Briefly explain the working principle of vacuum furnace
The working principle of the vacuum furnace mainly includes the following aspects:
**1. Establishment of vacuum environment**
The vacuum furnace extracts the air in the furnace through the vacuum pump system to achieve a high vacuum state. Commonly used vacuum pumps include mechanical pumps, diffusion pumps, molecular pumps, etc.
1. Mechanical pump: As a front pump, the pressure in the furnace is first reduced to a certain extent. It extracts the gas from the furnace and discharges it into the atmosphere through mechanical movements such as rotating blades or pistons. The vacuum degree that a mechanical pump can achieve is generally around 10⁻² - 10⁻³ Pa.
2. Diffusion pump: Further improve the vacuum degree on the basis of a mechanical pump. The diffusion pump uses a high-speed jet of oil vapor to bring gas molecules from the furnace to the outlet of the pump. The oil vapor flow is cooled and condensed at the outlet of the pump, while the gas molecules are discharged. The vacuum degree that a diffusion pump can achieve is generally around 10⁻⁴ - 10⁻⁶ Pa.
3. Molecular pump: For higher vacuum requirements, molecular pumps can be used. Molecular pumps transfer momentum to gas molecules through high-speed rotating blades and discharge gas molecules from the furnace. The vacuum degree that molecular pumps can achieve can be as high as 10⁻⁸ - 10⁻¹⁰ Pa.
**2. Heating system**
The main heating methods of vacuum furnaces include resistance heating, induction heating and microwave heating.
1. Resistance heating: Heat is generated after power is turned on by installing heating elements such as resistance wires or resistance belts in the furnace. The advantages of resistance heating are simple structure, low cost and relatively accurate temperature control. The disadvantages are that the heating speed is relatively slow and the heating elements are prone to aging.
2. Induction heating: Using the principle of electromagnetic induction, an alternating magnetic field is generated in the furnace, causing eddy currents to generate heat inside the heated object. The advantages of induction heating are fast heating speed, high efficiency and good temperature uniformity. The disadvantages are high equipment cost and certain requirements for the shape and size of the heated object.
3. Microwave heating: Use the energy of microwaves to make the molecules inside the heated object move violently and generate heat. The advantages of microwave heating are extremely fast heating speed, selective heating, energy saving and environmental protection. The disadvantages are complex equipment, high cost, and certain restrictions on the materials of the heated objects.
**Three, temperature control system**
The temperature control system of the vacuum furnace is mainly composed of temperature sensors, controllers and heating power supplies.
1. Temperature sensor: used to monitor the temperature in the furnace in real time. Common temperature sensors include thermocouples and thermal resistors. Thermocouples use the thermoelectric effect of two different metals to measure temperature, and have the advantages of fast response speed and wide measurement range. Thermal resistors use the characteristics of the resistance of metals changing with temperature to measure temperature, and have the advantages of high measurement accuracy and good stability.
2. Controller: According to the signal fed back by the temperature sensor, it is compared with the set temperature value, and the temperature in the furnace is adjusted by controlling the output power of the heating power supply. The controller usually adopts PID (proportional-integral-differential) control algorithm to achieve precise temperature control.
3. Heating power supply: According to the instructions of the controller, the appropriate current and voltage are provided to the heating element to control the heating power. The stability and accuracy of the heating power supply play a key role in temperature control.
**Fourth, cooling system**
After the vacuum furnace completes the heating process, it needs to be cooled to reduce the temperature in the furnace. The cooling methods mainly include natural cooling, forced air cooling and water cooling.
1. Natural cooling: Let the temperature in the furnace drop naturally. This method has a slower cooling speed and is suitable for situations where the cooling speed is not required.
2. Forced air cooling: Blow cold air into the furnace through a fan to accelerate the dissipation of heat in the furnace. Forced air cooling has a faster cooling speed, but it is necessary to prevent dust and impurities in the air from entering the furnace.
3. Water cooling: By setting water cooling pipes around the furnace wall or heating elements, circulating water is used to absorb the heat in the furnace. Water cooling has the fastest cooling speed, but it is necessary to prevent water leakage and scale formation.
**V. Process**
The process carried out in the vacuum furnace usually includes the following steps:
1. Loading: Place the material or workpiece to be processed into the vacuum furnace and ensure that the furnace is well sealed.
2. Vacuuming: Start the vacuum pump system to reduce the pressure in the furnace to the required vacuum degree.
3. Heating: According to the process requirements, set the appropriate heating temperature and heating rate, and start the heating system to heat the material in the furnace.
4. Insulation: After reaching the set temperature, keep it for a certain time to allow the material to fully react or achieve the required performance.
5. Cooling: According to the process requirements, select the appropriate cooling method to cool the material in the furnace.
6. Discharging: When the temperature in the furnace drops to a safe temperature, open the furnace door and take out the processed material or workpiece.
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