The equipment and accessories for dewatering, drainage and exhaust (steam) depending on vacuum pressure difference in the paper workshop can be classified as the vacuum system of paper machine. Besides the vacuum system of the dehumidification part, it also includes the vacuum device for discharging the condensate at the end of the steam condensate system and the matching vacuum device for the main vac. Roll, etc. The wet end vacuum system mainly includes: vacuum components, front stage gas water separator, vacuum pump (or fan), rear stage gas water separator, vacuum pipeline and other components.

Vacuum element

1) The vacuum elements of the forming part are set for paper web dehydration and forming, and the form and quantity of the vacuum elements are determined by the type of the former. In the early stage, the round net former (i.e. the round net slot former) with the speed less than 100m / min is not equipped with vacuum dehydration elements. The round net cage former developed from the round net slot former, such as vacuum round net former, Bristol former, super former, etc., is equipped with vacuum net cage, vacuum water suction tank, etc. its working speed can reach 500m / min or even higher.

The long net former is generally equipped with vacuum water suction tank, vacuum couch roll and other vacuum elements. The low-speed paper machine is often equipped with non vacuum dehydration elements such as case roll and case board, but with the increase of speed, the intensity of pressure vacuum pulse caused by case roll and case board in the process of dehydration increases sharply, which seriously affects the quality of paper web forming. In the high-speed paper machine, the case roll and the case board are gradually replaced by the vacuum dehydration board and the vacuum water suction tank.

There are also vacuum water suction tank, vacuum (V) roller and other vacuum dehydration elements in the forming machine. However, due to its short forming area, the number of vacuum dehydration elements is much less than that of the long net former.

It can be seen that in several main formators, the vacuum dehydration element is more used in the long net formator. For the long net former, the higher the speed is, the more the vacuum dehydration elements are. The vacuum degree of the vacuum dehydration element is higher and higher along the paper web direction.

2) The vacuum element of the pressing part includes a vacuum suction roll for sheet transfer, a vacuum press roll for sheet dehydration, and a vacuum suction water tank for woolen cloth washing and dehydration transfer. The vacuum degree of the press part is higher than that of the forming part.

Front stage gas water separator

The gas-water separator of the front stage separates the gas-water mixture discharged from the vacuum element as much as possible to avoid the white water entering the vacuum pipeline, so as to improve the working efficiency of the vacuum system. After the air-water mixture is separated by the separator, the white water overflows through the water seal tank or is pumped to the white water tank under the network, and then returns to the white water system of the workshop. The wet air is discharged by the vacuum pump through the vacuum pipeline.

The volume of the gas-water separator of the front stage is determined by the volume of the gas-water mixture discharged in unit time of the vacuum element. The diameter of drain pipe and exhaust pipe of gas water separator is determined by the volume of gas water mixture and the volume ratio of gas water. In the forming part, the air-water volume ratio increases with the increase of the sheet dryness along the sheet running direction, from 0.25 to 125. In the press section, the air-water volume ratio of the mixture discharged from the vacuum water suction tank of the wool cloth is determined by the structure of the water suction tank and the water content of the wool cloth, and the air-water volume ratio of the mixture discharged from other vacuum elements is also related to the paper web dryness.

The vacuum elements with the same vacuum degree can share a gas-water separator. A gas-water separator can be used for more vacuum elements in the front section of the forming section with a small gas-water volume ratio. However, a gas-water separator cannot be used for too many vacuum elements in the back section of the forming section and the pressing section where the gas-water volume is relatively large.

When the separator is arranged, on the premise that the inlet of the separator is lower than the outlet of the vacuum element, the height of the separator shall be increased as much as possible to ensure sufficient water leg height, and at the same time, the effective liquid level of the water seal tank and the effective volume of the off grid white water pool shall be increased. If sufficient water leg height cannot be ensured due to space and paper machine structure, drainage pump shall be provided. In general, the separator used for vacuum couch roll and woolen cloth vacuum water suction tank under the pressing section needs to be equipped with a drainage pump.

In order to ensure the stability of the system, the water leg of the separator shall be vertically inserted into the water seal tank, and an overflow partition shall be set in the water seal tank to ensure the stability of the liquid level. The inclination of the water leg will cause the air in the water leg, which will cause the fluctuation of the vacuum degree and affect the sheet forming.

Vacuum pump and exhaust fan

The centrifugal fan or Roots fan shall be selected as the vacuum generating device for the vacuum element with low vacuum degree (5-15kpa) at the beginning of forming area. For other vacuum components, roots vacuum pump or water ring vacuum pump shall be selected according to the vacuum degree requirements. Roots vacuum pump mechanical exhaust and noise, due to the existence of mechanical wear and maintenance work, but do not need working fluid, the system is simple. The water ring vacuum pump has small maintenance work, less power consumption than the former, and large single machine capacity, but it needs working fluid, so the system is relatively complex. The water ring vacuum pump is often used in the vacuum system of large paper machine. In the following discussion, the vacuum pump refers to the water ring vacuum pump.

When water ring vacuum pump is selected, one pump with diaphragm and two chambers can be selected. Two chambers correspond to different vacuum elements with different vacuum requirements and work under different vacuum degrees. The reduction of the number of vacuum pumps can not only save the equipment investment, but also reduce the total installed capacity of the motor, thus reducing the energy consumption and system operation cost. At the same time, we will reduce the floor space and capital construction investment.

The pumping speed of vacuum pump is determined according to the exhaust speed, vacuum degree, average pressure in the pipe, pipe diameter and pipe length of vacuum element. When arranging the vacuum pump, try to concentrate on the area close to the vacuum element, shorten the length of the vacuum pipe and reduce the energy consumption of the system.

Posterior stage