The main parameter of the design of the ventilation system of the drying part is the humidity of the water discharged by the hood. The structure form and sealing performance of the hood have a great influence on the humidity of the hood discharging water vapor. Low humidity operation of cadres requires higher exhaust volume and higher air supply volume. Reasonable structure and good sealing performance are the guarantee of efficient gas hood. So, what are the structural characteristics of open and closed gas hoods?

This is a relatively simple form of air hood, that is, a top cover is installed above the dry node. Many covers are made of only low insulation sheets and require high displacement to prevent condensation. There are also external thermal insulation panels to provide better insulation performance, and its humidity can be slightly improved. This type of hood is generally not tightly assembled to the drying part, leakage is a relatively large problem, in order to capture water and prevent condensation, the design value of the exhaust volume is very large.

The exhaust moisture range of the open top gas hood design is 0.04~0.07kg evaporated water /kg air. The simple cover exhaust humidity is about 0.04kg evaporated water /kg dry air. The exhaust humidity of the simple cover with thermal shield is about 0.07kg evaporated water /kg air. About 13kg of air is required for every 1kg of water evaporated, increasing energy consumption due to high air flow through the cadre. High exhaust volume must be balanced with the air supply of the heating building, thus increasing the heat load of the building. Low exhaust temperature also increases the cost of heat recovery, which is mostly undesirable. Open hoods are rarely used on new paper machines. Many old paper machines still use open hoods. They are gradually being replaced by more efficient, enclosed gases.

This form of hood is common in North America. The air cover is composed of a thermal insulation dense cover on the operating floor, with a lifting screen in front and a sliding screen in back as an entrance and exit of the drying part on the operating floor. The hood also includes a closed bottom plate which is not insulated on the bottom side of the drying part. The function of the airtight bottom plate is to prevent air leakage from the bottom plate. If there is no clotted floor, the clotted air hood on the operating floor will "boost pressure". The natural chimney effect in the drying department causes the "open pressure" effect, the sealing plate can limit the airflow and prevent the pressure boost effect, the early design of the closed air hood, exhaust humidity of about 0.09kg evaporated water /kg dry air. Most of the medium humidity gas currently operates in the range of 0.11~0.13kg/kg, evaporating about 10kg of air per kg of water. The typical exhaust temperature of this hood is 80℃ dry bulb temperature and 59℃ wet bulb temperature. The heat recovered by a medium humidity closed air hood is usually attractive to northern climates. The recovered heat is often used for pre-heat hoods to supply air, heating production water and heating buildings to replenish air.

The overall structure and layout of the high humidity closed air hood is basically the same as that of the medium humidity closed air hood, but it is more selective in manufacturing and material selection. The thermal insulation board of the tongue-and-groove joint used in the medium humidity closed air cover should not be used in a high humidity environment for a long time, and has the problem of weak sealing and sea leakage. The high humidity closed air cover uses materials such as high insulation value plates and high temperature resistant rubber, and can be well sealed. Basically solves the problem that the lifting door of the medium humidity closed air hood is not tightly sealed and the problem of the Cui polyester leakage. The high humidity sealed air hood reduces energy consumption and is very good for heat recovery. High humidity sealed air hood design exhaust humidity of about 0.16~0.17 kg water /kgd.a. To evaporate 1kg of water, about 7kg of air is consumed. The typical exhaust temperature is 85C dry bulb temperature and 62℃ wet bulb temperature. The main advantage of the high humidity closed air hood is to reduce the air flow through the hood and the impact of the air flow on the operation of the paper. The air flowing through the drying section can be better controlled to meet the needs of high-speed paper machine operation.

The drying part of the paper machine is equipped with an efficient closed air hood, which can improve the production environment, reduce the production energy consumption, and improve the efficiency and product quality of the paper machine. At present, domestic and foreign paper machine manufacturers have applied a lot of new technologies and new materials in the design of paper machine ventilation and gas hood, so that the performance of paper machine ventilation system and gas hood is becoming perfect, to meet the needs of high efficiency and high speed paper machine operation.