TM 5-852-5/AFR 88-19, Volume 5
characteristics are also poorer than for water. For
(M.I.) resistance cable, are approved for installation
example, a 50% mixture of glycol and water would
within water pipes or for submerged conditions. In-
require a 15% increase in flow rate to achieve the
line cables are more practical for long water supply
same heat transfer. Design information on these
transmission mains, but are impractical in the
heating systems is available from the American
distribution network because valves and other
Society of Heating, Refrigeration and Air-
fittings must be bypassed. They may be subject to
Conditioning Engineers (ASHRAE) (ethylene glycol
vibration damage when fluid velocity is greater than
is toxic and cross-contamination must be
4 feet per second, and the cables must be removed
prevented). Propylene glycol, which is non-toxic,
to clean the pipes and when some types of pipe
but more expensive, can be used. Glycol solutions
repairs are made. Heating cables are more
are corrosive to zinc and can leak through joints and
conveniently and commonly located on the outside
pump seals that will not leak water at the same pres-
pipe surface. The required capacity for heating
sures. Some boiler manufacturers void their war-
cables must be increased by a factor of 1.5 unless
ranty if glycol solutions are used. Mechanical seal
flat or wide heating strips or adequate contact
pumps should be utilized to prevent leakage. Special
between the cable and the pipe, preferably with heat
organic fluids can also be used instead of water-
transfer cement, is maintained. Exterior cables for
glycol solutions.
pre-insulated pipe are commonly installed within a
(2) Electric heat tracing. Electric heat tracing
raceway or conduit attached to the pipe surface,
systems are relatively easily installed and controlled.
which facilitates fabrication, installation, removal
They can be installed continuously on water and
and replacement. In this configuration, the air space
sewer pipelines, or only at freeze-susceptible
and poor contact of the heating cable with the pipe
locations, such as road crossings, service connec-
can further reduce the heat transfer efficiency and
tions or at appurtenances such as fire hydrants.
the heat input for plastic pipes may need to be two
Because of the relatively high cost of electrical
to six times that for a heating cable within the pipe.
energy, these systems are usually installed for
It is difficult to make the joints in the exterior
freeze-prevention or system restoration in the event
heating cable channel watertight, as is required for
of operating upset, such as a prolonged no-flow
most cables or their joints when used undergound.
(c) Overheat protection. Plastic pipes, insu-
condition, rather than as the primary method of
maintaining a minimum operating temperature in the
lations and the electric heat tracing system itself
system.
must be protected from overheating unless the self-
(a) Types. A variety of electric heat tracing
limiting heating cable is used. For conventional
systems and products are available from a number of
cables, a high temperature thermostat cut-off is
manufacturers. Resistance-type cables and wires are
usually installed and set at about 85 degrees F and
available for installation with pipes or for exterior
the sensor is placed on the surface of the heat cable.
(d) Controls. To provide freeze protection,
tracing. Small-diameter metal pipes, such as service
lines, can be heated or thawed by induction heating
automatic control systems must activate the electric
from an alternating current in a wire wrapped
heat tracing system at a set point above 32 degrees
around the pipe which induces eddy currents within
F to provide some lead time and allow for variances
the pipe. The most common electric heat tracing
in the temperature detection sensitivity of the
systems used are zone-parallel and self-limiting
thermostat and sensor. To provide economical
continuous parallel heating cables and strips. Since
operation, the controls also cut off the power supply
they contain separate conductor and resistance buss
when heating is not required. These controls are
wires or conducting material in the same casing,
often a major cause of malfunction and wasted
they produce a constant heat output per unit length,
energy. Mechanical thermostats with capillary tube
and can be conveniently cut to the desired length.
sensing bulbs are limited to about 16 feet in length
Maximum lengths are usually 250 feet to 650 feet.
and temperature control is only possible within a
One type has a carbon-filled polymeric heating
few degrees. Electronic thermostats are much more
element with self-adjusting properties that decreases
sensitive but they are expensive. The resistance
heat output as the temperature increases. This cable
sensors they use can be located any practical
will not burn itself out or overheat plastic pipes, and
distance from the controller and the system can be
the heat output modulates, to some extent, with
selected to maintain fluid temperatures within 0.1
temperatures along the pipe.
degree F. This type of system, which is commonly
(b) Location. Maximum heat transfer effi-
used in Greenland, allows the utility system to be
ciency will be obtained if the heating cables are
reliably operated at near-freezing temperatures. The
installed inside the pipe. However, the coating and
sensors must be located with care to provide proper
joints of only a few cables, such as mineral insulant
control, freeze-protection, and prevent the waste of
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