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At present, our country **advocates pursuing a path of sustainable development while ensuring steady economic growth**, and it sets increasingly higher requirements in terms of energy use and environmental protection policies. Steam is an essential power source and heat source for power generation and other modern industrial production processes. To improve the overall energy utilization rate and meet environmental protection requirements, cogeneration and centralized heating have become development directions encouraged by policy. As a result, industrial development zones across our country are increasingly adopting cogeneration to provide steam power for the enterprises located within these zones. In other words, the superheated steam that has already been used to generate power during the cogeneration process is utilized as a steam source for various downstream processing steps and steam-using equipment. Therefore, thermal power plants supply the heating network in the development zone with superheated steam at high pressure and temperature, whereas the process operations and steam-using equipment in ordinary factories – such as various heat exchangers, heating devices for cooking and concentration, as well as most air conditioning and cooling systems – require saturated steam. This necessitates the use of desuperheaters to convert superheated steam into saturated steam. I. Working principle of the pressure and temperature reduction device: After the high-temperature, high-pressure superheated steam supplied by the thermal power plant is delivered to various steam-consuming points, it must first pass through a pressure and temperature reduction device. There, the pressure and temperature of the superheated steam are reduced to values close to the desired saturated state (usually 3–5°C below the saturated temperature), before it is sent to heat exchange equipment for use. There are two most basic types of temperature reducers. 1. Non-contact cooling: The medium used for cooling steam does not come into direct contact with the superheated steam being cooled. Colder liquids, gases, and vapors can all be used as cooling media. The surrounding air can also be used as a cooling medium. This type of temperature reducer is similar to a shell-and-tube heat exchanger. Superheated steam enters one side of the heat exchanger, while the cooling medium enters the other side. By controlling the temperature of the superheated steam, it is possible to adjust either the flow rate of the inlet superheated steam or the flow rate of the cooling medium. 2. Direct contact type: The medium used to cool the steam (usually water) mixes directly with the superheated steam, as in the Venturi and direct injection type temperature and pressure reduction systems shown in the figure below. The superheated steam first enters the pressure reducer after being depressurized. The cooling water mixes directly with the superheated steam, absorbing its heat and evaporating into steam. While the superheated steam is cooled. A certain amount of cooling water is added through the atomization and mixing device inside the temperature reducer. The control of the amount of cooling water added is achieved by measuring the steam temperature downstream of the desuperheater. So it can produce dry steam. This prevents damage and erosion to the pipes and equipment below. All direct-contact type temperature reducers must break the incoming water into small droplets in order to increase the surface area-to-volume ratio of the water. The higher the surface area to volume ratio of water, the faster the evaporation rate of the water droplets, and the faster the steam cools down. The process of forming small water droplets is commonly referred to as “atomization”. The quality of the atomization of the water used for temperature reduction will directly affect the control performance of the temperature reduction system; different types of temperature reducers employ various methods for atomizing the water used for temperature reduction. It is worth noting that the mixing of physicalized water droplets with steam, as well as the evaporation of the water droplets (along with the cooling of the steam), is a process that takes time and does not occur instantly. Therefore, most of the temperature reduction process does not occur inside the desuperheater, but rather in the pipelines downstream of the desuperheater outlet. Therefore, for a good overall design, the piping layout downstream of it is also crucial. From the above, it is easy to understand why cooling water droplets and superheated steam need a period of time for proper mixing. If the mixing is poor, moisture cannot effectively absorb heat from the superheated steam; as a result, the evaporation of water droplets is incomplete, leading to water droplets escaping downstream of the desuperheater, and the temperature at the desuperheater outlet cannot be controlled. Therefore, the water droplets should remain suspended in the downstream pipeline for as long as possible. To ensure this, the downstream pipeline should maintain a relatively high flow velocity to sustain sufficient turbulence in it. This speed is higher than the steam flow rate of conventional steam distribution systems. This is why the temperature reducers and the corresponding pipes are usually (but not always) smaller than the pipes in the steam distribution system. Common sources of water used for cooling include: boiler make-up water, demineralized water, deionized water, and condensate water. Urban tap water or process water may also be used, depending on the hardness of the water supply. Scale may accumulate inside the cooling water nozzles of the desuperheater and on the inner wall surfaces of the pipes downstream of the desuperheater. Generally, the higher the temperature of the cooling water, the better, as hot water droplets absorb less heat to reach the evaporation temperature; as a result, they evaporate more quickly, thereby achieving a more efficient cooling effect. Using hot water also reduces the amount of water that falls onto the inner walls of the pipes. Therefore, the water supply pipes should be insulated. A pressure drop is required through the water control valve. We mentioned earlier that the water should be as hot as possible, but flashing caused by control valves should be avoided. To inject cooling water, the pressure at the desuperheater nozzle must be equal to or greater than the pressure of the steam in the pipeline. Different types of temperature reducers have varying pressure requirements, but the typical minimum pressure values are as follows: For jet-type temperature reducers, it is +0.5 bar relative to the steam pressure; for Venturi-type temperature reducers, it is +0.1 bar relative to the steam pressure. Steam atomization type temperature reducers require the same pressure as the steam pressure. For jet-type and Venturi-type temperature reducers, the highest pressure is needed at the maximum water flow rate. It is worth noting that the water flow rate is proportional to the square root of the pressure difference between the cooling water and steam. Therefore, if the water flow rate increases by 4 times, the pressure difference must increase by 4×16. If an independent or booster pump is used, a return system is required to ensure that water continuously flows through the pump. Calculation of cooling water consumption: Enough water must be added to cool the steam to the desired temperature ； If the water volume is insufficient, steam cannot be cooled adequately; on the other hand, if there is too much water, wet saturated steam may be generated, leading to erosion of downstream pipes and equipment. By using the following enthalpy balance equation, it is possible to calculate the required amount of cooling water conveniently and quickly: Ms x(hi—hd) = Mw x(hd-hw). Here, Mw = amount of coolant in kg/h; Ms = amount of superheated steam in kg/h; hi = enthalpy of superheated steam in kJ/kg; hd = enthalpy of steam after temperature reduction in kJ/kg; hw = enthalpy of coolant in kJ/kg. II. Structure and types of desuperheaters Desuperheaters must be able to effectively compensate for changes in environmental conditions as well as changes in steam temperature or flow rate. The choice depends on the following factors: operating pressure u, operating temperature u, steam flow rate u, superheat before and after temperature reduction u, required control ratio u, water pressure that can be provided (a booster pump may be needed if there is insufficient pressure), and the required accuracy of control over the final temperature. There are three different types of temperature reducers: (1) jet type temperature reducer, (2) Venturi type temperature reducer, (3) steam atomization type temperature reducer. The following will introduce the structure and characteristics of different types of temperature reducers. 1. Venturi-type desuperheater: The venturi-type desuperheater utilizes throttling to create high-speed zones and turbulence, thereby facilitating full contact between steam and cooling water and achieving the greatest desuperheating effect. The temperature reduction process is completed in three stages: The temperature reduction in the first stage takes place within the internal diffuser. Some of the steam accelerates inside the inner nozzle and atomizes the water injected therein. The second stage of temperature reduction is the mixing of the saturated mist from the internal diffuser and the remaining steam within the main diffuser. The main diffuser itself generates high speeds by restricting the steam flow, thereby creating intense turbulence in that area to accomplish the temperature reduction in the second stage. The third and final temperature reduction stage takes place in the pipeline downstream of the desuperheater. During this stage, the remaining water droplets suspended in the steam evaporate, thereby achieving the desired final temperature at a certain point downstream of the desuperheater. This device minimizes the possibility of cooling water coming into contact with the inner wall of the pipes. It results in minimal pipe erosion and the greatest temperature reduction effect. The Venturi cooler achieves the greatest high-speed mixing effect. The steam flow regulation ratio varies depending on the actual operating conditions; for horizontally installed desuperheaters, the regulation ratio is generally 4:1, while for vertically installed desuperheaters, it can exceed 5:1. When used with a well-designed pressure relief station, its steam flow regulation ratio can be improved to over 5:1. If the adjustment ratio of the steam flow exceeds the capacity of a single desuperheater, two desuperheaters can be used in parallel, with automatic switching taking place in response to changes in the steam flow. Features of the venturi-type overflow reducer: Its adjustment ratio is generally sufficient for most industrial applications, and the pressure remains relatively low in most cases. It’s simple to operate. There are no moving parts. The control is precise, and the saturation temperature TsAT+3℃ can usually be achieved. Suitable for applications with stable or variable steam conditions. 2. Complete jet-type desuperheater: A complete jet-type desuperheater is easy to install. It consists of a jet nozzle assembly, a heat shield, and a shell and cover connected by flanges. This is the simplest type of temperature reducer, where the coolant is sprayed into the steam stream through one or more atomizing nozzles. The atomization nozzle is located on the central axis of the desuperheater, and the coolant is sprayed into the steam in the same direction as the steam. This desuperheater includes a thermal sleeve. Superheated steam can pass through the annular surface between the thermal sleeve and the inner diameter of the shell. The heat sleeve provides a hot surface that enables the sprayed liquid to evaporate rapidly, while also protecting the shell of the temperature reducer from erosion. The operation of the heat sleeve enables the desuperheater to ensure the effective operation of the system when it is under light load and nozzle atomization is not most efficient. Features of jet-type temperature reducers: Simple operation. There are no moving parts. The cost is low. Zero vapor pressure drop. The low flow regulation ratio capability is lower than the ability to approach the saturated steam temperature (it is generally best to be at TsAT+5℃). It can easily cause erosion on the inner wall of steam pipes. Using an internal heat sleeve can overcome this problem. It also helps with the evaporation of moisture. Applications: Relatively stable steam load, relatively stable steam temperature, and relatively stable cooling water temperature. Steam atomization type desuperheater – This type of desuperheater uses auxiliary high-pressure steam to atomize the incoming cooling water within the desuperheater’s diffuser. The pressure of the auxiliary steam must be at least 1.5 times the pressure of the steam at the inlet to the desuperheater (gauge pressure). The minimum required pressure is 3 barg. Generally, the flow rate of the atomized steam is 2% to 5% of the main steam flow rate. The temperature reduction process is completed in two stages. Process 1 takes place inside the diffuser, where the cooling water is atomized by high-speed misted steam. In the second stage of temperature reduction, the saturated mist from the diffuser mixes with the steam in the main pipeline. The evaporation process takes place in the pipeline at the outlet of the desuperheater. The moisture remaining in this outlet pipeline is suspended in the steam and gradually evaporates, thereby achieving the desired final temperature at a certain point downstream of the desuperheater. Features of the steam atomization type desuperheater: The coolant is introduced from the center of the main steam flow. The coolant is sprayed onto the steam in the direction of the steam flow. Good regulation ratio. The adjustment ratio for steam flow can reach up to 50:1 at most. However, the most effective operation and control adjustment ratio is approximately 20:1. The cooling fluid adjustment ratio is the same for all data. Its structure is very compact, making it the shortest in length among all desuperheaters. The pressure drop can be ignored; it is suitable for applications where the steam flow varies greatly and a high regulation ratio is required. III. Installation orientation of the desuperheater and control components: The desuperheater can be installed horizontally or vertically; when installed vertically, the steam must flow upward. Spiza**strongly opposes vertical installation, where steam flows downward. For water-cooled desuperheaters installed horizontally, the ideal installation direction is with the cooling water connection ports facing downward (the same applies to the steam atomization ports of steam-atomization type desuperheaters). Satisfactory operation can also be achieved in other directions, but the drainage performance is poorer. For vertical installation, we recommend that the cooling water pipes (and, if necessary, the steam misting pipes) be connected from below to the corresponding connections on the temperature reducer. This arrangement ensures optimal drainage when closed. The distance between the temperature reducer and the pressure reducing valve: In applications involving pressure reduction and temperature reduction, the temperature reducer should be located at least 5 times the diameter of the pipe downstream of the pressure reducing valve, or 1.5 meters away (distance A on the system diagram). The pressure sensor shall be located at least 1.5 meters downstream of the desuperheater outlet flange. But the ideal location is for the pressure sensor to be at the point where steam is used. This way, the pressure control valve can compensate for any pressure losses in the pipes between the desuperheater and the point of use. The distance between the water spray point and the temperature sensor is crucial. If the sensor is too close to the water spray point, the evaporation of water will be insufficient, and the temperature sensor will give incorrect readings. The location of the temperature sensor depends on many factors. The most important among them is the value of the residual superheat. The table below can be used as a reference. Distance B (meters) indicated by the steam residual superheat in °C: 57.50, 106.80, 156.25, 305.00, 503.70, 1002.50. It is important to maintain a constant supply steam pressure. The steam temperature after the desuperheater controls the amount of water added. The higher the temperature, the more the control valve opens and the more water is added. The usual goal of temperature reduction is to lower the temperature of superheated steam to a value just above the saturation temperature of the steam. However, if the steam pressure supplied increases, its corresponding saturation temperature also increases. The set value of the controller remains unchanged; as a result, in order to reach the set temperature, the control system adds additional water, which makes the steam very humid and leads to many problems. In some applications, steam must be free of moisture; in such cases, it is recommended to install a vapor-water separator downstream of the temperature reducer. This protects the downstream pipelines and equipment from moisture damage in cases of control system failure or abnormal operations, such as during startup. In applications where the temperature of the cooled steam is close to the saturation temperature or where a large modulation ratio is used (for example, for spray-type desuperheaters, the modulation ratio > 2:1; for Venturi-type desuperheaters, the modulation ratio > 3:1; for steam-atomization type desuperheaters, the modulation ratio > 5:1), it is also recommended to install a steam-water separator downstream. The soda separation device must be installed downstream of the temperature sensor, so that there is enough time for the moisture to evaporate. The steam trap used in conjunction with the soda water separation device should be able to prevent air from causing blockages; the discharge pipe coming out of the steam trap should have sufficient capacity to carry away the cooling water, and it should be installed as vertically as possible. It is recommended to install filters on the cooling water supply pipes to prevent the small orifices of the control valves and temperature reducers from becoming clogged. It is also recommended to install a filter upstream of the superheated steam pressure control valve. In applications involving pressure reduction control, a safety valve should be installed downstream of the pressure reducing valve to protect the thermostat and the equipment downstream in cases such as overpressure occurring when the pressure control system fails, or overheating occurring when the pressure control system fails. The desuperheater and downstream equipment must be capable of withstanding the maximum temperature limits of the superheated steam, to ensure system safety in the event of a failure in the pressure and temperature control systems. Finally, it should be noted to the readers that the term \"regulation ratio\" is widely used to describe the performance of different types of temperature reducers. But when considering the entire system, we should keep in mind that the temperature reducer is just one component of the temperature reduction system. Obviously, if the control ratio of the installed control system is lower than that of the desuperheater, the control ratio of the entire desuperheating system also decreases. For example, in a specific temperature and pressure reduction system, if the adjustment ratio of the cooling water control valve is lower than that of the desuperheater, then the adjustment ratio of the cooling water control valve will limit the adjustment ratio of the entire pressure and temperature reduction system.