HEAT TRACE CONSIDERATIONS
1. TYPES OF HEATER CONTROL
There are two types of temperature control:
ambient (air sensing) and line sensing (pipe sensing). on small
projects, either of these types of control are achieved with indi-
vidual component temperature controllers. on larger projects
it may be advantageous, in terms of cost and maintenance, to
use larger central control cabinets with electronic control.
LINE SENSINg CONTROL
For line sensing control, a thermostat is used to sense the actual
pipe temperature. The heater is only energized when the pipe’s
temperature drops below the thermostat’s set point.
When controlling a heater circuit on a pipe that has both
flowing and non-flowing segments (laminar flow), the sensor
should be placed to best sense the temperature of non-flowing
segments. on critical temperature control processes, separate
heater circuits may be required.
Advantages of this system include more precise temperature
control and minimum energy usage. Initial control costs and
ongoing maintenance costs will rise in proportion to the number
of controllers used.
SENSINg AMBIENT AIR
For ambient control, the heater is turned “off” and “on” de-
pending on the temperature of the surrounding air. The setpoint
temperature to turn on the cable may be adjustable or preset
(40°F is a typical value). When energizing multiple heater loads,
a contactor may be used to perform the actual switching. Ad-
vantages of ambient control include simplified control wiring and
lower control maintenance costs. however, excessive energy
consumption and loss of precise temperature control may result
(since heaters may be on when pipe temperature is warm).
2. EFFECTS OF HEAT SINKS
Any thermally conductive material that penetrates through the
insulation pulls heat away from the pipe or vessel at a high rate. If
extra heater cable is not installed at these points, the pipe may
be colder in those areas, which may result in freeze-up or loss of
process temperature.
3. HEAT-UP REqUIREMENTS
Heat loss tables do not include adequate power to provide
rapid heat-up of pipes or vessels filled with product. Should rapid
heat-up be required, extra heat must be added. This is often
accomplished by using extra heaters that are turned on only in
heat-up situations. Contact Easy Heat for further information.
4. HAzARDOUS AREA DESIgN CRITERIA
heaters installed in hazardous (explosion hazard) areas
must have sheath temperatures that do not exceed the
ignition temperature of the hazardous gas or dust that is
present. The method of limiting this temperature varies with
different types of products:
pipe SenSinG
tHerMoStAt
HeAtinG cABLe
tApe BULB to pipe
LocaTe buLb aT
LeaST 90
o
From
heaTer
Self-regulating heaters may be used based on their maximum
“T Rating.” Under no conditions will they exceed those tem-
peratures.
Each heater installed in a hazardous area must have a metal
shield or sheath. This provides an effective return ground path as
well as providing added physical protection.
All connections and control equipment must meet the criteria
for hazardous area application. For Division I applications, please
consult Easy Heat.
5. NON-METALLIC SURFACES
Non-metallic pipes and vessels often have low
softening and melting points. Care must be taken in design not
to let the surface or heater reach that temperature.
TSR cables can be used safely without concern.
6. DESIgNINg SELF-REgULATINg HEATER
CABLES FOR PLASTIC PIPE
Plastic pipe is not very thermally conductive, which results in less
heat being transferred to plastic pipe than to metal pipe. There
are three methods of applying heater cable to plastic pipe:
a)
regular attachment at one-foot intervals—
Attachment of cable at one foot intervals along the pipe.
Sometimes adequate.
b)
foil over the cable—
Fasten the cable at one-foot intervals
(as above) and then cover with a layer of adhesive-backed
foil tape. Usually adequate
c)
foil over/under (sandwiched) cable—
Apply a layer of
adhesive-backed foil tape on the pipe.
Fasten the cable over the foil tape per
a)
above. Then ap-
ply another layer of foil tape over the cable. Almost always
adequate.
7. USE OF METAL FOIL TAPE TO LOWER
SHEATH TEMPERATURE ON METAL PIPE
metal foil tape can be used on all types of heaters to lower
sheath temperature. This should only be done to improve life
expectancy.
DO NOT USE THIS TECHNIqUE TO LOWER SHEATH
TEMPERATURES FOR HAzARDOUS APPLICATIONS.
©2009 Easy Heat
www.easyheat.com
