Heat and Mass Transfer
The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

Move actually
Affect the intervening medium
Do not move actually
Does not affect the intervening medium

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Heat and Mass Transfer
In counter flow heat exchangers

Both the fluids at inlet are in their hottest state
Both the fluids at exit are in their hottest state
Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
One fluid is in hottest state and other in coldest state at inlet

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

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