In the horizontal tube continuous cooking system, preheating screw (also known as preheater or screw preheater) is an important pretreatment equipment. It is mainly used to preheat and pre-impregnate the raw materials to improve the cooking efficiency and reduce energy consumption. The following are its specific functions:
1. Preheating of raw materials
● Preheating of raw materials (wood chips, straw chips, etc.) using the waste heat (such as steam condensate or secondary steam) generated by the cooking system to increase its initial temperature, shorten the cooking time and reduce steam consumption.
● Example: In sulfate pulping, preheating screw preheats wood chips from room temperature to 80~ 90℃, significantly reducing the steam demand of the cooking tube.
2. Chemical pre-impregnation
● During the preheating process, the cooking liquid (such as NaOH, Na ² S) is sprayed to the raw material to make it fully penetrate and create conditions for the subsequent cooking reaction.
● Example: In KP method pulping, the preheating screw sprays the liquid evenly on the surface of the wood chips to improve the delignification efficiency.
3. Raw material homogenization
● Through the stirring and propulsion of the spiral blades, the raw materials are uniformly heated and fully contacted with the liquid medicine to avoid local overheating or uneven reaction.
● Example: In the production of grass pulp, the preheating screw mixes the grass chips with the liquid medicine to ensure that each piece of grass is uniformly impregnated.
4. Removal of air and moisture
● During the preheating process, the air and part of the moisture in the raw material are discharged, reducing the gas resistance in the cooking tube and improving the heat transfer efficiency.
● Example: In the cooking of wood chips, the preheating screw discharges the air in the wood chips to avoid the formation of air resistance in the cooking tube.
5. Energy saving and consumption reduction
● Preheating the raw material through the waste heat of the recovery system, reducing the direct steam consumption of the cooking tube and improving the energy utilization efficiency. ● Example: In a large pulp plant, the preheating screw can reduce the cooking energy consumption by 10% to 20%.
