Exhaust gas heat recovery unit and exhaust gas heat recovery method

Abstract

An exhaust gas heat recovery unit includes a plurality of heat pipes ( 23 ) each of which includes a heater-side heat pipe portion ( 23 a ) positioned on the side of a heater ( 21 ) which heats pure water using heat of exhaust gas discharged from an engine and an evaporator-side heat pipe portion ( 23 b ) positioned on the side of an evaporator ( 22 ) which evaporates the pure water, heated at the heater-side heat pipe portion ( 23 a ), using the heat of the exhaust gas. The pitch of heater-side corrugated fins ( 28 a ) on the outer periphery of the heater-side heat pipe portion ( 23 a ) is set to substantially half the pitch of evaporator-side corrugated fins ( 28 b ) on the outer periphery of the evaporator-side heat pipe portion ( 23 b ).

1 . An exhaust gas heat recovery unit, comprising:
 a heat pipe that is provided in an exhaust pipe, and that includes a heater portion which heats a working fluid using heat of exhaust gas discharged from an engine and an evaporator portion which evaporates the working fluid, heated by the heater portion, using the heat of the exhaust gas, wherein   fins that receive a predetermined amount of heat of the exhaust gas are provided on an outer periphery of the heat pipe, and   a pitch of the fins on the outer periphery of the heater portion of the heat pipe is set to a pitch smaller than a pitch of the fins on the outer periphery of the evaporator portion of the heat pipe.   
     
     
         2 . The exhaust gas heat recovery unit according to  claim 1 , wherein the working fluid flows in the heat pipe. 
     
     
         3 . The exhaust gas heat recovery unit according to  claim 1 , wherein the pitch of the fins on the outer periphery of the heater portion of the heat pipe is set in such a manner that an amount of heat that is received from the exhaust gas within a predetermined period from cold starting of the engine is larger than an amount of heat that is required to increase a temperature of the working fluid in the heater portion of the heat pipe, which is exposed to an ambient temperature of equal to or lower than a freezing temperature of the working fluid at cold starting time of the engine, to a temperature equal to or higher than the freezing temperature. 
     
     
         4 . The exhaust gas heat recovery unit according to  claim 1 , further comprising:
 a condenser that is provided outside the exhaust pipe, and that condenses the working fluid, which has been evaporated at the evaporator portion of the heat pipe, by conducting heat exchange with an externally supplied heat transfer fluid; and   a fluid passage which is formed outside the exhaust pipe, which extends near the heater portion, and through which the working fluid condensed by the condenser is discharged from the condenser, wherein   the pitch of the fins on the outer periphery of the heater portion of the heat pipe is set in such a manner that an amount of heat that is received from the exhaust gas within a predetermined period from cold starting of the engine is larger than a sum of an amount of heat that is required to increase a temperature of the working fluid in the fluid passage, which is exposed to an ambient temperature of equal to or lower than a freezing temperature of the working fluid at cold starting time of the engine, to a temperature equal to or higher than the freezing temperature and an amount of heat that is required to increase a temperature of the working fluid in the heater portion of the heat pipe, which is exposed to an ambient temperature of equal to or lower than the freezing temperature of the working fluid at cold starting time of the engine, to a temperature equal to or higher than the freezing temperature.   
     
     
         5 . The exhaust gas heat recovery unit according to  claim 3 , wherein a length of the predetermined period is from 100 seconds to 200 seconds. 
     
     
         6 . The exhaust gas heat recovery unit according to  claim 3 , wherein a length of the predetermined period is 135 seconds. 
     
     
         7 . The exhaust gas heat recovery unit according to  claim 1 , wherein the pitch of the fins on the outer periphery of the evaporator portion of the heat pipe is set to a pitch large enough to offset an increase in exhaust gas flow resistance, which is caused due to the small pitch of the fins on the outer periphery of the heater portion of the heat pipe. 
     
     
         8 . The exhaust gas heat recovery unit according to  claim 1 , wherein the pitch of the fins on the outer periphery of the heater portion of the heat pipe is substantially half the pitch of the fins on the outer periphery of the evaporator portion of the heat pipe. 
     
     
         9 . The exhaust gas heat recovery unit according to  claim 1 , wherein a fluid level of the working fluid in a vertical direction at engine starting time is a boundary between the heater portion and the evaporator portion. 
     
     
         10 . The exhaust gas heat recovery unit according to  claim 1 , wherein the working fluid is pure water. 
     
     
         11 . The exhaust gas heat recovery unit according to  claim 4 , wherein the fluid passage extends near the heater portion while spirally curving. 
     
     
         12 . The exhaust gas heat recovery unit according to  claim 4 , wherein the working fluid passage linearly extends near the heater portion. 
     
     
         13 . (canceled) 
     
     
         14 . The exhaust gas heat recovery unit according to  claim 4 , wherein a length of the predetermined period is from 100 seconds to 200 seconds. 
     
     
         15 . The exhaust gas heat recovery unit according to  claim 4 , wherein a length of the predetermined period is 135 seconds.