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1. What is positive pressure? Negative pressure? Gauge pressure? Absolute pressure? Answer: If the pressure is higher than atmospheric pressure, it is positive pressure. If the pressure is lower than atmospheric pressure, it is negative pressure. The pressure indicated by the gauge is gauge pressure. The sum of gauge pressure and atmospheric pressure is absolute pressure. 2. What is vaporization? What is evaporation? What is boiling? What are the similarities and differences between evaporation and boiling? Answer: The process by which water turns into steam is called vaporization. The process of vaporization that occurs on the surface of water is called evaporation. The intense vaporization process that produces bubbles inside water is called boiling. Both evaporation and boiling are processes by which water turns into steam; this is what they have in common. However, there is a difference between the two; evaporation and boiling are two different forms of vaporization. Evaporation occurs at the surface of a liquid and can take place at room temperature as well. Boiling, on the other hand, is vaporization that occurs within a liquid, and it can only take place at the saturation temperature corresponding to that pressure. For example, the water in the water wall vaporizes while in a boiling state, and when washed clothes dry, it is vaporization that occurs in an evaporative state. 3. What is the latent heat of vaporization? Why does the latent heat of vaporization decrease as pressure increases? Answer: There is no essential difference between water molecules and water vapor molecules; it’s just that the distance between water molecules is very small, while the distance between water vapor molecules is large. This is why steam of the same weight occupies a much larger volume than water. Water molecules must possess considerable energy in order to overcome the attraction exerted on them by other water molecules and escape to the surface to become vapor molecules. Therefore, for water molecules to turn into steam molecules, they must absorb heat from the outside environment to increase their energy. The heat required to convert water at a certain pressure and saturated temperature into saturated steam per kilogram is called the latent heat of vaporization, with the unit being kJ/kg. The latent heat of vaporization decreases as pressure increases. For example, the pressure is 0.1 MP, and the latent heat of vaporization is 2259 kJ/kg ； The latent heat of vaporization at pressures of 1 Ma and 10 MPa is 2018 kJ/kg and 1327 kJ/kg respectively ； When the pressure rises to the critical pressure of 22.11 MPa, the latent heat of vaporization is 0. As pressure increases, the saturation temperature of water rises, and the kinetic energy of water molecules increases accordingly. Less heat is required from the outside to give these water molecules the energy needed to break free from the gravitational forces acting between them. Therefore, as pressure increases, the latent heat of vaporization decreases. 4. What is saturation temperature? Why does the saturation temperature increase as pressure increases? Answer: The temperature at which a liquid (water, unless otherwise specified) boils under a certain pressure is called the saturation temperature. There is a one-to-one relationship between saturation temperature and pressure, with the saturation temperature increasing as pressure increases. Water molecules in water must overcome the attraction between them as well as external pressure in order to escape to the surface. The greater the external pressure, the more kinetic energy the water molecules need to escape from the surface; temperature represents the intensity of molecular motion, so as pressure increases, the saturation temperature rises. For example, at one atmosphere of pressure, the saturation temperature of water is 100°C. The saturation temperature corresponding to an absolute pressure of 4.5 MPa in medium-pressure boilers is 255.5℃ ； In a high-pressure furnace with an absolute pressure of 11 MPa, the saturation temperature of water is 317°C. 5. What is dry saturated water vapor? Wet saturated water vapor? What is the dryness degree of saturated water vapor? Answer: Saturated water vapor without moisture is called dry saturated water vapor. For example, the steam obtained after the steam-water mixture from the water wall is separated by the steam-water separation device in the drum can be considered dry saturated steam. Saturated water vapor that contains moisture is called wet saturated water vapor. For example, the steam-water mixture in the water wall is wet saturated steam. The ratio of the weight of steam to the weight of the vapor-liquid mixture is called the dryness degree of saturated water vapor, denoted by χ. Obviously, χ=0 corresponds to saturated water ； χ=1 corresponds to dry saturated steam ； 0＜χ＜1 corresponds to wet saturated steam. In a boiler, the water in the drum water volume has χ=0, while the steam-water mixture in the water wall has 0＜χ＜1. The steam obtained from the separation of the soda mixture by the steam-soda separation device in the drum can be considered to have χ=1. 6. What is superheated steam? What is superheat? Answer: Steam with a temperature higher than the saturation temperature at the corresponding pressure is called superheated steam. The degree of superheating of steam is called superheat. Superheat is numerically equal to the temperature of superheated steam minus the temperature of saturated steam at the corresponding pressure. Taking the Y-130/39 boiler as an example: the absolute pressure of the superheated steam is 4 MPa, the saturation temperature is 249°C, and the temperature of the superheated steam is 450°C; therefore, the superheat degree = 450°C – 249°C = 201°C. 7. What are the various ways of heat transfer? Answer: In daily life and production, it is common to encounter heat being transferred from objects with higher temperatures to those with lower temperatures. Although the processes of heat transfer are diverse, no matter how complex a heat transfer process may be, it is always composed of three methods: convection, radiation, and conduction. In different situations, one or two of these methods play a dominant role, with the other methods serving as supplementary ones. Taking the heat transfer in the water-cooled walls of a furnace as an example, heat transfer from the flame to the outer surface of these walls occurs primarily through radiation, with convection playing a secondary role. Heat is transferred from the outer wall of the water wall tubes to their inner wall through conduction, while the heat transfer from the inner wall of the water wall tubes to the steam-water mixture inside the tubes occurs primarily through convection, with conduction playing a secondary role. 8. What is convective heat transfer? What are the advantages and disadvantages? Answer: The heat transfer mode in which the heated medium and the flow direction of the heating medium are the same is called convective heat transfer. Taking the heat transfer in the superheater as an example: the steam, which is the medium being heated, flows from left to right, and the flue gas, which is the heating medium, also flows from left to right. The inlet temperature of the steam is , the outlet temperature is , the inlet temperature of the flue gas is , and the outlet temperature is . During convective heat transfer, the inlet temperature of the heating medium is high, but the inlet temperature of the material being heated is low, resulting in a low wall temperature at the inlet. At the outlet, the temperature of the heating medium is high, but since the temperature of the heating medium itself is low, the wall temperature at the outlet remains low as well. Since the temperature of the entire tube wall is not high during convective heat transfer, it is possible to avoid using expensive alloy steels; ordinary, inexpensive carbon steel can suffice, thereby reducing equipment investment. This is the advantage of convective heat transfer. Along the flow direction of the heating medium and the medium being heated, the temperature of the heating medium continues to decrease, while the temperature of the medium being heated continues to increase, resulting in a decreasing temperature difference for heat transfer along the flow direction of the media. Therefore, with a constant heat transfer area, convective heat transfer results in a smaller amount of heat transfer ； With a constant heat transfer amount, a larger heat transfer area is required. Based on the characteristics of convective heat transfer, it is generally used in situations where the temperature of the medium to be heated is high, and it is necessary to avoid using alloy steel or more advanced materials. In high-temperature superheaters, convective heat transfer is more commonly used. 9. What is counterflow heat transfer? What are the advantages and disadvantages? Answer: Heat transfer in which the flow direction of the heating medium is opposite to that of the material being heated is known as counterflow heat transfer. During counterflow heat transfer, the temperature of the medium being heated is higher at the outlet, but it is in contact there with a heating medium of even higher temperature ； The outlet temperature of the heating medium is low, but the temperature of the material being heated here is even lower. Throughout the entire process of counterflow heat transfer, the temperature difference for heat transfer between the heating medium and the material being heated remains large. Compared to convective heat transfer, when the inlet and outlet temperatures of the heating medium and the material being heated are the same, counterflow heat transfer requires a smaller heat transfer area, which allows for cost savings in equipment investment; this is a major advantage of counterflow heat transfer. Another advantage of counterflow is that it can more effectively raise the temperature of the medium being heated and lower the temperature of the heating medium. During convective heat transfer, the outlet temperature of the medium being heated is definitely lower than the outlet temperature of the heating medium ； In counterflow heat transfer, the outlet temperature of the heated medium can be higher than that of the heating medium. Air preheaters generally use counterflow heat transfer, so the wind temperature at the outlet of the preheater is higher than the flue gas temperature. If convective heat transfer is used, not only must the temperature of the hot air be lower than that of the exhaust gas, but it is also not possible to reduce the exhaust gas temperature to a lower level. However, counterflow heat transfer also has disadvantages. In counterflow heat transfer, the temperature of the medium being heated is higher at the outlet, and since the temperature of the heating medium there is even higher, the wall temperature of the heat transfer surface is also higher. If the wall temperature exceeds the allowable 480°C for carbon steel, expensive alloy steel must be used. The higher the wall temperature, the greater the amount of alloying elements required in the alloy steel, and the higher the cost of the material. Generally, when the temperature of the medium being heated is low and the wall temperature does not exceed the allowable operating temperature for carbon steel, counterflow heat transfer is preferred to reduce equipment costs. The economizer is a typical example of counterflow heat transfer.