TYPES OF HEAT EXCHANGERS DOE-HDBK-1018/1-93 Heat Exchangers Parallel flow DT_lm  =                                            (200 80^oF)      (145  120^oF) ln                     (200 80^oF) (145 120^oF) 61^oF Inserting  the  above  values  into  heat  transfer  Equation  (2-2)  for  the  counter  flow  heat exchanger yields the following result. Q 70                BTU hr  ft² F (75ft²) (72 F) Q 3.8x10⁵               BTU hr Inserting  the  above  values  into  the  heat  transfer  Equation  (2-2)  for  parallel  flow  heat exchanger yields the following result. Q 70                BTU hr  ft² F (75ft²) (61 F) Q 3.2x10⁵               BTU hr The results demonstrate that given the same operating conditions, operating the same heat exchanger  in  a  counter  flow  manner  will  result  in  a  greater  heat  transfer  rate  than operating in parallel flow. In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow; they are usually a combination of the two or all three types of heat exchangers.   This is due to the fact that actual heat exchangers are more complex than the simple components shown in the idealized  figures  used  above  to  depict  each  type  of  heat  exchanger.     The  reason  for  the combination of the various types is to maximize the efficiency of the heat exchanger within the restrictions placed on the design.   That is, size, cost, weight, required efficiency, type of fluids, operating  pressures,  and  temperatures,  all  help  determine  the  complexity  of  a  specific  heat exchanger. One method that combines the characteristics of two or more heat exchangers and improves the performance of a heat exchanger is to have the two fluids pass each other several times within a single heat exchanger.    When a heat exchanger's fluids pass each other more than once, a heat exchanger is called a multi-pass heat exchanger.  If the fluids pass each other only once, the heat exchanger is called a single-pass  heat  exchanger.   See Figure 6 for an example of both types. Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more sets of "U" bends in the tubes.  The "U" bends allow the fluid to flow back and forth across the length of the heat exchanger.   A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger.   These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect. ME-02 Rev. 0 Page 8