A heat exchanger ( 50 ) is provided for transferring heat between first and second fluids ( 52 ) and ( 54 ) having a maximum operating mass flow rate through the heat exchanger ( 50 ) and mass flow rates that are substantially proportional to each other. The heat exchanger ( 50 ) provides essentially constant outlet temperatures for the first and second fluids ( 52,54 ) for all of the flow rates within the operating spectrum of the heat exchanger ( 50 ) without the use of an active control system. The heat exchanger ( 50 ) is of particular use in the fuel processing system ( 36 ) of proton exchange membrane type fuel cell systems.
1. A method of operating a recuperative heat exchanger that transfers heat between a first fluid and a second fluid having a maximum operating mass flow rate through the heat exchanger, the first and second fluids having mass flow rates that are substantially proportional to each other, the method comprising the steps of:
a) flowing a first fluid through a first pass of a first flow path of the recuperative heat exchanger;
b) flowing a second fluid through a second pass of a second flow path of the recuperative heat exchanger, the second pass of the second flow path adjacent the first pass of the first flow path and the temperatures of the first and second fluids closely approaching or attaining a common temperature at the exit of their respective passes at the maximum operating mass flow rate;
c) flowing the first fluid through a second pass of the first flow path of the recuperative heat exchanger after the first fluid has flown through the first pass of the first flow path; and
d) flowing the second fluid through a first pass of the second flow path of the recuperative heat exchanger prior to the second fluid flowing through the second pass of the second flow path, the first pass of the second flow path adjacent the second pass of the first flow path and the temperatures of the first and second fluids closely approaching or attaining a common temperature at the exit of their respective passes at the maximum operating mass flow rate.
2. The method of claim 1 further comprising the step of:
e) flowing the first and second fluids through additional passes of their respective first and second flow paths.
3. A method of operating a recuperative heat exchanger that transfers heat between a first fluid and a second fluid having a maximum operating mass flow rate through the heat exchanger, the first and second fluids having mass flow rates that are substantially proportional to each other, the method comprising the steps of:
a) flowing a first fluid through N passes of a first flow path of the recuperative heat exchanger: and
b) flowing a second fluid through N passes of a second flow path of the recuperative heat exchanger, with the first and second passes being arranged in N adjacent pairs, the first and second fluids having a concurrent flow relationship through each of the N adjacent pairs, the temperature of the first and second fluids closely approaching or obtaining a common temperature at the exits of their respective passes in each of the N adjacent pairs at the maximum operating mass flow rate, and the first fluid first encountering a first pair of the N adjacent pairs and flowing consecutively through the adjacent pairs to the Nth adjacent pair, and the second fluid first encountering the Nth adjacent pair and then flowing consecutively through the adjacent pairs to the first adjacent pair in an order opposite from the first fluid.
4. The method of claim 3 further comprising the step of selecting the number N of adjacent pairs as a function of the desired temperature ranges between respective inlet and outlet temperatures of the first and second fluids as they enter and exit the recuperative heat exchanger.