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9
Cold Re echnical Digest
No. 86-1, June 1986

An Introduction to
Heat Tracing
Karen Henry

This d;ci:mnt-j -,-,been aPproved

for PublL r,--J, ase and . sale; its
distribution is unlimited.
US Army Corps L
of Engineers
Gold Reqions R( ,w ir lh &
Enqn( ( rnq Ll-iboratory
Acce
GRO'
NTIS TAO
,)TIC
ed
,nannounC
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BY--- ~~
Availabilt oe__

Dis Se cial

CRREL's cold Regions Technical Digests

are aimed at communicating essential
technical Information in condensed form to
researchers, engineers, technicians, public
officials and others. They convey up-to-
date knowledge concerning technical prob-
lems unique to cold regions. Attention is
paid to the degree of detail necessary to
meet the needs of the intended audience.
References to background information are
ncuded 'r the soecialist.
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REPORT DOCUMENTATION PAGE_ RCOREAD INSTRUCTIONS

BEFORE COMPLETING FORM
I. REPORT NUMBER 12. GOVT ACCESSION NO. RECIPIENT'S CATALOG NUMBER
Cold Regions Technical Digest 86-1 "1-.D /7- 1 7
4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

AN INTRODUCTION TO HEAT TRACING

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(e) 8. CONTRACT OR GRANT NUMBER(&)

Karen Henry

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT, TASK
AREA & WORK UNIT NUMBERS
U.S. Army Cold Regions Research and
Engineering Laboratory
Hanover, New Hampshire 03755-1290
It. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

June 1986
13. NUMBER OF PAGES

20
14. MONITORING AGENCY NAME & ADORESS(if different from Controlling Office) IS. SECURITY CLASS. (of thie report)

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16. DISTRIBUTION STATEMENT (of this Report)

Approved for public release; distribution is unlimited.

17. DISTRIBUTION STATEMENT (of the aberact entered In Block 20, If different from Report)

IS. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue an faevor aide If neceeeamry nd identify by block number)

Cold regions Freezing

Construction Thawing
Fluid flow Water pipes
Fluids
24L A0IqRACr (CWin te msre i
Nan ewseer edfidenllfy by block nuinber)
Heat tracing is the generic term which refers to the application of heat to a pipeline or
vessel to prevent freezing, to thaw frozen fluid, to maintain viscosity or temperature of
a fluid, or for other reasons such as to keep components from separating or gas from con-
densing. The purpose of this digest is to summarize current heat tracing practices-their
principles of operation, advantages and disadvantages. Heat tracing methods covered in-
clude heat transfer fluids (glycols and heat process fluids), steam, and electricity (resis-
tance, skin effect, impedance and inductance).

DO I4
aoUnclansified TIO" OF NOV S1lI OSOLETE
4
SECUPITY CLASSIFICATrO OF THIS C0IAE 'WNv P818e FnrffP'r*
COLD REGIONS TECHNICAL DIGEST No. 86-1, JUNE 1986
USA Cold Regions Research and Engineering Laboratory
Hanover, New Hampshire 03755

An introduction
to heat tracing

Karen Henry

Introduction Heat tracing is the generic term which refers to the applica-
tion of heat to a pipeline or vessel to prevent freezing, to thaw
frozen fluid, to maintain viscosity or temperature of a fluid,
or for other reasons such as to keep components from sepa-
rating or gas from condensing. The term originated with the
placement of steam lines adjacent to process or transport
lines-the transporting pipeline was "traced" with the steam
line. Cold regions operations increase the demand for freeze
prevention and viscosity maintenance. Freeze protection is es-
pecially important, since freezing can damage pipes and
equipment.
Heat tracing can utilize the heat given off by a hot fluid line
near or touching a pipeline. Or it can use the electrical resis-
tance of materials to produce heat, either in the pipeline itself
or in a cable or pipe which is in contact with or placed inside
the pipeline.
The purpose of this digest is to familiarize the reader with
current heat tracing practices, and to survey the heat tracing
methods available. Each method is described, and its princi-
plea of operation, advantages and disadvantages are dis-
The eutho, a ONIcussed.
bar f,CRIEL' i This digest is not meant to be a heat tracing design guide; it
Enginewng Rf wh does not contain detailed calculations on heat transfer and
Bmrnch. economics. Appropriate design references are cited through-
 2 COLD REGIONS TECHNICAL DIGEST NO. 861

out the report. A rough screening of the literature was con-

ducted before the references were compiled, and a wide varia-
tion was noted in the quality of specific design guidance
given. Apart from small-scale water freeze protection, each
heat tracing system should be designed for its specific appli-
cation. Many manufacturers have design capability and will
provide the guidance needed.
Tables 1 and 2 (p. 10-12) list the temperature ranges of
heat tracing methods and the advantages and disadvantages
of each method. These tables may be used to find the most
likely candidates for a specific application. The text can then
be referred to for greater detail. Table 1 lists both the maxi-
mum fluid maintenance temperature for each heat tracing
method and the highest temperature to which the system can
be exposed without damaging it.
All heat tracing systems except steam are capable of close
temperature control. Thermostats are usually used with
freeze protection systems, and can be used with steam. A
thermostat is an on-off controller with a temperature sensor.
The power is activated when the ambient temperature falls
below a set point.
For close temperature control, there are sophisticated con-
trols and temperature sensors available, such as analog con-
trol devices and thermistors or thermocouples. Analog con-
trollers vary power input to a system as temperatures vary;
they are usually reliable when installed properly. Because
they moderate the power used for heat tracing, they are often
greater energy-savers than thermostats. Even though analog
controls are more expensive than thermostats, they are some-
times used with freeze-protection systems for reliability and
economy. IEEE Standards 622-1979 and 622A-1984 cover the
design and installation of electric pipe heating and control
systems.

Heat Steam has a high specific heat and a high latent heat of va-
tracing porization, making it an ideal fluid to carry and transfer heat.
fluids It is also universally available and nontoxic, and presents no
danger of arcing in explosive environments.
Steam Saturated steam is used for heat tracing. The heat is trans-
ferred to the medium being traced largely through condensa-
j tion. Steam can be applied internally, externally, or as jacket-
ing, where it completely surrounds the process or transport
pipe (Kohli 1979, Schilling 1983).
 AN INTRODUCTION To HEAT TRACING 3

TRACER TRACER.

1.Basic single tracer!

V Tpipe arrangements
HEAVY TRACING LIGHT TRACING .forheavy and light
SINGLE PROCESS MULTIPLE PROCESS SINGLE PROCESS MULTIPLE PROCESS steam tracing (Wter
Tum TUS ruse TUNS Oehlschlaeger 1976).

Internal tracing and steam jacketing are expensive and

complicated ways to use steam for heat tracing and are there-
fore rarely used. A pipe with internal steam lines is hard to
clean, and both internal tracing and jacketing become com-
plicated in the presence of valves or any other geometrical ir-
regularities (Kohli 1979).
An external steam tracing system consists of headers, the
supply line, traps, and usually a line to return the condensate
to the source. Steam tracing is available at temperatures up to
370 *C (700 *F) but it is usually not used above 200 C (400 F)
because of the high pressures involved (Luke and Miserlis
1977, Oehlschlaeger 1977).
The steam trace-line and product line are usually contained
in an insulated bundle. They can be in direct contact, which is
referred to as "heavy tracing," or they can be isolated by in-
sulation, which is referred to as "softening," or "light trac-
ing" (see Fig. 1). Softening makes steam use more efficient.
The product pipe is a much greater heat sink than the sur-
rounding insulation, and the contact with the pipe can be
somewhat irregular. The insulation redistributes the heat
more evenly along the line, and thus eliminates hot spots, al-
lowing efficient steam use over long runs. When high heat
loads are desirable, direct contact between the heat tracing
and product pipe is most suitable. Heavy tracing is the easiest
form of steam tracing to design and install (Luke and Miserlis
1977, Kohli 1979).
The most significant problem with steam tracing is an ef-
fect known as "tailing." As heat is given off to the fluid line
and to the insulation, the steam condenses and its pressure
drops; pressure is also lost to friction. The pressure loss along
the line is accompanied by temperature reduction, or "tail-
ing." This effect can be mitigated by wrapping the tracer
around the fluid line, increasing the number of turns per unit

Immm m m mm m mmm m m m md
4 COLD REGIONS TECHNICAL DIGEST NO.86-1

length as distance from the header increases. Accurate heat-

transfer calculations and testing are necessary to determine
the actual reduction in tailing accomplished (Schilling 1983).
Steam traps are also employed to expel condensate from
the lines and thus reduce tailing. This helps reduce build-up
of water film (which will lower the heat transfer characteris-
tics of the trace-pipe wall) and maintains the desired steam
flow rate. The use of steam traps, however, only reduces and
does not eliminate the tailing effect (Oehlschlaeger 1977,
Kohli 1979, Schilling 1983). In addition, steam traps are one
of the most expensive maintenance items on a steam tracing
system. They need to be repaired and replaced periodically to
prevent malfunction or failure. It has been estimated that
25% of the heat energy of steam is lost at steam traps (Ham-
mack and Kucklinca 1977), and unless the traps are self-
draining or are manually drained, the condensate in them
may freeze during a shut-down.
Saturated steam exists at unique combinations of tempera-
ture and pressure. If the pressure is reduced the steam be-
comes superheated, but the superheat in heat tracing applica-
tions is rapidly dissipated. For this reason, the basic control
of steam heat tracing temperature is a pressure-reducing
valve, and precise control of temperature is difficult to
achieve with this method. In addition, uneven contact be-
tween the steam line and product pipe results in uneven distri-
bution of heat. This effect becomes more significant if the
steam temperature is quite different from the fluid mainte-
nance temperature. Because of tailing, uneven distribution of
heat, and the use of pressure reducing valves, temperatures of
steam tracing methods typically will vary around a desired
temperature by ± 5 V2 C (10 *F) below ground or ±II C
(20"I) above ground (Clough 1984).
Steam jacketing can provide very close temperature con-
trol, but can only be used practically on short sections of
pipeline.
Steam tracing is used primarily for freeze and pour-point
protection. Steam is utilized most efficiently when the desired
line temperature is close to that of steam at reasonable or
available pressures.
Economic comparisons were found in the literature re-
viewed. Most of them show that steam is significantly more
expensive to install and maintain than electrical resistance
heat tapes, even though the energy cost is lower. This is be-

'I- m t
AN INTRODUCTION To HEAT TRACING 5

cause the control of energy input is greater with the electrical

resistance heat tapes so the electrical systems use less total
energy. Installation, maintenance and energy costs vary with
location, and the two systems are comparable in some situa-
tions (Luke and Miserlis 1977). The economics become more
favorable if the steam is a byproduct of a process already in
place. Some expense may be involved in assuring the quality
of the steam.
Several factors should be borne in mind when considering
the use of steam tracing. If an electric power source is not
available or reliable, steam tracing is the best option. Most
temporary repairs can be made on steam tracing while the
system is in operation, whereas most repairs on other heat
tracing systems require system shutdown. Steam can provide
many times the design heat loss for continuous operation to a
line, thus providing melt-out in a very short time. Moisture
and corrosion can affect some electrical systems, and some
can't be used in explosive atmospheres. Steam tracing pre-
sents few problems in such environments, provided the sys-
tem does not exceed the ignition temperature of vapors in the
atmosphere. Finally, some maintenance personnel are more
comfortable with steam tracing (mechanical) than with elec-
tric tracing with its electronic control systems.
Design and application of steam tracing systems are treated
by Bertram et al. (1972), Oehlschlaeger (1976, 1977), Luke
and Miserlis (1977), Kohi (1979) and Schilling (1983).

Other heat transfer fluids can be divided into two categor- Other heat
ies: glycols and "heat process fluids." (Heat process fluids transfer fluids
are also referred to as "high temperature organics" or "hot
oils.") Both types of fluids are used for a variety of heat
transfer purposes-heat tracing being only one. Heat process
fluids are usually used for high temperature applications
150 °-400 "C (300 R-750"F), and glycols are used at tempera-
tures up to 120"C (250 *F).
There is a scarcity of information available from users of
heat transfer fluids for heat tracing. Most of the following
discussion is based on Dow Chemical Company literature.
Heat tracing systems utilizing heat transfer fluids in the li-
quid phase can achieve as precise temperature control as ne-
cessary with proper design. Used in the liquid phase, they re-
quire only one steam trap (associated with the steam boiler
used to heat the fluid) and therefore avoid the related equip-
6 COLD REGIONS TECHNICAL DIGEST NO. 86-1

ment installation and maintenance cost. They do require spe-

cial pumps and reservoirs, however, which add to the cost.
Glycols. There are two glycols commonly used for heat
transfer: ethylene and propylene. The glycols manufactured
for heat transfer have specially formulated corrosion inhibi-
tors added. This, along with their properties as freezing point
depressants when added to water, and their wide range of op-
erating temperatures (-SO0 F to 250°F), make glycols attrac-
tive options for heat tracing.
Glycols are used in aqueous solutions ranging from 20%o to
6007%by weight, thus taking advantage of the specific heat of
water to enhance their heat transfer characteristics. The
trade-off for the lower freezing point obtained with glycol is a
less efficient heat transfer (lower specific heat). The freezing
point depressant characteristic is important because the sys-
tem is then protected from freezing even during intermittent
operation. Figure 2 is a graph of the freezing points of vari-
ous aqueous glycol solutions.
Ethylene glycol at a concentration of 60%Vo will remain li-
quid down to temperatures of -50 °C (-58 F), whereas a 20%
solution has a freezing point of -9 °C (16 IF). Propylene gly-
col remains liquid down to -33 0C (-28 *F) in higher concen-
trations. The upper temperature limit of glycol solutions used
for heating is 120 *C (250 °F).
Glycol systems require periodic checks and replacement to
ensure that the glycol concentration is at the necessary level.
Glycols are hygroscopic and the corrosion inhibitors break
down over time.

40 I I

20

0Ethylene Glycol

-20 - Propylene Glycol

-4 0 -- Ethylene Glycol -
2. Freezing points of -

aqueous glycol solu-

tions (redrawn from -60
Dow Chemical Com- 0 20 40 60 80 100
pany literature). Glycol Weight (%)
AN INTRODUCTION To HEAT TRACING 7

Propylene glycol is considered safe for use in foods,

whereas ethylene glycol is regarded as being too toxic for ap-
plications where there is a possibility of ingestion. For this
reason, it is best that propylene glycol be used for heat tracing
of potable water supplies unless double-walled heat exchang-
ers are employed.
The Alaska Area Native Health Services have had good re-
sults with propylene glycol systems for freeze protection of
water utilities in arctic installations. The glycol systems have a
better service record than electric heat tapes, which are easily
damaged during installation or become corroded (Farmwald
1984).
Some users have elected to retrofit steam systems with gly-
cols because of lower maintenance costs and because steam
traps often malfunction.
Heat ProcessFluids.The term "heat process fluids" refers
to a variety of special organic fluids developed for heat trans-
fer at -70°C (-100'F) to 260--400°C (500 0 -750 OF). As men-
tioned previously, these fluids are most often used for heat
transfer at temperatures above 150°C (300 0 F) and probably
towards the upper end of the 260 P-400 0C temperature limita-
tions. This is because they cost about three times as much as
undiluted glycol.
Heat process fluids can be used in the liquid phase or in the
vapor phase. The fact that the vapor pressures of saturated
heat process fluids are much lower than steam vapor pressure
at the same temperature reduces the cost of equipment. For
example, at 300 'C (575 *F) the pressure of saturated steam is
8590 kPa (1246 psi), whereas the vapor pressure of a heat
process fluid manufactured by Dow Chemical at the same
temperature is 882 kPa (128 psi) (Mrochek 1984).

There are many forms of electric heat tracing. They can be Electrical
divided into two categories: 1) electrical resistance heat tapes resistance
(or cables), whose mode of operation depends primarily on heat tracing
the electrical resistive properties of material when a current is
applied, and 2) all other forms of electrical heat tracing, in-
cluding skin effect tracing, impedance tracing and induction
heating. In the second category, the methods take advantage
of the magnetic inductive effects of an alternating current
source.
Electrical resistance heating cables or tapes are of two Tapes and
types: 1) those where the heating elements are connected in cables
 8 COLD REGIONS TECHNICAL DIGEST NO. 86-1

Bus
Heater
Lead Htre
Hae
,-Wire

0I _ _ _ _ ___

a. Series resistance. b. Parallelresistance, continu-

ous type.
Bus us
u'r
Wire Heate Heater

. Circuit diagramse
for resistance heat
tracing (after Bylin c. Parallelresistance, zonal d. Parallelresistance, continu-
and Hutzel 1979). type. ous self-limiting type.

series (i.e. one continuous resistor), and 2) those where the

heating elements are connected in parallel. Parallel resistance
heat tracing has two or more heating elements connected ac-
ross the voltage source. One type of parallel heat tape, called
self-limiting (or parallel modulating), is capable of regulating
its output locally along its length. Figure 3 shows circuit dia-
grams for various forms of electrical resistance heat tape.
Series Resistance Heat Tape. The most primitive forms of
electric heat utilized series power cables and heaters which
were adapted from other heating systems. Mineral-insulated
copper-sheathed power cable would be overloaded with cur-
rent, causing an PR heating effect* (Bilbro and Leavines
1969). Series resistance heating cables have a specific resistivi-
ty according to the user's specifications of length and re-
quired heat output. In most cases, a standard cable will be
provided and a certain spiraling ratio recommended. For
greater lengths, the manufacturer will produce a specific alloy
of the correct resistivity.
The conductor used in series resistance cable is bigger in di-
ameter than that used in parallel resistance heat tape; and
(unlike parallel heating elements) it doesn't need to be con-
nected to bus wires. These factors make series cable compara-
tively rugged and impact-resistant, as well as being capable of
carrying high heating loads. Some series cables are made of

I? I - current in amperes; R = resistance in ohms.

I.
AN INTRODUCTION To HEAT TRACING 9

special alloys and insulated with magnesium oxide. They are

referred to as "mineral-insulated" heat cable and can heat up
to a temperature of 590 *C (1100 *F) (Lonsdale 1981, Fenster
1984).
Some disadvantages of series resistance heat tape are: 1)
Any break results in complete loss of the circuit. 2) It is fairly
stiff and unwieldy. 3) It requires skilled personnel for correct
installation. 4) There is a risk of burn-out and fire if the tape
is accidentally crossed over itself or subjected to local heat-
ing, or if poor contact is made with the process pipe which
serves as a heat sink for sheath temperature.* 5) Magnesium
oxide is quite hygroscopic, so that if mineral-insulated cable
is exposed to moisture, the resulting corrosion can cause
short-circuiting (Fenster 1984, Stewart 1977, Bylin and Hut-
zel 1979).
Temperature modulation of series resistance heat tape can
be achieved during operation by varying the current made
available to the circuit.
Parallel Resistance Heat Tape. Parallel resistance heat tape
has heating elements which are electrically connected in par-
allel, either uniformly along a bus bar or in zones (see Fig. 3).
Because the resistive elements are connected in parallel, the
heat tapes can be manufactured at set wattages so they can be
cut to length in the field, easily accommodating last-minute
changes in the lengths required. Therefore, they are signifi-
cantly less expensive to design and purchase than series heat
tapes. The other major advantage in the use of parallel heat
tapes is that if mechanical damage is incurred, only part of
the heating circuit is lost.
The main disadvantage of the use of parallel heat tape
stems from the resistive wires, which are thin and fragile. The
thinner wires are also unable to carry high heating loads with-
out melting, the upper temperature limits being 200 °C
(400 F). The connections between the heating elements and
the bus wires are also easily damaged during installation and
operation.
Continuous parallel heat tapes have more fragile connec-
tions between the bus wire and heating element than do zone
heaters, making them more susceptible to mechanical dam-
age. When a zone heater is damaged, however, more of the
circuit is lost.
The sheath of an electric cable is the outermost covering of the heat trac-
ing cable.

t!,>
10 COLD REGIONS TECHNICAL DIGEST NO. 861 A
Table 1. Temperature imitations of beat trac
Maximum fluid Maximum-
Heat tracing maintenance exposure
method temperature temperature

A. Fluids

1. Heat transfer fluids

a. Glycols 150 C (250 OF) 160 °C (325 OF) See Figu
glycols.,
tures, hil
quired t(
sition.

b. Heat process fluids 260 0 -4000 C (500°.-75 0 "F) None Most rer
(organics) temperal
(-500 to
mainteni
on the p
used.

2. Steam About 2000C (400 0F); higher if the None This lim
vapor pressure can be tolerated the vapo

B. Eiectriciy

1. Series resistance 590°C (1100O) Varies (see below) Above 2

lated cat

2. Parallel resistance
a. Continuous and I
PVC insulated: 65 *C (150 SF) 220(C (250 o)
zonal Teflon insulated: 65 C (1501F) 2000 C (400OF)
Silicone rubber insulated: .90 DC 150C (300oF)
(200)
b. Self-limiting PVC insulated: 65"C (150 F) 85 RC (185 OF)
Teflon insulated: ISOaC (3000F) 190 0C (370*F)

3. Skin effect , 150C (300 0 1F) , 1500C (300*F) Maximu

limited I
tor insul

4. Impedance Up to melting point of metal being None Method

heated above C
loss of I

S. Inductance Up to Curie point of metal being None This typ

heated melt me
molten .
 86-1
. AN INTRODUCTION To HEAT TRACING 11

I mPeratre limitations of heat tracing methods.

Maimurt
EXPsure Source of
temperature Comments information

r 160 0 C (325 OF) See Figure 3 for freezing points of Dow Chemical Co. literature
glycols. At high exposure tempera-
C tures, high circulation rates are re-
quired to prevent thermal decompo-
sition.

None Most remain pumpable down to Dow Chemical Co. literature

temperatures of -45 o to -70 C
o (-500 to -100 0F). Maximum fluid
maintenance temperature depends
on the particular heat process fluid
used.

p the None This limit is a practical one due to Luke and Miserlis (1977)
5ss the vapor pressures involved.

Varies (see below) Above 200C (400*1) mineral insu- Lonsdale (1981)
u lated cable is used.

220C (250 F) Manufacturer's representative

200 0 C (400-F) (Ricwil)
1C 150 C (300 *F)

83-C (185 F) Manufacturer's representative

190-C (370 F) (Ricwil)

= 150*C (300 ) Maximum exposure temperature is

it limited by that of electrical conduc-
tor insulation.

*ng None , Method isnot recommended for use Smith (1960), IEEE (in prepara-
above Curie point of metal, due to tion)
loss of heating efficiency.

g None This type of heating is often used to IEEE (in preparation)

melt metals and keep them Ina
molten state.

....................
12 COLD REGIONS TECHNICAL DIGEST NO. 861

Table 2. Advantages and disadvantages of available beat tracing methods.

A. Flis Advasitag Didvaateges
1. Heat transfer fluids Precise temperature control.
a. Glycols Can retrofit a steam system to use Needs a circulating system.
aqueous glycol solutions. Depress-
es freezing point of water.
b. Heat process fluids Precise temperature control. Wide Relatively expensive. Needs a cir-
(organics) temperature range. Low freezing culating system.
temperatures.
2. Steam Can take advantage of waste Non-uniform distribution of heat.
steam. Rugged. No danger of arc- Expensive to install and maintain.
ing in explosive environments. Temperature control is not pre-
High heat transfer rates are possi- cise. Not always practical above
ble (can provide rapid melt-out). 2000C (400 F) due to high vapor
Does not need a reliable electric pressures involved.
power source.
3. Fiscvdty Precise temperature control. Needs a reliable electric power
Various temperature control options source.
1. Resistance Relatively inexpensive. Exposure to high temperatures
and/or moisture will damage some
insulation and cables.
a. Series Rugged. Capable of high tempera- Cannot be field-cut. One break in
tures. the cable causes an open circuit.
Will burn out if crossed over it-
self.
b. Parallel Can be field-cut. If a resistor Relatively fragile.
fails, heating circuit is still main-
tained.
1) Continuous and Will burn out if crossed over it-
zonal self.
2) Self-limiting Will not burn out if crossed over Somewhat more expensive than
itself. Responsive to local heat other forms of parallel heat tape.
demands.
2. Skin effect Simple components (i.e. easy to Impractical for applications less
construct and repair). Rugged. than 130 m (500 ft) long.
Needs relatively few energy inputs.
Can be part of prefabricated in-
sulated pipe bundle.
3. Impedance High heat transfer rates are possi- Need to insulate pipe surface in
ble. Can be easily retrofitted on order to avoid electrical hazard to
existing metal pipeline systems. personnel. May need to electrical-
High temperatures are possible. ly isolate flanges and pipeline
Heating element (pipeline) cannot from support structure. Requires
burn out. specific design for each applica-
tion.
4. Inductance High temperatures are possible. Very expensive. Irregularite such
High heat transfer rates are possi- as valves and flanges difficult to
ble. Heating element cannot burn design for. Requires specificdesign
out. for each application.
AN INTRODUCTION To HEAT TRACING 13

Bus Wires
Self-limiting Conductive 4. The construction
Material4.Tecntuio
a. Electrical Insulations of self-limiting elec-
trical resistance heat
Metallic Shield tape (a) and a dia-
gram of the self-
Carbon Chains Before H . limiting action (in
Thermal Expansion the conductive mater-
b. ial) on a molecular
Carbon Chains After level (b) (after Speer
Thermal E~parision and Kucklinca 1975).

Continuous and zonal parallel heat tape, like series resis-

tance heat tape, varies the temperature by varying the cur-
rent. Parallel heat tape can burn out if crossed over itself or if
exposed to high temperatures along the pipeline.
Self-limiting parallel resistance heat tape consists of two
conductors which act as bus wires with a continuous sheet of
semi-conductive material between them (see Fig. 4). The ma-
terial between the conductors consists of graphite particles
embedded in a matrix of a cross-finked copolymer. When the
temperature of the heat tape increases, the matrix expands,
increasing the distance between the conductive graphite parti-
cles. This increases the electrical resistance and reduces the
amount of heat output at that location. Self-limiting heat
tracing thus provides heat only as it is needed. Self-limiting
heat tape cannot maintain a temperature above 150 C
(300*F) and cannot be exposed to high temperatures without
damaging the system (Fenster 1984).
Self-limiting heat tracing can be crossed over itself without
the possibility of burn-out; it can respond to local heating de-
mands; it is flexible and can be field-cut. The low likelihood
of burn-out makes it inherently safe. But as with any electri-
cal tracing, caution must be exercised to ensure that the con-
trol system is safe and that the entire system is installed prop-
erly. Self-limiting heat tracing can burn out with a constant
current source, but this problem is rare since constant voltage
sources are most often used.
Although self-limiting heat tracing is more rugged than
other forms of parallel resistance heat tracing, it is less rugged
than series resistance heat tracing.
For design of electrical resistance heat tracing see Luke and
lserlis (1977), Stewart (1977), Bylin and Hutzel (1979),
Lonadale (1981), and IEEE (1983).
14 COLD REGIONS TECHNICAL DIGEST NO. W6.1

5. Current density
distribution due "-Current Density
to skin effect CustrtbDniit
(after Burpee and Distribution
Carson 1977). Inner Conductor

Methods based Skin Effect Tracing. Skin effect refers to the tendency of
on alternating alternating current to flow in the outer part of the current-
current effects carrying conductor. At power frequencies (60 Hz) in common
copper or aluminum conductors, the size and properties of
the conductor are such that skin effect is insignificant. If fer-
romagnetic mild steel is used as a conductor, the skin effect is
an order of magnitude greater and becomes significant.
The second inductive effect which is utilized by skin effect
tracing is known as the "proximity effect"-the attraction
between two opposite-flowing currents located close to-
gether. If two conductors are arranged coaxially, and the cur-
rents are flowing in opposite directions as with skin effect
tracing, the current on the outer cylindrical conductor will be
attracted to the inside. Figure 5 illustrates the current density
distribution in coaxial conductors with currents flowing in
opposite directions.
The direction of current flow in the skin effect circuit is
such that the current flows out through a small low-resistance
conductor which is located inside of a large cylinder made of
ferromagnetic mild steel. The outer cylinder is also the return
current path (see Fig. 6). The inner conductor transmits the
current with only slight heat generation (PR loss). The prox-

PIPELINE PIP]zr A

a ITEEL TUBE
"lMEAT TUBE)
POWER 44
SOURCE

6. Cross-sectional
view of skin effect A
heat tracing (modi- INTERNAL CABLE
fied from Ando and
Kawahara 1976).
AN INTRODUCTION To HEAT TRACING 15

imity of the inner conductor causes the skin effect current in

the outer conductor, or "heat tube," to be concentrated on
the interior of its surface. This pronounced skin effect causes
a large generation of heat. The system is relatively safe since
the current is carried on the interior of the tube (Ando and
Kawahara 1976). The tube is attached to the product pipe by
intermittent welding or heat transfer cement to provide a
thermally conductive path. Very few energy inputs are re-
quired along a line-one power station can service up to 48
km (30 miles) of line.
Skin effect current tracing is constructed of ordinary, low-
cost materials and can be installed using standard construc-
tion techniques. As a result, skin effect tracing is relatively in-
expensive to install, very rugged, and easy to repair. The
manufacturer will assist with design, and it can be purchased
as part of a prefabricated pipe, heat tracer and insulation
bundle.
Given a reliable power source, the disadvantages of skin ef-
fect tracing are few. The upper temperature limit of just
above 150 *C (300 *F) is set by the maximum exposure temper-
ature of the insulation of the electrical conducting wire locat-
ed inside the heat tube. Skin effect tracing is not practical or
economical to use on lengths of pipeline less than 150 m (500
ft) long. Skin effect tracing's main application is in petroleum
product transport on long lines requiring good temperature
control. It has also been used to prevent water from freezing
in long water mains.
Skin effect was rated by IEEE as having a high system effi-
ciency and high system power factor.* It is described by An-
do (its inventor) and Kawahara (1976) and by IEEE (in prep-
aration).
Impedance Heating. Impedance heating is the term applied
to a heat tracing system in which an electrical connection is
made directly to the (metal) process or carrier pipe. The pipe
itself is the heating element and its effective resistance is the
source of heat.
The significant benefits of impedance heat tracing are its
high temperature capabilities and the fact that because the
pipeline is the heating element, it cannot burn out (although
r Powerfactor is the ratio of the actual power consumed in an AC circuit
to the product of the RMS (root mean square) voltage and amperes in the
du~crcuit. A high to
in# equipment power factor is at
be operated desirable
optimumsince It allows the power-WPerat-
efficiency.
16 COLD REGIONS TECHNICAL DIGEST NO.1

Magnetic Flux
Power Cable--,

7. Current and (N etd ae~

magnetic flux lines
of impedance heatins i I
StandPI le -
system (after Koester
1978). Standard Pipeh A-C Current

the supply cable or connections can). Impedance heating also

has high heat transfer rates and uniform heat distribution,
leading to exceptionally good temperature control. Simple
on/off thermostatic control (utilizing thermocouples) can
maintain temperatures within 2 C (3 04 'F). Finer control
can be obtained with analog controllers. Another outstanding
advantage of impedance heating is that it can be retrofitted to
an existing pipeline without disturbing its thermal insulation.
Even though the elements of an impedance heating system
are simple, each system needs to be individually engineered to
meet the design criteria, and the engineering can become
complicated. If a piping system has many branches, electrical
balance is hard to obtain. Flanges may require electrical isola-
tion if more than one power source, such as a transformer, is
used to obtain electrical balance. The pipeline itself needs to
be electrically isolated from its support structure (that is, op-
erated "ungrounded" except at one point). Since currents ex-
ist on all external surfaces of the pipeline, they need to be
guarded from contact by personnel. Pipelines are normally
operated at 30 volts or less as an added precaution.
Impedance heating requires more energy input than skin
effect but less than induction heating. IEEE (in preparation)
rates both the system efficiency and the system power factor
of impedance heating as moderate. The only application of
impedance heating found recorded in the literature is viscosi-
ty maintenance, as required by the petrochemical industry
(Koester 1978, Smith 1980).
IEEE (in preparation) provides a review of the special con-
siderations involved in the design and installation of impe-
dance heating systems.
Induction Heating.Induction heating is the process of cre-
ating heat in an electrical conductor by placing it in the mag-
netic field of an alternating current source. This is accom-
 AN INTRODUCTION To HEAT TRACING 17

plished by winding a low-resistance wire or series of wires

around a conductive pipeline or vessel with high magnetic
permeability so that heat is generated without actual electrical
contact between the wire and the structure. The coils are con-
nected to an AC voltage source. The alternating current flow-
ing through the coils induces eddy currents and hysteresis
losses in the pipeline material. The heat generation associated
with these effects is the source of heat.
Aside from very high temperature capabilities, the primary
advantage of induction heating is the absence of thermal re-
sistance between the heat tracing and pipeline. The pipeline
can therefore be heated quite rapidly. (Incidentally, a pri-
mary design consideration is that the coils being used often
need to be thermally protected from the pipeline.)
The disadvantage of induction heating is its expense. Each
system needs to be specially designed. Power inputs are re-
quired at short intervals along the pipeline and material re-
quirements are very large.
Achieving uniform temperature distribution would be a
painstaking design process; coil spacing would have to be ad-
justed for every pipeline irregularity such as a valve or flange.
Care would have to be taken to ensure that any uneven heat-
ing caused by irregularities in the wall, coil or external flux
path was tolerable (Erickson 1984, IEEE in preparation).
Induction heating is not a likely candidate for most heat
tracing applications. IEEE (in preparation) rates induction
heating as having moderate system efficiency and a low sys-
tem power factor. Induction heating is most frequently used
for melting metals and maintaining them in a molten state.
No other mention of its use was found in the literature re-
viewed. It is reviewed here for its potential in high-tempera-
ture applications.
For more information on induction heating, see IEEE (in
preparation) and/or any text on induction heating.

The following people contributed to this digest: Charles Acknowledgments

Korhonen, Gary Phetteplace, Ronald Atkins and David Cole
* of CRREL; Neal Fenster of Ricwil, Inc.; and David Bor,
Terry Wenger and John Beyrau of Dow Chemical Co., Inc.
18 CoLDo REGIONS TECHNICAL DIGEST NO. W1

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AN INTRODUCTION To HEAT TRACING 19

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p
20 CowD REGIONS TECHNICAL DIGEST NO. 86l

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LMEI