Types of Degradation
There
are two ways that heat
transfer fluids degrade.
Oxidation occurs
when hot fluid comes in contact with air for a long
period of time. The acids formed
have poor thermal stability and will
further degrade at relatively low
operating temperatures (400°F) .
Severe oxidation will result in
sludge, heater buildups and
extremely high viscosity (fluid gets
thicker) at ambient temperature.
Overheating occurs
when a fluid is
heated above the maximimum film
temperature (the fluid temperature
on the inside wall of the heater tube
or the surface of the electric
element) that the fluid manufacturer
has specified. The high temperature
causes the fluid molecules to break
apart (or crack) which decreases the
viscosity (fluid gets thinner). Such
cracking also can cause pump
cavitiaiton. If there is severe
overheating, solid carbon particles
will form.
Causes and Cures
An
annual fluid analysis is the most
effective way to determine if your
heat transfer fluid is degrading. The
analysis should include an Acid
Number to measure oxidation and a
Distillation Range (or Boiling Point)
test to measure the amount of
cracked molecules (Low Boilers).
Once the problem is identified, the
causes and cures are pretty
straightforward.
Hot Expansion Tank/Reservoir
Oxidation only occurs in the
expansion tank (or reservoir). It is
normal for the tank to get hot during
start-up as the fluid expands into it.
Once the system reaches operating
temperature, the tank should be cool
enough to place a hand on it (for
most people the threshold of pain is
around 140°F). Smoke from the vent
or a popping sound from inside are
also signs that the tank is too hot.
Make sure that all of the warmup/
boilout lines running into the tank
are valved off when the heater outlet
temperature reaches 250°F. There
should only be one line connecting
the tank to the main loop during
normal operation. If the system has
a combination deaerator/expansion
tank, make sure to follow the
manufacturer’s instructions
regarding what to insulate. When all
else fails, install a nitrogen blanket
on the tank head space.
Burner Problems
One common cause of overheating is
flame impingment.
In most fired heaters, the radiant
zone (the coil surface that actually
faces the flame) is less than 30% of
the total heated surface area but
accounts for 50% to 70% of the total
heat transfer. This high heat-to-surface-
area ratio produces the
highest film temperature in the
heater. If the burner is not aligned
properly or if the flame disperser is
corroded, flame may impinge directly
onto the coil's surface. This causes
an extremely high film temperature
that can exceed the maximum
recommended film temperature for
the fluid. Heater tube failure can
result if the problem is not
corrected promptly.
Low
Flow
Reduction in flowrate through the
heater is another cause of
overheating. Low flow affects the
heat transfer from the heater tube
wall (or electric element surface) in
two ways:
1. There is less volume of fluid to
remove the heat so the overall
temperature of the fluid increases.
2. Reduction in turbulent flow also
reduces the mixing of the hot
fluid, at the tube wall or element
surface, with the cooler fluid.
This results in an increase in film
temperature.
Some of the more common causes
of low flows are plugged y-strainers
and malfunctioning or improperly
set bypass valves.
Degradation: Special Case
Contamination of thermal fluid systems is always self-inflicted. It happens
when a fluid with poor thermal stability (hydraulic fluid and glycol
are prime suspects since they are often stored in drums in the same area)
is added by mistake. An immediate reaction may be pump cavitation or
a “geyser” from the expansion tank vent. Many times, however,
the only indication that contamination has ocurred is an increase in
the amount of carbon solids in the fluid.
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