Thermal oil is currently the most appropriate technological alternative for all types of industry where indirect heating is required. The durability of the equipment and installations is practically unlimited and can be applied in all types of sectors.
Thermal oils were introduced on the market in 1930. The composition of the thermal fluids is based on eutectic mixtures, diphenyl oxides, biphenyls, etc. They have high boiling points. Their practical uses are in the liquid phase in the range of 200°C to over 400°C.
Types of heat transfer fluids in industry Although the most widely used heat transfer fluids are oils, other substances are also used.
Thermal oil properties:
Good heat transfer properties Good oxidation resistance Good anti-foaming properties Chemical stability Long lifetime Weak vapour pressure Low volatility Low volatility Low viscosity Low melting point Low oxidising power: must not attack metals Must not be toxic or Low dangerous price For all these properties it is the heat transfer fluid used in solar thermal power plants.
Degradation is caused by three phenomena: Contamination Oxidation Cracking Contamination It occurs when a fluid with poor thermal stability is mistakenly added (excessive water, or hydraulic fluids) to the system. The immediate result can be cavitation of one of the pumps.
Oxidation Reaction with oxygen causes the formation of carboxylic acids, which form sludge. The reaction is favoured by the presence of oxygen, by the temperature and by the presence of catalysts (certain metals such as Cu, or the sludge itself).
Sludge has several effects:
They clog filters Erosion of pumps, if they pass through the filters They are deposited on valve seats They catalyse the oxidation reaction They lower the flash point Hydrocarbon cracking This is the thermal or catalytic utilisation of hydrocarbons to form lighter or even heavier hydrocarbons (by joining two light hydrocarbon molecules).
The process produces three types of substances:
Coal Low molecular weight hydrocarbons High molecular weight hydrocarbons All of them lower the flash point. Coal and high boilers can deposit on the exchangers, reducing the heat transfer capacity.
The four most critical tests to perform to check if our oil is degraded are:
Acid number, shows the current level of oil oxidation, being able to anticipate dirt and sludge problems.
Viscosity, determines if the oil has become too viscous for efficient heat transfer.
Insolubles, analyses the possible alteration in the composition of the fluid, caused by cracking, which can form deposits.
Flash point, accepting a variation of 10% of the initial value.
If the tests conclude that the oil has suffered degradation, this may be due to equipment or operational anomalies. Problems should be corrected before they cause more serious problems, saving money and increasing system efficiency.
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