Abstract:
A means for mitigating or precluding formation of ice in water stored in large storage tanks generates large mixing bubbles toward the bottom of the tank, causing mixing of thermally stratified layers of water in the tank through turbulence created as the bubbles rise through the tank. Incipient stratification of water along thermoclines is detected and the mixer is engaged only when temperatures of portions of stratified water in the tank approach freezing.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional application Ser. No. 61/127,376, filed May 12, 2008, entitled WATER SUPPLY MIXING PROCESS. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and apparatus to mix thermally stratified potable water supplies to prevent freezing. 
     2. Description of the Related Art 
     When a large capacity tank is underutilized, differential thermal conditions in the tank can cause the contents to stratify in thermoclines, where warmer layers of water meet cooler layers. If, as is often the case, a tank with stratified contents is both filled and emptied from a limited portion of the tank, water supplied by the tank will be from recently filled, fresher strata, while the remaining strata in the tank may age, unused and relatively undisturbed. For non-insulated or under-insulated tanks in colder regions during winter months, the aging, unused strata of water in such tanks may lose sufficient heat to freeze. When ice forms in a water storage tank, the effective liquid capacity of the tank is reduced by the volume of ice in the tank. If a sufficient volume of ice is formed, the effective liquid capacity of the tank may be reduced by such an amount that it is not sufficient for water supply needs. 
     Managers of water supply systems, such as municipalities, have employed various means to minimize formation of ice in water supply tanks, with varying degrees of success. In some systems, heating is used to prevent ice formation. In some such systems, water is heated. In some cases, liquid water from the tank is pumped and circulated through a heat exchanger to raise its temperature. In other cases, at least some of inflowing water is heated during filling of the tank. In yet other cases, steam is injected into the liquid water in the tank to raise its overall temperature. Such water tank heating systems are expensive and require considerable maintenance. 
     In other systems, small sparging bubbles are provided to water in lower portions of the tank, exchanging heat from the air trapped in the bubbles to the water surrounding them as the bubbles rise through the tank. If the air provided to form the bubbles is significantly warmer than the water through which the bubbles pass, and if a sufficient quantity of air is bubbled through the tank, such sparging can elevate the temperature of some of the liquid water and somewhat reduce ice formation. Because the heat capacity of air is relatively small, however, to be effective such systems must provide a very large volume of sparging bubbles and, preferably, the air forming the bubbles must be heated to a considerably high temperature. For tanks with significant ice formation tendencies, such systems are either ineffective or very expensive. 
     What is needed is a method of preventing or remediating stratification of water in storage tanks to preclude ice formation in the first place. As will be understood by those in the art, stratification can be obviated by sufficient vertical mixing of water in the tank, mixing warmer water from recent fillings with cooler water from prior fillings. Such mixing can also assist in melting ice already formed from water previously thermally stratified in the tank. The effectiveness of such mixing for ice remediation may be enhanced by providing heat to water that is to be mixed. 
     A number of means for mixing liquids are available to de-stratify stored water. A mechanical mixer, comprised of a screw or blade that is turned by a motor, is commonly employed to mix various liquids. Mechanical mixers, however, are subject to a number of shortcomings for mixing drinking water in storage tanks. 
     Mixing the strata in a typical large water storage tank with a mechanical mixer requires a large amount of energy relative to the amount of water that is actually mixed. Further, agitation of the water in the tank by mechanical mixers can disturb sediment settled in the bottom of the tank, resulting in suspended sediment degrading the aesthetics of the water for drinking. Further still, mechanical mixers are often inefficient, mixing some but not all strata in a storage tank. In addition, acquisition costs can be high for a mechanical mixer having sufficient capacity to mix all the strata in a large storage tank. Yet further, costs are high to retrofit an existing water storage tank with a mechanical mixer, retrofitting further often entailing a need to drain the tank or otherwise temporarily remove the tank from the water distribution system. What is needed are more economical and efficient means of mixing water to eliminate stratification with minimal disturbance to sediment in the tank. What is needed further is such means that can be retrofitted to a water storage tank operation economically and without a need to take the water tank off-line. 
     For economy, it is further desirable that the mixer that is used to obviate stratification be engaged only when needed, i.e. only when thermal stratification is taking place and cooler strata are at risk of freezing. Accordingly, it is desirable to have a means for determining when mixing is needed and for engaging the mixer only at such times. 
     It is further desirable that the mixer system be easy to install and easy to operate. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a means for mixing drinking water stored in large storage tanks, preventing thermal stratification of the water to reduce risk of freezing, by generating large mixing bubbles toward the bottom of the tank, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank. Embodiments of the present invention detect incipient stratification of water along thermoclines and engage the mixer only when temperature of water strata within the tank approach freezing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing objects, as well as further objects, advantages, features and characteristics of the present invention, in addition to methods of operation, function of related elements of structure, and the combination of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein: 
         FIG. 1  is a diagram of an embodiment of the present invention in a rectangular storage tank; 
         FIG. 2   a  is a diagram of a bubble forming plate according to an embodiment of the present invention; 
         FIG. 2   b  is a diagram of an alternative embodiment of a bubble forming plate; 
         FIG. 3  is a diagram illustrating mixing of drinking water in a storage tank by turbulence caused by rising bubbles according to an embodiment of the invention such as illustrated in  FIG. 1 ; 
         FIG. 4  is a diagram illustrating an alternative embodiment of the present invention; 
         FIG. 5  is a diagram illustrating a thermocline detection arrangement according to an embodiment of the invention; and 
         FIG. 6  is a flow chart for operation of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view of a water storage tank  18  in which is installed a mixer according to an embodiment of the invention. The tank  18  is filled with drinking water from below by standpipe  25 . 
     Located in tank  18 , a mixer  26  injects a gas safe for drinking water, such as air, to generate large mixing bubbles. As further discussed in reference to  FIG. 3  below, the mixing bubbles are large enough to move a substantial amount of water as they rise toward the water&#39;s surface. For effective and efficient mixing of drinking water, bubbles generated by the present invention should be large, from approximately one half meter to several meters in diameter. The mixing current, resulting from turbulence from displaced water as the large bubbles rise, mixes the water to obviate stratification. 
     The mixer  26  includes a forming plate  28  to form mixing bubbles from the injected gas, and a valve  29  to permit or prevent the gas from reaching the forming plate  28 . The mixer  26  also includes a distribution line  30  to supply the forming plate  28  with the gas when the corresponding valve  29  is open, and a controller  32  to open and close the valve  29 . For example, in one embodiment, the mixer  26  includes five forming plates  28 , five valves  29 , and five distribution lines  30 , and the controller  32  includes a memory (not shown) and a processor (not shown) to allow a user to input data to control when and how long each valve. 
     Each forming plate  28 , one embodiment of which is shown in  FIG. 2   a , includes an orifice  36 . When the valve  29  is opened, air flows through the distribution line  30  toward the forming plate  28 , and then exits the distribution line  30  through the orifice  36 . The forming plate  28  prevents the air from rising toward the surface of the water until the valve  29  injects more air than the forming plate  28  can hold, at which time most of the air escapes from under the forming plate  28  and forms a large mixing bubble. The large mixing bubble then rises toward the surface of the wastewater. When the valve  29  is closed, air does not flow through the orifice  36 . 
     By outfitting with strong, permanent magnets, plates  28  may be installed in an active, filled water storage tank that is comprised of ferromagnetic material. In such a case, distribution lines  30  are flexible and plates  28  are simply dropped into tank  18 . Referring now to  FIG. 2   b , illustrated is plate assembly  14 , comprising plate  28  operatively connected to flexible distribution line  30 . Plate  28  is further attached to permanent flat magnet  10  via support legs  12 , providing a space between plate  28  and magnet  10 , thereby elevating forming plate  28  some distance above the bottom of tank  18  so that movement of bubbles from the underside of plate  28  is sufficiently distant from the tank bottom so as not to disturb sediment in the bottom of tank  18 . As will be appreciated by those of skill in the art, the required length of support legs  12  will vary depending upon the depth and density of sediment in tank  18 . In embodiments using magnets for this purpose, it is important that magnet  10  be sufficiently strong to retain plate assembly  14  on the bottom of tank  18  against the buoyancy of both distribution line  30  and plate assembly  14  when large bubbles are formed on the underside of plate  28 . 
       FIG. 3  illustrates the mixing caused by the large bubbles generated by a mixer such as that illustrated in  FIG. 1 . The mixing bubbles  40  generate the mixing currents indicated by the arrows  42  ( 28  arrows shown but only  5  labeled with the reference number  42  for clarity) that mix the water  50 . The strength of the mixing currents  42  depends on the size of each bubble  40  and the speed at which each mixing bubble  40  travels through the water and the speed at which each mixing bubble  40  travels through the water. Small bubbles, such as generated by prior art sparging systems, effectively generate no mixing currents. Large bubbles generated by the present invention can generate strong currents effective to cause considerable mixing of the water. 
     The speed of the mixing bubble  40  depends on the density of the gas employed in the invention relative to the density of water  50 , and the bubble&#39;s shape. The greater the difference between the densities of water  50  and the gas, the faster the mixing bubbles  40  rise through water  50 . The more aerodynamic the shape of the bubble  40  becomes the faster the bubble  40  rises through water  50 . For example, in one embodiment, the bubble  40  forms an oblate spheroid—a sphere whose dimension in the vertical direction is less than the dimension in the horizontal direction. In other embodiments, the bubble  40  forms a squished sphere having the trailing surface—the surface of the bubble  40  that is the rear of the bubble  40  relative to the direction in which bubble  40  moves—that is convex when viewed from the direction that the bubble  40  moves. 
     The size of the mixing bubble  40  depends on the flow rate of the gas into water  50 . The flow rate depends on the size of the orifice  36  and the gas&#39;s injection pressure. As one increases the gas injection pressure, one increases the amount of gas injected into water  50  over a specific period of time that the valve  29  is open. And, as one increases the area of the orifice  36 , one increases the amount of gas injected into water  50  over a specific period of time that the valve  29  is open. As one increases the diameter of the forming plate  28  one increases the amount of gas the forming plate  28  can hold before the gas escapes it. For example, in one embodiment the size of the bubble  40  is approximately 0.5 meters across its largest dimension. In other embodiments, the bubble  40  is approximately 3 meters or greater across in largest dimension. 
       FIG. 4  illustrates an alternative embodiment of the present invention. In this embodiment for elevated oblate spheroid water tank  402 , valves  29  under control of controller  32  provide a supply of pressurized gas through distribution line  30 , which disperses the pressurized gas through orifice  33  into the lower portion of tank inlet standpipe  404 . Because of the high pressure of the head of water over the lower portion of standpipe  404 , bubbles  40  emitted at orifice  33  are initially small and spherical. However, as they rise through standpipe  404  to enter tank  402 , the pressure diminishes with diminishing head of water and bubbles  40  therefore become larger, assuming an oblate shape as they travel upward. As will be appreciated by those in the art, this growth in size of bubbles  40  is more pronounced the lower orifice  33  is placed in tank standpipe  404  and the higher the pressure and rate of gas delivered by orifice  33  to water in the tank. By the time bubbles  40  enter tank  402 , they have become large, on the order of 0.5 to 3 or more meters in diameter along the largest dimension, providing mixing currents as indicated by arrows  42  just as in the embodiment discussed in reference to  FIG. 3 . 
     For some tanks  402 , standpipe  404  serves as both an inlet and an outlet pipe. Preferred operation of the present invention takes place when there is no net outflow in standpipe  404 . Accordingly, for such tanks, it is preferred to add a sensor (not illustrated) for water flow in standpipe  404  so that controller  32  opens valves  29  to provide pressurized gas to tank  402  only when there is no net outflow from the tank in standpipe  404 . 
       FIG. 5  illustrates a cylindrical tank  502  in which a thermocline  504  has developed. Embodiments of the present invention detect the presence of thermocline  504  by comparing temperature readings from sensors  506 ,  508 . The presence of thermocline  504 , separating strata of water in tank  502  is indicated by significant difference between temperature readings from sensors  506  located in the upper portion of tank  502  and sensors  508  located in the lower portion of tank  502 . While the actual value of a temperature difference indicating a thermocline will vary with tank configuration, prevailing weather conditions, etc., a temperature difference of 4 to 10 degrees C. between different levels of water in the tank may indicate the presence of thermocline  504 . When the environmental temperature is below the temperature of inlet water, older strata will be colder than strata comprised of water from more recent fillings. Under such conditions, since depicted tank  502  is filled from the bottom, with the development of thermocline  504  water below thermocline  504 , comprised in part of water from recent fillings, will be warmer than the water above thermocline  504 , comprised mostly of water from earlier fillings which has lost heat through tank  502  to the environment. As will be appreciated by those of skill in the art, such thermal inversions are commonly observed in bottom-filled tanks when the environmental temperature drops to approximately 36 deg. Fahrenheit or below. 
     When the temperature of colder strata approaches freezing, risk of ice formation is present. In the depicted embodiment, when the difference in temperature indicated by upper sensors  506  and lower sensors  508  indicate the presence of a thermocline and the temperature indicated by an upper sensor  506  approaches freezing, controller  32  directs valves  29  to provide pressurized gas to supply line  30 , providing gas to form bubbles under plates  28  as discussed above in reference to  FIG. 1 . 
     As will be appreciated by those of skill in the art, sensors  506 ,  508  may be any form of electronic sensor, such as a thermistor, capable of measuring temperatures in the range of 0 to 100 degrees C. For tanks comprised of ferromagnetic material, sensors  506 ,  508  may comprise a means of magnetic adhesion, for ease of installation. For tanks without substantial thermal insulation, temperature sensors  506 ,  508  may adhere to the exterior of tank  502 . 
       FIG. 6  is a flow chart for operation of an embodiment of the invention for tanks having a single standpipe for inlet and outlet of water, such as that depicted in  FIG. 5 . If sensors indicate stratification  602  and sensors further indicate risk of freezing  604  in at least one stratum, valves are opened  608  only if no outflow is detected  606 . 
     Although the detailed descriptions above contain many specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various other embodiments and ramifications are possible within its scope, a number of which are discussed in general terms above. 
     While the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. Accordingly, the present invention is not intended to be limited to the specific forms set forth herein, but on the contrary, it is intended to cover such alternatives, modifications and equivalents as can be reasonably included within the scope of the invention. The invention is limited only by the following claims and their equivalents.