1. The Advantages of Steam as a Heat Transfer Medium w Steam has a high heat capacity. Compared with liquid heat transfer medium, the smaller the steam system piping when delivering the same amount of heat, which means lower costs. w The weight of the steam line is relatively light, which can reduce the support cost of the pipeline. w In a steam pipe, there is a flow as long as there is a pressure drop, so there is no need for a circulating pump. w The steam system is flexible and can increase or decrease the load within a certain range as needed. w Steam can be controlled using two-way control valves, thus avoiding the use of three-way valves. w From the point of view of use, the heat transfer coefficient of steam to the medium is usually twice that of water to the medium, thus making the heat transfer equipment more compact. In addition, there is no temperature gradient on the surface of the steam heat transfer, and the same temperature prevails in the steam filled space. 2. Superheated steam If the saturated steam through the hotter surface of the heat exchanger, the steam temperature will be higher than its evaporation temperature, steam at this time is called superheated steam, higher than saturated steam temperature is called superheat. Superheated steam has its own application areas, such as the turbine used in the generator set, through the nozzle to the motor, to promote the motor rotation. However, superheated steam is seldom used in the heat transfer process of industrial processes because the superheated steam must be cooled to saturation temperature before condensation releases the enthalpy of evaporation. Obviously, the superheated steam is cooled to the saturation temperature compared to the enthalpy of vaporization of saturated steam The amount of heat released is small, which reduces the performance of the process equipment. 3. Steam dryness at a certain pressure boiling point temperature of the steam is called dry saturated steam, the dryness of 1 at this time. However, it is difficult to produce 100% dry steam in practical applications, usually with a certain amount of water droplets. If the steam contains 10% by mass of water, the steam is 90% dry, that is, the steam dryness is 0.9. The actual enthalpy of vaporization for wet steam is therefore not the hfg shown on the steam table, but the product of the dryness x and hfg: actual enthalpy of evaporation = enthalpy of vaporization x dryness 4. The concept of steam and enthalpy Steam is the vapor form of water . Generating steam requires heating the water to boiling point, boiling water to steam if heating is continued. These two processes involve the following energy terms: w Liquid enthalpy or sensible heat This portion of energy refers to the amount of heat required to heat water to the boiling point, where the heat of heating only changes the temperature of the water, usually indicated by the symbol hf . The heat that heats 1 kg of water from 0 ° C to the boiling point can be read from the steam table. For example, at atmospheric pressure (0 bar g), the boiling point of water is 100 ° c, and the heat required to heat 1 kg of water from 0 ° C to 100 ° c is 419 kj. w Evaporation enthalpy or latent heat of evaporation refers to the amount of heat required to convert all water at boiling temperature to steam, usually indicated by the symbol hfg. When heat is added, the temperature of the water / steam mixture is unchanged and all heat is used to convert liquid (water) to gaseous (vapor) phase change. The amount of heat required to evaporate 1 kg of water at boiling temperature can be read from the steam meter. For example, at atmospheric pressure (0 bar g), 2257 kj of heat is required to evaporate 1 kg of water at 100 ° c into 1 kg of steam at 100 ° c. w Total heat refers to all the energy in the steam, which is the sum of liquid enthalpy and enthalpy of evaporation expressed in hg. Total heat can also be read from the steam meter. For example, at atmospheric pressure (0 bar g), the total heat energy required for complete conversion of 1 kg of water at 0 ° c to 100 ° c is 419 + 2257 = 2676 kj. When steam transfers heat in the process, the enthalpy of evaporation is first passed, which is also the amount of heat that can be actually applied. At this point the steam condenses into water at the same temperature, called condensate.
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