On the influence of temperature difference on steam consumption
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Assume there is a reboiler; originally, the temperature difference between the superheated steam and the material in the tower bottom was 60 degrees. Now that the steam has become saturated steam, the temperature difference between this steam and the material in the tower bottom is 20 degrees (with all other conditions remaining unchanged). Will the steam consumption remain essentially the same? In terms of the use of energy as latent heat, the steam consumption should remain essentially unchanged. However, as the temperature difference decreases, the heat transfer rate drops, and the amount of heat transferred is affected. Can it then be assumed that only sensible heat has a heat transfer rate, while latent heat has no heat transfer rate? I hope fellow sailors can help clarify this.
Why isn’t anyone answering? Could an expert help explain this?
The heat transfer rate depends not only on the temperature difference but also on the heat transfer coefficient. When heated by saturated steam, a film of condensed liquid forms on the tube wall. Since the temperature difference is caused by thermal resistance, the temperature of the pure saturated steam in its gas-liquid phase is consistent; in other words, there is no temperature difference from the gas phase to the surface of the film of condensed liquid. As a result, almost all of the thermal resistance associated with heat transfer due to condensation is concentrated within the film. When superheated steam is used, there is a thermal resistance in the gas phase, resulting in a heat transfer coefficient that is much lower than that of saturated steam condensation.
This post was last edited by DXJ122 on 2010-10-7 at 19:21. 1 The steam consumption remains basically constant, as the heat from the superheated portion accounts for only a very small fraction of the total heat. 2. Talking again about the temperature difference, when calculating heat transfer in a reboiler, only the saturation temperature is considered, while the superheat is not taken into account. According to your requirement, whether it is overheated or saturated, it remains at 20 degrees.
This post was last edited by DXJ122 on 2010-10-7 at 22:11, in response to the original poster: I can’t remember much about heat transfer theory. The latent heat you mentioned, that is, the superheated portion, since it exists, certainly has its own laws of heat transfer, that is, a transmission rate. It goes from the superheated temperature T2 to the saturated temperature Tb; in this case, heat is transferred from the vapor phase to the tube wall, and the heat transfer coefficient is very low (I estimate it to be only a few hundred), unlike the case of saturated steam, where heat is transferred through condensation onto the tube wall, and the coefficient in that case is usually over 10,000. Inside the reboiler tubes, there is \"boiling heat transfer,\" and the temperature Tf results in a very high coefficient. As we all know, K represents the total contribution of fouling inside and outside the tube, as well as on the metal wall and on both sides. It is possible to calculate separately the K value for superheating (superheat) and the K value for the condensation of saturated steam. The average temperature difference in the superheated portion is determined by calculating the logarithmic temperature difference between T2-Tb and Tf, while for the saturated portion it is the 20 degrees you mentioned. Just writing these out of thin air.
The original poster’s statement isn’t entirely accurate; the heat transfer temperature difference has only decreased slightly – by less than 1 degree. For example, when superheated steam was used, the average logarithmic temperature difference was 21 degrees, while when saturated steam was used, deltaTm was still around 20 degrees. Just calculate it using software like TASC and you’ll know. The reason is simple: 1. The enthalpy difference between superheated steam and saturated steam is not small; for the enthalpy of phase change, this difference can be as much as two orders of magnitude. Therefore, superheated steam turns into saturated steam after traveling a short distance along the pipe, and it is the saturated steam that carries out heat exchange over 95% of the pipe’s surface area. 2. Once the previous point is understood, it actually makes no sense to discuss further changes in the heat transfer coefficient. Q=K*A*deltT. Since the latter two terms remain unchanged, K also remains unchanged. If one insists on getting technical about it, then well, the K value of superheated steam is definitely lower than that of saturated steam. If you look up the principles of chemical engineering, you’ll find the answer: there is no liquid film, no turbulence.