Discussion on problems in the condensation process of vacuum condensers
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Regarding the actual condensation process of water vapor in a vacuum condenser, I think the whole process is quite complex and contradictory; I would appreciate it if everyone could help analyze it. (1) The vacuum condenser is a shell-and-tube type condenser; the cooling water flows through the shell side (entering at the bottom and exiting at the top), while the vacuum gas flows through the tube side (entering at the top and exiting at the bottom as condensed water and residual uncondensed gas), with complete counterflow. The inlet temperature of the cooling water is 32°C, and the outlet temperature is 37°C. The gas composition entering the condenser is: 200 kg/h of water vapor, 10 kg/h of dry air, at a temperature of 100°C and a pressure of 80 mbarA ； The gas composition exiting the condenser is: 14.48 kg/h of water vapor, 10 kg/h of dry air, with a temperature of 35°C and a pressure of 80 mbarA. The gas outlet of the condenser is connected to a water ring vacuum pump, whose inlet pressure is 80 mbarA. To facilitate problem analysis, it is assumed that the pressure drop on the gas side (the tube side) of the vacuum condenser is 0. (2) In fact, when designing the condenser, certain assumptions were made; it was assumed that the condensation temperature of water vapor is 35°C. Based on this, the vapor pressure of water vapor at the exit of the condenser was calculated to be 56 mbarA, while the partial pressure of dry air was 24 mbar ； Since the amount of dry air remains constant, the flow rate of water vapor calculated is 14.48 kg/h ； Based on the amount of water vapor condensed, it is possible to calculate the required flow rate of cooling water and the area of the condenser. (3) My biggest confusion is: does water vapor start to condense at 41°C or at 35°C. When the gas enters the condenser, based on its composition, it can be calculated that the partial pressure of water vapor is 77.6 mbarA, and the dew point corresponding to this pressure is 41°C. When the gas at 100°C enters the condenser, its temperature gradually drops; when it reaches 41°C, condensation begins, without having to wait until 35°C. However, when designing the condenser, we assume condensation at 35°C; the resulting condenser is also capable of meeting the requirements of the operating conditions. (4) Is it the vapor pressure of water at the inlet of the condenser that determines the temperature at which condensation begins, or is it the vapor pressure of water at the outlet of the condenser? Since the difference between these two partial pressures is quite large, according to the example given above, the inlet partial pressure is 77.6 mbar (corresponding to a dew point of 41°C), while the outlet partial pressure is 56 mbar (corresponding to a dew point of 35°C). (5) If the condenser is large enough, even when condensation occurs at 35°C, will the gas temperature continue to drop after condensation is complete? If the gas temperature does continue to drop, will new condensation occur? Then the aforementioned assumption would be incorrect, wouldn’t it (the assumption of condensation at 35°C)? Please help analyze it. The more detailed the description of the entire condensation process, the better. Thank you.
Is this the project I seemed to have been in touch with? 1) The vacuum condensation process is complex; the key factor is the relatively low pressure (unlike processes with high pressure, where the pressure drop can be ignored) ； Vacuum condensers typically operate at around 100 mbar, so a pressure drop of 5–10 mbar cannot be ignored! ), your process still doesn’t take into account the pressure drop! 2) The 35 degrees you assumed is generally not sufficient for condensation to occur, unless your condenser has a large area and sufficient length (even with a large area, insufficient length will prevent condensation from taking place)! Condensing to 35 degrees is very uneconomical. A temperature as low as around -40 degrees is typical for economical designs ； 3) The cooling and condensation process is relatively complex; specialized manufacturers use dedicated software to carry out accurate calculations, and ordinary heat exchanger manufacturers are likely not capable of doing this. First, it is cooled to a water vapor partial pressure of 97 mbar, corresponding to 45.2 degrees; then, as it continues to cool, condensation occurs, and the pressure also changes. In short, it’s a rather complex process ； The final condensation cools down to 40–42 degrees (for a cost-effective condenser)! 4) At 80 mbar, with water at 32 degrees Celsius as the cooling fluid, no condensation will occur – only cooling!
The dew point corresponding to 80 mbar is 41.5°C, and the temperature of the cooling water is 32°C; condensation occurs when the steam cools below 41.5°C. Why is it said that only cooling occurs, with no condensation taking place? Please provide clarification as well.
1) The effect of the tubes must be taken into account; even with plates that are 0.1 mm thick in a plate heat exchanger, it is not possible to cool the fluid to the temperature of the cooling water inlet, and a temperature difference of 2 degrees is still required for heat exchange between water and water! Moreover, when considering heat exchange with water vapor, the gas resistance caused by non-condensable gases must also be taken into account. The wall thickness is around 1 mm, which is far greater than 2 degrees; furthermore, since the gas is water vapor, a temperature rise will occur as soon as it condenses. 2) To cool down to the theoretical limit through condensation, a longer tube length is required, which results in a greater pressure drop; therefore, the temperature that can be achieved, the pressure drop, and the economic efficiency of the equipment need to be considered together ； 3) It’s clear that you have considerable expertise in oil refining, deodorization, decolorization, and vacuum systems. If you need information regarding the internal design, feel free to contact me via private message.
I have studied it; I have conducted a few simulations of negative pressure flash evaporation, and the results still need further verification.