Selection and oil filling of thermal oil for electrically heated thermal oil heaters:
When selecting a thermal oil manufacturer, it's practical to choose brands like Great Wall, Kunlun, or Mobil for synthetic thermal oil. The boiling point of the thermal oil should exceed the system's high operating temperature. As different brands of thermal oil are strictly prohibited from being mixed, it's recommended to consider ease of procurement or purchase a surplus for backup. After the entire heating system is installed and inspected, perform system oiling. Add the thermal oil through the filling port of the expansion chamber. When oiling, open all valves in the system (except the drain valve), and then slowly inject the thermal oil into the system via the expansion chamber. Stop oiling when the liquid level shows about 1/2 of the expansion chamber height. After oiling, cover the filling cap, connect the overflow pipe, and carefully check the operation of all valves in the pipeline. It is strictly forbidden for there to be any oil leakage or seepage in the system.
The electrically heated thermal oil boiler, as a new type of special industrial boiler that is safe, energy-saving, low-pressure, and capable of providing high-temperature heat energy, is gaining rapid and widespread application.
Common accident隐患prevention and handling measures for electrically heated thermal oil heaters:
1. Prevention of potential hazards caused by the decline in thermal oil quality: Early deterioration of thermal oil quality due to poor operation and management of production processes is one of the common issues in electrically heated thermal oil furnaces. Some thermal oils degrade severely in performance after just one or two years of use. Prolonged use of poor-quality thermal oil leads to an increasing accumulation of carbon on the heating surface tube walls, causing a reduction in the internal diameter of the tubes, which in turn lowers the flow rate of the thermal oil. This gradual increase in circulation pump resistance results in a continuous decline in heat transfer efficiency, eventually leading to fouling, blockages in the furnace tubes, and ultimately to catastrophic events such as overburning, deformation, and pipe explosions.
One cause of thermal oil quality degradation is local overheating leading to thermal cracking, and the other is oxidation of the thermal oil. When thermal oil exceeds its specified high operating temperature, it can locally overheat, causing thermal decomposition and condensation, resulting in the precipitation of residual carbon, a decrease in flash point, deepening color, reduced viscosity, and decreased heat transfer efficiency, leading to coking and aging. When thermal oil comes into contact with oxygen in the air, it undergoes oxidation reactions, forming organic acids and condensing into a paste, increasing viscosity, not only reducing the service life of the medium but also causing acidic corrosion of the system. Currently, domestic thermal oil manufacturers have different quality standards, and the quality of the thermal oil produced varies greatly. For thermal oil in use, the "Technical Supervision Regulations for the Safety of Organic Heat Carriers" stipulates that the kinematic viscosity, flash point, residual carbon, and acid value should be tested at least once a year.
2. Hidden dangers of automatic control system failure: Currently, the domestic thermal oil electric heating element technology uses programmable logic controller (PLC) as the core control for the thermal oil control system. Under the working pressure of the circulating pump, the thermal oil at a certain temperature enters the heater from the inlet, where it is heated in the heating chamber of the heater, gradually increasing in temperature, and then flows out from the outlet, thus achieving the required process temperature for the thermal oil, further enhancing the automation level of the equipment.
In the control system, thermal oil is pumped throughout the system by a gear pump. Upon reaching the set level, the magnetic flip paddle level controller automatically shuts off the gear pump. At this point, the circulation pump and electric heater are activated to begin heating. Once the outlet oil temperature reaches the set value, the oil enters the user system for circulation. After being used by the equipment, the oil returns to the heater for reheating, repeating the cycle. This continuous operation constitutes the working state of the thermal oil.
These automatic control systems are the effective safeguards for the safe operation of electrically heated thermal oil boilers, yet it is often these safeguards that fail in practice due to a lack of safety awareness or economic reasons.
3. Potential Issues with Air and Moisture Ingress: During the process of refueling, oil replacement, and maintenance in electrically heated thermal oil heaters, air and moisture are prone to enter easily. During the heating process, these air bubbles expand and water evaporates, with their volume changes far exceeding the thermal expansion of the thermal oil itself. When the relative pressure is zero, the volume expansion of water vapor during evaporation is about 1,600 times its original size, and air expands approximately 5 times when heated to 270°C. If these air and moisture cannot be discharged in time, they will inevitably lead to an increase in operating pressure, posing a serious threat to system safety. Therefore, it is essential to strictly control the moisture and other components in the thermal oil. During the heating start-up process, the exhaust valve should be opened repeatedly to exhaust air, water, and the mixture of steam in the system with the thermal oil, and attention should be paid to controlling the rate of thermal oil temperature rise.
4. Leak隐患 between pipe flanges or valves: The connection points of pipe flanges or valves are prone to leakage in the thermal oil system. Due to the thermal oil's characteristics of being easily permeable and flammable, especially its strong adhesion to insulating materials made of porous materials. If a leakage occurs in the thermal oil and is not detected in time, it can easily lead to a fire. Therefore, it is essential to regularly inspect the integrity of the flange and valve connections and replace them promptly if any damage is found. Throughout the shutdown to operation process of the thermal oil furnace, due to significant temperature changes, flanges are prone to loosening and frequent leakage, which should be taken seriously. All sealing components in the pipeline system should be selected from flexible and metal-wound products with good sealing and flame-retardant properties, avoiding those containing rubber components. Insulating materials should be made of materials with low porosity and not easily soaked by organic heat carriers to prevent the insulating layer from catching fire in case of organic heat carrier leakage.
5. Potential隐患 of Electroheating Rod Coking: When using an electroheating rod to heat thermal oil, the rod's surface will quickly clog and carbonize, ultimately leading to the destruction of the rod and the waste of the thermal oil. The severe coking can be analyzed through heat transfer calculations:
For a 220V 4kW electric heating rod with an area of 0.125㎡，the thermal conductivity coefficient K is 67-114 W/℃·㎡ under natural convection, taking the higher value K=114 W/℃·㎡. Calculating Q=KF△t, the temperature difference between the rod surface and the heat transfer oil must be greater than 275℃ to transfer 4kW of heat to the oil. If the average temperature of the oil is 300℃, the rod surface temperature must reach 575℃. However, for a heat transfer oil with an allowable usage temperature of 340℃, the allowable film temperature in the heating furnace tube (equivalent to the rod surface temperature) must not exceed 370℃. Now, with the oil contacting the rod surface at 575℃, carbonization and agglomeration is certainly severe, and the lifespan of the oil is significantly shortened.
To prevent severe carbon accumulation on the surface of electric heating rods, the method is to increase the surface area of the rods, keeping the surface heat load within an appropriate range. By calculating, converting the two 2kW resistors in parallel within a 4kW electric heating rod into series, the total power of the resistors is 1kW when the terminal voltage is 220V. The surface area of the electric heating rod remains at 0.125m², and the required temperature difference for heat transfer is 69°C. Similarly, when the average oil temperature is 300°C, the surface temperature of the electric heating rod should be 369°C, which is sufficient to transfer 1kW of heat to the conductive oil. 369°C is close to the allowable film temperature of 370°C for the conductive oil, potentially alleviating coking issues. A good solution is to design and manufacture electric heating rods with a surface area that keeps the surface temperature below the allowable liquid film temperature of the conductive oil.
Electrically heated thermal oil furnace thermal oil heater system temperature control (Please refer to the temperature controller manual for detailed information)
1. Set parameters of temperature control instruments according to process requirements for heating and maintenance of temperature control.
2. Once the heat-conducting oil heater is in use, inspect the pipeline connections of the circulating system when the heat-conducting oil reaches 200°C or the required operating temperature (below 200°C). Be cautious to prevent oil leakage due to the expansion of fasteners heated. Secure the fasteners at the heating element's wiring connections while in a heated state to prevent overheating and damage to the connection terminals caused by expansion.
Attention! Heat-tightening the heating element fasteners should be done with the power off. Monitor and adjust the temperature control instrument's relevant parameters, implement automatic temperature control operation, until stable control is achieved.
