Environmental Engineering Reference
In-Depth Information
Energy in form of heat Q is linked with the heat flow Q :
(3.1)
Every temperature change
Q. The change in
heat can be calculated with the specific heat capacity c and the mass m of the
affected material:
∆ϑ
also causes a heat change
(3.2)
Some confusion can occur owing to the use of different temperature scales.
The Fahrenheit scale is generally not used in scientific work. However, the
coexistence of the temperature
given in degrees Celsius and the absolute
temperature T given in Kelvin is problematic. The conversion of Celsius to
Kelvin is given by:
ϑ
(3.3)
The formula for the conversion of Fahrenheit to Celsius and Kelvin can be
found in the appendix. The numerical value of the temperature difference
∆ϑ
in degrees Celsius (°C) is the same as the temperature difference
T in Kelvin
(K). For a correct balance of units, the temperature difference in the equation
above for the heat change should be given in Kelvin. The same goes for
equations that are introduced in the following section. However, since the
Celsius scale is more common than the Kelvin scale, the Celsius scale is
used for most of the temperature differences and equations of this chapter.
The heat flow Q , which causes the heat change for a constant heat capacity
c , is:
(3.4)
For heat capacities of various materials see Table 3.2.
Figure 3.1 shows a construction of n layers with a surface area A . On one
side there is a temperature
ϑ 2 . This temperature gradient
generates a heat flow through the layers given by:
ϑ 1 , on the other
(3.5)
This heat flow Q causes a heat increase at the side with the lower temperature
and a heat decrease on the other side until both sides have the same
temperature. If the heat content of one side is much bigger than the other, the
temperature change at the side with the high heat content can be neglected.
For instance, the heat content of the environment surrounding a building is
much higher than that inside the building. The heat flow through the building
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