Heat and Mass Transfer
Heat flows from one body to other when they have

Different temperatures
Different specific heat
Different atomic structure
Different heat contents

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

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Heat and Mass Transfer
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)

k. (dx/dT)
k. (dT/dx)
k. A. (dT/dx)
k. A. (dx/dT)

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