Introduction to Thermal Oils
Thermal oils or heat transfer fluids are widely used to carry thermal energy in process heating, metal working and machine cooling applications.
They are mainly used in high temperature process applications where the optimum bulk fluid operating temperatures of between 150ºC and 400ºC&
They are safer and more efficient than steam, electrical, or direct fire heating methods.
High temperature heat transfer oils can be categorized by chemical structure into two primary groups:
Synthetics: also referred to as ‘aromatics’, are man-made fluids, specifically tailored for heat transfer applications. They are formulated from alkaline organic and inorganic compounds.
“Hot Oils”: Hydrocarbon fluids, which are either standard, solvent refined mineral oil based fluids or fluids based on hydrocracked or synthesized hydrocarbons.
SYNTHETICS or “HOT” thermal Oils
The Two Process of Thermal Fluid Degradation
Over time, oil tends to break down by reacting with dissolved atmospheric oxygen. This oxidation starts a chain reaction which increases acidity and viscosity, darkens oil colour and leaves surface deposits and varnishes.
A more viscous fluid will be more difficult to pump, have poorer heat transfer characteristics as well as an increased chance of coke formation.
Oxidation is also accompanied by an increase in the acidity (TAN) of the fluid.
Thermal degradation occurs when oil is overheated past its boiling point. As the fluid boils, much like water, it produces lighter components in the form of vapours. Excessive overheating or cracking can cause reduced viscosity as well as pose safety concerns with the creation of the lighter components which in turn reduces the overall flash point, fire point and auto-ignition temperatures.
What happens during thermal degradation?
When thermal degradation occurs at extremely high temperatures, the effect is not only to break carbon-carbon bonds but to separate hydrogen atoms from carbon atoms and form coke. In this case, fouling of the heat transfer surfaces is very rapid and the system will soon cease to operate.
In heat transfer terminology, the two types of degradation products are known as "low boilers" and "high boilers".
The effect of the low boilers is to decrease the flash point and viscosity of the fluid as well as to increase its vapour pressure. The effect of the high boilers is to increase the viscosity of the fluid as long as they remain in solution. However, once their solubility limit is exceeded, they begin to form solids which can foul the heat transfer surfaces.
The Consequences of Oxidative & Thermal degradation
- Loss of System Efficiency & Increased Energy Costs
- Production Loss due to maintenance downtime
- Increased Maintenance Costs
- Increased thermal fluid costs due to premature degradation
- Increased Disposal Costs
National Costs of Thermal Fluid System Fouling
- US $14.5 Billion
- UK $2.5 Billion
- Germany $4.9 Billion
- France $2.4 Billion
- Japan $10 Billion
- Australia $463 Million
- New Zealand $64 Million
Symptoms of Thermal Fluid Degradation
- Reduced Flow Rates
- High Sludge Content In Filters
- Increase In Carbon Residue Content In Oil Analysis
- Reduction In Flash Point In Oil Analysis
- Increase In Oil Viscosity In Oil Analysis
- Increase In Tan (total Acid Number) In Oil Analysis
- Reduced System Efficiency
What Happens When Thermal Fluid Degrades
- Degraded thermal fluid becomes more viscous and the consequent reduction in system flow is the most common cause of overheating.
- As flows decrease, fluid velocity and turbulence also decrease, and the heat transfer fluid remains in prolonged contact with the heated surfaces.
- Even though the system’s bulk fluid temperature may not change much, the film temperature can rise dramatically-and quickly. This extra heat is no longer transferred as rapidly to the bulk of the fluid, and the fluid decomposes at the heated surface.
- As molecules at the film layer degrade, carbon can be formed. This carbon adheres to the heated surfaces and bakes on–thickening as the process repeats itself and successive layers are added. Difficult to remove, the carbon coating acts as an insulator both in heater tubing and on electrical heater elements, and can severely affect flows. The carbon that escapes or breaks away from the heated surface is carried throughout the system, and can lodge in restrictions, clog small channels and hang up control valves-further contributing to the problem.
- In addition, the fluid is now increasingly more viscous. Not flowing as well, it remains in contact with the heated surface longer, picking up even more heat and continuing to overheat, degrade and foul.
- Flows may decrease for a variety of reasons-contaminants lodged in valves, lines or strainers, bypass valves hanging up, pump problems, out-of-spec components or the wrong valves mistakenly closed.
Damage to pump volute casing caused by suspended particles in the thermal fluid
- Photo of the combustion area inside a thermal fluid heater. The black marks at the bottom are where the coil has leaked due to poor flow caused by high carbon contaminates in the oil and carbon deposits inside the coil, causing poor heat transfer from the flame.
- This caused hotspots and thermal fatigue, resulting in failure of the coil. Oil has subsequently been forced out of the fracture and burnt by the burner flame creating a further build-up of carbon deposits.
- Massive sludge build-up in an 11,000 litre thermal fluid system.
- This photo shows the socket where the level switch is installed on the side of the CDX tank.
- The thermal fluid had not been changed after it had severely degraded over time due to system maintenance issues.
- When the oil was eventually drained over 1 tonne of sludge was found in the CDX tank.
Importance of Removing Contaminants
- When Thermal fluid needs to be replaced, the common practice is to re-fill the system with Flushing Oil, run it for up to 150 hours before draining it out, cleaning the filters and replacing with new Thermal Fluid.
- The disadvantage of this method is that it only flushes the system out. It does not remove any of the carbon deposits and varnishes that are adhering to the surfaces and reducing the system efficiency.
- The use of an Oil UK Flushing Additive Concentrate is a more effective method in relation to both system cleanliness and costs.
Comparison of System Cleaning Methods