Fundamental physical variables
Symbol
Designation
Unit
n
Quantity
l
Length
mm; m
r
Radius
mm
d
Diameter
s
Wall thickness
u
Velocity
m/s
A
Area
mm²; m²
V
Volume
m³
V̇
Volume flow rate
m³/h
V̇n
Standard volume flow rate
mn³/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
pabs
Absolute pressure
bara
ρ
Density
kg/m³
ρn
Standard density
kg/mn³
h
Specific enthalpy
kJ/kg
Specific evaporation enthalpy
cp
Specific heat capacity
kJ/kgK
Combustion
Hi
Net calorific value
kWh/kg; kWh/mn³
Hs
Gross calorific value
α
Flue gas condensation ratio
kg/kg
λ
Excess air
Efficiency
ƞ
%
qA
Flue gas loss relative to Hi
ηa
Annual degree of utilisation
Heat transfer
Heat conductivity
W/mK
Heat transfer coefficient
W/m²K
k
Heat transition coefficient
Fluid mechanics
ζ
Pressure loss coefficient
Pipe friction coefficient
Other
Coefficient of thermal expansion
mm/(m ⋅ 100K)
L
Electrical conductivity
μS/cm
