Use of superheated steam
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Why is the heat exchange effect poor when superheated steam is used directly without temperature reduction?
This post was last edited by li*ch1968 on 2010-5-8 at 18:55. Superheated steam needs to be cooled first before heat exchange can take place, similar to the non-condensable gases in a system
This relates to engineering thermodynamics, and it provides a very detailed explanation in it
The gas-phase heat transfer coefficient of superheated steam is low; only during the condensation process is the heat transfer coefficient high. When cooling superheated steam to saturated steam, heat transfer area is wasted, resulting in an unsatisfactory overall heat transfer efficiency.
In the superheating section of superheated steam, the temperature is very high, but the enthalpy is low; in most heat exchange processes, a large amount of heat is released during the process of vapor turning into condensed water, as heat of vaporization is released. The enthalpy value in the condensation section is high.
The heat transfer coefficient of superheated steam is low. Compared to the corresponding saturated steam, it seems to be many times higher. In the superheated saturation region, the enthalpy drop is very low and can generally be ignored. But it affects the heat exchanger area. If the pressure is increased to achieve saturation, it places high demands on the material of the heat exchanger.
This post was last edited by li*ch1968 on 2010-5-8 at 18:55. So that’s how it is; in other words, it can be understood that as the temperature decreases, the heat released during phase changes becomes less intense.
Superheated steam turns into saturated steam, and the heat released in this process is sensible heat; since there is no phase change, the amount of heat released is less. When saturated steam turns into condensed water, latent heat is involved, and because there is a phase change, more heat is released. Do you understand now?
The heat transfer coefficient of superheated steam is low; it is several times lower compared to that of saturated steam. Moreover, the enthalpy value of superheated steam is much lower than that of saturated steam.