What is thermal oil?

Heat transfer oil, also called thermal oil, is the oil that circulates through a loop of pipes and carries heat from a heater to the machines, such as the press, the tank or the roll, well above 100 °C. It circulates like the water in a car engine, but it brings heat to the machine instead of taking it away, and it stays liquid far above the boiling point of water, without high pressure. Below: how the loop works, why the oil wears out and how to check yours.

What is heat transfer oil used for?

Heat transfer oil is used to heat industrial processes above 100 °C without the high pressure that water would require. To see why that matters, think of the kitchen.

In an ordinary pot, water stops getting hotter at 100 °C, because it boils and turns to steam. To go beyond that, a pressure cooker traps the steam, and the pressure inside rises. A plant works the same way: heating a process to 200 °C with water would require a whole piping system built for high pressure, with vessels, valves and pipes designed for it.

Many people assume the oil carries more heat than water. Water actually carries more heat in each liter; its problem is pressure. Heat transfer oil reaches those temperatures still liquid, with no pressure cooker: the pressure in the loop stays low, little more than what the pump needs to move the oil.

In many uses the oil delivers its heat through a heat exchanger, the equipment where the hot oil flows on one side of a metal wall and the product on the other, without mixing. Where heat transfer oil shows up:

• plywood, MDF and rubber presses, with platens heated from inside;

• asphalt plants, to keep the binder (the asphalt itself, the black pitch) fluid in tanks and lines;

• rolls and calenders (heated rolls that press and smooth the sheet) for paper, fabric and plastic;

• reactors and tanks in the chemical, resin and paint industries;

• continuous fryers and food ovens, through a heat exchanger, with no contact with the food.

In the food industry, the project may require a fluid approved for incidental food contact; in that case, confirm with the Technical Team.

If you want the number: water stays liquid at 200 °C under about 15.5 bar absolute, roughly 15 times the air pressure at sea level (IAPWS-IF97 steam tables).

What is heat transfer oil?

Heat transfer oil is an oil formulated to circulate hot and deliver heat for years. Think of cooking oil that withstands frying, now made to stay hot all the time.

It is also called thermal oil or heat transfer fluid, and it has two parts. The first is the base oil, which makes up almost the whole product. The second is the additives, chemicals in small doses that give or reinforce a quality.

Most heat transfer oils use mineral base oil, taken from petroleum and highly refined to last under heat (see how base oil is refined).

There are also synthetic heat transfer oils, built by chemical reaction from chosen molecules. Synthetic is for when the process goes beyond what mineral oil can take. If the maximum temperature of your loop, given by the heater manufacturer, is above the use limit stated for mineral oil, the case belongs to synthetic (see the difference between synthetic, semi-synthetic and mineral oil).

The main additive is the antioxidant, which slows the reaction of the oil with oxygen in the air. From the finished heat transfer oil, the plant expects five things:

1. stay liquid and give off little vapor at working temperature;

2. carry a lot of heat in each liter;

3. pass that heat quickly to the tube wall;

4. flow easily through the pump;

5. withstand thousands of hours hot without breaking down.

Kelpen Oil formulates the Thermmo line of heat transfer oils, in mineral and synthetic versions.

If you want the number: API 1509 splits base oils into five groups. Solvent extraction usually gives Group I, and hydrogen refining gives Groups II and III. Group IV gathers the polyalphaolefins, which are synthetic, and Group V everything else, such as naphthenic oils and the synthetic aromatics common in heat transfer fluids.

How does a heat transfer oil system work?

The system is a closed loop of pipes in which the same oil heats up, delivers heat and returns, thousands of times. It is like blood, which leaves the heart, carries heat to the whole body and always returns the same way. The path of the oil:

1. The pump pushes the oil to the heater (on the plant floor, often called the thermal oil boiler), a pipe wound in a spiral (the coil) heated by a burner or by electric resistance.

2. The oil leaves hot and goes to the consumers: the press, the tank, the roll.

3. There it gives up heat through the wall of the equipment and returns cooler, through the return line.

4. The oil goes back to the pump and the cycle starts again.

Off this path, the loop has an expansion tank, at the highest point, connected to the return line (the cooler side) before the pump. What is it for? Hot oil takes up more space than cold oil (this is thermal expansion). That extra volume goes to the tank, which also keeps the pump always full of oil.

The tank also has a connection on top, to the air or to a gas; what sits on that side changes the life of the oil, and you will see this further on.

Figure 1. The heat transfer oil loop. The pump pushes the oil to the heater; the hot oil (red) gives up heat to the consumer and returns cooler (blue). The expansion tank sits at the highest point, connected to the return line before the pump.

The oil passes through the heater thousands of times, and that is where it suffers most.

Where does heat transfer oil suffer most inside the loop?

Heat transfer oil suffers most in the layer touching the wall of the heater tubes. Think of a frying pan on the stove: the oil at the bottom heats up sooner and more than the oil on top, and stirring spreads the heat. In the heater the same happens, and the tube wall is hotter than the oil flowing through the middle.

That thin layer of oil against the wall is the hottest, and its temperature is called the film temperature: the highest the oil faces in the loop.

What keeps that layer under control is the flow rate, how much oil passes per second. With good flow, the oil runs agitated (turbulent flow). Even so, a nearly still layer remains against the metal. The agitation keeps that layer thin, and it exchanges heat quickly with the rest of the oil.

Figure 2. The layer of oil against the tube wall is the hottest. With good flow it stays thin; with low flow it thickens and gets hotter.

Four situations heat that layer too much:

1. low flow, from a worn pump, a clogged filter or a valve not fully open;

2. a poorly adjusted burner, with the flame hitting the tube wall;

3. deposit stuck inside the tube, which heats the metal and holds still oil against the wall (the origin of this deposit comes right below);

4. stopping the pump while the heater is still hot: the oil sits still inside the hot tube and cooks there. That is why the burner goes off first, and the pump keeps running until the oil cools to a safe temperature.

If you want the standard: the ISO 6743-12 classification of heat transfer fluids gives the use limit by bulk temperature, the average temperature of the oil leaving the heater. In the heater, the film temperature is always higher than this average, and it is what limits the fluid.

Why does heat transfer oil degrade?

Heat transfer oil degrades for three reasons: excess heat, oxygen from the air and contamination. It is what happens to frying oil used too many times, which darkens and starts to smoke sooner.

1. Excess heat. Heat breaks the molecules apart: this is thermal cracking. The small pieces evaporate easily, and the oil thins and starts to give off flammable vapor at a lower temperature. That limit is called the flash point: the lowest temperature at which the oil vapor ignites for an instant when a flame is brought near.

   Other pieces join into large molecules, which thickens the oil. When those molecules go beyond what the oil can keep dissolved, they stick to the hot wall and become coke, a hard crust of carbon.

2. Oxygen from the air. Hot oil reacts with oxygen, and this reaction is called oxidation. It is the same one that turns butter rancid outside the fridge, sped up by heat, and it leads to acids, thicker oil and sludge, a pasty residue that sticks to pipes and valves.

   Parts made of copper, brass or bronze in contact with the hot oil speed up this reaction, so the loop avoids those alloys.

3. Contamination. Water or process product (such as asphalt) gets in through a leaking exchanger, and water also gets in through the expansion tank. Another contamination is a different oil added by mistake when topping up. Water suddenly turns to steam inside the hot oil and hammers the pump and the piping.

   That is why the first start-up, and every start-up after maintenance, heats the system in stages, so the water leaves through the expansion tank.

Coke also closes a vicious cycle. It insulates the wall: the tube metal gets hotter, which can deform or burst the tube, and the oil trapped and stagnant in the deposit breaks down even more, which forms more coke.

Figure 3. Heat splits the oil molecule. The light pieces evaporate and lower the flash point; the heavy ones thicken the oil and become coke on the wall, which heats the tube metal and forms more coke.

How this shows up in your plant:

• the press or the tank takes longer to reach temperature;

• the burner stays on longer;

• the pump makes more noise or draws more current;

• the sample comes out dark and smells burnt;

• more vapor than before comes out of the top connection of the expansion tank.

Open or closed system: what changes in the choice?

The difference is in what fills the space at the top of the expansion tank: air in the open system, or gas in the closed one. With air, the oil oxidizes more and its vapor meets oxygen, so the choice of oil must also look at the flash point.

In the open system, the expansion tank has a vent, an opening to the air, and the oil touches the air there. In the closed system, the tank is sealed, and the empty space receives an inert gas, one that does not react with the oil. It is almost always nitrogen (N₂), the same gas that makes up most of the air we breathe. What harms hot oil is the oxygen (O₂) in the air, which reacts with it; nitrogen does not.

Figure 4. In the open system, the vent lets air touch the oil in the expansion tank. In the closed system, the tank is sealed and nitrogen fills the space above, through a gas connection. Each tank connects to the return line of the circuit in Figure 1.

The two consequences:

• Oxidation: in the closed system, without oxygen, the reaction that forms acids and sludge hardly happens, and the charge lasts longer.

• Safety: in the open tank, the oil vapor meets the air. For that vapor never to catch fire, the flash point of the oil must stay well above the temperature of the oil in the tank.

That is why the tank is connected to the return line, the cooler side, and the oil inside it hardly circulates: cold, it oxidizes less and gives off less vapor. In practice, the margin is checked in the design and again in every analysis, because the flash point drops when the oil cracks.

One safety rule applies to every loop. In the heater and in the lines, the oil may work above its own flash point. Inside the tube, with no air, it does not catch fire.

The danger is a hot leak, especially one that soaks the thermal insulation, the blanket wrapped around the pipes: there the oil reacts slowly with the air, heats up and can ignite by itself, with no flame. A leak is fixed at once.

If you want the number: in the specification of new oil, the flash point is usually measured in a Cleveland open cup, in which the oil is heated in an open cup and a flame passes over it (ASTM D92 and NBR 11341, the Brazilian standard for products with a flash point from 79 °C to 400 °C). The margin over the tank is always checked by the same test method. The open cup is used to follow the oil in service because what matters is the trend: the result is compared with that of the new oil, measured by the same method.

What is the viscosity of heat transfer oil?

The viscosity of heat transfer oil is the one the heater manufacturer indicates, and it is usually low. Viscosity is the resistance of the oil to flowing: honey has high viscosity, water has low.

Industry expresses viscosity by the ISO VG grade, the number in the name of the oil (for example, 46 or 68), measured at 40 °C (learn what ISO VG means).

At working temperature, well above those 40 °C, every heat transfer oil becomes very thin. The difference between grades shows up more during heating: the oil, still cold, is thick, the pump moves less oil, and the burner is already lit. The heater design counts on the indicated grade so that the flow can cope with this. A thicker grade, put in on your own, reduces the flow and heats the layer at the tube wall.

The Thermmo line is offered in different ISO VG grades; the Kelpen Oil Technical Team confirms the one for your project.

If you want the standard: under ISO 3448, the ISO VG grade is the kinematic viscosity at 40 °C in cSt (centistokes), with a tolerance of 10% on each side of the midpoint. For example, ISO VG 32 runs from 28.8 to 35.2 cSt.

How can you tell whether the heat transfer oil is still good?

You know whether heat transfer oil is still good by analyzing a sample and comparing it with the new oil. It works like a routine blood test, which finds the problem before the symptom.

The sample is taken with the oil circulating, always from the same place, through a sampling point with a cooler, so that the oil comes out cold. Hot oil burns the skin and, in contact with the air, can catch fire. It also loses its light compounds in the bottle, and the analysis ends up showing a better oil than the one in the loop.

The reference is the new oil: each reading is compared with it, and what counts is the direction of change from one sampling to the next, the trend (see the types of oil analysis). Record every top-up of new oil, because it dilutes the used oil and changes the reading.

One of the readings is acidity. Oxidation forms acids, and the laboratory measures how much acid is in the oil, the total acid number (TAN). Together with viscosity, flash point and water, it makes four readings that cross-check like this:

1. viscosity and acidity rising together: oxidation;

2. viscosity rising with acidity unchanged: large molecules from cracking, on the way to coke, or mixing with a heavier product;

3. flash point and viscosity falling: light compounds from cracking, or mixing with a lighter product;

4. water present: ingress through the expansion tank or a leaking exchanger, with a risk of steam surges.

Figure 5. The cross-reading of the analysis, always compared with the new oil: each combination of results points to a cause.

The charge is replaced according to its condition: when the readings leave the range agreed with the oil supplier and with the heater manufacturer, or when the trend accelerates. The calendar alone does not show that.

If you want the standard: DIN 51529 covers the testing and evaluation of used heat transfer fluid. Laboratories that follow this standard measure the flash point in a closed cup (ASTM D93, Pensky-Martens): the covered cup traps the vapor, and so the test detects the light ends from cracking better and gives a lower value than the open cup. Always compare results from the same method.

These laboratories also measure:

• insolubles (the solid dirt that does not dissolve in the oil);
• carbon residue, which together with the insolubles shows coke forming;
• the boiling range (the temperatures at which the oil starts and finishes boiling), which separates light and heavy ends;
• TAN, which in Brazil is measured by NBR 14448, based on ASTM D664.

How does Kelpen Oil check heat transfer oil in service?

Kelpen Oil analyzes heat transfer oil in service in its own laboratory in Arujá (SP), the same one that approves every batch it formulates. There are 25 analyses under ASTM, NBR and ISO standards, in the logic of the lubricant analysis laboratory. Analysis of oil in service is a paid service.

Along with the sample, it is worth sending the product name and, if there is one, a sample of the new oil from the same charge, which serves as a reference. In heat transfer oil, five tests tell the story of the charge:

Which heat transfer oil should you use in each system?

First comes the maximum process temperature, which tells whether mineral oil is enough; then the type of loop. The ISO VG grade is always the one the heater manufacturer indicates, never changed on your own.

The Kelpen Oil Thermmo line has mineral and synthetic versions; the Technical Team indicates the version and the grade for your loop.

Frequently asked questions

Can I use hydraulic oil as heat transfer oil? It is not recommended. Anti-wear hydraulic oil carries additives made to protect pumps and valves at moderate temperature; on the hot wall of the heater, above the temperature they were designed for, they decompose and form deposit. Heat transfer oil is formulated to stay hot all the time.

When should heat transfer oil be changed? By analysis: when viscosity, flash point or acidity leave the range agreed with the oil supplier and the heater manufacturer, or when the trend between two samplings shows wear accelerating. When changing, drain with the system warm: the oil still flows well and will not burn anyone. Afterwards, check whether there is coke or sludge to clean out before the new charge.

Can I top up the level with new oil? Yes, with the same product as the charge, recording every volume added. High consumption points to a leak or to light compounds leaving through the top connection of the expansion tank; topping up without recording hides that signal, and the new oil dilutes the used oil, which changes the reading of the next analysis.

Is heat transfer oil suitable for an asphalt plant? Yes. The plant uses heat transfer oil to keep the asphalt fluid in tanks and lines. The same care applies as in any loop: know the type of expansion tank, respect the design grade and follow the charge by analysis.

Glossary

• Film temperature: temperature of the oil layer against the heater tube wall, the highest in the loop.

• Thermal cracking: breaking of the oil molecules by excess heat.

• Flash point: lowest temperature at which the oil vapor ignites for an instant when a flame is brought near.

• Coke: hard crust of carbon that sticks to the hot tube wall.

• Sludge: pasty residue from oil oxidation, which sticks to pipes and valves.

• TAN (total acid number): measure of how much acid is in the oil, which rises with oxidation.

Talk to the Technical Team

The Kelpen Oil Technical Team evaluates the type of system, the maximum process temperature, the grade requested by the heater manufacturer and the history of the charge, and indicates the version and the grade of the Thermmo line. If the oil is already in service, the Arujá (SP) laboratory performs the analysis of the oil in service, as a paid service.

Before buying the next charge, note three things about your loop: whether the expansion tank has a vent to the air or receives nitrogen, the ISO VG grade the heater manual asks for and the temperature of the oil at the heater outlet. With them, the choice comes out right. Send this article to whoever looks after the heater in your company.