Symbols
Physical symbols
Fundamental physical variables
Symbol 
Designation 
Unit 
n 
Quantity 

l 
Length 
mm; m 
r 
Radius 
mm 
d 
Diameter 
mm 
s 
Wall thickness 
mm 
u 
Velocity 
m/s 
A 
Area 
mm²; m² 
V 
Volume 
m³ 
V̇ 
Volume flow rate 
m³/h 
V̇_{n} 
Standard volume flow rate 
m_{n}³/h 
m 
Mass 
kg; t 
ṁ 
Mass flow rate 
kg/h; t/h 
Q 
Heat quantity 
kJ; kWh; MJ; MWh 
Q̇ 
Heat flow rate 
kW; MW 
t 
Time 
s; h; a 
T 
Temperature 
K; °C 
p 
Excess pressure 
Pa; mbar; bar 
p_{abs} 
Absolute pressure 
bara 
ρ 
Density 
kg/m³ 
ρ_{n} 
Standard density 
kg/m_{n}³ 
h 
Specific enthalpy 
kJ/kg 
r 
Specific evaporation enthalpy 
kJ/kg 
c_{p} 
Specific heat capacity 
kJ/kgK 
Combustion
Symbol 
Designation 
Unit 
H_{i} 
Net calorific value 
kWh/kg; kWh/m_{n}³ 
H_{s} 
Gross calorific value 
kWh/kg; kWh/m_{n}³ 
α 
Flue gas condensation ratio 
kg/kg 
λ 
Excess air 
kg/kg 
Efficiency
Symbol 
Designation 
Unit 
ƞ 
Efficiency 
% 
q_{A} 
Flue gas loss relative to H_{i} 
% 
η_{a} 
Annual degree of utilisation 
% 
Heat transfer
Symbol 
Designation 
Unit 
λ 
Heat conductivity 
W/mK 
α 
Heat transfer coefficient 
W/m²K 
k 
Heat transition coefficient 
W/m²K 
Fluid mechanics
Symbol 
Designation 
ζ 
Pressure loss coefficient 
λ 
Pipe friction coefficient 
Other
Symbol 
Designation 
Unit 
α 
Coefficient of thermal expansion 
mm/(m ⋅ 100K) 
L 
Electrical conductivity 
μS/cm 