Thermal Oil Heat Recovery
As we have already learnt in our last technical info, water has excellent properties as a heat transfer medium.
Unfortunately, these advantages can only be used in a relatively small temperature range.
A common alternative for high- but also low temperatures are “organic heat carrier fluids”, the so-called “Thermal Oils”.
These media are designed to transport extreme temperatures in liquid condition. The following table shows the wide temperature range that Micocal Heat Transfer Fluids can cover.
Regardless of what kind of heat recovery concept you have in mind, the target is always to recover as much heat as possible. Therefore, the waste heat must be cooled down as much as possible. Unfortunately, there is not always a heat consumer available, that operates at low temperatures. Therefore, also for heat recovery plants, we often have to transfer high temperatures and that is the area where “Organic Heat Transfer Fluids” are able to fully exploit their strengths.
Since the temperatures for these systems are usually above the acid dew point or the freezing point of the heat carrier, we must pay particular attention to the upper limits, such as: “Admissible Film Temperature” and “Admissible Operation Temperature” of the fluid. While the film temperature is more a theoretical figure, used to describe the temperature of the oil film, which is in contact with the pipe wall, the operation temperature is one of the main criteria for the evaluation of the right heat carrier fluid. Other important figures are viscosity, specific heat capacity, steam pressure and heat conductivity, toxicological properties and hazard classes. Depending on the process, which will be heated, other specific properties like “Food Grade” or “Halal” certification can be another important parameter for the selection of the thermal oil.
In contrast to water, at which a high pressure builds up above 100°C, organic heat transfer media can be heated very strongly without a significant vapor pressure developing. Temperatures of up to 350°C can be transferred in liquid condition, in non-pressurized systems with these media. Theses capabilities open a range of advantages in high-temperature applications. Starting with the lower costs for the installation of non-pressurized systems through simplified approval procedures to outstanding flexibility and simple but exact temperature control. There are no chemicals or thermal energy needed for water treatment, organic heat transfer media are non corrosive, usually do not freeze and so on…
Additionally, in many cases, the heating surfaces in consumers cannot withstand the high pressures that would arise at similarly high temperatures in hot water steam systems.
Besides of all the advantages of organic heat transfer media, hydrocarbon chains can be decomposed by thermal overload or oxidation. But these potential disadvantages can be avoided by a professional and accurate design of the heat transfer system. The internationally applicable standard for the construction and equipment of such system is DIN 4754. Here, the materials to be used, dimensioning of piping and apparatus, system design as well as the safety engineering execution are regulated in detail.
If the plant is well designed and the maintenance is carried out properly, the service life of the Thermal Oil (Heat Carrier Oil) can easily be as long as the service life of the plant, which can easily reach more than 20 years.
Please note, that the “Operation Temperature” is the average temperature of the oil in you plant (feed line temperature + return line temperature / 2). If your plant is operated for example at 280°C feed line temperature and the return line temperature is 250°C, the average oil temperature would be 265°C.
If, for example, the admissible temperature of your oil is 320°C (normal figure for mineral heat carrier oil), the service life of the oil would already be more than 32 years.
In principle, Thermal Oil Heat Recovery Plants are very similar in design to Warm Water Heat Transfer Plants. The heat transfer fluid is circulated in a closed circuit by a pump from the heat recovery unit to the consumers and back. The thermal expansion of the oil is compensated by an expansion tank, which also provides the static pressure to the pump. Depending on the system concept, a safety valve may be required to protect the heat recovery unit from overpressure.
Due to the fact, that thermal oil plants are not pressurized, standard materials that do not have to withstand high pressures can be used for the pipes and apparatus. Since it is a closed circuit, no treatment is required. refilling the oil is only necessary to compensate for decomposition and minimal losses due to the mechanical seal of the pump.
As mentioned before, the measures to maintain the correct temperatures and to avoid oxidation is very important for the lifetime and to avoid unplanned downtime of Thermal Oil Plants. In one of our next technical information letters we will describe the important criteria and the best equipment for the design and execution of Heat Transfer Plants, with organic media, in detail.
PT Maxxtec Teknologi Indonesia has been established 2014 to bring innovative European Technologies to South East Asia. Our aim is to improve energy efficiency of local productions in order to increase cost efficiency and to help protect the environment.
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