Mineral
insulated heating cables (MIHC)
These are composed of a highly compressed mineral
insulant which locates one or two resistance wires
within an outer metal sheath of copper, cupro-nickel,
stainless steel or nickel-based alloy (Fig 2a).
Single wire cables have a termination at each end
and generally have to be formed into "hairpins~'
to allow the circuit connections to be made at only
one end of the pipe system.
They are often laid onto a pipeline in a triple
run and star- connected across a 415V, 3-phase supply.
Double wire cables allow supply connections to be
made at one end only,
Spiralling of MIHC around the pipework should be
avoided to minimise the possibility of induced currents
in the pipework.
The heat transfer efficiency can be less than that
of a flat-form heating tape because of the contact
limitations imposed by fitting one circular cross
section to another (unless a form of heat transfer
medium is used). Likewise on some installations
it is practicable to cover over the fully installed
heating cables with aluminium foil prior to the
application of thermal insulation. This will prevent
the heating cables becoming encapsulated in the
lagging with subsequent overheating.
Like all types of electric trace heating, it is
important to match the cable resistance to the circuit
length and power output. This can determine using
Ohms Law.
Outputs of heating cable are shown as having a power
density rated in watts/metre:
max sheathed temperature of
200°C up to 30 watts/metre for copper
sheathed cable max sheathed temperature of
400°C up to
100 watts/metre for cupro-nickel sheathed
cable max sheathed temperature of 600°C
up to 225 watts/metre for stainless steel
sheathed cable |
Thermostatic control is essential with this system,
especially when operating at high temperatures.
Series circuit heating
tapes and cables (constant watt output)
These are composed of two or more resistance wire
elements connected to form a series or series-parallel
electric circuit insulated and/or covered in a
suitable insulating material
(Fig 4a).
This type of heating cable/tape is normally supplied
in predetermined, factory-terminated lengths and
has a constant power output at rated value when
energised. Series circuit heaters are most commonly
used in a 3 phase configuration for heating of
long pipelines.
They are available with optional metal wire braid
outer coverings for increased mechanical and electrical
protection and for hazardous area use.
Series circuit heating units are available for
operating temperature applications up to 400°C
and at power ratings up to 150W/metre.
Temperature control is normally provided by a
thermostat or sensor which can be combined with
resistance monitoring current sensing and RCD
protection.
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Parallel
circuit heating tapes and cables (contstant watt
output)
These are constructed from a single
resistance wire element, wrapped or spiralled around
two insulating bus-bar conductor
which run the length of the heating
tape or cable.
The resistance wire is designed to make electrical
contact with alternate bus-bars at predetermined
intervals, eg 600mm. These connections are made
at the 'NODE' point where the nickel- chrome resistance
heating wire is either welded or connected by rivets
to the bus-bars forming the parallel resistance
conductors. As a result a sequence of electrically
parallel heating circuits are formed along the length
of the heating unit .
Parallel circuit heating tapes and cables may be
cut to any convenient length within the limits imposed
by the maximum heating circuit length (typically
50 to 100m) and minimum bus- bar connection interval.
In practice this type of heating tape or cable is
suitable for complex pipework systems and/or for
maintenance purposes, since lengths of new tape
can be added in the field.}
Because of the parallel circuit design feature,
the power output per unit length is constant, regardless
of the overall length of the heating unit.
Parallel circuit tapes and cables are available
for operating temperature applications up to 425°C
and with power density outputs up to 130W/m.
Temperature control is normally by the use of thermostats
or other sensors whilst a tinned copper braiding
can be used with an RCD for electrical protection
but at the same time also providing mechanical protection.
Self-limiting or self-regulating
heating tapes and cables (variable watt output)
The flfst parallel circuit self-regulating heating
tape was introduced to the industrial and commercial
markets in 1970.By using a technology consisting
of a conductive carbon polymer to form a cross-linked
self-regulating heating element which was extruded
onto two bus-bars running the length of the heating
units to create an infmitely parallel circuit.
The self-regulating core is in essence an infinite
number of parallel resistors which permits the cable
to be cut to any length without creating cold sections.
Because it is self-regulating and infinitely parallel,
the output varies along the length of the cable
depending upon the temperature of the core of the
heater. An optional metallic braid covering can
be provided for additional mechanical or electrical
protection and for hazardous areas.
As it is flexible it can be overlapped without burnout,
making it easy to trace flanges, valves and instruments.
Maximum circuit lengths are typically 50 to 200m.
Self-limiting heating tapes are used primarily for
frost protection but types now exist having a maximum
continuous operating temperature of about 150 DC
with a maximum withstand temperature of around 200
DC. These tapes are available at a power density
rating of up to 60-80W 1m at 10°C pipe temperature,
but power declines above this temperature.
Self-limiting heating tapes can be used in many
applications where temperature control by thermostat
or sensor is not required. However, higher pipework
temperature will be produced in response to rising
ambient temperature, and since the power output
does not fall to zero when this temperature is reached
an additional temperature controller, ie thermostat
is required.
Circuit breakers must have sufficient capacity to
allow for the inrush current of initial start-up,
a feature unique to self-limiting heating tape,
because of the variable power output/temperature
characteristic. The lower the selected start-up
temperature is, the higher the power and thus the
bigger the electrical protection size would be.
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