Perfomance analysis of industrial vacuum furnace

---

The performance of the vacuum furnace is mainly reflected in the following parameters: degassing, extraction speed and ultimate vacuum.

degassing
Degassing is the phenomenon of slow evaporation of materials in vacuum, which is the main problem to be considered in vacuum performance. In a vacuum system, the surface of any solid material may have layers of molecules formed by the accumulation of liquids and gases. These layers gradually evaporate as the pressure drops, because the vacuum emits more energy than the surface. Inert gases, nitrogen, and volatile solvents are degassed relatively quickly, so water vapor and oil stick to the surface and take several hours to evaporate. Dust particles, porous materials, and other foreign substances increase the surface area of the system, which can lead to more degassing.

Temperature and radiation provide enough energy for some of the absorbed molecules to escape the surface. Raising the oven temperature gradually releases molecules that would otherwise stay on the surface at low temperatures. Therefore, as the furnace heats, degassing will increase. This can be reduced by lowering the furnace temperature or by maintaining a constant temperature.

Keeping it clean is the key to reducing degassing. Cleaning vacuum parts with solvent removes water and oil. In addition, purge with inert gas or dry nitrogen to remove internal air and prevent the formation of water vapor film on the surface of the system when exposed to air. After the furnace chamber is opened, the internal heating can also achieve this effect. Try to minimize the number of times the chamber is opened, especially in wet weather, when not in use to maintain a vacuum. These measures can reduce the phenomenon of degassing and improve the performance of vacuum furnace.

Take time to speed
The pumping speed is the time when the vacuum valve is opened and the vacuum chamber reaches the specified pressure when the furnace chamber is at atmospheric pressure. It is the method to measure the system performance under any pressure (including pumping heavy and mass gas). For reproducibility, the furnace chamber should be cleaned, emptied, and degassed, and each vacuum pump has been running long enough to achieve full pumping speed.

Ultimate vacuum
The ultimate vacuum is the vacuum state obtained after the actual degassing is completed, at which point the gas flow caused by the leakage and the vacuum suction system balance the capacity of the specified pressure exhaust. To be correct, the vacuum measuring instrument must be sensitive and accurately calibrated.

Raise the furnace to its maximum temperature to speed up the degassing of the system. When the furnace is heated in a vacuum and cooled to room temperature, the true limiting vacuum and leakage rate can generally be measured. The ultimate vacuum of vacuum pump is called “ultimate low pressure”, which is a good way to obtain the isolation performance of vacuum pumping system.

We need to test the performance of the vacuum furnace regularly, mainly to detect the ultimate vacuum degree and leakage rate.

Only when these two parameters meet the technical requirements can the machining quality of the parts be guaranteed:

Leakage velocity refers to the rate of pressure rise in the vacuum chamber when the vacuum valve is closed, including two aspects: leakage velocity and degassing leakage velocity are the pressure rise caused by the flow of gas through the actual leakage point in the chamber, which is constant; Degassing (including virtual leakage) is the gradual approach to 0 as the tank empties. Virtual leakage is the volume contained in the vacuum bulkhead that slowly leaks gas into the chamber. If the rate of rise is unacceptable, additional measurements are taken to confirm the previous reading. No change in velocity means actual leakage rather than degassing.

The leakage rate is calculated in micron mercury (mHg) per hour and is usually measured in 10-15 minutes. The leakage rate in the general all-metal heating zone furnace is not more than 2 mHg (0.266Pa) per hour, while the leakage rate in the soft packing heating zone is not more than 5 mHg (0.655Pa). When measuring the rising speed of pressure in the furnace chamber, empty the furnace chamber until the pressure remains constant (ultimate vacuum), then close all the vacuum valves and press the stop button to isolate the furnace chamber. This minimizes the impact of closing the valve and balances the system. After a proper interval, the vacuum is measured again and the pressure rise per unit time is calculated. For example, if the initial pressure measured is 10 mHg and 12 mHg (1.57pa) after 15 minutes, then the rate of rise is 2 mHg (0.266pa) every 15 minutes or 8 mHg (1.06pa) per hour. To ensure the accuracy of the inspection, measure for at least 10 minutes, preferably 30 minutes.

If the leakage speed of the equipment is found to exceed the specified value of the equipment, it means that there is leakage of the equipment, then we need to conduct vacuum leakage inspection

There are three common ways to locate a vacuum leak (preferably with a helium mass spectrometer) : (1) apply a helium mass spectrometer: spray helium where a leak is suspected, and then use a leak detector. Helium is a small molecule that can easily pass through the leak. The leak detector is usually connected to the valve of the mechanical vacuum pump. If it looks like an internal leak, drain all the water from the internal cooling lines and put helium in. This procedure can be used to check for leaks in the furnace water jacket.
(2) solvent leak check: a simple and effective way to find a medium-size leak is to use a solvent such as acetone or ethanol, only if the system pressure is less than 200 mHg (27Pa). Be careful to spray a thin stream of volatile solvent over the area in question and watch for pressure breaks (rise or fall). The lower the pressure, the more sensitive the method. Care should be taken to avoid solvent sputtering on the painted surface, as acetone can dissolve some types of paint.
(3) soap bubble test: use soapy water to find large leakage points in the pressurization system. After brushing the soap solution, if there is a leak, bubbles will form.