How to extend Electric Process Heater Life?
HongMing Qi Chief engineer of HuaNeng WuXi Electrothermal Co.
Offering no pollute, easy control and low maintenance, More and more electric
process heaters are widely used in the chemical process industries, electric power
generation and offshore oil productions. However there are no standard for EPH (electric
process heater), it is difficult to size and specific this heater, as they combine aspects of
pressure vessel, tube-sheet heater exchanger, chemical process and electrical &
mechanical design. Here some tips that help customer to understand and operate EPH
effectively.
1. Choose the right sheath material and watt density
In order to prevent EPH corrosive, we must choose the right sheath material to match
the fluid being heated. On this point, normally the manufacturer can provide related informations. Watt density is technical term of heat flux, watt density is very important to
electric heater, lower watt density could extend the heater life, however the cost is
increased, too high watt density can result in: 1.failure of the heater.2.Damage to the
material being heated. Typical values are 12-15w/in2 for heat transfer medium, 20-30w/in2
for gas and 50-60w/in2 for water. However typical value is only Guide. Ideal watt density
are calculated based on fluid flow, fluid specific heat, heat conductive, and viscosity of
fluid .in fact, watt density ‘s low or high direct reflect heater sheath temperature value, we
must keep the EPH cool at all process condition, not to overheat or damage material being
heated.
2. Keep EPH are sizing turbulent flow
The illustration below shows the effect of fluid velocity on film temperature and
indicates the importance of this heater design
requirement. In electrical heaters, all the heat delivered
by the elements passes into the fluid. The maximum watt
density at the surface of the heat source and the fluid
velocity over it should be in proper balance to avoid
excessive film temperature. From left picture, we know,
the high flow velocity will increase the heat transfer
coefficient and lower the heater sheath (wall) temperature
and extend the heater life, as flow velocity decreases, the
sheath (wall) temperature increase exponentially. Flow
type can be divided into three type based on Reynolds
number.(laminar: Reynolds less than 2300;transitional
(2300<=Re<=10000); turbulent:(Re>10,000)), Reynolds
increase as flow velocity increases, a good circulation
EPH design should keep it turbulent flow, As a general
rule of thumb, we design EPH via baffle or decrease
vessel diameter or multi heater in series to increase flow velocity. However we must pay
attention to if Reynold not exceed 2300, maybe there is a “dead zone” behind the baffles,
so we want to increase heat transfer via add baffles, it must be keep the heater turbulent
type in all process conditions.
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3. Calculate vessel wall temperature
When heating air or gases in insulated vessel or circulation heaters, radiation from
EPH causes the vessel wall temperature exceed the outlet gas temperature. Radiation
heat transfer from heating elements equal to radiation heat transfer absorbed by gas adds
convective heat transfer by vessel wall. Excessively high wall temperature can create an
unsafe or dangerous condition, though radiation heat from heating elements decrease
heating elements temperature , it also raises vessel design temperature, we know,
material stresses decrease with temperature increase, it raise vessel cost.
In order to reduce vessel cost, in most cases (particularly heating air or gas),
, we could add radiant heat shield between shell and bundle, Heat shields shall be
stainless steel in the normally conditions. Heat shield shall not be permitted to contact the
shell they protect or the elements, the minimum clearance between element and shield
shall be equal to the ligament between elements, the clearance between the vessel
minimum internal diameter and the heater element support structure and/or baffles shall
not be less than twice that recommended by TEMA, There shall be a positive method of
maintaining the shield/shell clearance (e.g., by the use of spacers).Unless a detailed
thermal analysis is performed to establish the heater/pipe shell temperature (Ts), the
following simplified method may be used to provide a conservative estimate for Ts at any
point along the shell:
Ts (NO heat shield) = (element sheath temperature + local gas mean temperature)/2
Ts (with heat shield) = (heat shield temperature + by-pass gas temperature)/2
Where heat shield temperature = (element sheath temperature + local gas core
temperature)/2
The by-pass gas temperature is the main gas mean temperature at the start of the shield.
The outlet temperature will be the weighted average of the bypass and core gas
temperatures.
4. Prevent heating elements failure
Moisture element: Magnesium oxide, known as “MgO” or “mineral insulation”, is a fine,
granular powder in bulk form, it is “filled” between the resistance elements and the heater
sheath, its advantage is goods thermal conductivity and great dielectric strength.
But one major disadvantage is hygroscopic, hygroscopic means that MgO absorbs any
humidity or moisture which comes in contact with it, moisture reduces MgO’s dielectric
strength, if enough moisture is absorbed, the resistance element could short out to the
metal sheath and destroy the heater or blow fuse .moisture can be removed by following
ways:
1. Heating up stand-off section
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Use a soft flame, e.g. propane burner, heating up the stand-off section to 248℉.
2. Placed desiccant bags in terminal box
Placed a lot of desiccant bags in the terminal box for some days or one week, then
recheck the insulation resistance again, if still exist problems, please replace it with new
desiccant bags. Keep some days until the insulation resistance rise up
3. Low –volts heating
Applying low-volts (such as SCR with angle fired) to heating elements, it also helps to
remove the moisture.
4. Heating heater bundle by fluid
Heating up the switched off heater by process fluid to 248℉
5. Blanket purge nitrogen to terminal box
Blanket purge nitrogen to terminal box, it help to get rid of moisture.
6. Pull out the heater bundle, place into oven
Finally, if above ways is still not solve the moisture question, we could pull the heater
bundle and place into oven “baking” to get rid of the water.
7. Replace the failure elements by spare parts
Normally condition, manufacturer provide 10% spare parts in the heater bundle, we could
disconnect the failure elements and replace it by spare parts.
Open and short circuits: Open and short circuits are common failure ways, open circuits
are deemed the operation resistance temperature exceeds the maximum withstand
temperature of resistance wire, the main reasons are resistance wire watt density is too
high or control way improper. Short circuits are normally deemed MgO absorb moisture.
Tube rupture: At present a relating small amount of information has been published about
EPH tube rupture, the primary reason is because the inside of EPH is filled with
compressed MgO, common viewpoints are:
1.Fouling built up on the heating elements, for example, when heating water with
bicarbonate, calcium carbonate will built up on the heating elements surface, however the
expanding coefficient is difference between the calcium and elements, when heating,
some day the calcium carbonate crack suddenly, however the temperature of element is
still high, water contact with high temperature elements
through crack suddenly, the water become vapor rapidly
then burn a hole or rupture the elements, it is like we
pour the water into high temperature iron pot ,the pot
often been burned a hole.
2. Vibration and attrition: If pump pressure is too high or
fluid specific weight too big, it will cause the heater
bundle vibration, and further cause attrition between
elements. See left picture is pump pressure too high
cause heater tube rupture (heating molten salt).
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5. Choose reasonable construction
According to different applications, we recommended different heater construction,
please see following two construction:
High temperature process flange closure:
Heating elements are tig welded to process flange, suit standoff distance is allowed for
Process Flange
Process Flange
Epoxy seal
Epoxy seal
Tig Weld
hemetic plug
MgO
Tig Weld
Heating elements
MgO
hemetic plug
Tig Weld
Riser Sleeve
Terminal Box
sleeve
Terminal Box
Flexible Lead wire
cold pin
cold pin
STAND-OFF
STAND-OFF
HIGH TEMPERATURE PROCESS FLANGE CLOSURE
sleeve
Flexible Lead wire
HIGH PRESSURE PROCESS FLANGE CLOSURE
cooling of terminal box connections. Elements are seal welded to back side of box. This
Construction avoids any possibility of condensation of corrosive vapor in standoff zone.
High pressure process flange closure:
Heavy gauge “riser” intermediate tube is tig welded to process flange, and extends into
the standoff zone. At end of “riser” a tig weld seals to the heating element under good
welding conditions to insure integrity of seal under frequent cycling and difficult
temperature/pressure requirements.
Increase creepage Gap between cold pin and element sheath
As I above mentioned, MgO is hygroscopic
Wire to Pin 360°
and absorbed moisture easy cause current
Fusion Weld
leakage between cold pin and sheath, two
way could reduce this phenomenon:
1.Add sleeve at the end of element, then
filled with epoxy squeezed in place with a
ceramic plug.
Nickel-Chromium
2. Adopt bludgeon cold pin, one end
Resistance Wire
matching coil, see right picture, another end
Cold Pin
is small diameter cold pin for connection.
Above two way increase creepage gap
between cold pin and element sheath
Metal Sheath
Threaded Stud
MgO Insulation
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6. Routine Maintenance
Periodically check all electrical connections, including field and factory-made
connections for tightness, and insulation resistance are test at least once a year.
Periodically inspect for leakage and retighten Immersion Heater flange bolts when required.
Inspect the terminal box and conduit connections for evidence of water leaks or moisture
collection. Tighten connections as required. Clean up any corrosion. Do not continue using
a heater with signs of damage. Where the heater is installed in cold climates, consider
safety precautions to prevent damage due to freezing fluid.
When the heater is not in service. Where buildup of solids on the heating elements or
significant corrosion is expected, periodically remove the Immersion Heater to inspect the
vessel and heating elements. Do not continue using a heater with signs of damage. Before
removing, note the orientation of the flange relative to the vessel. Place a reference mark
on the vessel and Immersion Heater or use other methods, such as drawing a sketch
showing the conduit hub orientation, to ensure proper orientation when reinstalling.
7. Choose right control way
On/off cycling will cause heating elements’ expansion and construction, the thin wire
inside the heating element weakens, using SCR, cycling less than 1 seconds, stabilize the
elements’ temperature and increase the heater life. On the other hand, we must decrease
the sensor response time, adopting two loop control, the temperature of heating element
surface act as primarily control, the set point is adjusted by the outlet temperature
controller, improve sensor response rate and effectively eliminates temperature excursion.
Electric process heater is a set system need all aspect support, need mechanical engineer
construction design, need electrical control design and need chemical process fluid
process analyze, above tips hope it is useful to our customer and manufacturer.
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