service life of a heat transfer fluid and the operating
efficiency of the process can be increased by minimizing
thermal cracking, oxidation and contamination*.
All fired and electric immersion type heaters are
capable of exceeding the maximum recommended film temperature
of the heat transfer fluid under certain condition. Exceeding
the maximum film temperature for a period of time can
cause excessive cracking and premature fluid failure.
Excessive cracking can be minimized as follows:
Maintain Design Fluid Velocity at all Times
through the Heater - Pressure drop across the entire system
should be calculated when sizing pumps. System by-pass
valve response should be tuned to maintain design fluid
flow rate under all process conditions. Filters and strainers
should be properly located and monitored to prevent blinding.
Bring Cold Systems Up to Temperature
Slowly - Cold fluid can overheat if heater operates
at full power from start-up. Fluid temperature should
be increased in 25°F (15°C) increments until
fluid viscosity is less than 10 CPS (check fluid property
tables). Make sure this procedure does not heat up
the system more rapidly than manufactures recommendations.
Avoid Sudden Shutdowns - Allow
fluid to circulate until the heater outlet temperature
is a maximum of 250°F. If repeated power failures
occur, consider connecting pump to auxiliary power
Maintain System Instrumentation
- Failure of high temperature and/or low flow alarms
not only can cause overheating but can create potential
for equipment fires.
Check the Combustion Chamber
- Improper flame propagation or burner alignment can
cause overheating but can create potential for equipment
Severe fluid oxidation can create significant
equipment problems. In many cases, fouling or corrosion
of the expansion tank are the first signs that a problem
exists if routine fluid analyses have not been performed.
Minimizing oxidation is relatively simple:
Keep the Expansion Tank Below 140°F
- Smoke emitting from the expansion tank vent line
is usually a sign that the expansion tank is too hot.
Install either a cold seal pot on the expansion tank
vent or blanket the tank with a low pressure inert
gas. Never continuously operate a system with the
warm up valve open. The tank itself and lines leading
to it should be left uninsulated. A thermal loop seal,
reduced diameter piping (1/3 main pipe diameter) between
the expansion tank and main loop or thermal buffer
tank will reduce thermal currents.
Minimize Exposure of Hot Fluid to
Air - install either a cold seal pot on expansion
tank vent or blanket the tank itself with low pressure
Maintain Positive Net Pump Suction
Head (NPSH) at All Times - High vacuum due to
flow restrictions (such as plugged strainers) can
cause seal wear and allow air to be sucked into fluid.
Contaminants can promote fluid degradation
as well as cause operational problems. Contaminants can
enter the system in several ways:
New Systems - Make sure that
all fabrication debris (such as mill scale, weld spotter
and slag) or protective coatings are removed before
assembly. Pressure test system with either heat transfer
fluid or inert gas - Never pressure test with water.
System Cleaning** - Complete
draining from all system low points is required when
using organic based solvents or flushing fluids. At
least one full charge of new heat transfer fluid should
be used to flush the system if complete draining is
not possible. Water based cleaners must be completely
rinsed from the system with fresh water. Residual
water should be removed by draining and then purging
with hot, dry nitrogen down to a -20°F dew point
or vacuum. Boiling off residual water through the
expansion tank is not recommended since it will cause
Daily Operation - Always use
fresh fluid to top off system. Fluid “burped”
out the vent or collected in drip pans should be discarded.
Don’t mix fluids - if you have a problem, call
MultiTherm® at 800-225-7440.
*See Fluid Analysis
for more detail on mechanism and detection of fluid degradation.
** See System Cleaning Information for a review of cleaning
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