因为平均流速不是由同一时间各测点流速取平均值得出的，而是由一段时间内各点的流速取平均值得到的。
The average flow velocity can not get from the average flow velocity of every monitoring point in the same time, but by a period of time.
虽然板式换热器各板间流速不等，但在换热和流体阻力计算时，仍以平均流速进行计算。
Although velocity varies among different plate, plate heat exchanger in the change of heat and fluid resistance calculation, is still at an average velocity to calculate.
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