Roxul Industrial Insulation Process - Manual (Page 123)

2.3 Bases for thermal calculations
2. Theory
The following sections outline a number of definitions
and approaches to heat transfer calculations.
Detailed calculation processes are outlined in the
ASTM C680 and VDI 2055, and the EN 12241
standards, as well as in various standards, such as
ASTM C680 and BS 5970. The calculation bases are
similar in all the standards. In Europe, the VDI 2055 is
the most widely used and accepted definitions and
calculation basis. In North America ASTM standards
C168 and C680, as well as ASHRAE methods, are the
most widely used.
The calculation of multiple-layer insulation
constructions is to some extent quite complex, as
iterative calculation processes need to be carried out.
The procedures outlined in the following sections are
only suitable to obtain an approximate calculation of
insulation constructions. The thermo-technical
engineering program "ROCKASSIST" (coming soon)
can be used for detailed calculations. Another tool
available is NAIMA's "3E Plus
® Insulation Thickness
Computer Program".
For technical assistance and the most up-to-date
data for heat transfer and thermal modelling contact
the ROXUL
® Technical Services Team.
2.3.1 Heat Transfer – ASTM C168
and C680 (North American basis
and terms)
During heat transfer, thermal energy is transported
as a result of a temperature difference. Heat transfer
occurs through conduction, convection or radiation.
Thermal conduction is the transport of heat from
one molecule to another, as a result of a difference
in temperature. In solid substances, the average
distances between individual molecules remain the
same, but in liquids and gases it increases with
temperature.
Thermal convection is the transport of heat in
liquids and gases through flow processes. A
distinction is drawn between free convection
(natural convection), in which the movement occurs
as a result of variations in density, and forced
convection where the flow is generated by external
forces such as wind or blowers.
Thermal radiation is the transfer of heat by
electromagnetic radiation to and from exposed
surfaces separated by radiation permeable media,
such as air.
Terms
Heat (Energy)
Heat is the quantity of thermal energy that is supplied
to or dissipates from a body. The unit of heat is BTU
(Joule).
Heat flow rate, Q
The heat flow rate, Q, is the quantity of heat
transferred to or from a system in unit time. The unit
of heat flow is BTU/hr (Watt).
Heat flux, q
Heat flux, q, is the heat flow rate through a surface of
unit area perpendicular to the direction of heat flow.
The units are expressed in BTU/hr.ft
2 (W/m 2 ) for
surfaces or BTU/hr.ft (W/m) for pipes (referring to the
surface of the insulation).
Mean Specific Heat
The quantity of heat required to change the
temperature of a unit mass of a substance one
degree. The units are expressed as BTU/lb.°F
(J/kg.K).
Thermal Capacity (heat capacity)
The quantity of heat required to change the
temperature of the body one degree. The unit of heat
capacity is BTU/°F (J/K).
Conductivity
Thermal conductivity is a material property defining
heat flow through a homogeneous material, “λ” (or
“k“). It indicates the heat flow rate “Q” through unit
area of material “A” induced by unit temperature
gradient “∆T / L” in a direction perpendicular to that
unit area (Heat-Flux per unit temperature difference
120