Selection and Maintenance of Heat Transfer Oil

 

Hot transfer oil (GB/T4016-83) is a kind of heat transfer medium. Because of its features of heating uniformity, accurate temperature control and can produce high temperature under low vapor pressure, as well as good heat transfer effect, saving energy, easy to convey and operate, it has been widely used in all kinds of occasions in recent years. Along with the multi usages and increasing consumption, heat transfer oil is developing in a direction of high efficiency, energy saving, cost less, service long and convenient to operate.

 

Main performance indexes when using heat transfer oil:

1. Kinematic viscosity (CST): the ratio of dynamic viscosity to density of heat transfer oil at a certain temperature is called the kinematic viscosity of the temperature.

The kinematic viscosity of heat transfer oil reflects its resistance to motion and make all difference of the fluidity of the oil at certain temperature. Requirement of kenimatic viscosity of heat transfer oil is on the basis of thermal stability, initial boiling point, flash point and other important indexes up to standard, and at the same time with low viscosity and high fluidity at both high and low temperature.

2. Flash point (℃): oil is heated up in a specified-structural container and vaporizes to form an ignitable mixture in air. When the air mixture reaches to a certain concentration, it will burn when meeting an ignition source. The transient combustion process is called flash burn and the lowest temperature at which flash burn happens is called flash point.

Flash point is an important safety and volatility index of keeping, conveying and using flammable fluids which reflect the possibilities of formation of high volatile and combustible vapors. Combustion liquid with low flash point features high volatile, easy to catch fire and poor safety.

3. Autoignition temperature (℃) of the oils is the lowest temperature at which they will spontaneously ignite in a normal atmosphere without an external source of ignition.

Heavy oils are much easier to get oxidized. So the lower the autoignition temperature is, the lighter the oil is, while the lower the flash point and ignition point, the higher the autoignition temperature.

4. Ignition temperature (℃) of a flammable fluid is the temperature at which its air mixture will continue to burn (at least for 5 seconds) after ignition by an open flame.

The difference between autoignition temperature and ignition temperature is that the latter needs open flame, so the former is higher than the latter.

Usually, for the same oil: autoignition temperature > ignition temperature >flash point

5. The maximum service temperature (℃) refers to the highest permissible service temperature of the main fluid detected at the heating outlet.

If the actual detected temperature is higher than this value, the most part of the main fluid will split. The maximum service temperature subjects to thermal stability test and determined by the performance indexes such as flash point, ignition point. Usually, splitting rate of heat transfer oil under use should be controlled within 10%, that is to say, the quantity of supplementary heat transfer oil caused by splitting should be less than 10% of the total quantity.

6. Other performance indexes such as density, pour point, initial boiling point, carbon residual, acid value, appearance, sulphur content and moisture are all should be taken into consideration.

 

Heat transfer oil selection of mould temperature controller

At present, the heat transfer oils of Shini mould temperature controllers mainly are: Mobil 605 (maximum working temperature is 200℃), Gaudi HT-32A (maximum working temperature is 300℃), Surgol DH603 (maximum working temperature is 300℃), South Sea MCH32 (maximum working temperature is 200℃), among which Surgol DH603 is still in trial period and waiting for further conformation.

The heat transfer oil selection should be based on several aspects of the machine such as working condition, economy, environmental protection, energy saving and all the above-mentioned performance indexes. The working temperature of Shini mould temperature controller is 20~30℃ lower than its oil flash point, when choosing the oil, there are several points for you to pay attention to:

1. Oil maximum service temperature

Get to know how is your oil maximum service temperature decided and whether there is a test report issued by authorized institution. Generally, working temperature of mould temperature controller can not exceed the oil maximum service temperature.

2. Evaporability and Safety of the oil

The flash point of chosen oil must meet the requirements; the initial boiling point is no less than the maximum service temperature and ignition temperature will be high. Thus, the safety of the heat transfer oil can be assured.

3. Oil quality

High quality heat transfer oil is light yellow liquid, clear transparent and without any impurity precipitation or suspended matter. It features excellent storage stability and there is no color change and sediment after being illuminated. 

4. Oil low-temperature fluidity

Low-temperature performance index should be based the located area and environment temperature of the equipment. Normally the pour point is not higher than - 9 ℃, low temperature kinematic viscosity (0 ℃ or even lower) is relatively low. Oil low-temperature fluidity asks for good liquidity under low temperature.

5. Heat-transfer

Oil with low viscosity, high density, high specific heat capacity and high coefficient of heat conductively can ensure good fluidity and heat transfer efficiency.

6. Choose quality guaranteed products

When purchasing heat transfer oils, the operation requirements, environment protection and economy should be taken into consideration. Domestic brand oils is inexpensive but substantial with good performances which can meet the requirements, the name brands are Great Wall, Kunlun, Shell Tongyi, CNPC,et.. For higher quality, you can choose foreign brand products, such as Solutia, Mobil, BP, Shell and Esso.

 

           Note: (1)ASTM：American Society for Testing and Materials;

                      (2)CST：kinematic viscosity units: 1CST=1mm2/s.

 

 Heat transfer oil maintenance

Some maintenance procedures must be taken during the operation to prevent heat transfer oil from going bad and prolong its service life. To ensure the system run safely and stably, please put the following practices into your regular preventive maintenance schedule:

1. Regularly inspection of leakage

Strengthen the field monitoring and make sure that heat transfer system is in good condition with no leakage. Regular check and repair the system to avoid oil aging caused by leakage or serious accidents caused by oil insufficient.

2. Make sure there is no coking

When the operation temperature is higher than the maximum service temperature, coking will appear in the oil pipes and with the coke increases; the oil pipe temperature goes higher and causes faster coking, then oil pipe temperature will go higher and higher, the thick oil pipe will lead to bad heat transfer performance or even explosion. Therefore, the temperature of heat transfer oil at the heat carrier outlet must be strictly controlled. The film temperature of heat transfer oil should never exceed the permissible value. Usually, the heating pipe mould temperature controller uses is made of copper in the market. Because coke is easily formed on the surface of the copper pipe which influences heat transfer efficiency and accelerate the ageing of the heat transfer oil, heating pipes made of stainless steel are produced to solve this problem.

3. Oxidation of heat transfer oil must be avoided

Heat transfer oil is easily oxidized and goes bad during high temperature operation, so it is often used in a hermetic closed system to avoid the high temperature oil meets the air and prolong its service life. While the temperature of air contact part oil must not be too high, or the oil will degrade quickly and its lifespan will be shortened.

4. No water or other impurities are allow into the heat carrier

With the heating of the heat carrier, the water impurities in the oil will rapidly evaporate which will lead to the pressure in the pipe increase sharply and lose control, in succession explosion will happen. So, the water and other impurity in the heat transfer oil should be removed before it is put to use. The oil will dewater when its temperature rise to 95℃, and control the heat rate within 5℃/h; keep the temperature for 24 hours when it rises to 110~140℃ to dewater and another 4 hours when it rises to 180~200℃ to remove the distillate. During the dewatering process, the pressure gauge pointer will oscillate frequently and the temperature at the outlet will fluctuate widely. Then you should pay attention to the oil level of the expander and refill to keep a normal oil level. After dewatering, the pressure gauge and temperature meter will tend towards stability.