Analysis and calculation of the steam pressure and temperature reduction process

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This post was last edited by speedhorse09 on 2022-11-8 at 21:44. Steam is the most commonly used heat source in factories. The specifications of the steam entering the factory or equipment may not be sufficient to meet the heating requirements of those devices; therefore, it is necessary to reduce its temperature and pressure before using it in heating equipment. Analyzing the process of steam pressure and temperature reduction is important for accurately determining the design parameters of relevant equipment and pipelines, as well as for calculating the amount of water required for temperature reduction. Calculation and analysis are carried out in conjunction with mainstream process simulation software. The pressure-enthalpy diagram makes it clear to see how the temperature of saturated steam changes during pressurization. Within a certain range (where the pressure is not too high), the temperature of saturated steam decreases after depressurization, becoming higher than the saturation temperature corresponding to that pressure (as can be seen from the red isotherms). The superheat of the steam relative to the saturation line represents the enthalpy value after depressurization (since the process is adiabatic and isentropic, the enthalpy remains constant); therefore, the difference in enthalpy between this value and the enthalpy value on the saturation line corresponds to the latent heat required for adding water. The required amount of make-up water can be calculated by referring to a chart or a table of saturated steam enthalpies, and using the enthalpy difference to determine the latent heat of vaporization of the make-up water. Use Aspen and PROII respectively to calculate the state after depressurization of saturated steam, and compare it with the pressure-enthalpy diagram. https://52yunfan.com/data/attachment/forum/202103/27/205307loloxtdwhr3xzqqj.jpg Taking 1.6 MPaG saturated steam (100 kmol/h) as an example, the superheating temperature after depressurizing to 0.6 MPaG steam. https://52yunfan.com/data/attachment/forum/202103/27/205414jeyy85tjt6erky2y.png Aspen: Use a flash tank model to carry out adiabatic pressure reduction ； https://52yunfan.com/data/attachment/forum/202103/27/205452qlbujhthjjq0*il.jpg
https://52yunfan.com/data/attachment/forum/202103/27/205453yicntqz94s469n64.jpg
After execution: https://52yunfan.com/data/attachment/forum/202103/27/205534qgxsxjkikjjezjsd.png
https://52yunfan.com/data/attachment/forum/202103/27/205609d4666r4ewxxl0411.jpg
So, how is the amount of water needed for temperature reduction calculated? The conditions for water addition are 25°C and 1.6 MApaG (the pressure of the feed water ultimately depends on the pressure drop across the atomizing nozzles in the temperature reducer). The conditions of the flash tank are adjusted to those of saturated steam; this represents a non-adiabatic process. The amount of water required is calculated based on the latent heat associated with phase changes. https://52yunfan.com/data/attachment/forum/202103/27/205756p9l77l7v7z9d53e6.jpg https://52yunfan.com/data/attachment/forum/202103/27/205827m4y3oeecet8ky3gw.jpg Method 1: Use utility calculations by creating a new utility condition and specifying the conditions for inlet and outlet, as shown in the image: https://52yunfan.com/data/attachment/forum/202103/27/205949bxvja2hrqrey7pjp.jpg At this point, an additional flow of water is added to the flash tank, with parameters of 1.6 MpaG, 25°C, and 21.2321 kg/h; https://52yunfan.com/data/attachment/forum/202103/27/205952ko8z53589cc5girj.jpg The parameters of the flash tank are then adjusted to represent an adiabatic process. https://52yunfan.com/data/attachment/forum/202103/27/210122guzz8rc8hruthcl9.jpg Sensitivity analysis can also be used to introduce a flow of water: low flow rate, 1.6 MpaG, 25°C, 1 kg/h; https://52yunfan.com/data/attachment/forum/202103/27/210123bh5y0mu8okby68rz.jpg There are many variables here, and it depends on the initial conditions – the flash tank is adiabatic. The outlet temperature is set to the target value (corresponding to the saturation temperature), with an infinitesimally small flow rate of the liquid phase. https://52yunfan.com/data/attachment/forum/202103/27/210249vypmnykgzkx5x58n.jpghttps://52yunfan.com/data/attachment/forum/202103/27/210249ap3xp1op05o3oypx.jpghttps://52yunfan.com/data/attachment/forum/202103/27/210249zfn85y1ju8q5yfhr.jpghttps://52yunfan.com/data/attachment/forum/202103/27/210324tyc0mlxs7owoklsl.jpg When overheating occurs, the degree of freedom of steam is 2, requiring dual-parameter settings; Aspen’s design rules make it difficult to achieve convergence, and even if convergence is achieved, it is similar to trial-and-error calculations. The PROII calculations are shown in the attachment; my understanding may not be correct, so I would like to discuss this with everyone. If there are any mistakes, I hope you can point them out.