|By Richard Hartfelder - Process Systems Product
Many plant engineers do not give much thought
to the heaters operating within their processes and applications -
unless those heaters fail, require significant maintenance or cause
other problems. Unfortunately, heaters play an integral role in many
applications. Therefore, heater problems can easily snowball and
lead to much larger headaches.
Following a few simple guidelines will not only reduce the
likelihood of heater-related issues, but can actually have a
significant positive impact on the efficiency of systems and reduce
maintenance requirements and costs. Below are 10 ways to maximize a
heater's service life and performance.
Tip 1: Guard against heater contamination
Contamination is the most frequent cause of heater failure (see
images). As heaters expand and contract during cycling, they often
draw in organic or conductive materials. This can lead to an arcing
failure between individual heater windings or between heater
windings and the electrically grounded outer heater sheath. When
allowed to collect at the lead end of a heater, contaminants can
also cause electrical shorts between power pins or terminals.
Therefore, it is important to keep lubricants, oils, low-temperature
tapes or processing materials out of contact with the lead end of
the heater. Employing seals will help.
2: Protect leads and terminations from high temperatures and
Standard fiberglass-insulated lead wire may be used in applications
with ambient temperatures up to approximately 260°C (500°F). If a
lead is exposed to higher temperatures, high-temperature lead wire
or ceramic bead insulation should be used. An unheated section of
the heater, extending away from the heated region of the system,
enables the leads to run at a beneficially cooler temperature.
When heaters are mounted in moving machinery, it is essential to
anchor the leads to prevent them from being damaged. A lead
protection option should be specified and used for optimum
protection against lead damage.
Tip 3: Heater selection and sizing are important
A heater's wattage should be matched as closely as possible to the
application's actual load requirements to limit ON/OFF cycling (see
tip 6). For fitted-part applications, specify the hole or an
alternative application feature size to ensure an optimal fit
between the heater and application feature. A tight fit minimizes
air gaps and reduces the instances of hot spotting.
Tip 4: Ground the equipment
It is common sense and safe practice to electrically ground all
equipment on which the heater is used. Grounding equipment helps
protects plant and personnel in the event of an electrical failure
in the heating system.
Tip 5: Regulating voltage ensures the rated heater
voltage matches voltage supply
It is essential to ensure a heater's rated voltage matches the
available voltage supply because wattage increases (or decreases) at
the square of the change in voltage applied to a heater. For
example, if a heater is rated for 120V/1000W and is connected to a
240V supply, it will generate four times the rated wattage output or
4000W. This will cause a heater to fail relatively quickly and can
also cause significant damage the attached equipment.
Tip 6: Prevent excessive heater cycling
Excessive temperature cycling is very detrimental to the life of a
heater. The most detrimental is the cycle rate that allows full
expansion and contraction of the heater resistance wire at a high
rate (30 to 60 seconds' power ON and power OFF). This causes severe
stress and oxidation of the resistance wires inside a heater. A bad
temperature cycle is typically found when thermostats are used.
Thermostats respond slowly to temperature changes and have large
switch ON/OFF temperature differentials. An improvement, but a
somewhat more expensive solution, is to use ON/OFF or PID
controllers with mechanical relays. It is crucial to not switch the
frequency or cycle time too rapidly (somewhere between 3 to 10
seconds), because the relay contacts can wear out quickly.
The most effective way to minimize heater element temperature
cycling, and the most expensive solution, is to use solid state
relays (SSRs) and SCR power controllers coupled to PID temperature
controllers. This combination provides the best performance for both
your thermal system as well as for the heater itself. Solid state
switching devices cycle power to the heater very rapidly (from one
second with a SSR, down to milliseconds with phase-angle fired
SCRs). This fast-power cycling dramatically reduces heater element
wire temperature excursions and substantially extends heater life.
Tip 7: Ensure that the sheath material and watt density
ratings are compatible with the material being heated
This is absolutely critical to ensure long heater life and healthy
processing equipment. When heating solids, such as metals, the
operating temperature and heater-to-part fit drive sheath material
and watt density choices. Carbon steels, aluminum, silicone rubber
sheath materials are fine for lower temperatures (a few hundred
degrees). However, as temperatures increase beyond this point,
sheath material choices become limited to galvanized or stainless
steels and other higher temperature metal alloys. As temperature
also increases, the watt density must decrease accordingly to
prevent internal resistance wires from oxidizing quickly and failing
prematurely. A good heater-to-part fit ensures proper heat transfer
and does not force the resistance wires to overheat.
When heating gases, the operating temperature and flow rates
dictate what sheath material and watt density can be used. For
example, you can run higher watt densities when heating hydrogen
versus nitrogen, but hydrogen requires Incoloy 800 sheaths, whereas
304 Stainless Steel will work for many nitrogen applications.
Increasing flow and turbulence across the heater elements means
better heat transfer, which raises watt density values. For liquid
heating, the prime driver for materials and watt density selection
is the fluid material and flow rate. Water can easily handle 42.52
to 70.87W/cm2 (60 to 100W/in2) using a copper
sheath, whereas a 50/50-water/glycol mix can only handle 21.26 W/cm2
(30 W/in2) and must use a steel sheath.
Tip 8: Mount immersion tank heaters horizontally near the
Heaters should be placed horizontally and near tank bottoms to
maximize convective circulation. Vertical mounting is only advisable
when limitations, such as space restrictions, prohibit horizontal
placement. Regardless of whether a heater is mounted horizontally or
vertically, it is essential to place it high enough to avoid any
sludge and debris buildup in the bottom of the tank. Likewise, for
both mounting methods, the entire heated length of the heater must
be immersed at all times - one reason vertical mounting is rarely
recommended. It is also important to avoid placing heaters in
restricted spaces that limit convective flow and/or where free
boiling or steam traps can occur.
Tip 9: Prevent build-up and sludge on the heater elements
Scale, coking and sludge build-up on heater sheaths must be
minimized. Any accumulation should be periodically removed or at
least minimized, to avoid inhibiting heat transfer to the liquid.
Periodic cleaning prevents heater elements being forced to operate
at higher temperatures, which can lead to early heater failure.
Extreme care should also be taken to avoid getting silicone
lubricant on the heated section of a heater. Silicone will prevent
the "wetting" of the sheath by the liquid, act as an insulator, and
possibly cause the heater to fail.
Tip 10: Ensure proper, tight temperature control and
safety limit protection
Matching the appropriate temperature control system to the heater is
imperative to strong heater performance and life. Each process
application should, at the very least, include a process temperature
sensor (to sense the material being heated) and a limit sensor (to
sense the heater sheath temperature). The process sensor should be
directly immersed into the material to be heated, or snugly inserted
into a thermowell inside the fluid itself. For safety reasons, two
separate control systems should be used - one for process
temperature control and one for high limit control. PID type process
temperature controllers offer more stable control and faster
response than ON/OFF switching controls or thermostats. The trade
off is that PID control is often more expensive than ON/OFF types
and not always necessary for applications that do not require highly
accurate temperature control.
Incoloy is a registered trademark of the Special Metals