In the calculation of thermal performance of heat exchanger, the mean temperature differences are often adopted. This study is carried out to investigate the logarithmic and arithmetic mean temperature differences. In the experiments, two calculating methods are adopted, which are measuring wall temperature (MWT) and logarithmic-mean temperature difference (LMTD). The comparisons between these two calculating methods are conducted on the basis of experimental and theoretical analysis. It is shown that the results gained by these two methods are different when calculating the thermal performance of heat transfer. The differences come mainly from the difference between LMTD and the arithmetic-mean temperature difference (AMTD). Experimental results show that there are large difference between LMTD and MWT in small Reynolds number region, whereas in turbulent flow area, the results gained by LMTD and MWT have small difference. It is shown that in small Reynolds number region, adopting LMTD may induce error.

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