Why is it necessary to reduce the pressure in the steam system?

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As we all know, steam should be transported at high pressure and used at low pressure. But why is that? 1. Advantages of high-pressure transmission: A. The higher the steam pressure, the greater the density, and thus less space is required per unit mass of steam. Therefore, steam boilers often operate at high pressures; even smaller boilers can generate a large amount of steam, which is then delivered to the points where it is needed through relatively small pipes. B. The higher the pressure, the higher the temperature, and the slower the rate of steam condensation; therefore, the heat transfer efficiency must be high. 2. Reasons for steam pressure reduction (using low-pressure steam): A. The lower the pressure, the higher the heat enthalpy of the steam; at 1.6 MPa, the heat enthalpy is 1922.64 kJ/kg, at 0.8 MPa it is 2030.31 kJ/kg, and at 0.3 MPa it is 2133.33 kJ/kg. Therefore, using low-pressure steam can reduce the steam load on the boiler and save energy. To reduce the size of the boiler, most steam boilers are designed with relatively high pressures, and should not operate at lower pressures, as this would cause the steam to contain water and result in a decrease in its dryness. For this reason, the more economical approach is usually to have the boiler produce steam at a higher pressure and then reduce the pressure at the point of use. Only by adding pressure reduction stations can high-quality, stable low-pressure saturated steam be obtained, which is required by the equipment process. B. The lower the steam pressure, the more suitable it is for use in equipment; it allows for lower pressure ratings for valves, equipment, pipes, and accessories in the downstream systems. The purchase cost of low-pressure equipment is much lower than that of equipment that uses high-pressure steam, and such equipment also has a longer lifespan. C. The lower the pressure, the lower the temperature of the condensate water; as a result, the proportion of flash evaporation of the condensate water behind the steam trap decreases, and the amount of heat lost also decreases accordingly. D. Because the physical properties of saturated steam are such that pressure and temperature are in a one-to-one relationship, in certain processes it is possible to indirectly control the temperature by controlling the pressure, as pressure control is simpler and allows for precise temperature regulation. This is quite common in sterilization ovens, contact dryers, vulcanizing machines, and calenders, as it is difficult to measure the surface temperature of these devices using temperature sensors. Therefore, pressure control is also the basis for heat exchanger temperature control. E. Sometimes, reducing pressure is also done to stabilize it. Now, more and more factories are required to use steam supplied centrally by thermal power plants, as there are many units that require steam, and their production activities often take place during the day; few factories operate at night. As a result, pressure is usually high at night, reaching its peak, and it is necessary to reduce this pressure in order to meet the needs of the factories. Even during the day, when the steam pressure drops, it remains quite unstable; therefore, a pressure regulator is also needed to stabilize the pressure. 3. Pressure reduction device: An automatic pressure reducing valve or a pneumatic control valve can be used to accurately control the steam pressure. Self-acting pressure relief valves include: pilot-operated pressure relief valves and directly-acting pressure relief valves ； Pneumatic control valves require high-temperature pressure sensors and pressure controllers to be used in conjunction with them. 3. Energy-saving benefit analysis using low-pressure steam 1. For example: one boiler with a capacity of 35 t/H and a pressure of 9 barg, while the pressure required in the workshop is 5 barg; the energy that can be saved can be calculated based on this. 2. Benefit calculation: This value is obtained from the saturated steam tables (note that in these tables, the absolute pressure equals gauge pressure plus 1). At a pressure of 9 barg, the enthalpy of the steam is 2014.44 kJ·kg-1; at a pressure of 5 barg, it is 2085.64 kJ·kg-1. Therefore, by reducing the pressure from 9 barg to 5 barg, an additional amount of heat of 2085.64–2014.44 = 71.2 kJ·kg-1 can be generated. For a boiler with a capacity of 35 t/h, this means an additional 35000 kg/h * 71.2 kJ·kg-1 = 2492000 kJ/h of heat can be produced. Given a fixed demand for steam, it is possible to produce less heat, meaning that such an amount of heat can be saved. Now, we convert this heat amount into the volume of steam produced. Our boiler is capable of generating steam at a pressure of 9 barg; therefore, the heat mentioned earlier is equivalent to 2492000 (kJ/h) / 2014.44 (kJ·kg-1) = 1237 kg/h of steam at 9 barg pressure. This means that 1237*24 = 29688 kg of steam can be saved per day. Thus, the proportion of coal that can be saved, or the increase in thermal efficiency, is 1237/35000 = 3.5%. The thermal efficiency of the boiler is 80%, and the total heat energy of saturated steam at 9 bar is 2777.12 kJ/kg – 1 = 2777.12/4.2 = 662.9 Kcal/kg. Therefore, 662.9 Kcal/kg * 35000 kg/h = 23201500 Kcal/h. Typically, three-stage coal combustion is used, with a calorific value of only 6308 Kcal/kg; hence, the coal consumption by the boiler is 23201500/6308 = 3678 kg/h, which is equivalent to 3.678 (T/h). Therefore, by using low-pressure steam at 5 bar, the amount of coal that can be saved per day is: 3.678*24*3.5% = 3.1 T. The current price of coal for level 3 combustion is 780 yuan per T; thus, the savings per day amount to 3.1*780 = 2418 yuan. Calculated on the basis of 360 working days per year, a total savings of 2418*360 = 870,000 yuan can be achieved. It can be seen that for devices operating at low pressures, it is necessary to use pressure regulators to reduce the steam pressure to the level required by the devices, which can **reduce energy consumption**.