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Maximum operating temperature

When the heat transfer system is operating, the operating temperature is the average main liquid temperature of the conduction oil measured at the exit of the heater; the maximum operating temperature is the maximum main liquid temperature of the conduction oil at the heater exit. The recommended maximum operating temperature is determined by measuring the thermal degradation rate of the transfer fluid at high temperature. This is done by measuring the thermal aging of constantly heated transfer fluid at prescribed temperatures. The deterioration rate measured must conform to the standard value.
The maximum operating temperature is closely related to the degradation rate of the transfer fluid, which is a major factor affecting the service life of the oil. The degradation rate is also related to the oil composition at the high and low boiling points. The annual degradation rate at the maximum operating temperature from the economic viewpoint is usually regarded as 5%. If you use SCHULTZ’s transfer fluid within the recommended maximum operating temperature and maximum film temperature, and no pollution or oxidation occurs, your SCHULTZ’s transfer fluid will provide a long service life.


Maximum liquid film temperature

Liquid film temperature refers to the transfer fluid temperature on the heater’s heating surface, i.e. the temperature inside the transfer fluid boundary layer in contact with the furnace tube wall. The maximum allowable liquid film temperature is the maximum allowable temperature of the transfer fluid inside the boundary layer, and this temperature should not be exceeded anywhere in the system. The liquid film temperature is usually 20–30°C (30–50°F) higher than the main liquid temperature. If the liquid film temperature exceeds the maximum temperature, the thermal cracking rate of the transfer fluid inside the boundary layer will be high, and the furnace tube heating surface will overheat due to coking.


Auto-ignition temperature

The auto-ignition temperature of the transfer fluid is the minimum auto-ignition temperature without any external heat source (e.g. flame or spark) under barometric pressure. The auto-ignition temperature is a vital safety performance index for heat transfer fluid, indicating the transfer fluid’s auto-ignition tendency in the air in case of accidental oil leakage during operation.



The acidity levels indicate the total content of organic acid in the transfer fluid, and can be used for judging the degree of metal corrosion in the system. High acidity usually indicates arts and crafts material leakage in the system that pollutes the transfer fluid. If the expansion drum is not shielded by inert gas, the organic acid generated by the oxidation reaction between the transfer fluid and the air will raise the acidity level. The organic acid generated in this process forms sludge sediment, increasing the fluid viscosity, slowing down the flow velocity, and reducing the heat transfer efficiency. If the transfer fluid is contaminated by pollution and oxidation, the fluid should be eliminated from the system, and the system cleaned thoroughly to remove any residual acid or pollutant. The new replacement transfer fluid should be checked that it meets the specified acidity level.



Viscosity reflects the flow resistance of the transfer fluid in the system, and relates to the fluidity of the oil and its pumping ability at a certain temperature. It is directly related to the heat transfer effect. A change in oil viscosity usually means pollution, overheating or oxidative degradation of the system. High viscosity and poor fluidity will make the circulatory system difficult to start-up, and lower the heat transfer rate; the circulating pump will require more power and the system will consume more energy, increasing the operation costs. When the viscosity of the transfer fluid exceeds the prescribed value, the oil should be replaced or new oil added to dilute the existing oil. Long-term application of high-viscosity transfer fluid will degrade the oil quality. A transfer fluid of low viscosity indicates a high content of low-boiling-point substance, which will destabilize the system, producing air resistance and cavitation of the pump. The low-boiling-point substance can be removed by using an expansion drum exhausting with thermal fluid.



The moisture of the transfer fluid is a key index related to the smooth running of the heating equipment system. If moisture exists in the transfer fluid, the volume will expand abruptly due to gasification during heating, leading to pump cavitation, unstable pressure and unsmooth operation, and even blowout and ignition. A new system that has been washed will have residual water; the water can penetrate into the storage tank and the oil conduction system via the ventilation opening; increased moisture when the system is running indicates water leakage into the conduction oil. Water vapor can be produced by slowly heating the moisture from the expansion drum. If a large quantity of moisture has leaked from the system it should be replaced after the transfer fluid is removed and after drying.


Flash point

The flash point is the ignition temperature when water vapor and air mix during oil evaporation. The flash point of heat transfer fluid is an index indicating the product’s volatility and safety in relation to high-volatility decomposition products that are generated during system operation. The flash point indicates the temperature of oil vapor ignition in case of open fire; the lower the flash point, the lower the temperature at which the material will evaporate and be ignitable, and the poorer the safety. The flash point can be determined by two methods – open and closed cup: open cup flash point (Cleveland open cup method ISO 2592 and ASTM D92); close cup flash point (Pensky-Martens closed cup method ISO 2719, ASTM D93). A low-boiling-point substance in the system can lower the flash point of the transfer fluid. The flash point can be increased by expansion drum exhausting.


Low/high boilers

The gas composition at the low and high boiling points was determined using gas chromatography, based on the ASTM D 8887 method, and the distribution of the boiling scope or the organic heat transfer distillation curve was derived.