Comparison of the Types of Heat Exchangers
Each of the three types of heat exchangers has advantages and disadvantages. But of the three, the counter flow heat exchanger design is the most efficient when comparing heat transfer rate per unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that the average T (difference in temperature) between the two fluids over the length of the heat exchanger is maximized, as shown in Figure 4. Therefore the log mean temperature for a counter flow heat exchanger is larger than the log mean temperature for a similar parallel or cross flow heat exchanger. (See the Thermodynamics, Heat Transfer, and Fluid Flow Fundamentals Handbook for a review of log mean temperature). This can be seen by comparing the graphs in Figure 3, Figure 4, and Figure 5. The following exercise demonstrates how the higher log mean temperature of the counter flow heat exchanger results in a larger heat transfer rate. The log mean temperature for a heat exchanger is calculated using the following equation.
Heat transfer in a heat exchanger is by conduction and convection. The rate of heat transfer, “Q”, in a heat exchanger is calculated using the following equation.
Consider the following example of a heat exchanger operated under identical conditions as a counter flow and then a parallel flow heat exchanger.
Inserting the above values into heat transfer Equation (2-2) for the counter flow heat exchanger yields the following result.
Inserting the above values into the heat transfer Equation (2-2) for parallel flow heat exchanger yields the following result.
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.