Principle of vacuum drying oven and common problems

Operating principle of vacuum drying oven and analysis of common problems

1. Reasons for the difference between the meter reading of vacuum drying oven and that of glass rod thermometer in vacuum chamber:

The general electric vacuum drying box USES the way of heating the wall of the vacuum chamber first and then radiation heating from the wall to the workpiece. In this way, the temperature sensor of the thermograph can be arranged on the wall outside the vacuum. The sensor can receive convection, conduction, and radiant heat simultaneously. However, a glass rod thermometer in a vacuum chamber can only receive radiant heat. Since the blackness of the glass rod cannot reach 1, a considerable part of the radiant heat is refracted, so the temperature value reflected by the glass rod thermometer is certainly lower than the temperature reading of the instrument. Generally speaking, it is normal for the temperature reading of the meter to differ within 30℃ from that of the glass rod thermometer at 200℃. If the temperature sensor of the thermometer is placed in a vacuum chamber, the difference between the temperature value of the glass rod thermometer and the temperature reading of the meter can be reduced, but cannot be eliminated, and the sealing reliability of the vacuum chamber increases a potentially unreliable link. If you do not want to see this difference from the perspective of practical operation, you can use the thermostat table specific display correction function to solve.

2. The reason for vacuum drying chamber to pump vacuum and then heat up

1) If we follow the procedure of heating first and then vacuuming, when the heated air is drawn out of the vacuum pump, the heat will inevitably be brought to the vacuum pump, resulting in too high temperature rise of the vacuum pump, which may reduce the efficiency of the vacuum pump.

2) The heated gas is directed to the vacuum pressure gauge, which will produce a temperature rise. If the temperature rise exceeds the specified operating temperature range of the vacuum pressure gauge, it may cause an indication error.

The correct way to use it is to vacuum and then heat it up. After the rated temperature is reached, if the vacuum degree is found to decrease, then pump appropriately. This is beneficial to extend the service life of the equipment.

3) The workpiece is put into the empty chamber for vacuum pumping in order to pump out the gas components that can be pumped out of the workpiece material. If you heat the workpiece first, the gas will expand when it is heated. As the tightness of the true chamber is very good, the great pressure generated by the expansion gas may make the toughened glass of the observation window burst. This is a potential danger. This danger can be avoided by following the procedure of first vacuuming and then heating.

3. The reason why the temperature uniformity parameter is not set in the vacuum drying oven

General electric (blast) drying box is equipped with temperature uniformity parameters: natural convection drying box is the upper limit of working temperature by 3%, forced convection drying box is the upper limit of working temperature by 2.5%. Only the electric vacuum drying oven does not set the temperature uniformity parameter, this is why? It is almost impossible to achieve uniform temperature in a vacuum drying chamber by the movement of gas molecules. Therefore, we can no longer conceptually apply the temperature uniformity defined by the usual electric (blast) drying oven to the vacuum drying oven. It doesn't make sense to have this index in a vacuum. The amount of heat radiation is inversely proportional to the square of the distance. The radiant heat received by an object 20cm from the heating wall is only 1/4 of that received by an object 10cm from the heating wall. There's a big difference. This phenomenon is similar to the winter sun, when the sun side is very warm, the sun side is colder. In terms of the structure, it is difficult for the vacuum drying oven to achieve uniform radiant heat at all points in the three-dimensional space of the studio, and there is also a lack of authoritative evaluation method, which may be the reason why temperature uniformity parameter is not set in the standard of electric vacuum drying oven.

I. Structural Features:

1. The inner tank of the box is made of mirror stainless steel plate or drawing plate by argon arc welding, and the outer shell is made of high-quality cold-rolled steel plate with electrostatic spraying, with beautiful and novel appearance.

2. The insulation system adopts the super-fine glass fiber to fill the insulation area, and the joint of the inner and outer bile adopts the non-metallic high temperature resistant material to effectively reduce the high temperature conduction.

3. Door opening is adopted on the side of the circuit system to facilitate maintenance and overhaul.

4. The tightness of the box door can be adjusted, and the silicone rubber sealing ring can be formed as a whole to ensure high vacuum in the box.

5. Storage, heating, testing and drying are carried out in an environment without oxygen, so there is no oxidation.

6. Cuboid studio maximizes the effective volume.

7. Toughened and bulletproof double glass door can be easily observed in the working room.  

8. Microcomputer temperature controller with timing function, accurate and reliable temperature control.

Ii. Control System

1. Touch key temperature controller, digital LED display, PID intelligent control instrument;

2. Pt100 platinum resistance temperature sensor;

3. Control mode: heat balance temperature adjustment mode;

Iii. Protection System

The protection system consists of overtemperature protection and alarm device. Ensure the execution of components and test pieces intact.

Iv. Technical Parameters

1. Temperature range: RT+10℃ ~ 250℃ (RT+6℃ ~ 65℃)

2. Temperature resolution: ±0.1℃

3. Constant temperature fluctuation: ±1℃ (±0.5℃)

4. Achieve vacuum degree: < 133Pa

5. Working environment temperature :+5℃~40℃

6. Power source :AC1 220V 50Hz

7. Carrier bracket (standard configuration) : 1~3 pieces

8. Timing range: 1-9999 min

V. Optional Parts:

1. Intelligent program temperature controller (30 programmable segments), digital display film button or LCD touch screen;

Vacuum pump: 2×Z-2 (2L/S),2×Z-4 (4L/S);

3. Printer, which can print test data in real time;

4. RS485 communication interface and software for computer control;

5. Dry the filter tank;

6. Gas-liquid separator.