A method for evaluating heater availability in a shaft seal overflow energy recovery system

Abstract

The present invention discloses a method for evaluating the availability of a heater in a shaft seal overflow energy recovery system, comprising the following steps: inputting heater operating conditions; determining the pressure of the gas phase components at the hot flow side inlet; assuming that the limit temperature is the initial value of the hot flow side outlet temperature and the loop count N=0; determining the pressure of the gas phase components at the hot flow side outlet; calculating the steam flow rate at the hot flow side outlet; calculating the heat exchange capacity at the hot flow side; calculating the cold flow side outlet temperature; determining the heat transfer driving temperature difference; calculating the heat exchange capacity of the heat exchanger; if N=0 and the heat exchange capacity difference is greater than 0, the heater is unavailable; otherwise, calculating the absolute value of the relative error in heat exchange capacity; if it is greater than 0.01%, adjusting the hot flow side outlet temperature and setting N=N+1, and recalculating the heat exchange capacity; otherwise, exiting the loop, calculating the flow rates and end difference of each phase at the hot flow side outlet; if all are not greater than the allowable value, the heater is available; otherwise, the heater is unavailable. The present invention can comprehensively and quickly evaluate the availability of the heater in a shaft seal overflow energy recovery system under relatively few known conditions.

Description

Technical Field

[0001] The invention belongs to the technical field of shaft seal overflow energy recovery, and in particular relates to a heater availability evaluation method applied to a shaft seal overflow energy recovery system. Background Art

[0002] Steam turbine units often utilize a self-sealing system for their shaft seals. When the unit is operating at low load, steam is supplied by the shaft seal header. As the unit load increases, the high- and medium-pressure shaft seals overflow, supplying steam to the low-pressure shaft seal. As the load continues to rise, the overflow volume increases, and excess overflow steam is diverted to the condenser, resulting in energy loss. To address this, many thermal power plants have proposed recovering the shaft seal overflow energy. The primary method is to divert the excess overflow steam to a heater to heat the condensate. Therefore, heater availability is a key factor in evaluating the feasibility of a shaft seal overflow energy recovery system.

[0003] Currently, heater availability assessments in shaft seal overflow energy recovery systems are crude, neglecting the phase transition of steam on the hot stream side, the changes in the outlet gas and liquid phases, and whether the heater's end differential meets requirements. In light of this, the present invention proposes a calculation method for effectively and rapidly assessing heater availability in shaft seal overflow energy recovery systems. Summary of the Invention

[0004] The purpose of the present invention is to remedy the defects of the existing evaluation method and provide a method for evaluating the availability of a heater of a shaft seal overflow energy recovery system.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for evaluating heater availability of a shaft seal overflow energy recovery system comprises the following steps:

[0007] Step 1: Input the heater operating conditions according to the operating environment of the heater in the shaft seal overflow energy recovery system;

[0008] Step 2: Calculate the partial pressures of water vapor and air at the inlet of the heater on the hot flow side according to Dalton's law of partial pressures;

[0009] Step 3: Given a cycle count N = 0, calculate the outlet limit temperature at which all steam on the hot flow side is condensed as the initial value of the hot flow side outlet temperature;

[0010] Step 4: Determine the saturated water vapor pressure corresponding to the outlet temperature of the hot flow side based on the working fluid physical parameters. Calculate the air partial pressure at the hot flow side outlet based on the principle that the hot flow side pressure must be maintained constant when the heater is working.

[0011] Step 5: Calculate the steam flow rate at the hot flow side outlet according to the gas phase component pressure at the hot flow side outlet;

[0012] Step 6: Based on the pressure and temperature of the gas phase components at the outlet of the heat flow side and the physical properties of the working fluid, determine the specific enthalpy values ​​of the outlet saturated water vapor, saturated water, and air, and further calculate the heat transfer on the heat flow side;

[0013] Step 7: Calculate the outlet temperature of the cold stream side based on the heat exchange balance between the cold stream side and the hot stream side and the constant pressure specific heat capacity and inlet temperature of the heater operating conditions input in step 1;

[0014] Step 8: Calculate the heat transfer driven temperature difference using the logarithmic heat transfer temperature difference formula and correction coefficient based on the cold flow side outlet and hot flow side inlet and outlet temperatures;

[0015] Step 9: Calculate the heater heat transfer capacity based on the input total heat transfer coefficient, heat transfer area, and the heat transfer driving temperature difference obtained in step 8;

[0016] Step 10: Determine based on the value of the loop count N: if N=0, go to step 11; if N≠0, go to step 12;

[0017] Step 11: Calculate the difference between the heat transfer rate on the hot flow side obtained in step 6 and the heater heat transfer rate obtained in step 9. If the difference is greater than 0, the steam cannot be completely condensed, and the heater is concluded to be unusable, and the evaluation ends. If the difference is less than or equal to 0, proceed to step 12.

[0018] Step 12: Calculate the absolute value of the relative error between the heat transfer side heat transfer obtained in step 6 and the heater heat transfer obtained in step 9. If the absolute value is less than or equal to 0.01%, proceed to step 13; if the absolute value is greater than 0.01%, proceed to step 15.

[0019] Step 13: Obtain the heater end difference based on the difference between the hot flow side outlet temperature and the cold flow side inlet temperature; Calculate the hot flow side outlet gas phase and liquid phase flow rates based on the heat flow measured flow rate and component mass fraction in the heater operating conditions input in step 1 and the hot flow side outlet steam flow rate obtained in step 5;

[0020] Step 14: If the heater end difference, hot flow outlet gas phase, and liquid phase flow rates obtained in step 13 are all less than the corresponding allowable values ​​specified in the heater operating conditions input in step 1, the conclusion is drawn that the heater is usable and the evaluation ends; otherwise, the heater is unusable and the evaluation ends;

[0021] Step 15: Loop count N=N+1, update the outlet temperature of the heat flow side according to the adjustment strategy, and its outlet temperature must be less than the initial value of the heat flow outlet temperature given in step 3; restart from step 4 until the loop is exited and the evaluation is completed.

[0022] Furthermore, the heater is a partition-type heat exchanger.

[0023] Furthermore, in step 1, the operating conditions of the heater include:

[0024] Cold flow side: the working fluid is liquid water, pressure, inlet temperature, flow rate, and specific heat capacity at constant pressure of the working fluid;

[0025] Hot flow side: the working medium is a steam-air mixture, with pressure, inlet temperature, flow rate, and steam mass fraction;

[0026] Heater total heat transfer coefficient, heat transfer area;

[0027] The maximum allowable end difference of the heater, the maximum allowable gas flow rate and liquid flow rate at the hot flow side outlet.

[0028] Furthermore, the hot flow side working medium steam-air mixture can be replaced by other condensable and non-condensable gas mixtures.

[0029] Furthermore, the liquid water on the cold flow side can be replaced by other single-phase fluids.

[0030] Furthermore, in step 3, the outlet limit temperature at which the steam on the hot flow side is completely condensed is 99.9% of the saturated steam temperature corresponding to the partial pressure of the steam at the hot flow side inlet.

[0031] Furthermore, in step 5, the calculation method of the steam flow rate at the hot flow side outlet is:

[0032]

[0033] In the above formula, m v,out1 is the steam flow rate at the hot flow side outlet, p v,out1 is the outlet steam pressure on the hot flow side, p air,out1 is the outlet air pressure on the hot flow side, M v is the relative molecular mass of steam, M air is the relative molecular mass of air, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side.

[0034] Furthermore, in step 6, the heat transfer amount on the heat flow side is calculated as follows:

[0035] Q1=h air,out1 (1-w v,in1 )m in1 +h w,out1 (w v,in1 m in1 -m v,out1 )+h v,out1 m v,out1 -h air,in1 (1-w v,in1 )min1 -h v,in1 w v,in1 m in1

[0036] In the above formula, Q1 is the heat transfer on the heat flow side, h air,out1 is the specific enthalpy of the air at the outlet of the hot flow side, h v,out1 is the specific enthalpy of steam at the hot flow side outlet, h w,out1 is the specific enthalpy of saturated water at the outlet of the heat flow side, h air,in1 is the specific enthalpy of the air at the hot flow side inlet, h v,in1 is the specific enthalpy of steam at the hot flow side inlet, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side, m v,out1 is the steam flow rate at the hot flow side outlet.

[0037] Furthermore, in step 13, the calculation method of the gas phase flow rate at the hot flow side outlet is:

[0038]

[0039] Calculation method of liquid flow rate at the hot flow side outlet:

[0040]

[0041] In the above formula, m q,out1 is the gas phase flow rate at the outlet of the hot flow side, m l,out1 is the liquid phase flow rate at the hot flow side outlet, p v,out1 is the steam pressure at the outlet of the hot flow side, p air,out1 is the outlet air pressure on the hot flow side, M v is the relative molecular mass of steam, M air is the relative molecular mass of air, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side.

[0042] Furthermore, in step 15, the adjustment strategy includes but is not limited to a global or local optimization algorithm.

[0043] By adopting the above technical solution, the present invention has the following advantages:

[0044] 1. The present invention determines whether the heater meets the operating requirements by using four indicators: steam condensation on the hot flow side of the heater, outlet gas and liquid flow rates, and end difference. This provides a comprehensive and effective evaluation method for determining the availability of the heater in the shaft seal overflow energy recovery system.

[0045] 2. The heat transfer calculation method provided by the present invention takes into account the steam dissolved in the non-condensable gas at the heat transfer outlet, making the evaluation of the heat transfer on the heat transfer side more accurate;

[0046] 3. The present invention defines the hot flow side outlet limit temperature for the first time and uses it as the initial value of the hot flow side outlet temperature. This allows for quick determination of whether the heater is unusable due to inability to condense all steam without requiring iteration.

[0047] 4. When cyclically updating the heater outlet temperature, the present invention adopts global / local optimization algorithms as adjustment strategies, which can effectively reduce the number of iterations and improve calculation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is a flow chart of the method of the present invention;

[0049] Figure 2 Schematic diagram of the structure and internal flow of a heater in a shaft seal overflow energy recovery system of a thermal power plant in the embodiment;

[0050] Figure 3 Graph showing the change in outlet temperature on the hot flow side during the cycle in the embodiment;

[0051] Figure 4 Graph showing the change in the absolute value of the relative error in heat transfer during the cycle in the embodiment. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the present invention are further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] like Figure 1 As shown, the present invention provides a method for evaluating the heater availability of a shaft seal overflow energy recovery system, comprising the following steps:

[0054] Step 1: Based on the operating environment of the heater in the shaft seal overflow energy recovery system, enter the heater operating conditions, including:

[0055] Cold stream side: The working fluid is liquid water, with the following information: pressure, inlet temperature, flow rate, and specific heat capacity at constant pressure. Hot stream side: The working fluid is a steam-air mixture, with the following information: pressure, inlet temperature, flow rate, and steam mass fraction. The heater's total heat transfer coefficient and heat transfer area must be specified. The maximum allowable end-to-end difference between the heater and the maximum allowable hot stream outlet gas and liquid flow rates must be specified. The cold stream side working fluid can also be other single-phase fluids, while the hot stream side working fluid can also be other condensable and non-condensable gas mixtures.

[0056] Step 2: Calculate the steam and air partial pressures at the inlet of the heater on the hot flow side according to Dalton's law of partial pressures.

[0057] Step 3: Given a cycle count N=0, the outlet limit temperature at which all steam on the hot flow side is condensed, i.e., 99.9% of the saturated steam temperature corresponding to the hot flow side inlet steam partial pressure, is used as the initial value of the hot flow side outlet temperature.

[0058] Step 4: Determine the saturated steam pressure corresponding to the outlet temperature of the hot flow side according to the physical properties of the working fluid. Calculate the air partial pressure at the outlet of the hot flow side based on the principle that the pressure on the hot flow side must be kept constant when the heater is working.

[0059] Step 5: Calculate the steam flow rate at the hot flow side outlet according to the gas phase component pressure at the hot flow side outlet, that is:

[0060]

[0061] In the above formula, m v,out1 is the steam flow rate at the hot flow side outlet, p v,out1 is the outlet steam pressure on the hot flow side, p air,out1 is the outlet air pressure on the hot flow side, M v is the relative molecular mass of steam, M air is the relative molecular mass of air, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side. The unit of pressure is Pa, and the unit of flow rate is kg / s.

[0062] Step 6: Based on the pressure and temperature of the gas phase components at the outlet of the heat flow side and the physical properties of the working fluid, determine the specific enthalpy values ​​of the outlet saturated water vapor, saturated water, and air, and further calculate the heat transfer on the heat flow side, namely:

[0063] Q1=h air,out1 (1-w v,in1 )m in1 +h w,out1 (w v,in1 m in1 -m v,out1 )+h v,out1 m v,out1 -h air,in1 (1-w v,in1 )m in1 -h v,in1 w v,in1 m in1 In the above formula, Q1 is the heat transfer heat flow side, h air,out1 is the specific enthalpy of the air at the outlet of the hot flow side, h v,out1 is the specific enthalpy of steam at the hot flow side outlet, h w,out1 is the specific enthalpy of saturated water at the outlet of the heat flow side, h air,in1 is the specific enthalpy of the air at the inlet on the hot flow side, h v,in1 is the specific enthalpy of the steam at the hot flow side inlet. The unit of specific enthalpy is J / kg.

[0064] Step 7: Calculate the outlet temperature of the cold stream according to the heat exchange balance of the cold stream and hot stream and the constant pressure specific heat capacity and inlet temperature of the cold stream input in step 1, that is:

[0065]

[0066] In the above formula, t out2 is the outlet temperature of the cold flow side, m in2 is the inlet flow rate on the cold flow side, C p is the constant pressure specific heat capacity of the working fluid on the cold flow side (unit: J / (kg·℃)), t in2 is the inlet temperature of the cold flow side (unit: °C).

[0067] Step 8: Based on the cold flow side outlet and hot flow side inlet and outlet temperatures, use the logarithmic mean heat transfer temperature difference Δt m The product of the temperature difference Δt driven by heat transfer is determined by multiplying the temperature by the correction coefficient ψ.

[0068] For co-current heat exchangers:

[0069]

[0070] For counter-flow heat exchangers:

[0071]

[0072] Among them, t in1 is the inlet temperature of the hot flow side, t out1 is the outlet temperature of the hot flow side.

[0073] Correction coefficient calculation formula:

[0074]

[0075] Where R and P are dimensionless parameters, and the calculation formula is:

[0076]

[0077]

[0078] Step 9: Calculate the heater heat transfer capacity based on the input total heat transfer coefficient, heat transfer area, and the heat transfer driving temperature difference obtained in step 8, that is:

[0079] Q3=AkΔt

[0080] In the above formula, A is the heat exchange area (unit: m 2 ), k is the total heat transfer coefficient (unit: W / (m 2 ·℃)).

[0081] Step 10: Determine the value of N calculated in the loop: if N=0, execute step 11; if N≠0, execute step 12.

[0082] Step 11: Calculate the difference between the heat transfer rate on the hot flow side obtained in step 6 and the heater heat transfer rate obtained in step 9. If the difference Q1-Q3>0, it is concluded that the overflow steam cannot be completely condensed and the heater is unusable, and the evaluation ends; if the difference Q1-Q3≤0, execute step 12.

[0083] Step 12: Calculate the absolute value of the relative error between the heat transfer on the heat flow side obtained in step 6 and the heat transfer on the heater obtained in step 9 If ε≤0.01%, go to step 13; if ε>0.01%, go to step 15.

[0084] Step 13: The heater end difference is obtained based on the difference between the hot flow side outlet temperature and the cold flow side inlet temperature, that is, ΔD = t out1 -t in2 ; Calculate the gas phase flow rate and liquid phase flow rate at the hot flow side outlet according to the hot flow side inlet flow rate, component mass fraction and hot flow side outlet steam flow rate:

[0085] Step 14: If the heater end difference, hot flow side outlet gas phase, and liquid phase flow rates obtained in step 13 are all less than the allowable values ​​entered in step 1, the heater is available and the evaluation ends; otherwise, the heater is unavailable and the evaluation ends.

[0086] Step 15: Loop count N = N + 1. Use the global / local optimization algorithm as the adjustment strategy to update the outlet temperature of the heat flow side. The outlet temperature must be less than the initial outlet temperature value given in Step 3. Restart from Step 4 until the loop is exited and continue until the evaluation is completed.

[0087] In order to further verify the technical effects of the present invention, the technology and features of the present invention are described in detail below through specific embodiments, but these embodiments are not intended to limit the scope of protection of the present invention.

[0088] Example 1:

[0089] Taking the heater in the shaft seal overflow energy recovery system of a thermal power plant as the object, its structure and internal flow diagram are shown in the figure below. Figure 2 As shown, the working medium on the hot flow side is a mixture of water and steam, and the working medium on the cold flow side is liquid water. The method proposed in the present invention is used to evaluate the availability of the heater, and the specific method includes the following steps:

[0090] Step 1: Based on the operating environment of the heater in the shaft seal overflow energy recovery system, enter the heater's operating conditions: cold stream side: pressure, inlet temperature, flow rate, and constant-pressure specific heat capacity; hot stream side: pressure, inlet temperature, flow rate, and component mass fraction; heater total heat transfer coefficient, heat transfer area; maximum allowable end differential of the heater, and maximum allowable gas and liquid flow rates at the hot stream outlet. Specific parameters are shown in Table 1.

[0091] Step 2: According to Dalton's law of partial pressures, the steam partial pressure at the inlet of the hot flow side of the heater is 70.93 kPa; the air partial pressure is 23.37 kPa.

[0092] Step 3: Given the loop count N as 0, the initial value of the hot flow side outlet temperature is calculated to be 90.19°C.

[0093] Step 4: Based on the working fluid's physical properties, determine the saturated steam pressure corresponding to the hot-side outlet temperature. Based on the principle that the hot-side pressure must be maintained constant during heater operation, calculate the air partial pressure at the hot-side outlet. When cycle count N = 0, the hot-side outlet steam and air partial pressures are 70.69 kPa and 23.61 kPa, respectively.

[0094] Step 5: Calculate the steam flow rate at the hot flow outlet based on the gas phase component pressure at the hot flow outlet. When the cycle count N=0, the steam flow rate at the hot flow outlet is 0.6025 kg / s.

[0095] Step 6: Based on the pressure and temperature of the hot-side outlet gas phase components and the working fluid physical properties, determine the outlet saturated steam specific enthalpy, saturated water specific enthalpy, and air specific enthalpy, and further calculate the heat transfer rate on the hot-side. When the cycle count N = 0, the obtained hot-side outlet steam, saturated water specific enthalpy, and air specific enthalpy are 2659.84 kJ / kg, 377.84 kJ / kg, and 364.29 kJ / kg, respectively; the heat transfer rate Q1 on the hot-side is 206.62 kJ / s.

[0096] Step 7: Calculate the cold stream outlet temperature based on the cold and hot stream heat exchange balances and the cold stream constant pressure specific heat capacity and inlet temperature input in Step 1. When the loop count N = 0, the cold stream outlet temperature is 44.132°C.

[0097] Step 8: Based on the cold stream outlet and hot stream inlet and outlet temperatures, calculate the heat transfer driving temperature difference using the logarithmic heat transfer temperature difference formula and the correction factor. When the cycle count N = 0, the heat transfer driving temperature difference is 94.379°C.

[0098] Step 9: Calculate the heater heat transfer capacity Q3 based on the input total heat transfer coefficient, heat transfer area, and the heat transfer driving temperature difference obtained in Step 8. When the cycle count N = 0, Q3 is calculated to be 2497.04 kJ / s.

[0099] Step 10: Determine the value of N calculated in the loop: if N=0, execute step 11; if N≠0, execute step 12.

[0100] Step 11: Calculate the difference ΔQ between the heat transfer rate on the hot stream side obtained in Step 6 and the heater heat transfer rate obtained in Step 9. If the difference is greater than 0, conclude that the overflow steam cannot be fully condensed and the heater is unusable, and the evaluation ends. If the difference is less than or equal to 0, proceed to Step 12. When the loop count N = 0, the calculated ΔQ < 0, and proceed to Step 12.

[0101] Step 12: Calculate the absolute value of the relative error ε between the heat transfer side heat transfer obtained in Step 6 and the heater heat transfer obtained in Step 9. If this value is less than or equal to 0.01%, proceed to Step 13; if this value is greater than 0.01%, proceed to Step 15. When the loop count N = 0, the calculated ε > 0.01%, and Step 15 is executed.

[0102] Step 13: Obtain the heater end difference based on the difference between the hot flow side outlet temperature and the cold flow side inlet temperature; calculate the hot flow side outlet gas phase and liquid phase flow rates based on the flow rate and component mass fraction input in step 1 and the hot flow side outlet steam flow rate obtained in step 6.

[0103] Step 14: If the heater end difference, hot flow side outlet gas phase, and liquid phase flow rates obtained in step 13 are all less than the allowable values ​​entered in step 1, it is concluded that the heater is available and the evaluation ends; otherwise, the heater is unavailable and the evaluation ends.

[0104] Step 15: Loop count N = N + 1, and update the hot flow outlet temperature according to the adjustment strategy. Execute again from step 4 until the loop exits and the evaluation is complete.

[0105] In this example, the generalized descent gradient method is used as the adjustment strategy for updating the outlet temperature of the heat flow side. Figure 3 、 Figure 4 These are the change in the outlet temperature of the heat flow side and the change in the absolute value of the relative error in heat transfer during the cycle. This example exits the loop after the 21st iteration and continues with steps 13 and 14. The evaluation result indicates that the heater is operational. Table 2 shows some of the heater parameters and results at the completion of the evaluation.

[0106] Table 1

[0107]

[0108] Table 2

[0109]

[0110] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific embodiments. Any other implementation methods derived from the technical solution of the present invention or the application of this technical solution to other technical fields also fall within the scope of protection of the present invention.

Claims

1. A method for evaluating heater availability of a shaft seal overflow energy recovery system, characterized in that: The steps include: Step 1: Input the heater operating conditions according to the operating environment of the heater in the shaft seal overflow energy recovery system; Step 2: Calculate the partial pressures of water vapor and air at the inlet of the heater on the hot flow side according to Dalton's law of partial pressures; Step 3: Given a cycle count N = 0, calculate the outlet limit temperature at which all steam on the hot flow side is condensed as the initial value of the hot flow side outlet temperature; Step 4: Determine the saturated water vapor pressure corresponding to the outlet temperature of the hot flow side based on the working fluid physical parameters. Calculate the air partial pressure at the hot flow side outlet based on the principle that the hot flow side pressure must be maintained constant when the heater is working. Step 5: Calculate the steam flow rate at the hot flow side outlet according to the gas phase component pressure at the hot flow side outlet; Step 6: Based on the pressure and temperature of the gas phase components at the outlet of the heat flow side and the physical properties of the working fluid, determine the specific enthalpy values ​​of the outlet saturated water vapor, saturated water, and air, and further calculate the heat transfer on the heat flow side; Step 7: Calculate the outlet temperature of the cold stream side based on the heat exchange balance between the cold stream side and the hot stream side and the constant pressure specific heat capacity and inlet temperature of the heater operating conditions input in step 1; Step 8: Calculate the heat transfer driven temperature difference using the logarithmic heat transfer temperature difference formula and correction coefficient based on the cold flow side outlet and hot flow side inlet and outlet temperatures; Step 9: Calculate the heater heat transfer capacity based on the input total heat transfer coefficient, heat transfer area, and the heat transfer driving temperature difference obtained in step 8; Step 10: Determine based on the value of the loop count N: if N=0, go to step 11; if N≠0, go to step 12; Step 11: Calculate the difference between the heat transfer rate on the hot flow side obtained in step 6 and the heater heat transfer rate obtained in step 9. If the difference is greater than 0, the steam cannot be completely condensed, and the heater is concluded to be unusable, and the evaluation ends. If the difference is less than or equal to 0, proceed to step 12. Step 12: Calculate the absolute value of the relative error between the heat transfer side heat transfer obtained in step 6 and the heater heat transfer obtained in step 9. If the absolute value is less than or equal to 0.01%, proceed to step 13; if the absolute value is greater than 0.01%, proceed to step 15. Step 13: Obtain the heater end difference based on the difference between the hot flow side outlet temperature and the cold flow side inlet temperature; Calculate the hot flow side outlet gas phase and liquid phase flow rates based on the heat flow measured flow rate and component mass fraction in the heater operating conditions input in step 1 and the hot flow side outlet steam flow rate obtained in step 5; Step 14: If the heater end difference, hot flow outlet gas phase, and liquid phase flow rates obtained in step 13 are all less than the corresponding allowable values ​​specified in the heater operating conditions input in step 1, the conclusion is drawn that the heater is usable and the evaluation ends; otherwise, the heater is unusable and the evaluation ends; Step 15: Loop count N=N+1, update the outlet temperature of the heat flow side according to the adjustment strategy, and its outlet temperature must be less than the initial value of the heat flow outlet temperature given in step 3; Execute again from step 4 until the loop is exited and the evaluation ends.

2. The heater availability evaluation method for a shaft seal overflow energy recovery system according to claim 1, characterized in that: The heater is a partition type heat exchanger.

3. The heater availability evaluation method for a shaft seal overflow energy recovery system according to claim 1, characterized in that: In step 1, the operating conditions of the heater include: Cold flow side: the working fluid is liquid water, pressure, inlet temperature, flow rate, and specific heat capacity at constant pressure of the working fluid; Hot flow side: the working medium is a steam-air mixture, with pressure, inlet temperature, flow rate, and steam mass fraction; Heater total heat transfer coefficient, heat transfer area; The maximum allowable end difference of the heater, the maximum allowable gas flow rate and liquid flow rate at the hot flow side outlet.

4. The heater availability evaluation method for a shaft seal overflow energy recovery system according to claim 3, characterized in that: The hot flow side working medium steam-air mixture can be replaced by other condensable and non-condensable gas mixtures.

5. The heater availability evaluation method for a shaft seal overflow energy recovery system according to claim 3, characterized in that: The working fluid liquid water on the cold flow side can be replaced by other single-phase fluids.

6. The method for evaluating heater availability of a shaft seal overflow energy recovery system according to claim 1, characterized in that: In step 3, the outlet limit temperature at which the steam on the hot flow side is completely condensed is 99.9% of the saturated steam temperature corresponding to the partial pressure of the steam on the hot flow side inlet.

7. The method for evaluating heater availability of a shaft seal overflow energy recovery system according to claim 1, characterized in that: In step 5, the calculation method of the steam flow rate at the hot flow side outlet is: In the above formula, m v,out1 is the steam flow rate at the hot flow side outlet, p v,out1 is the outlet steam pressure on the hot flow side, p air,out1 is the outlet air pressure on the hot flow side, M v is the relative molecular mass of steam, M air is the relative molecular mass of air, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side.

8. The method for evaluating heater availability of a shaft seal overflow energy recovery system according to claim 1, wherein: In step 6, the calculation method of the heat transfer amount on the heat flow side is: Q1=h air,out1 (1-w v,in1 )m in1 +h w,out1 (w v,in1 m in1 -m v,out1 )+h v,out1 m v,out1 -h air,in1 (1-w v,in1 )m in1 -h v,in1 w v,in1 m in1 In the above formula, Q1 is the heat transfer heat flow side, h air,out1 is the specific enthalpy of the air at the outlet of the hot flow side, h v,out1 is the specific enthalpy of steam at the hot flow side outlet, h w,out1 is the specific enthalpy of saturated water at the outlet of the heat flow side, h air,in1 is the specific enthalpy of the air at the hot flow side inlet, h v,in1 is the specific enthalpy of steam at the hot flow side inlet, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side, m v,out1 is the steam flow rate at the hot flow side outlet.

9. The method for evaluating heater availability of a shaft seal overflow energy recovery system according to claim 1, characterized in that: In step 13, the calculation method of the gas phase flow rate at the hot flow side outlet is: Calculation method of liquid flow rate at the hot flow side outlet: In the above formula, m q,out1 is the gas phase flow rate at the outlet of the hot flow side, m l,out1 is the liquid phase flow rate at the outlet of the hot flow side, p v,out1 is the outlet steam pressure on the hot flow side, p air,out1 is the outlet air pressure on the hot flow side, M v is the relative molecular mass of steam, M air is the relative molecular mass of air, w v,in1 is the steam mass fraction at the hot flow side inlet, m in1 is the inlet flow rate on the hot flow side.

10. The heater availability evaluation method for a shaft seal overflow energy recovery system according to claim 1, characterized in that: In step 15, the adjustment strategy includes but is not limited to a global or local optimization algorithm.

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