Information

arranged in the form of a spiral
permit much smaller surface loads
since the heat dissipating wire
surface is strongly reduced as
compared with freely stretched
wires.
In case of a bobbin being densely
wound with resistance wire, the
surface of this structure can be
taken as the reference quantity for
the value “Watt per cm²”. This
means that the diameter of the
wound bobbin can be taken as
“wire diameter”. The result is that
for such a structure at a given
surface load in W/cm² the surface
temperature will be considerably
higher than for a single wire.
Fig. 3 shows the possible current-
carrying capacity in Watt/cm² for
different temperatures dependent
on the “wire” diameter. Bobbins
should be referred to in this
diagram by the bobbin diameter.
Since the current-carrying capacity
in Watt per cm² is given, this
diagram applies to all Isabellenhütte
alloys. The interrelationship
between current loads (Amps) and
resulting temperature for an
ISOTAN
®
wire of 0.5mm diam. can
be seen from Fig. 4. It must be kept
in mind, however, that for this type
of presentation the curves for
materials with different resistivities
are also different, unlike the
previous figures.
page 6
As shown in Fig. 1, a given surface
load – expressed in Watt per cm² -
causes equal wire temperatures for
every alloy. Therefore the current
load values for equal wire
diameters can be converted by the
following formula:
Fig. 2
P = constant, thus i
1
² ⋅ R
1
= i
2
² ⋅ R
2
Thus : i
1
= i
2
⋅
1
2
R
R
and : : i
1
= i
2
⋅
1
2
ρ
ρ
The following tables show, with
ISOTAN
®
as an example, the
geometrical data between 0.02 and
6.3mm diam. as well as the current-
carrying capacity values in Amps
for 40/60/80/100/200/300/400/500
and 600°C. In accordance with the
above formula, the current values
are converted in accordance with:
x
ISOTANx
ii
ρ
49.0
⋅=
where i
x
refers to the current for a
wire of an alloy with the resistivity
ρ.
It must be kept in mind that for ρ the
values valid for
ρ the respective
temperature must be used (see the
tables on pages 7 + 8).
The current-carrying capacity tables
all refer to bare wire; due to better
heat dissipation, oxidized wires
(only possible for the alloys ISA-
CHROM60
®
, ISA-CHROM80
®
and
ISOTAN
®
) can withstand a load
increase of up to 20%, expressed in
Watt per cm², mainly at higher
temperatures.
Fig. 3
The current-carrying capacities of
enamelled wires are about the
same as those of bare wires are
about the same as those of bare
wires. The heat insulation effect of
the enamel is compensated by the
increase of the effective diameter
and good heat dissipation
properties. Silk-covered wires
exhibit strongly varying loading
capacities, depending on the kind
of manufacturing process and type;
the respective value must be
determined individually.
Fig. 4
Fig. 4: Current-carrying capacity of ISOTAN
®
wire of
0.5mm diam. in dependence on overtemperature of the
wire against air at 20°C (100°C overtemperature
correspond to a real temperature of 120°C).
Comparison of the share of radiation and
convection effects in heat dissipation of
resistance wires in air at 20°C. In the dark
boundary area heat dissipation by convection
about equals that by radiation. On the right of
the boundary area radiation prevails; on the left
convection has the larger share.