Heat exchanger

Abstract

Provided is a heat exchanger capable of suppressing formation and adhesion of scale while achieving a higher heat transfer rate, and having a longer life. A heat exchanger 10 includes throttle passages 37 and 47 having smaller flow passage cross-sectional areas than another portion between an upstream space 25 a and a downstream space 25 b, and a plate heater 20 configured in such a manner that a heat generation density is lower in a portion closer to a water outlet 25 b than in a portion closer to a water inlet 25 a. This makes it possible to suppress a temperature from rising to a high temperature at which local boiling will take place, even in a boundary layer between a portion of the plate heater 20 which is closer to the water outlet 25 b, where the temperature of the washing water tends to be high, and washing water contacting the portion of the plate heater 20 which is closer to the water outlet 25 b, while improving the heat transfer rate. As a result, generation of air bubbles is suppressed, and the generated air bubbles are guided quickly to the water outlet 25 b. Thus, a heat exchanger which can prevent formation of scale and adhesion of the scale onto the plate heater 20 and has a longer like is provided.

1 . A heat exchanger comprising:
 a casing having a water inlet and a water outlet;   a heater disposed inside the casing and having surfaces serving as heat transfer surfaces; and   a passage space provided inside the casing, to guide a fluid which has flowed into the heat exchanger through the water inlet to the water outlet, the fluid being guided to the water outlet while exchanging heat with the heat transfer surfaces of the heater;   characterized in that the heater is configured in such a manner that a heat generation density in a portion closer to the water outlet is lower than a heat generation density in a portion closer to the water inlet.   
     
     
         2 . The heat exchanger according to  claim 1 ,
 wherein the heater is a plate heater placed to be oriented in a direction substantially parallel to a vertical direction, and has an obverse main surface and a reverse main surface which are the heat transfer surfaces; and   the passage space extends from the water inlet at a lower portion to the water outlet at an upper portion along the obverse and reverse heat transfer surfaces of the plate heater.   
     
     
         3 . The heat exchanger according to  claim 1 ,
 wherein the heater is a ceramic heater including a ceramic base body, a heat generation resistive element formed by printing a pattern of a resistive element on the ceramic base body, and an electrode, and the printed pattern has a line width greater in a portion of the heater which is closer to the water outlet than in a portion of the heater which is closer to the water inlet.   
     
     
         4 . The heat exchanger according to  claim 1 ,
 wherein the heater is a ceramic heater including a ceramic base body, a heat generation resistive element formed by printing a pattern of a resistive element on the ceramic base body, and an electrode, and the printed pattern has a line interval greater in a portion of the heater which is closer to the water outlet than in a portion of the heater which is closer to the water inlet.   
     
     
         5 . The heat exchanger according to  claim 2 ,
 wherein the passage space includes an upstream space including an opening of the water inlet and a downstream space including an opening of the water outlet, and a throttle passage having a smaller flow passage cross-sectional area than another space is provided between the upstream space and the downstream space.   
     
     
         6 . The heat exchanger according to  claim 5 ,
 wherein the passage space at one heat transfer surface side of the plate heater and the passage space at the other heat transfer surface side of the plate heater are symmetric.   
     
     
         7 . The heat exchanger according to  claim 5 ,
 wherein the downstream space has a greater volume than the upstream space.   
     
     
         8 . The heat exchanger according to  claim 5 ,
 wherein the throttle passage has a horizontal throttle passage extending substantially horizontally from a portion of the heat exchanger which is closer to the water inlet, to flow the fluid in an upward direction toward the downstream space.   
     
     
         9 . The heat exchanger according to  claim 8 ,
 wherein the throttle passage has a vertical throttle passage extending substantially vertically upward, from one end portion of the horizontal throttle passage which is farther from the water inlet than the other end portion of the horizontal throttle passage, to flow the fluid horizontally toward the downstream space.   
     
     
         10 . The heat exchanger according to  claim 5 ,
 wherein the throttle passage has a slit shape; and   the throttle passage has an increased-width portion having a greater opening width than another portion.   
     
     
         11 . The heat exchanger according to  claim 5 ,
 wherein an agitating wall for agitating the fluid is provided in the downstream space to extend in a substantially upward and downward direction along the plate heater; and   the agitating wall is corrugated to have a horizontal amplitude.   
     
     
         12 . The heat exchanger according to  claim 5 , wherein a buffer wall extending substantially horizontally along the plate heater is provided in the downstream space. 
     
     
         13 . The heat exchanger according to  claim 12 ,
 wherein the buffer wall has a plurality of buffer walls arranged in the upward and downward direction; and   the buffer walls are provided with remaining portions formed by cutting portions of the buffer walls such that positions of the remaining portions are different between adjacent upper and lower buffer walls when viewed from above.   
     
     
         14 . The heat exchanger according to  claim 5 , comprising:
 a pair of passage forming members disposed to sandwich the plate heater;   wherein each of the passage forming members includes a base portion of a plate shape disposed to face the plate heater, and a rib protruding from a surface of the base portion which faces the plate heater; and   the slit-shape throttle passage is defined by the rib and the plate heater which faces the rib, and is provided between the rib and the plate heater which faces the rib.   
     
     
         15 . The heat exchanger according to  claim 1 ,
 wherein the heater is a plate heater placed to be oriented in a direction substantially parallel to a vertical direction, and has an obverse main surface and a reverse main surface which are the heat transfer surfaces; and   the passage space is formed in a sinuous passage extending from the water inlet at a lower portion to the water outlet at an upper portion along the obverse and reverse heat transfer surfaces of the plate heater.   
     
     
         16 . The heat exchanger according to  claim 15 ,
 wherein the sinuous passage is defined by a plurality of wall portions arranged vertically to extend substantially horizontally;   the sinuous passage has, in a range from the water inlet to the water outlet, a passage for guiding the fluid in one direction in a substantially horizontal direction, and a passage for guiding the fluid in an opposite direction in the substantially horizontal direction such that the passages are arranged alternately from a lower side to an upper side; and   a bypass passage is provided in an upward and downward direction on a portion of each of the wall portions in a longitudinal direction thereof to provide communication between adjacent upper and lower passages.   
     
     
         17 . The heat exchanger according to  claim 16 ,
 wherein the sinuous passage at one heat transfer surface side of the plate heater and the sinuous passage at the other heat transfer surface side of the plate heater are symmetric; and   the bypass passage at one heat transfer surface side of the plate heater and the bypass passage at the other heat transfer surface side of the plate heater are symmetric.   
     
     
         18 . The heat exchanger according to  claim 16 ,
 wherein bypass passages formed on the plurality of wall portions substantially conform in position to each other when viewed from above.   
     
     
         19 . The heat exchanger according to  claim 16 , comprising:
 a pair of passage forming members disposed to sandwich the plate heater;   wherein each of the passage forming members includes a base portion of a plate shape disposed to face the plate heater, and a plurality of ribs protruding from a surface of the base portion which faces the plate heater to form the wall portions; and   a remaining portion formed by cutting a portion of each of the plurality of ribs is provided on a portion of each of the plurality of ribs in a longitudinal direction thereof such that a tip end of the rib is dented with respect to another portion to form the bypass passage.   
     
     
         20 . The heat exchanger according to  claim 19 ,
 wherein the remaining portion of the rib has a tapered shape when viewed from above such that a cut portion width of the remaining portion decreases as a cut portion depth of the remaining portion increases.   
     
     
         21 . The heat exchanger according to  claim 19 ,
 wherein the remaining portion of the rib has a circular-arc shape when viewed from above such that a cut portion depth of a center portion of a cut portion width is greater.   
     
     
         22 . The heat exchanger according to  claim 19 ,
 wherein the remaining portion of the rib provided at a relatively upper side has a greater cut portion width than the remaining portion of the rib provided at a relatively lower side.   
     
     
         23 . The heat exchanger according to  claim 19 ,
 wherein the remaining portion is not provided on the rib provided at a relatively lower side; and   the remaining portion is provided on the rib provided at a relatively upper side.