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_n³/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_n³
h	Specific enthalpy	kJ/kg
r	Specific evaporation enthalpy	kJ/kg
c_p	Specific heat capacity	kJ/kgK

Combustion

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

Other

Symbol	Designation	Unit
α	Coefficient of thermal expansion	mm/(m ⋅ 100K)
L	Electrical conductivity	μS/cm