fahrenheit and that the bacteria was not present at temperatures of 135 degrees Fahrenheit and above. The World Health Organization recommends that hot water temperature be maintained above 131 degrees Fahrenheit. 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 85 to 105 degrees F. 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. 
     Reference is made to U.S. Pat. Nos. 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 131 degrees F. 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 U.S. Pat. No. 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. U.S. Pat. No. 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 also not a viable solution. 
     The study reported in 2011 confirms that the bottom circumferential cavitated area between the bottom end section of the tank side wall and the dome-shaped bottom wall and this cavity gets progressively narrower down to its lower end and creating stagnant water niches for sediments to accumulate and form a culture bed and permissive environment for biofilm formation and proliferation of microorganisms, including free-living amoebas and  Legionella . A solution to this problem thus becomes an urgent need. 
     In our recently filed U.S. patent application Ser. Nos. 15/731,020, filed Apr. 10, 2017 and 15/731,956, filed Sep. 1, 2017 there is described various heating means to heat the dome shaped bottom wall of the tank to prevent bacteria proliferation in the cavitated area. We also disclose the recirculation of hot water from the uppermost region of the tank where the water temperature is in the environment of 140 degrees F. to the bottom area of the tank for a predetermined time period to sanitize the bottom end of the tank during certain time periods and periodically. Because the prevention of the  Legionella  bacteria in such tanks is of utmost importance for people&#39;s health continuous research is ongoing in an attempt to find a solution to eradicate this public risk. 
     SUMMARY OF THE INVENTION 
     It is a feature of the present invention to provide an improved tank construction for electric water heaters and which substantially eliminates the danger zone in the cavitated circumferential area surrounding the dome-shaped bottom wall of the water holding tank where there is a risk of bacteria proliferation and wherein the improvement is economical and simple. 
     It is a further feature of the present invention to provide a tank construction for electric water heaters and wherein a filler material isolates the danger zone in the cavitated circumferential area of the tank surrounding the dome-shaped bottom wall from the tank interior. 
     A still further feature of the present invention is to provide a tank construction for electric water heaters and wherein the tank bottom wall is modified by a filler material which creates a bottom tank inner surface on which sediments can disperse and which has a substantially smooth and generally planar surface or surfaces free from cavities in which bacteria can proliferate. 
     Another feature of the present invention is to provide a method of constructing a water holding tank for electric water heaters and wherein at least a bottom portion of the cavitated circumferential area about the dome-shaped bottom wall is filled with a filler material which sets therein to isolate at least a substantial portion of the cavitated circumferential area and to form a bottom tank inner surface which has smooth and generally planar surfaces on which sediments can disperse and which isolates at least the lowermost portion of the cavitated circumferential area from the tank interior. 
     According to the above features, from a broad aspect, the present invention provides a water holding tank for an electric water heater and which comprises a cylindrical side wall, a top wall and a dome-shaped bottom wall. A cavitated circumferential area is defined between a lower end section of the cylindrical side wall and the dome-shaped bottom wall. A filler material is set in at least a lowermost portion of the cavitated circumferential area to fill and isolate the at least lowermost portion of the cavitated circumferential area from the interior of the water holding tank. 
     According to another broad aspect of the present invention there is provided an electric water heater which comprises a water holding tank having a cylindrical side wall, a top wall and a dome-shaped bottom wall. A hot water conduit extracts hot water from an upper portion of the tank. A cold water inlet releases water under pressure in a lower portion of the tank. Two or more resistive heating elements heat water in the upper and a lower portion of the tank. Temperature sensing and control means is provided to operate the resistive heating elements to heat water within the tank portions to a pre-set desired temperature. A cavitated circumferential area is defined between a lower end section of the tank cylindrical side wall and the dome-shaped bottom wall. A a filler material fills at least a lowermost portion of the cavitated circumferential area to isolate the at least lowermost portion of the cavitated circumferential area from the lowermost region of the tank. 
     According to a still further broad aspect of the present invention there is provided a method of constructing a water holding tank for an electric water heater. The method comprises the steps of securing a dome-shaped bottom wall to a cylindrical side wall as part of the water holding tank. The dome-shaped bottom wall and a lower end section of the cylindrical side wall form an internal cavitated circumferential area about the dome-shaped bottom wall. A predetermined quantity of a settable fluid filler material is applied in at least a lowermost portion of the cavitated circumferential area and distributed substantially uniformly thereabout to fill and isolate the at least lowermost portion of the cavitated circumferential area from the interior of the water holding tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention and modifications thereof will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a simplified fragmented side view of an electric water heater showing basic components thereof and in which sediments build-up is illustrated in the cavitated circumferential area about the dome-shaped bottom wall and further illustrating a recirculating pump mounted on the top wall of the casing of the water heater; 
         FIG. 2  is an enlarged cross-sectional view of the cavitated circumferential area about the dome-shaped bottom wall showing a filler material set therein; 
         FIG. 3  is an enlarged cross-sectional view of the cavitated circumferential area illustrating a modification wherein a heating element is incorporated in the filler material; 
         FIG. 4  is an enlarged cross-sectional view of the cavitated circumferential area illustrating a still further modification wherein a silver-copper salt or other anti-bacterial substance is incorporated into the upper surface of the filler material; 
         FIG. 5A  is a schematic view of the water holding tank illustrating a construction stage at which a filler material is deposited in the cavitated circumferential area, and 
         FIG. 5B  is a schematic view of the water holding tank showing the tank sitting vertically on a vibrating platform to cause the filler material to evenly distribute about the cavitated circumferential area. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings and more specifically to  FIG. 1 , there is shown generally at  10  a water holding tank of an electric water heater and basic component parts thereof. The water holding tank  10  is formed of steel and has a cylindrical side wall  11 , a dome-shaped bottom wall  12  and a top wall  13  all secured together by welding. A hot water conduit  14  is secured to the top wall  13  and projects into the upper portion  15  of the tank to extract hot water therefrom when demanded by appliances connected to the hot water distribution line  16 . A dip tube  17  is secured to the top wall  13  and extends to the bottom portion  18  of the tank and spaced a predetermined distance above the dome-shaped bottom wall  12 . A pipe connection  23  interconnects the domestic water supply line  24  to the dip tube through a shut-off valve  25 . Instead of the dip tube  17 , the cold water inlet can also be fed through a coupling adjacent the bottom end of the cylindrical side wall as used with some tank construction. As herein illustrated a top resistive heating element  19  is secured to the tank side wall  11  and extends into the upper portion  15  of the tank. Likewise, a bottom resistive heating element  20  is secured to the tank side wall  11  and extends into the bottom portion  18  of the tank to heat water therein. The resistive heating elements  19  and  20  are controlled by respective temperature sensing and control thermostats  21  which are pre-set to heat water within their respective portions to a desired water temperature, usually 140 degrees Fahrenheit. 
     With further reference to  FIG. 2 , it can be seen that the dome-shaped bottom wall  12  and the lower end section  11 ′ of the cylindrical side wall  11  form a cavitated circumferential area  22  all around the outer portion of the dome-shaped bottom wall  12 . Also, this cavitated area diminishes in width towards its bottom end  22 ′ which is very narrow. Over time, sediments from the domestic water supply line  24  enters the tank  10  and deposit on the dome-shaped bottom wall and precipitate into the bottom end  22 ′ of the cavitated area due to the slopping shape of the bottom wall and forms a sedimentary bed  26  in which bacteria, such as the  Legionella  bacteria, can survive and propagate for the reason that this area  22 ′ is spaced furthest away from the bottom resistive heating element  20  where water within the tank is often stagnant and at its lowest temperature. As mentioned in the research discussed above,  Legionella  bacteria can be present in water temperatures of about 130 degrees Fahrenheit and slightly higher and this sedimentary bed  26  can constitute an ideal culture bed location for such bacteria to develop. 
     In order to prevent the formation of such sedimentary culture bed  26  the present invention is a simple solution which is simply to isolate the cavitated circumferential area  22  from the bottom end of the tank  10  by introducing therein a suitable filler material  30  to isolate, at least the lowermost portion  31  of the cavitated circumferential area  22  from the lowermost region  18  of the tank. By doing so, the bottom surface of the tank presents a smooth planar surface section  32  in the cavitated circumferential area  22  without crevices or cavities in which sediments can stack-up. Also, the dome shaped area  32 ′ is now reduced to a very shallow dome shape form. Accordingly, sediment deposits  26 ′ will disperse on such surfaces and become free moving thereon and because the bottom surface section  32 , in the cavitated area is now closer to the bottom heating element  20 , the water temperature at this surface section will be higher and above 135 degrees Fahrenheit at which the  Legionella  bacteria cannot survive. 
     In our co-pending U.S. patent application Ser. No. 15/731,021, filed Apr. 10, 2017 there is described the use of a water pump to introduce hot water from the upper portion  15  of the tank into the lower portion  18  and above the cavitated circumferential area  22  whereby to bring the water temperature in the cavitated area  22  to a temperature sufficiently high, about 140 degrees Fahrenheit, to kill any bacteria that may live in the cavitated area. Such is illustrated in  FIG. 1  where it can be seen that a pump  35  is mounted on the top surface of the water heater casing  34  and has a conduit  36  extending inside the tank to the upper portion  15  thereof where it draws hot water, usually at a temperature of about 140 degrees Fahrenheit, and releases it to the bottom region  18  of the tank through an open end  38  of a conduit  37  secured to the tank top wall  13 . Because the bottom end of the tank now has a mostly smooth planar-like surface  32 , or surface sections, this hot water becomes more uniformly distributed over the surface  32  forming a hot water strata that sanitizes the bottom end of the tank preventing bacterial proliferation. To sanitize the tank this pump is placed in operation for 3 to 4 hours and the pump circulates a gentle water flow not to create a turbulence to cause the sediments to flow upwards towards the upper region  15  of the tank where water is drawn as such sediments may cause problems in the mixing valves of faucets connected to the hot water distribution line  16 . As shown in  FIG. 2 , the filler material  30 , as herein illustrated, fills about 80% of the cavitated circumferential area  22 . However, filling at least the lowermost portion, about 50% of the depth of the cavitated area, would provide a viable solution for the reason that the crevice portion of the cavitated circumferential area is eliminated and the remaining cavitated area is wider, flat and shallow and extends closer to the apex  39  of the dome-shaped bottom wall  12 . Of course, the circumferential cavitated area could be filed entirely with a filler material to create a smooth bottom surface across the tank. The location of the drain outlet  40  of the tank is also re-positioned to lie slightly above the bottom surface of the filler material  30 . A drain valve, not shown but obvious to a person skilled in the art, is connected to the drain outlet  40 . 
     The filler material  30  may consist of various suitable materials such as an epoxy resin or a cement material. As shown in  FIG. 3  an epoxy resin material  41  is used as the filler. Also, a modification is herein shown where a curved metal conduit, preferably a copper tube  42  having a heating wire  43  therein is disposed in the cavitated circumferential area  22  prior to pouring the epoxy therein to embed the tube  42  therein to heat the epoxy or cement to 135 degrees F. or above to raise the temperature of water thereover to a safe temperature. Such a filler material has thermal conductive properties. The tube  42  and wire  43  are mounted in such a way as to permit the removal and connection of the wire in the lower thermostat area described in one of our co-pending applications referred to herein. Also, a heating film  44  having a heating wire  45  supported therein can be mounted in a cavity  46  formed on the outer surface of the foam base  47  and located in contact with the dome-shaped bottom wall  12  adjacent the cavitated area  22 , as also described in one of our co-pending applications. Suitable heat conductive metal particles  48  may also be mixed with the epoxy to improve its conductivity. 
       FIG. 4  illustrates a still further modification wherein the filler material  30 , either an epoxy or cement material, has a silver-copper salt  49  applied on the surface thereof and adhered thereto when the filler hardens and sets. An anti-microbial agent or zinc powder particles or combinations of any of the particles and the salts may also be applied to the surface to prevent bacteria growth, provided any of such salts or agents or combinations thereof are not hazardous to public health. The cement filler material is a low viscosity cement having superior body strength which will not decompose, retract, crack or disconnect when hard set and exposed to water at various temperatures including water temperatures up to about at least 190 degrees Fahrenheit. 
     The method for constructing the tank  10  is relatively simple as is the proposed solution of eliminating the cavitated circumferential area to prevent the formation of a culture bed in which bacteria can proliferate. The method comprises, after the dome-shaped bottom wall  12  is welded to the cylindrical side wall  11 , of inserting a predetermined quantity of the filler material  30  in a substantially fluid state in the cavitated circumferential area  22  from the inside of the cylindrical side wall and distributed about the cavitated area  22  to fill and isolate at least the lowermost portion of the cavitated area, as shown in  FIG. 5A . The filler material  30  is preferably, although not exclusively a quick-set filler material. Thereafter, as shown in  FIG. 5B , the tank is dispose vertically on a vibrating platform  50  and is imparted vibrations by a motor  51  to cause the fluid filler material to substantially evenly distribute about the cavitated circumferential area  22  and rid itself of any air pockets and form a smooth flat filler surface. 
     It is within the ambit of the present invention to cover all obvious modifications of the preferred embodiment described herein provided such modifications fall within the scope of the appended claims.