Thermal oils as a heat transfer media is gaining acceptance replacing steam in many applications. This article is a brief on its advantages over steam , specifications and applications.
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What is Thermal Oil?
Thermal oils are heat transfer fluids that transfer the heat from one hot source to another process. This could be from a combustion chamber or from any exothermic process. The main application is in fluid phase heat transfer.
They are available in chemically different forms as:
Synthetic Oil, which are aromatic compounds.
Petroleum based oils, which are paraffin’s.
Synthetic glycol based fluids.
Thermal oils are available in a wide range of specifications to suit the needs of various processes. Currently available thermal oils have a maximum temperature limit of around 400 °C. There are Thermal oils that are in use in Cryogenic fields up to -100 °C.
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Steam vs. Thermal Oil
A heat transfer medium is required where direct heating of a process is not practically possible.
Steam was the prime choice as a heat transfer medium. The advantages of steam were the availability, low cost of water, with no environmental issues.
Effective Heat transfer by steam uses the latent heat. The saturation pressure dictates the temperature at which heat transfer takes place. To obtain higher temperatures the pressures have to be higher. For 350 °C, you will require a pressure of 180 bar. This will require higher thickness for the heat exchangers tubes. This increases the weight and thermal stresses and will require special manufacturing techniques. All this leads to higher cost.
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Thermal oil scores on this point. Even at temperatures of 350 °C, the pressure requirements are just sufficient to overcome the system pressure drops. This also decreases the pumping cost.
The system is simple in that it requires only a pump, an expansion and storage tank and the heat exchangers. A steam system on the other hand requires demineralized makeup water supply, drains, traps, safety valves, chemical additions and blow downs.
Using thermal oils eliminates all these complications along with issues of corrosion, scaling, fouling, and deposits in the heat transfer areas.
This is why thermal oil finds many uses in the process industry. All this leads to considerable reduction in cost.
Unlike steam, thermal oils also find use in applications where the temperatures are below zero °C.
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Thermal Oil Heaters
Thermal oil heaters functionally are similar to steam boilers. The combustion chamber that burns fuel oil, bio fuel, coal or any other fuel with all necessary safety devices is the same. Since there is, no evaporation the oil passes through simple heater coils placed in the radiant or convection zones. The oil pumped through these coils heats up and flows to the process heat exchanger. The cooler oil returns to a tank and then back to the pump. An expansion tank takes care of the thermal expansion. The tanks have provisions to prevent the oxidation and vaporization of the oil.
Marine applications use very compact heaters with helical coils.
Heaters downstream of gas turbines or diesel engines can function as heat recovery systems for use in suitable downstream process.
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Applications of Thermal Oil
Thermal oil finds use in a wide range of industrial applications.
This is in use the oil gas and chemical industry for almost thirty years and is part of many chemical reactions.
Storage and transportation of items like asphalt, which have low solidification temperatures, use thermal oil heaters . This is especially suitable for marine transportation of these materials.
Solar Thermal systems or Concentrating Solar Power (CSP) use thermal oils as a heat storage medium. This allows the power plants to produce power even when there is no sunshine.
Heating Biodiesel for transesterification, paper and board manufacturing, and noble metal extraction also makes use of this.
Thermal oil finds use in large scale baking and frying in industrial kitchens. The high and even temperatures that are possible help in retaining taste.
Industrial laundries are converting to the use of thermal oils instead of steam for most of the applications.
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Specifications of Thermal Oil
Thermal oils are available in a wide range of specifications. Different manufacturers produce different oils under different brand names. Different specifications suit various needs of the process and applications. Some of the commonly used brands are
Therminol from Solutia Inc.
Dowtherm from Dow Chemical Co.
Exceltherm from Radco Industries Inc.
Paratherm from Paratherm Corp.
The most important characteristic is the maximum temperature of service. The oil is thermally stable till this temperature. It should be borne in mind that the cost increases exponentially with increase in the maximum temperature limit. Currently the thermal oils are available up to 400 °C. Oils are also available for cryogenic applications up to -100 °C.
Apart from the temperature other characteristics that determine which type or brand of fluid to be used are the
Heat transfer co-efficient.
Environmental issues including toxicity, shipping restrictions and disposal methods.
Oxidation and degradation potential.
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Problems with Thermal Oil
As with any system, thermal oil heaters too have their share of problems.
Sudden trips or unplanned shutdowns of the system may cause the oil to overheat.
Overheating causes degradation of the oil and will form sludge.
This will necessitate cleaning and replenishing the oil.
Leaks especially in the combustion area can cause fire hazards.
Valves, gaskets and packings should be suitable for thermal oil use.
Piping and supports design has to take care of thermal expansion and thermal fatigue. Because this is low pressure application theses are sometimes not taken care of.
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As manufacturers develop and produce thermal oils with higher temperature limits and specifications industry is finding increasing applications, improving energy efficiency and reducing cost.