1. The vacuum degree is controlled by stopping and opening the vacuum pump. Generally speaking, the control system will set an upper limit and a lower limit to mark the vacuum pump on or off. This scheme requires that there is a valve between the vacuum pump and the equipment that can be closed with the vacuum pump closed. When the measured vacuum value reaches the lower limit of the set value, the vacuum pump and valve stop working at the same time. Due to the sublimation of water vapor, the vacuum value in the box will increase. As time goes on, when the measured vacuum value reaches the upper limit of the set value, the vacuum pump will be turned on again to keep the vacuum value in the box within the set range. The advantage of this method is that the energy consumption will be reduced after the vacuum pump stops working, and the service life of the vacuum pump will be prolonged.
2. The vacuum is controlled by controlling the air intake. Similarly, the control system will set an upper limit and a lower limit to mark the opening or closing of the solenoid valve. When the measured vacuum value reaches the lower limit of the set value, the control solenoid valve starts to intake air. Due to the sublimation of water vapor, the vacuum value in the tank will increase. As time goes on, when the measured vacuum value reaches the upper limit of the set value, the valve will be closed to keep the vacuum value in the tank within the set range. The control method is relatively simple. A more accurate control system is required. According to the difference between the vacuum value and the set value, a valve that can automatically adjust the flow rate can be installed to control the air intake volume, making the control more accurate.
3. The vacuum is controlled by adjusting the size of sublimation channel. This method requires an adjustable butterfly valve or mushroom valve between the box and the cold trap. When the vacuum value is too high, the vacuum value of the box can be increased by closing the sublimation channel. When the vacuum of the box is too low, the vacuum is increased by increasing the sublimation channel. This method has high requirements for control system.
4. The vacuum is controlled by adjusting the temperature of the cold trap. Because the temperature control precision of the refrigeration system is not very high, and the frequent start of the refrigeration system is also a reason to restrict the control mode. This scheme is not recommended for general small systems, but suitable for large screw parallel units and large food freeze-drying units.
Each of the above plans has its own advantages and disadvantages. Users can choose their own appropriate scheme according to the actual needs.
appendix
Conversion between different vacuum units
|
|
Pa（N/m2）
|
bar
|
mbar
|
Toor（mmHg）
|
Micron
mTorr
|
atm
|
Psi
|
|
1 pa=
|
1
|
10X10-5
|
1X10-2
|
7.5X10-3
|
7.5
|
9.87X10-6
|
1.45X10-4
|
|
1 bar=
|
1X105
|
1
|
1X10-5
|
750
|
7.5X105
|
0.987
|
14.5
|
|
1 mba
|
100
|
1X10-3
|
1
|
0.75
|
750
|
9.87X10-4
|
1.45X10-2
|
|
1 Torr
|
133
|
1.33X10-3
|
1.33
|
1
|
1000
|
1.32X10-3
|
1.93X10-2
|
|
1 micron
|
0.133
|
1.33X10-6
|
1.33X10-3
|
1X10-3
|
1
|
1.32X10-6
|
1.93X10-5
|
|
1 atm
|
1.1X105
|
1.013
|
1013
|
760
|
7.6X105
|
1
|
14.7
|
|
1 psi
|
6.89X103
|
6.89X10-2
|
68.9
|
51.71
|
5.17X104
|
6.8X10-2
|
1
|
