Patent Publication Number: US-2011048344-A1

Title: Water heating apparatus

Description:
BACKGROUND OF THE INVENTION 
     This application claims the entire benefit of Japanese Patent Application Number 2009-195836 filed on Aug. 26, 2009, the entirety of which is incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a latent heat-recovery type water heating apparatus comprising a first heat exchanger and a second heat exchanger. 
     DESCRIPTION OF RELATED ART 
     In a latent heat-recovery type water heating apparatus, there are provided a first heat exchanger for exchanging heat between sensible heat contained in an exhaust gas and water flowing through a heat exchanger tube, and a second heat exchanger for exchanging heat between latent heat contained in the exhaust gas and water flowing through the heat exchanger tube. The first heat exchanger is arranged upstream of an exhaust gas passage through which an exhaust gas from a burner flows, and downstream of a water flow passage through which water flows, whereas the second heat exchanger is arranged downstream of the exhaust gas passage and upstream of the water flow passage, so that high thermal efficiency is achieved. 
     However, according to this water heating apparatus, when used in winter or in a cold district, there may be a possibility that water remaining in a heat exchanger tube of a heat exchanger freezes, which leads the water heating apparatus to become unavailable. In particular, water is apt to freeze in the second heat exchanger because water remains, even after drainage of water, inside the heat exchange tube of the second heat exchanger whose diameter is smaller for the purpose of recovering latent heat. Japanese Laid-open Patent Publication No. 2008-151473 discloses a second heat exchanger having a plurality of heat exchanger tubes surrounded with a frame-type heat conduction member. The frame-type heat conduction member is in contact with the heat exchanger tubes and with a heater, so that heat is transmitted from the heater to the heat exchanger tubes through the heat conduction member to thereby prevent the heat exchanger tubes from freezing. 
     However, this arrangement of the second heat exchanger requires to heat the additional heat conduction member and the heater for a long period of time, which increases the cost. Further, the heat conduction member can only locally heat the heat exchanger tubes, and thus heat is not transmitted entirely over the heat exchanger, resulting that an antifreeze effect is not sufficiently performed. 
     In view of the above, the present invention seeks to provide a water heating apparatus which is simple in structure without requiring a heater and a heat conduction member and which can immediately eliminate a failure due to freezing water in the second heat exchanger. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention as embodied and described herein as a first embodiment, a water heating apparatus comprising: an exhaust gas passage through which an exhaust gas from a burner flows; a water flow passage through which water flows; a first heat exchanger configured to exchange heat between sensible heat contained in the exhaust gas and water flowing through a heat exchanger tube, and arranged upstream of the exhaust gas passage and downstream of the water flow passage; a second heat exchanger configured to exchange heat between latent heat contained in the exhaust gas and water flowing through a heat exchanger tube, and arranged downstream of the exhaust gas passage and upstream of the water flow passage; and a bypass pipe arranged in the water flow passage in such a position as to bypass the second heat exchanger and designed to allow a minimum amount of water required for a burner combustion operation to flow therethrough into the first heat exchanger. 
     Preferably, as a second embodiment, in the water heating apparatus according to the first embodiment, a water drain pipe may be connected in the water flow passage at an intermediate portion between the second heat exchanger and the first heat exchanger, and the bypass pipe may be connected between an upstream side of the second heat exchanger and the water drain pipe in an up-and-down direction. 
     Preferably, as a third embodiment, in the water heating apparatus according to the first or second embodiment, at least at a downstream end of the bypass pipe, the bypass pipe may have an inner diameter to allow the minimum amount of water. 
     With the above configuration of the water heating apparatus according to the first embodiment by simply providing the bypass pipe, it is possible to use the burner even if the second heat exchanger freezes, so that the heat exchanger tube of the second heat exchanger can be entirely heated by the exhaust gas from the burner. Therefore, even if the second heat exchanger freezes, the second heat exchanger can be defrosted quickly by the combustion heat of the burner, which results no influence on the usability. 
     With the above configuration of the water heating apparatus according to the second embodiment, in addition to the above advantages of the first embodiment, water does not remain in the bypass pipe after drainage of water from the water drain pipe. 
     With the above configuration of the water heating apparatus according to the third embodiment, in addition to the above advantageous effect of the first or second embodiment, it is possible to prevent water from remaining in the bypass pipe while ensuring the minimum amount of water supply to the first heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above aspect, other advantages and further features of the present invention will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic circuit diagram of a bath/hot water supply system; 
         FIG. 2  is a perspective view illustrating a structure for bypassing a second heat exchanger; and 
         FIG. 3  is a perspective view illustrating a modification of the structure for bypassing the second heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the accompanying drawings, one exemplary embodiment of the present invention will be described. 
     As seen in  FIG. 1 , a bath/hot water supply system  1  includes a hot water supply circuit  2  as a water heating apparatus and a bath circuit  3 . A hot water supply combustion chamber  4  which comprises hot water supply burners  5 , a first heat exchanger  6 , and a second heat exchanger  7  is disposed in the hot water supply circuit  2 , so that tap water is heated to supply as hot water. The bath circuit  3  is disposed in a bath combustion chamber  20  and comprises a bath burner  21  and a bath heat exchanger  22 . The bath circuit  3  is used for reheating water in the bath, etc. 
     In the hot water supply circuit  2 , a water supply pipe  8  for supplying tap water from waterworks is connected at an inlet of the second heat exchanger  7 . A hot water supply amount sensor  9  is provided in the water supply pipe  8 . An outlet of the second heat exchanger  7  and an inlet of the first heat exchanger  6  are connected through a connecting pipe  10  for supplying hot water heated by the second heat exchanger  7  to the first heat exchanger  6 . A hot water supply pipe  11  for supplying hot water heated by the first heat exchanger  6  is connected at an outlet of the first heat exchanger  6 . A hot water supply thermistor  12  is provided in the hot water supply pipe  11 . A reference number  13  indicates a hot water tap. In the hot water supply combustion chamber  4 , the first heat exchanger  6  is arranged upstream of an exhaust gas passage through which exhaust gases from the hot water supply burners  5  flow, and the second heat exchanger  7  is arranged downstream of the exhaust gas passage, so that heat is exchanged between water flowing through heat exchanger tubes of the first and second heat exchangers  6 ,  7  and the exhaust gases from the hot water supply burners  5 . Gas piping  14  for supplying gas to the hot water supply burners  5  is provided with a gas cock solenoid valve  15 , a gas supply proportional control valve  16 , and hot water supply switching solenoid valves  17  each of which is positioned in a branch pipe extending to the corresponding hot water supply burner  5 . The gas cock solenoid valve  15 , the gas supply proportional control valve  16 , and the hot water supply switching solenoid valves  17  are arranged in this order from the upstream side of the gas piping  14 . A fan  18  is provided at a lower side of the hot water supply combustion chamber  4 , which supplies combustion air to the hot water supply burners  5  and the bath burner  21 . 
     In the bath circuit  3 , return piping  24  and supply piping  27  are connected between the bath heat exchanger  22  and a bath tub  23 . The return piping  24  is provided with a circulating pump  25  and a bath thermistor  26 . Further, the return piping  24  is connected with a water filling pipe  28  which is branched from the hot water supply pipe  11  and equipped with a hot water supply solenoid valve  29  and a filling water quantity sensor  30 . When the hot water supply solenoid valve  29  provided in the water filling pipe  28  is opened, hot water heated in the hot water supply circuit  2  can be supplied to the bath tub  23 . A switching solenoid valve  31  is provided in a branch pipe extending between the gas piping  14  and the bath burner  21 . Reference numbers  32 ,  33 , and  34  indicate a controller, a hot water supply remote control, and a bath remote control, respectively. 
     A bypass pipe  35  for bypassing the second heat exchanger  7  is connected to a water flow passage in the hot water supply circuit  2 . To be more specific, as best seen in  FIG. 2 , a joint  36  is provided at an intermediate portion of the connecting pipe  10 , and a water drain pipe  37  is connected to the joint  36 . The bypass pipe  35  is connected in an up-and-down direction between the water drain pipe  37  and the water supply pipe  8 . The inner diameter of the bypass pipe  35  is determined such that an upper end portion (i.e., downstream end) thereof that is adjacent and connected to the water drain pipe  37  has a diameter (e.g., 3.0 mm) for obtaining a minimum amount of water (e.g., 2.5 L/min) required for burning operations of the hot water supply burners  5  in the hot water supply circuit  2  and the remaining portion of the bypass pipe  35  has a diameter (e.g., 4.8 mm) greater than that of the upper end portion. The reason for reducing the diameter of the upper end portion is that if the inner diameter is too large, the amount of water supplied to the second heat exchanger  7  becomes smaller, resulting in a decrease in the thermal efficiency. On the contrary, the reason for increasing the diameter of the remaining portion is that if the diameter is too smaller throughout the entire length of the bypass pipe  35 , drainage of water from the bypass pipe  35  may not be performed reliably. 
     According to the bath/hot water supply system  1  configured as described above, a normal hot water supply is performed in the following manner. 
     The hot water tap  13  provided in the hot water supply pipe  11  is first opened, so that water flows into equipment. When the hot water supply amount sensor  9  detects the flow of water, the controller  32  causes the fan  18  to operate for a predetermined period of time to discharge (i.e., prepurge) exhaust gases remaining in the hot water supply combustion chamber  4  and the bath combustion chamber  20 . Thereafter, the gas cock solenoid valve  15  and the respective hot water supply switching solenoid valves  17  provided in the gas piping  14  are opened, and the gas supply proportional control valve  16  is opened at a predetermined degree of valve opening, so that gas is supplied to the hot water supply burners  5 . An igniter  19  is then actuated and the hot water supply burners  5  are lit. 
     Exhaust gases from the hot water supply burners  5  flow first through the first heat exchanger  6  where heat can be exchanged with water flowing inside the heat exchanger tube of the first heat exchanger  6  and second through the second heat exchanger  7  where heat can be exchanged with water flowing inside the heat exchanger tube of the second heat exchanger  7 , and finally discharged to the outside. Accordingly, sensible heat is recovered at the first heat exchanger  6  and latent heat is recovered at the second heat exchanger  7 , so that heated water can be supplied through the hot water tap  13 . Drainage generated at the second heat exchanger  7  is neutralized by a neutralizing device  38  before discharge. 
     The controller  32  monitors the temperature of hot water by means of the hot water supply thermistor  12  provided in the hot water supply pipe  11 . The controller  32  further controls opening and closing of the hot water supply switching solenoid valves  17 , adjusts the gas supply proportional control valve  16  to a desired degree of valve opening, and controls the rotation speed of the fan  18  to continuously adjust the amount of air supply, such that the temperature of supplied hot water is adjusted to a setting temperature that is set through the hot water supply remote control  33  or the bath remote control  34 . When the hot water tap  13  is closed, the controller  32  detects that the hot water supply amount sensor  9  stops sending a signal and closes the gas cock solenoid valve  15  and the hot water supply switching solenoid valves  17  to extinguish the hot water supply burners  5 . The controller  32  further operates the fan  18  for a predetermined period of time (i.e., post purge). 
     Meanwhile, in the bath circuit  3 , when an automatic bath fill button on the hot water supply remote control  33  or the bath remote control  34  is pressed, the controller  32  opens the hot water supply solenoid valve  29  in the water filling pipe  28  for allowing water to flow into the hot water supply circuit  2  and then the hot water supply burners  5  is operated. Hot water from the hot water supply pipe  11  flows through the water filling pipe  28  and the return piping  24 , and is supplied to the bath tub  23 . When the filling water quantity sensor  30  provided in the water filing pipe detects that the amount of water reaches the setting amount, the controller  32  causes the hot water supply solenoid valve  29  to be closed to stop the supply of hot water, and then extinguishes the burners  5 . 
     Next, the circulating pump  25  operates to circulate hot water from the bath tub  23  within the bath circuit  3 . During the circulation of hot water, the bath thermistor  26  provided in the return piping  24  monitors the temperature of the hot water, and when this temperature becomes the setting temperature or lower, the bath burner  21  is lit. Accordingly, bath circulating water circulating between the bath heat exchanger  22  and the bath tub  23  is heated up to the setting temperature. When the temperature of the hot water reaches the setting temperature, the combustion using the bath burner  21  is stopped and the circulating pump  25  is also stopped. 
     In order to prevent freezing in the winter-time, a valve (not shown) provided in the water drain pipe  37  can be opened. This makes it possible to discharge water in the second heat exchanger  7  to the outside through the connecting pipe  10 , the joint  36 , and the water drain pipe  37 . However, since the second heat exchanger  7  employs a thin heat exchanger tube, water is not completely drain out and some water remains in the heat exchanger tube, so that the remaining water may freeze. In this state, even if the hot water tap  13  is opened, tap water from the water supply pipe  8  does not flow into the second heat exchanger  7 . According to the invention, since the bypass pipe  35  bypasses the second heat exchanger  7 , tap water flows directly into the first heat exchanger  6  from the bypass pipe  35  through the connecting pipe  10 , so that a flow of water within the equipment can be ensured and the hot water supply burners  5  can ignite for combustion. Therefore, hot water can be supplied even if the second heat exchanger  7  freezes. 
     When the hot water supply burners  5  ignite and start the combustion, the second heat exchanger  7  is entirely heated and the frozen portion is defrosted. This can restore a flow of water through the second heat exchanger  7  and therefore latent heat can be recovered. Since water flows through the bypass pipe  35  having a smaller diameter, the supply of hot water is restricted until the frozen second heat exchanger  7  is restored. However, the second heat exchanger  7  can be defrosted for a short time and the amount of hot water supply recovers quickly, which has little effect on the usability. 
     According to the bath/hot water supply system  1  configured as described above, the bypass pipe  35  is arranged in the water flow passage in such a position as to bypass the second heat exchanger  7 , and is designed to allow at least the predetermined minimum amount of water required for the burner combustion operation to flow therethrough into the first heat exchanger  6 . Therefore, by simply providing the bypass pipe  35 , it is possible to use the hot water supply burners  5  even if the second heat exchanger  7  freezes, so that the heat exchanger tube of the second heat exchanger  7  can be entirely heated by the exhaust gas from the burners  5 . Therefore, in case where the second heat exchanger  7  freezes, it could be defrosted quickly by the combustion heat of the hot water supply burners  5 , which does not affect on the usability. 
     Further, the water drain pipe  37  is connected in the water flow passage at an intermediate portion between the second heat exchanger  7  and the first heat exchanger  6 , and the bypass pipe  35  is connected in the up-and-down direction between the upstream side of the second heat exchanger  7  and the water drain pipe  37 . Therefore, water does not remain in the bypass pipe  35  after drainage of water from the water drain pipe  37 . 
     Furthermore, the downstream end of the bypass pipe  35  has a smaller inner diameter than that of the other part to allow the predetermined minimum amount of water. Therefore, it is possible to prevent water from remaining in the bypass pipe  35  while ensuring the predetermined minimum amount of water supply to the first heat exchanger  6 . 
     According to the above exemplary embodiment, the bypass pipe  35  is connected to the connecting pipe  10  through the water drain pipe  37 . However, in the case where the water drain pipe is arranged in a different position, as best seen in  FIG. 3 , the bypass pipe  35  can be directly connected to the joint  36 . Further, the inner diameter of the bypass pipe  35  is not limited to the reduced diameter that is reduced only at the downstream end of the bypass pipe  35  corresponding to the minimum amount of water. As long as water may not remain in the bypass pipe  35 , the bypass pipe  35  may have the same inner diameter throughout its length corresponding to the minimum amount of water. 
     Other modifications may be available. For example, a normally closed solenoid valve may be provided in the bypass pipe and when the second heat exchanger  7  freezes, the controller  32  may open the solenoid valve. In this instance, it is possible to determine that the second heat exchanger  7  is in a frozen state if the flow rate detected at the water filling pipe  28  is zero or lower than a predetermined value after the automatic bath fill button on the remote controller  33  or  34  is pressed, and then the controller  32  opens the solenoid valve. When the detected flow rate exceeds the predetermined value, the controller  32  may determine that second heat exchanger  7  is defrosted and the solenoid valve is closed. 
     Other than the above, the bath/hot water supply system  1  according to the above embodiment includes the hot water supply circuit  2  provided with a plurality of hot water supply burners  5 . However, the hot water supply circuit may have only one hot water supply burner  5 . Of course, the present invention is not limited to the bath/hot water supply system  1  as described above including the bath circuit  3  in addition to the hot water supply circuit  2 , and may be applicable to a single water heating apparatus without a bath circuit  3 .