TAILIEUCHUNG - Mechanical Engineering Systems 2008 Part 4

Tham khảo tài liệu 'mechanical engineering systems 2008 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Thermodynamics 67 Key points Wet steam cannot be superheated. Steam containing water droplets is called wet saturated steam. Steam not containing water droplets but which is at saturation temperature is called dry saturated steam. Steam above saturation temperature is known as superheated steam. The degree of wetness of saturated steam is given by its dryness fraction x. This is a value between 0 and 1. The higher the value the drier the steam and the more heat energy the steam will contain. Dryness fraction is the ratio of the mass of pure steam to the total mass of the steam sample and indicates what is called the quality of the steam. Key points Types of steam Most of us see steam only when we boil the kettle but we can learn something even from this. We observe that because it is able to lift the kettle lid steam can do work. Because we can scald our hands with it we know that it can transfer heat energy. Also if we look closely at the steam coming out of the spout we can see water droplets or a water mist within it indicating it is what we call wet steam . All boilers of whatever size are doing the same thing as the kettle . boiling water. For low grade applications such as heating we may need only hot water. For turbines we are interested in much higher temperatures and pressures to produce steam of the right quality. We have already said that the steam produced by a kettle is wet steam. This sort of steam is made up of water droplets and pure steam . steam which does not have water droplets. If the steam was taken away from the water from which it is produced and more heat energy added some of the droplets would change into pure steam and the steam would be drier. Eventually all the water droplets would have changed state and the steam would be dry. The steam produced from the boiling water is at the same temperature as the water. This is called the saturation temperature ts. The steam cannot rise above this temperature until all the water droplets .

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