At present, many large-scale paper drying equipment have begun to adopt the closed hood system. In the drying section composed of double row drying cylinders and double rod nets, a relatively closed area is composed of upper dry mesh and paper web, which is also called "pocket area". A large amount of air is carried in the dry fabric, and a positive pressure area is formed between them. The positive pressure air flow flows into the pocket area through the dry fabric to improve the air volume in the pocket area.
High speed hot air closed hood
In order to avoid the paper sheet shaking, the low permeability dry screen is used to prevent the air entering the pocket area. However, the low permeability dry screen has many disadvantages, such as the humidity in the pocket area is increased, the humidity at the edge of the web is lower than that in the middle part, which reduces the drying rate, and the paper defects such as uneven drying.
The suction effect of dry fabric must be controlled to reduce the adverse effect of suction, and dry air should be sent into pocket area to achieve pocket area balance.
The ventilation effect of pocket area is affected by the production speed and paper grade. Generally, the paper with low speed and high brightness uses high permeability dry mesh and relatively simple pocket area ventilation device to avoid paper sheet shaking.
Requirements for ventilation device in high speed pocket area:
1.) balance and control the air flow in pocket area
The ventilation device of high-speed pocket area requires that the pocket area must be balanced, and the high permeability dry mesh should be used without paper sheet shaking. Therefore, during the production process, the air brought in by the dry mesh must be blocked at the entrance and can not enter the collection area of the dry mesh and fabric roll.
The upper air nozzle blocks the air flow brought by the dry mesh, while the lower air nozzle blocks the air flow brought by the fabric roll. A pressure area is formed between the air nozzles, and a certain amount of dry air is sent into the pocket area, and the wet air is discharged from the pocket area by natural suction.
The air flow in the ppocket area can not cause the paper sheet to flow. The air entering the pocket area is dry air, not the wet air from the high-speed hot air sealed hood.
2.) the humidity in the pocket area is low and uniform
Generally, the drying cylinder pressure is lower than 207kpa, and the pocket area humidity is 0.18kgh2o/kg dry air. When the pressure is high, the humidity in pocket area is 0.2kgh2o/kg dry air, and the horizontal fluctuation is not more than 0.02kg/kg. Therefore, the humidity in the pocket area is low and uniform, which increases the drying rate of the paper sheet, and the horizontal moisture distribution of the paper sheet is uniform.
The air demand of most paper machines is 14 m3 / (min. m width of the paper machine).
3.) send hot air to the hood
The airtight hood needs hot air supply. The air is distributed to the place where the airtight hood is installed. The well-designed pocket area ventilation system, such as air nozzle and hot-air roll, is required.
The function of the airtight hood is relatively obvious, especially for the reasonable design, we can finally have a better use effect. If you have any need, please feel free to contact us.
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