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2. Overall Heat Transfer

A overall heat-transfer is named the process of thermal energy flow from one fluid
to other through the one layer or multi-layered wall of free-form. The heat-transfer is a

complicate process of heat transference. It includes a heat conduction, convection and
radiation. The typical examples of a overall heat-transfer is a heat flow from hot water

to room air through the walls of heater batteries of the central heating, transmission of

heat from room air to outside air through the walls.

A overall heat-transfer from inside air to outside air there is a complex process:

- inside → convection;

- in the wall construction → conduction ;

- outside → also convection;

The heat energy flow with overall heat transfer coefficient:

4
I
Q  k  dF (T  T 2 4 ) (W) (2.1)
1
2
 k - the overall heat transfer coefficient W/(m •K)
2
 dF – surface of the wall, m

 T 1 – temperature of the inside fluid, K or °C

 T 2 - temperature of the outside fluid, K or °C

The overall heat transfer coefficient , in thermodynamics and in mechanics, is the

proportionality coefficient between the heat flow and the thermodynamic driving force

ΔT for the heat flow

q W
k  , (2.2)
 T m 2 K

2
where: q - heat flow, W/m ;
ΔT - difference in temperature between the solid surface and surrounding fluid

area, K.

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