Low Flow in Thermal Fluid Heaters
One of the most important factors for a thermal fluid heating system is to maintain the proper flow through the heater, especially when a burner is firing into it. The flow profile inside the coils of a heater is like water flowing in a river. The water in the center of the river moves faster than the water near the riverbanks. The oil moving inside a pipe coil will act the same way. Oil in the middle of the pipe will flow with good turbulence but the oil near the inside of the pipe wall will move slower and thus absorb more heat from the heat source. A properly designed heater will take this “film temperature” into consideration when determining the nominal flow rate of a heater. All thermal fluids have temperature limits and the maximum recommended operating temperature (sometimes referred to as maximum bulk temperature) and the maximum film temperature will be listed in the physical characteristics for that fluid. This is the responsibility of the heater manufacturer and they should provide calculations to prove that film temperature calculations were made and accounted for.
If a heater operates with a flow rate below the design point, the film temperature will increase. If the low flow condition causes the maximum film temperature to be exceeded, the oil will overheat and “crack”. When oil cracks, it leaves hard carbon deposits on the inside wall of the pipe coil. This is called coking. This has two detrimental effects. One, the carbon layer on the inside wall of the pipe tends to act as insulation thus requiring a higher firing rate to achieve the same feedline temperature. Two, this carbon layer also reduces the inside area of the pipe coil thus increasing the resistance to flow. These two conditions, if left unchecked, will worsen over time and is sometimes referred to as the “death spiral” for the heater.
What to Do?
Correcting low flow conditions early is like treating cancer early. There is a better chance to “save the patient”. To state the obvious, low flow is corrected by increasing flow. The pump responsible for maintaining proper flow through the heater is called the primary pump. If you have determined that your flow is low by 5-10%, a larger impeller may be installed in the pump if it does not already have the maximum size impeller. You should look at the pump curve to see what effect this will have on the power requirement. Usually, the primary pump motor does not operate at its maximum power and there may be room to increase the impeller size. If the primary pump has the maximum size impeller, a VFD can be added to speed up the pump slightly. This may require an inverter duty motor. Sometimes, a larger motor will be required. If none of these options are viable, the primary pump should be replaced with a larger one. This will probably involve some piping changes at the pump.
The above recommendations are primarily associated with coking inside the heater coils. There are basically two ways that thermal fluid becomes problematic. One is coking as described above due to overheating. The other is oxidation which causes sludge deposits. Some manufacturers of thermal fluid offer cleaning fluids. These can remove sludge but will not remove the hard carbon deposits from inside heater coils.
I should mention that not all low flow conditions are caused by the heater. If the system has not been properly designed or if the system has been modified so that there is a substantial increase in pressure drop through the system, some system modifications must be considered. If the heater is in relatively good condition and the piping system is the problem, a user bypass can be added to ensure proper flow through the heater until the piping system can be corrected or cleaned.
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