HEAT TRANSFER | ||

With the cooperation and under supervision of our past president Ir. H. Ghys | ||

In practice heat transfer is the sum of the heat transfer that takes place through conductivity, convection and radiation. The relation below applies: | ||

q = k x A x∆T x t | ||

q : The quantity of heat ( j )
k : total heat transfer coeff. ( W/m ^{2} x K) |
A : area (m^{2})
∆T = temperature difference |
t : time ( s ) |

Heat transfer frequently occurs between two bodies,
separated by a wall.
For such a (clean) flat wall the relation below applies: |
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1/k = 1/α_{1} + d/λ + 1/α_{2} |
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k : total heat transfer coeff. ( W/m^{2}
x K)
α = heat transfer coefficient on respective sides of the wall ( W/m ^{2} x K ) |
d = thickness of the wall ( m ) | λ = coefficient of thermal conductivity for the wall (W/m x K) |

The transferred quantity of heat in a heat exchanger, is at each point a function of the prevailing heat difference and the total heat transfer coefficient. Applicable to the entire heat transfer surface is: | ||

Q = k x A x LMTD | ||

Q = tranferred quantity oh heat ( Watt )
k = total heat transfer coeff. (W/m ^{2} x K) |
A = heat transferring surface (m^{2}) |
LMTD = logarithmic mean temperature diff. ( K ) |

The logarithmic mean temperature difference is defined as the relation between the temperature difference at the heat exchanger's two connection sides according to the expression: | ||

LMTD = (Δ_{1}- Δ_{2 }/ ( In Δ_{1} / Δ_{2 }) |
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LMTD = logarithmic mean tempt. diff.(K) |
Δ_{1} = the tempt. difference ( K ) |
Δ_{2 = the temperature difference ( °K )} |