Abstract:
A gas water heater includes a water tank, a combustion chamber, a burner, a heat exchanger tube at least partially within the water tank, and a fan. The burner receives fuel gas and primary air to create a primary air-fuel mixture for combustion in the combustion chamber with secondary air. The primary air-fuel mixture has a gas concentration above the upper explosive limit of the fuel gas. Secondary air is supplied to the combustion chamber through a secondary air conduit that is separate from the burner. The heat exchanger tube receives the products of combustion, transfers heat from the products of combustion to water in the tank, and exhausts of the products of combustion. Only air flows through the fan; no fuel-air mixture flows through the fan. The water heater utilizes partial premix combustion with low excess air ratio and low contaminant emission. The water heater utilizes a high power burner such that the water heater can supply hot water continuously during low flow rate draws and hot water stored in the tank supplements the hot water supply of the water heater during higher flow rate draws.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to Chinese Patent Application No. 200910033216.7 filed Jun. 16, 2009. 
       BACKGROUND 
       [0002]    The present gas water heater may be classified as a storage gas water heater and a tankless gas water heater based on the hot water demand characteristics. 
       SUMMARY 
       [0003]    A tankless gas water heater is sometimes also referred to as an instant available water heater, and is common in Japan and more recently in China. The combustion system of the common products in the market is a sealed partial premix combustion system and the input power may be up to 32 kw (109,262 BTU/hr).  FIGS. 1   a  and  1   b  (collectively, “FIG.  1 ”) illustrate a typical construction for a tankless gas water heater. In  FIG. 1 , a tankless gas water heater  10  includes a primary air-supply channel  15 , a secondary air-supply channel  20 , a heat exchanger  25 , a burner  30 , a fan  35 , a cold water inlet  40 , a gas inlet  45 , and a hot water outlet  50 . The combustion system is located below the heat exchanger  25  and the flame burns from bottom up. Generally, several harmonica-like burners  30  are used, as shown in  FIG. 1   b . Japanese patents JP 2008-25985, 03-64314 and 01-144616 as well as Chinese patents 200720051450, 20062001192, 9324025 and 97240226 describe tankless water heaters. 
         [0004]    A type of heating equipment similar to this is a wall-mounted dual-purpose gas boiler used for domestic hot water supply and heating. The combustion system of such gas boilers is similar to the tankless gas water heater, but some high energy-efficient products of them have adopted a premix combustion system. 
         [0005]    The word “sealed” in the term “sealed partial premix combustion system” implies that the flow path from the air inlet of the fan to the flue gas outlet of the water heater is a single channel and that the combustion system has only one air inlet. A traditional non-sealed combustion system places the burner in communication with the atmosphere and generally there is no fan or the fan is at the end of the system. The pressure head is produced by the density difference between cold and hot air or the pumping of the fan, so a negative pressure is formed in the combustion chamber and the air required for combustion is drawn into the combustion chamber naturally. The sealed combustion system is generally in a turbulent combustion condition and the combustion intensity is high, while the non-sealed combustion system creates a laminar combustion condition. So under the same combustion chamber volume and working conditions, the output power of the non-sealed combustion system is less than the sealed combustion system. 
         [0006]    Partial premix combustion generally implies that the system divides the combustion air supplied by the fan into two parts, one part entering the burner directly is called primary air, and the other part entering the combustion chamber by the external channel outside the burner is called secondary air. The primary air is evenly mixed with the gas in the burner, which is called premix gas. If the air in the premix gas is not adequate to support complete combustion of the fuel gas, a portion of the fuel gas in the premix gas burns around the outlet of the burner at first which forms the inner flame, and the remaining fuel gas burns with the secondary air by diffusion and forms the outer flame. If the provided secondary air is still not enough to support the complete combustion of the fuel gas in the combustion chamber, it will result in the emission containing contaminants such as CO. One way to control the emission level of the contamination such as CO in a partial premix combustion system is to provide an air volume far more than theoretical value (i.e., well above the stoichiometric ratio), namely the excess air ratio must be much more than 1. In general, the designed excess air ratio is up to 1.8-2.5 (namely 80%-150% more than the theoretical air volume is needed). However, the excess air increases the requirements of the fan, makes the combustion noise higher, and reduces the heat exchange efficiency. 
         [0007]    A premix combustion system is generally regarded as a more advanced combustion mode. The premix combustion system fully premixes the fuel gas and the combustion air before the burner and generally controls the excess air ratio between 1.2-1.5. The premix gas burns out immediately after being sprayed from the burner, therefore the burning velocity is high and the contaminant emission level is low. The fuel gas and air mixture in a premix combustion system is close enough to the stoichiometric ratio to permit immediate combustion. As a result, the fan, combustion chamber, and other components must be sealed to comply with typical regulations for the prevention of external flammable vapor ignition. Consequently, premix combustion systems may be more expensive than partial premix combustion systems. 
         [0008]    The storage gas water heater is prevalent in the United States of America. In general, the volume of such kind of water heater is above 200 liters (52.83 gallons) in order to satisfy the large quantity of water required, for example, by a bathtub. The storage gas water heater better satisfies the requirements for larger quantities of hot water, but its relatively large volume is less suitable for families with a small living space, so the living conditions of, for example, the Chinese limit its development in China. There are similarly situated families in the United States and other countries who have relatively small living space, and large storage gas water heaters present a similar problem for these families. Thus, how to increase effective hot water supply with reduced water tank volume is an urgent problem to be solved. 
         [0009]    The common combustion system used with a storage gas water heater is a premix combustion system or a non-sealed partial premix combustion system. The storage water heater using a premix combustion system has the cost shortcomings set forth above. The storage water heater using a non-sealed partial premix combustion system generally includes a cast iron disk burner, and because of the inherent characteristic of the non-sealed combustion system, the output power of the storage water heater adopting such combustion mode is low, and it may need a relatively long time (more than 1 hour in some instances) to restore hot water following a large draw. To increase the input power of such kind of burner, more combustion air and a bigger combustion chamber are required, but it is challenging or impossible to increase input power beyond certain capacities in view of the natural, atmospheric air supply. 
         [0010]    In conclusion, to design a kind of gas water heater combining the advantages of tankless and storage gas water heater is a valuable task to be researched. The Chinese patent with patent number 200720080101 tried to design a gas water heater which not only has the two advantages of tankless gas water heater&#39;s “quick and continuous hot water supply” and the storage gas water heater&#39;s “constant-temperature hot water supply,” but also has greater storage and energy regulating capacity. The Chinese patent with patent number 200410024980 also made some attempt in this field. However, the two methods are all simple addition of storage water heater and tankless gas water heater and have no breakthrough in technology. The complicated system and big volume is unsuitable to small family customers. 
         [0011]    In one embodiment, the invention provides a storage gas water heater including a water tank, a fan, a burner, and a heat exchanger and combustion tube. An air inlet of the fan is open to air outside of the water heater and an outlet of the fan is connected with an air-supply opening. The air-supply opening is connected with the burner by a first air-supply channel and is connected with a combustion chamber of the heat exchanger and combustion tube by a second air-supply channel. The first and second air-supply channels together define a totally-sealed channel from the air inlet of the fan to the heat exchanger and combustion tube. At least part of the heat exchanger and combustion tube is inside the water tank. One end of the heat exchanger and combustion tube is connected with an outlet of the burner and an opposite end is open to air outside of the water heater. 
         [0012]    In another embodiment, the invention provides a storage gas water heater including a water tank, a fan, a burner and a heat exchanger and combustion tube. An outlet of the fan is open to air outside of the water heater. An air-supply opening is connected with the burner by a first air-supply channel and is connected with a combustion chamber of the heat exchanger and combustion tube by a second air-supply channel. At least part of the heat exchanger and combustion tube is inside the water tank. One end of the heat exchanger and combustion tube is connected with an outlet of the burner and an opposite end is connected with an air inlet of the fan. The first and second air-supply channels together define a totally-sealed channel from the air-supply opening to the fan outlet. 
         [0013]    In another embodiment, the invention provides a gas water heater including a water tank, a combustion chamber, a gas supply conduit, a burner, a heat exchanger tube, and a fan. The water tank is adapted to contain water to be heated. The combustion chamber is positioned at least partially within the water tank and adapted to receive a primary air-fuel mixture and secondary air, and is adapted to contain the complete combustion of the primary air-fuel mixture in the presence of the secondary air for the creation of products of combustion. The gas supply conduit provides fuel gas. The burner includes a burner inlet for the receipt of fuel gas from the gas supply conduit and primary air and a burner outlet communicating with the combustion chamber for delivery of the primary air-fuel mixture to the combustion chamber. The fuel gas and primary air mix in the burner to create the primary air-fuel mixture. The heat exchanger tube is positioned at least partially within the water tank, the heat exchanger tube communicating at a first end with the combustion chamber for the receipt of the products of combustion, the heat exchanger tube transferring heat from the products of combustion to water in the tank, and the heat exchanger tube having a second end for the exhaust of the products of combustion. The fan causes the primary air to flow into the burner, the secondary air to flow into the combustion chamber, and the products of combustion to flow through the heat exchanger tube and out of the second end. 
         [0014]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1   a  is a front view of a tankless gas water heater. 
           [0016]      FIG. 1   b  is a perspective view of a burner used in the tankless gas water heater of  FIG. 1   a.    
           [0017]      FIG. 2  is a front sectional view of a storage gas water heater. 
           [0018]      FIG. 3  is a sectional view of the storage gas water heater of  FIG. 2  along line  3 - 3 . 
           [0019]      FIG. 4  is a perspective view of a portion of the storage gas water heater of  FIG. 2 . 
           [0020]      FIG. 5  is a front sectional view of a storage gas water heater. 
           [0021]      FIG. 6  is a front sectional view of a portion of a storage gas water heater. 
           [0022]      FIG. 7  is a front sectional view of a storage gas water heater. 
           [0023]      FIG. 8  is a graph comparing a storage gas water heater and similar existing products. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0025]      FIGS. 2-4  illustrate a storage gas water heater  60  including a water tank  65 , a fan  70 , a burner  75 , and a heat exchanger and combustion tube  80 . The heat exchanger and combustion tube  80  is composed of a combustion chamber  85  and a heat exchanger tube  90 . The burner  75  is composed of a set of harmonica-like burners. The heat exchanger tube  90  is composed of a central main tube  95  and a spiral tube  100  which is connected with and revolves around the main tube  95 . The combustion chamber  85  is connected with the main tube by a reducer coupling  105 . 
         [0026]    The fan  70  includes an air inlet or fan inlet  110  and an outlet or fan outlet  115 . The water heater  60  includes an air-supply opening  120 . The air inlet  110  of fan  70  is open to (i.e., communicates with) the air outside the water heater  60 , and the outlet  115  is connected with the air-supply opening  120 . The air-supply opening  120  is connected with (i.e., communicates with) the burner  75  by a first air-supply channel  125  ( FIG. 3 ), and at the same time the air-supply opening  120  is connected with (i.e., communicates with) a burner inlet  130  of the combustion chamber  85  by a second air-supply channel  135 . The first air-supply channel  125  provides primary combustion air to the burner  75  for premixture with fuel gas, and the second air-supply channel  135  provides secondary combustion air to the combustion chamber  85 . A gas supply line or conduit  140  introduces fuel gas into the first air-supply channel  125  or burner  75 . The primary air is evenly mixed with the fuel gas in the burner  75  to form the premix gas. 
         [0027]    With reference to  FIG. 4 , the second air-supply channel  135  starts from a secondary air distributing plate  145  between the air-supply opening  120  and the combustion chamber  85 . The second air-supply channel  135  is further defined by and connected with the combustion chamber  85  through gaps  150  between the set of harmonica-like burners. As a result, a totally-sealed channel is defined from the air inlet  110  of the fan  70  to the heat exchanger and combustion tube  80 . The heat exchanger and combustion tube  80  may be at least partially inside water tank  65 , or totally submersed as illustrated. One end of the heat exchanger and combustion tube  80  is connected with an outlet  155  of the burner  75 , and an outlet  160  of the heat exchanger and combustion tube  80  is open to (i.e., communicates with) air outside the water heater  60 . 
         [0028]    One advantage of this partial premix combustion system lies in that the combustion air needed for the combustion enters combustion chamber  85  as two parts, and the combustion mode is sealed combustion. The burning system has only one air-supply opening  120  and one exhaust vent  160 . The mixed gas (i.e., the mixture of primary air and fuel gas) may in some embodiments be too rich for combustion (i.e., it is over the upper explosive limit), and does not become fully combustible until it enters the combustion chamber  85  and is further mixed with secondary air. In this regard, the system is designed to reduce the likelihood that a fully combustible mixture will be created outside of the combustion chamber  85 . 
         [0029]    Generally, if the concentration of the fuel gas in the air is lower than the lower explosive limit L low , the heat produced by the oxidation reaction is not enough to make up the lost heat, so burning can&#39;t continue; and if the concentration is over the upper explosive limit L high , burning also can&#39;t take place because of oxygen deficiency. Both of the explosive limits L are calculated as: L=V gas /(V air +V gas ). Where, V gas  is the volume of the fuel gas and V air  is the volume of the air. 
         [0030]    To make combustion normally ongoing, the fuel gas concentration shall be ensured to between the upper explosive limit and the lower explosive limit. Because this invention adopted a structure to separate primary air and secondary air, it may make the fuel gas concentration in the burner higher than the upper explosive limit, namely the fuel gas concentration in the burner is higher than the fuel gas concentration in the combustion chamber. In the combustion chamber, due to the addition of the secondary air, the fuel gas concentration falls into the explosive limit. For example, the upper explosive limit of methane in the air at normal temperature and pressure is 15%, thus, the fuel gas concentration in the burner shall be more than 15% as for the stated water heater used methane as fuel gas. 
         [0031]    In combustion, to completely burn out one cubic meter (1 m 3 ) of fuel gas, the required air volume calculated according to the reaction equation is called theoretical air requirement V o , while in actual combustion apparatus, if air is supplied only according to the theoretical air requirement, it is very difficult to fully mix the fuel gas and air, so the oxygen in the air can&#39;t thoroughly take part in the reaction and makes the combustion inadequate. So the actual air supply is generally more than the theoretical air requirement. The ratio of actual air supply V to theoretical air requirement V o  is defined as excess air ratio α, namely: α=V/V 0 . For hydrocarbon fuel C n H n , the relationship between the theoretical air requirement V o  and the fuel quantity V gas  participating in the action is: V 0 =n+0.25 m/0.21V gas . 
         [0032]    The excess air ratio α adopted by this invention is less than 1.3, which is at the same level with common existing premix combustion system. Study shows that this value may ensure high heat exchange efficiency of the system and low contaminant emission level, especially if the CO value is less than 50 PPM under rated load. Correspondingly, if the same combustion system is used in a tankless gas water heater, the CO value is generally higher than 100 PPM and the excess air ratio is more than 2. 
         [0033]    Compared with a non-sealed partial premix combustion system, the advantage of the above-mentioned system lies in that the excess air ratio is lowered by 40% and the CO emission amount has also great reduction. Under the conditions of similar water tank volume, the input power of the above-mentioned system may reach 30 kw (102,433 BTU/hr), while the traditional non-sealed partial premix combustion system is only 20 kw (68,289 BTU/hr) in general. The higher input power improves the restoring time of the water heater and provides the possibility of large quantity of water supply. Or with the similar hot water supply capacity, it may reduce the water heater&#39;s volume and broaden the usable range of storage gas water heater. Compared with premix combustion system, the advantage of the above-mentioned system is cost reduction potentially over 50%. 
         [0034]    The following table shows the comparison of a typical working parameter of the storage gas water heater  60  with an ordinary storage water heater, a tankless water heater, and a premix combustion system. The results show that the combustion efficiency of the storage gas water heater  60  may reach the level of the premix combustion system, and has great improvement compared with the ordinary tankless water heater and the ordinary storage water heater. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Flue Gas 
                   
                   
               
               
                   
                 Rated 
                 Thermal 
                 Temperature 
                 CO 
                 Excess air 
               
               
                   
                 Heat Load 
                 Efficiency 
                 ° C. 
                 ppm 
                 ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Storage Gas 
                 26 
                 107 
                 37 
                 61 
                 1.46 
               
               
                 Water Heater 
               
               
                 Ordinary 
                 26 
                 88 
                 150 
                 260 
                 2.35 
               
               
                 Tankless 
               
               
                 Premix System 
                 24 
                 101 
                 63 
                 91 
                 1.44 
               
               
                 Ordinary 
                 20 
                 80 
                 150 
                 200 
                 1.7 
               
               
                 Storage Type 
               
               
                   
               
             
          
         
       
     
         [0035]      FIG. 8  displays a comparison of the hot water supply capacity between the storage gas water heater  60  (shown as line  165 ), a tankless water heater (shown as line  170 ), and a traditional storage water heater (shown as line  175 ). The storage gas water heater  60  has an input power of 26 kw (88,775 BTU/hr), a water supply quantity of 10 L/min (2.64 gal/min) and a volume of 80 L (21.13 gal.). The tankless water heater has an input power of 26 kw (88,775 BTU/hr) and a water supply quantity of 13 L/min (3.43 gal/min) at 25° C. (77° F.) temperature rise. The traditional storage water heater has an input power of 20 kw (68,289 BTU/hr) and volume of 80 L (21.13 gal.). Line  175  is the water supply curve of the traditional storage water heater, which can continuously supply hot water with 40° C. (104° F.) temperature rise at flow rate of 10 L/min (2.64 gal/min) for 8 minutes, and then supply hot water with 25° C. (77° F.) temperature rise constantly. The line  165  is the water supply curve of the storage gas water heater  60 , which can supply water at flow rate of 10 L/min (2.64 gal/min) for 10 minutes and then continuously supply hot water of 36° C. (96.8° F.) temperature rise, namely like a tankless water heater. The line  170  is the water supply curve of the tankless water heater, which can only continuously supply hot water with 32.5° C. (90.5° F.) temperature rise. The height difference between a portion  180  of line  165  and line  170  is caused by the efficiency difference between the storage gas water heater  60  and the tankless water heater. As shown in  FIG. 8 , the storage gas water heater  60  has advantages over the traditional storage gas water heater with similar volume and the tankless water heater with similar input power. It combines the advantages of the traditional storage water heater&#39;s storage tank and the high power of the tankless water heater. 
         [0036]      FIG. 5  illustrates a storage gas water heater  185 , whose basic structure is the same as the storage gas water heater  60  shown in  FIGS. 2-4 . The air-supply opening  120  is connected with burner  75  by the first air supply channel  125  and at the same time is connected with combustion chamber  85  by the second air supply channel  135 . At least part of the heat exchanger and combustion tube  80  is inside the water tank  65 , and a first end  190  of heat exchanger and combustion tube  80  is connected with the outlet  155  of the burner  75 . The difference is that the outlet  160  of the heat exchanger and combustion tube  80  is connected with the air inlet  110  of fan  70 , which composes of a totally-sealed channel from the air-supply opening  120  to the fan outlet  115  with the outlet  115  open to the air. Because the fan  70  is positioned after the heat exchanger and combustion tube  80 , negative pressure is formed in the system while working, so air is drawn in through the air-supply opening  120  and the flue gas is discharged by the outlet  115  of the fan  70 . 
         [0037]      FIG. 6  illustrates a portion of a storage gas water heater  195 , whose basic structure is the same as the storage gas water heater  60  shown in  FIGS. 2-4  except for the combustion system, so there is no need for a detailed explanation of the rest of the water heater  195 . The combustion system includes a cylindrical burner  200 . One advantage of the water heater  195  is that the structure of the burner  200  is simpler than the harmonica-like burner  75  of the storage gas water heater  60 . The system needs only one burner  200  and the burner&#39;s length may be changed together with the load. If the load is increased, the length of the burner  200  and the combustion chamber  85  can also be increased. A gas line  205  is located at a front part  210  of the burner  200  and a gas line outlet  215  is positioned at a central line  220  of the burner  200 . The gas is sprayed along the central line  220  of burner  200 . The second air-supply channel  225  does not extend through gaps in the burner  200 , but is instead defined only by the secondary air distributing plate  230 . 
         [0038]      FIG. 7  illustrates a storage gas water heater  235 , whose basic structure is the same as the storage gas water heater  60  shown in  FIGS. 2-4 . The air inlet  110  of the fan  70  is open to the air and the outlet  115  is connected with the air-supply opening  120 . The air-supply opening  120  is connected with the burner  75  by the first air-supply channel  125  and at the same time connected with the combustion chamber  85  by the second air-supply channel  135 . The second air-supply channel  135  starts from the secondary air distributing plate  145  and is connected with the combustion chamber  85  through the gaps  150  between the burners  75 . The heat exchanger and combustion tube  80  is positioned inside the water tank  65 , and one end of it is connected with the outlet of the burner  75  and the other end is open to the air. The heat exchanger and combustion tube  80  is composed of horizontal combustion chamber  85  and the heat exchanger tube  90 . The heat exchanger tube  90  is composed of a main tube  240  and a Z-shaped elbow  245  connected with the main tube  240 . Because the water tank  65  is horizontal, the other components are also horizontal. One advantage of the storage gas water heater  235  is that the horizontal structure is suited to the available space in small living quarters and the water heater can be hung on the wall to save space. 
         [0039]    Except for the above-mentioned embodiments, this invention may have other embodiments. Any technology adopting identical substitution or equivalent alteration also belongs to the protection domain claimed by this invention.