Heat and Mass Transfer
The most commonly used method for the design of duct size is the

Dual or double method
Equal friction method
Static regains method
Velocity reduction method

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Heat and Mass Transfer
The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)

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Heat and Mass Transfer
According to Prevost theory of heat exchange

Heat transfer by radiation requires no medium
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
It is impossible to transfer heat from low temperature source to t high temperature source
All bodies above absolute zero emit radiation

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