Est 1967
95 Second Street, Booysens Reserve, Johannesburg
South Africa
PO Box 38653, Booysens 2016
Tel 27-11-835 2127
Fax 27-11-496 1263
E-mail vm@netdial.co.za
Heat Loss Calculation Principles
Factors Affecting Heat Transfer and Thermal Performance
Factors affecting heat transfer and thermal performance through fibrous materials occur in a combination of the following three mechanisms:
Factors affecting heat transfer and thermal performance through fibrous materials occur in a combination of the following three mechanisms:
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CONDUCTION - through the fibres and across the air spaces
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RADIATION
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CONVECTION
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The physical characteristics of the mineral wool fibres (Rockwool or Glasswool) and their orientation in the insulation material will affect the thermal performance. Thus density, fibre diameter, fibre orientation and shot content, all influence the overall conductivity. In general, the thermal conductivity will increase with temperature, as the component heat transfer mechanisms increase, but the rate of increase and the final value at any temperature will depend on the density and the quality of the fibre in the insulating material.
Symbols
Q Heat loss through the insulation - per square metre (W/M²)
Q' Heat loss through the insulation - per linear metre of pipe run (W/m)
th Hot face temperature, i.e. temperature of vessel/pipe (ºC)
tc Cold face temperature, i.e. temperature of outer surface of insulation (ºC)
ta Ambient temperature of still air (ºC)
tj Temperature at junction/inter-face of two layers of insulation (ºC)
t1 Temperature of lst interface (between lst and 2nd layer) of insulation (ºC)
t2 Temperature of 2nd interface (between 2nd and 3rd layer) of insulation (ºC)
tn Temperature of nth inter-face (between nth and n+l layer) of insulation (ºC)
x Overall thickness of insulation (m=01001 mm) (m)
xl Thickness of lst (inner) layer of insulation (m=0,001 mm) (m)
x2 Thickness of 2nd layer of insulation (m=0,001 mm) (m)
xn Thickness of nth layer of insulation (m=0,001 mm) (m)
k Thermal conductivity of insulation (W/mK)
kl Thermal conductivity of lst (inner) layer of insulation (W/mK)
k2 Thermal conductivity of 2nd layer of insulation (W/mK)
kn Thermal conductivity of nth layer of insulation (W/mK)
R Thermal resistance of insulation (m²/KW)
Rs Thermal resistance of outer surface of insulation (m²/KW)
dp Outer diameter of pipe (m=0,001 mm) (m)
ds Outer diameter of total insulation (m=0,001 mm) (m)
d1 Outer diameter of lst (inner) layer of insulation (m=0,001 mm) (m)
d2 Outer diameter of 2nd layer of insulation (m=0,001 mm) (m)
dn Outer diameter of nth layer of insulation (m=0,001 mm) (m)
f Surface heat transfer coefficient (W/m²K)
Q' Heat loss through the insulation - per linear metre of pipe run (W/m)
th Hot face temperature, i.e. temperature of vessel/pipe (ºC)
tc Cold face temperature, i.e. temperature of outer surface of insulation (ºC)
ta Ambient temperature of still air (ºC)
tj Temperature at junction/inter-face of two layers of insulation (ºC)
t1 Temperature of lst interface (between lst and 2nd layer) of insulation (ºC)
t2 Temperature of 2nd interface (between 2nd and 3rd layer) of insulation (ºC)
tn Temperature of nth inter-face (between nth and n+l layer) of insulation (ºC)
x Overall thickness of insulation (m=01001 mm) (m)
xl Thickness of lst (inner) layer of insulation (m=0,001 mm) (m)
x2 Thickness of 2nd layer of insulation (m=0,001 mm) (m)
xn Thickness of nth layer of insulation (m=0,001 mm) (m)
k Thermal conductivity of insulation (W/mK)
kl Thermal conductivity of lst (inner) layer of insulation (W/mK)
k2 Thermal conductivity of 2nd layer of insulation (W/mK)
kn Thermal conductivity of nth layer of insulation (W/mK)
R Thermal resistance of insulation (m²/KW)
Rs Thermal resistance of outer surface of insulation (m²/KW)
dp Outer diameter of pipe (m=0,001 mm) (m)
ds Outer diameter of total insulation (m=0,001 mm) (m)
d1 Outer diameter of lst (inner) layer of insulation (m=0,001 mm) (m)
d2 Outer diameter of 2nd layer of insulation (m=0,001 mm) (m)
dn Outer diameter of nth layer of insulation (m=0,001 mm) (m)
f Surface heat transfer coefficient (W/m²K)