Factors affecting the drying thermal efficiency of ventilation system

The process of air supply and exhaust of the hood is as follows. The air supply of the hood in the drying section mainly draws in fresh air through the air blower, exchanges heat with the air through the hood exhaust, and then heats the steam heater to 100-120 ℃. The air is sent to the bag area and the blower at the bottom of the paper machine through the air supply pipe of the hood. The wet air with evaporated water vapor from the paper sheet is discharged from the hood through the exhaust system of the hood through the air collection pipe at the top of the hood, and the heat is recovered through the heat recovery system of the air-to-air heat exchanger on the exhaust pipe, and the recovered waste gas is discharged to the outdoor.

From this, we can see that the factors affecting the drying thermal efficiency of the ventilation system mainly need to control the following three aspects:

1. The air supply and exhaust of the ventilation system must be balanced enough. Generally, there is a temperature difference between the supply air and the exhaust air. If the air temperature is low, the thermal efficiency of the drying section will be reduced. The higher the temperature of the fresh air inhaled by the forced draft fan or the higher the temperature after passing through the hood exhaust air heat exchanger, the less steam energy is consumed by the hood ventilation. In this way, we add an air heat exchanger with the condensation water of the steam condensation system as the hot coal after the hood exhaust air heat exchanger (usually only 45 ℃), and the air heat exchanger from the hood exhaust air After the first heating air heating again (the effect can reach 60-75 degrees Celsius). In this way, the temperature of fresh air entering the air supply fan can be effectively increased, and the steam consumption when the fresh air of the hood is heated to 100-120 ℃ can be reduced.

2. When the wet air discharged from the installation bag area of the ventilation system is mixed with the wet air with temperature difference in the hood, the air temperature will drop below its dew point, and part of the water vapor will condense on the surface of the hood, resulting in water dripping. (the moisture content of the air in the front of the hood can reach 0.25-0.3kg water / kg dry air, and the moisture content of the air in the back of the hood can reach 0.02-0.04kg water / kg dry air). That's why there is condensation in the separation layer of the pressing part. We organize the ventilation in the hood in this way, and adjust it through the air supply branch pipe and the air exhaust valve in the hood. The hot air with low moisture content at the rear end flows from the rear end of the hood to the front end, and is discharged from the hood together with the air with low temperature and high moisture content at the front end through mixing.

3. The air supply speed of the hood is relatively higher than the exhaust speed, and positive pressure is generated in the hood. The hot air will flow out from the place where the hood has an outlet. When the hot and wet air meets the cold air inside the hood, it will condense on the external surface of the rack and the hood. When the air supply speed of the hood is relatively lower than the exhaust speed, negative pressure will be generated in the hood, resulting in low thermal efficiency of the drying part, and condensate will be generated in the hood. Condensation water in the hood will seriously affect the product quality, so it is necessary to make sure that the air supply in the hood is greater than the exhaust air volume to form hot air leakage in the hood.

The adjustment of the air system in the hood of the drying section of the paper machine is the key to ensure the normal papermaking of the paper machine and the even and stable moisture content of the paper web. Generally, it is no longer adjusted at will under the condition of good adjustment. However, with the improvement of the speed and production capacity of the paper machine, the corresponding adjustment of the air system in the hood of the paper machine is required to adapt to the new production conditions, so as to achieve the balance. Increasingly strict energy conservation and emission reduction policies, fierce market competition and severe resource shortage make our paper industry have to produce delicately to realize economy, efficiency and environmental protection.