Currently, the steam supply system in the paper machine drying section is mainly divided into two basic types: three-stage steam supply and heat pump heating. Three-stage steam supply is a passive series steam heating system, where new steam is reduced to the required supply pressure for the high-temperature drying cylinder using a regulating valve. The same regulating valve is used for the medium- and low-temperature drying cylinders, resulting in wasted steam energy quality and quantity. Heat pump heating, on the other hand, is an active parallel steam heating system. Heat pumps are installed in each drying cylinder section of the paper machine drying section, and the heat pump supply pressure is set according to the temperature rise curve of the drying cylinder. The effective energy difference between the new steam and the steam used in the drying cylinder is converted into the working power of the heat pump. However, when the operating conditions of the paper machine change, the opening of the new steam regulating valve at the heat pump inlet decreases, resulting in a loss of steam effective energy and reducing the efficiency of the heat pump. This project applies an adjustable heat pump heating system to the paper machine drying section, thereby increasing the adjustment range of the heat pump heating system in the paper machine drying section, ensuring the steam supply pressure and drainage pressure difference of the drying cylinder, recovering secondary steam, reducing the condensate discharge temperature, and increasing the surface temperature of the drying cylinder and the water vapor partial pressure on the paper web surface, thus enhancing heat and mass transfer during the paper web drying process. 1 Steam Jet Heat Pump 1.1 Working Principle of the Heat Pump The heat pump in the paper machine drying section adopts a steam jet heat pump (or steam compressor). The steam jet heat pump utilizes the energy difference between the steam pressure supplied by the thermal power plant or boiler and the steam pressure used in the drying cylinder to convert it into the power of the heat pump. The working fluid is drawn into the receiving chamber 2 through the nozzle 1 to draw in low-grade fluid. After mixing and energy exchange between the working fluid and the ejected low-pressure fluid in the mixing chamber 3, the mixture gradually fills the entire cross-section, establishing a uniform velocity field. The velocity of the mixed fluid after compression in the diffuser 4 decreases, and the pressure of the mixed fluid increases to the steam supply pressure required by the heat user. 1.2 Classification of heat pumps Steam jet heat pump heating systems must be adjusted according to the steam parameters required by the heat users. Steam jet heat pumps can be divided into two types according to the adjustment and control method: mass regulation and flow regulation. (1) Steam mass regulation heat pump (also known as non-adjustable heat pump). The working steam main of the non-adjustable heat pump is equipped with a regulating valve. The heat pump itself is not equipped with a regulating device. When the operating conditions of the paper machine, i.e. the supply steam pressure and flow rate, change, the regulating valve on the working steam main of the heat pump inlet needs to be adjusted. During the adjustment process, the change of the opening degree of the steam regulating valve will change the working steam pressure entering the heat pump, thereby changing the ability of the new steam at the heat pump inlet to do work. In particular, when the steam load of the paper machine decreases, the opening degree of the steam regulating valve decreases, the actual new steam pressure supplied to the heat pump inlet decreases, resulting in a decrease in the pressure difference between the working steam at the heat pump inlet and the steam at the heat pump outlet, a decrease in the working capacity of the heat pump, and a decrease in working efficiency. (2) Steam flow regulation heat pump (also known as adjustable heat pump). The adjustable heat pump does not require a regulating valve on the inlet working steam main; the heat pump itself has an actuator and valve positioner. During paper machine operation adjustments, only the nozzle cross-sectional area needs to be adjusted. The inlet steam pressure remains constant, and the work capacity per unit flow rate of steam also remains constant. The heat pump maintains high efficiency under various operating conditions. 2. Actual Production Operation of the Adjustable Heat Pump Heating System 2.1 Paper Machine Technical Parameters The following analysis uses the 2640mm paper machine (producing offset paper) of Chibi Chenming Paper Industry Co., Ltd. as an example. The steam for the paper machine is supplied by the plant's thermal power plant at pressures of 0.85 and 0.5 MPa, respectively. The steam condensate is returned to the thermal power plant via mechanical return water. This paper machine is equipped with a sizing machine, 31 drying cylinders before the sizing machine, 10 drying cylinders after the sizing machine, 2 cooling cylinders, and an open steam hood. The main technical parameters of the paper machine are: paper width 2640 mm; drying cylinder diameter 1500 mm; wire width 3100 mm; product basis weight 60-80 m²; sizing method: double-sided sizing; nominal capacity 100 t/d. 2.2 Heat Pump Heating Process: Based on the temperature rise curve requirements of the drying cylinders, the drying cylinders before the sizing machine are divided into low-temperature, medium-temperature, and high-temperature sections, and the drying cylinders after the sizing machine are divided into medium-temperature and high-temperature sections. Adjustable heat pumps 101a and 101b are used for the high-temperature drying cylinders before and after sizing, and adjustable heat pump 102 is used for the medium-temperature drying cylinders. The steam pressure for the low-temperature drying cylinders is relatively stable, so a standard heat pump 103 is used. The steam system uses 0.85MPa high-grade steam as the working steam for the adjustable heat pump to control the drainage pressure difference of the drying cylinders. 0.50MPa low-grade steam is used to control the pressure of the drying cylinders in the medium and high temperature sections and to supplement insufficient steam consumption; it also serves as the working steam for heat pump 103. The steam condensate system uses a pump-free, residual pressure gravity-flow return system between the steam-water separators in each section of the drying cylinders, and the terminal condensate tanks are closed-loop systems without steam permeable pipes. The paper machine's heat pump heating system is equipped with a comprehensive non-condensable gas exhaust system to improve the heat transfer coefficient of the drying cylinders. 2.3 Operation of Adjustable Heat Pump Heating System The paper machine adopts an adjustable heat pump heating system in the drying section. The control system adopts interlocking optimized control, which meets the requirements of large quantitative changes. It is easy to operate, runs stably, and significantly improves product output and quality. Its characteristics are as follows: (1) Adjustable heat pump adapts to changes in the quantitative of paper machine products Analysis of experimental data shows that the adjustable heat pump heating system adapts to changes in the operating conditions of the paper machine. When the quantitative of paper machine products increases and the output increases, the steam inlet pressure, steam inlet volume and steam cylinder drainage pressure difference of each drying cylinder are reliably guaranteed. All the sprayed steam and secondary steam discharged from the drying cylinder are recovered and reused, and the pressure of the condensate tank is zero. (2) The speed and output of the paper machine have been significantly improved. Before this technical transformation, the drying section of the paper machine used a non-adjustable heat pump heating system. The comparison of the changes in the speed and output of the paper machine before and after the transformation when producing the same basis weight product shows that after the paper machine adopted the adjustable heat pump heating system, the output of 70 m2 product increased by 1.12 times and the output of 100 g/m2 product increased by 1.24 times. (3) The steam pressure of the drying cylinder was reduced and the drainage pressure difference of the drying cylinder was increased. The data shows that when the paper machine reaches the same speed and output by replacing the non-adjustable heat pump with an adjustable heat pump, the steam pressure of the drying cylinder and the pressure of the steam-water separator are reduced, and the drainage pressure difference of the drying cylinder will increase. (4) The effective energy of steam is fully utilized. Calculations show that when the saturated steam pressure is 0.85 MPa and 0.30 MPa, its effective energy is 842.39 kJ/kg and 718.10 kJ/kg, respectively. The three-stage steam supply is actually an ineffective devaluation of high-grade steam energy. For example, if the steam pressure of 0.85 MPa is reduced to 0.30 MPa, its effective energy loss is 124.49 kJ/kg, accounting for 14.8% of the new steam effective energy. Compared with the traditional three-stage steam supply system, the adjustable heat pump heating system makes reasonable use of both the grade and quantity of steam energy. 3. Average Drying Intensity per Unit Effective Area of ​​Drying Cylinder The drying intensity per unit effective area of ​​drying cylinder refers to the parameter of the drying cylinder configured when the paper machine has its maximum production capacity. Correctly obtaining this data is crucial for determining the number of drying cylinders during the modification of existing paper machines and for the economical and rational configuration of paper machine equipment. There is limited literature and experimental data available both domestically and internationally. Currently, the average drying intensity per unit effective area of ​​drying cylinders in domestic double-sided offset paper machines is typically around 20 ks/(m²•h), which is conservative. The average drying intensity per unit effective area of ​​the drying cylinder is calculated using the following formula: m = QG/F Where: m is the drying intensity, ks/(m²•h); G is the paper production rate, kg/h; Q is the dewatering rate, kg water/kg paper; F is the effective area of ​​the drying cylinder, m². This paper machine's drying section uses an adjustable heat pump heating system to produce products of 70, 90, 100, and 120 g/m². The average drying intensity per unit effective area of ​​the drying cylinder was calculated based on the paper machine's operating speed and paper production rate. The paper machine uses an open hood and simple bag ventilation. Experimental data shows that the drying intensity of the drying cylinder reaches more than 40 ks/(m2•h). Data comparison shows that the use of an adjustable heat pump heating system creates the prerequisites for increasing the paper machine speed, increasing output, and economically and rationally equipping the drying cylinder. 4. Conclusion Traditional three-stage steam heating methods waste the effective energy of steam. With non-adjustable heat pump heating systems, when paper machine speed or product changes, the only way to meet the paper web drying process requirements is to adjust the new steam pressure. Because the opening of the working steam regulating valve decreases, the pressure of the working steam entering the heat pump is forced to drop, resulting in a reduction in the heat pump's work capacity. Adjustable heat pump heating systems, used in the drying section of paper machines producing various products, can fully utilize the effective energy of new steam, maximize the work capacity of high-grade steam, improve heat and mass transfer in the drying cylinder, and overcome the shortcomings and deficiencies of traditional three-stage steam heating methods and conventional non-adjustable heat pump systems. They can fully utilize the grade and quantity of steam heat energy, significantly increase paper machine speed, increase output, improve quality, and reduce steam consumption.