Arithmetic mean temperature drop

Individual heat-transfer coefficient, W/m -°C or Btu/ft -h-°F h, based on arithmetic mean temperature drop average over inside of tube h , for outside of tube or particle local value h i, at trailing edge of plate h, for fully developed flow in long pipes hg, local value outside tube... 

Temperature drop, °C or °F AT), from inner wall of pipe to fluid A7, from outside surface to fluid distant from wall A7, arithmetic mean temperature drop ATj, logarithmic mean temperature drop Parameter in Eq. (12.26), (3n -f l)/4n ... 

In laminar flow the average coefficient h depends strongly on heated length. The average (arithmetic mean) temperature drop AT is used in the equation to calculate the heat-transfer rate q. 

The rate of temperature drop of a fluid as it flows along the length of a heat exchanger is not constant. In order to take account of this nonlinear relationship, the logarithmic mean temperature difference (EMTD) is used. If the inlet and outlet temperatures do not differ widely, an arithmetic mean can be used, because the relationship is considered to be linear. 

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