Steam condensate can flow in pipes. Is its driving force derived from uncondensed steam or the vapor pressure of the steam condensate itself?
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This post was last written by Guo Zhenyu_UiJDg on 2026-6-2 09:24 The editor would like to ask everyone a question, that is, the steam condensate in the condensate recovery main pipe can flow in the pipeline. Is its driving force derived from uncondensed steam or the vapor pressure of the steam condensate itself? I asked Bean Bao, and Bean Bao said "The main driving force comes from the vapor pressure of the steam condensate itself, not the uncondensed steam." Then I asked again. Qianwen, but Qianwen said that "it is mainly due to the residual pressure (residual pressure) or gravity difference of the uncondensed steam, rather than the vapor pressure of the condensate itself." I personally understand that Doubao is right, because usually a trap is installed. The trap blocks the steam and can only flow by its own vapor pressure. I would like to ask what everyone thinks.
The analysis in the thread on this question is already in place. The essence is pressure difference driving. Whether the residual pressure of uncondensed steam or the flash steam pressure of the condensate itself is dominant depends on the actual working conditions. I would like to add two points of my own understanding.: In fact, the most critical variables in this problem are system back pressure and condensate temperature. If the condensate temperature is higher than the saturation temperature under this back pressure, it will definitely flash. At this time, its own vapor pressure (flash steam) is the main driving force. ； If the condensate temperature is lower than the saturation temperature under back pressure, it will not flash and can only be pushed by the residual pressure of the uncondensed steam at the front end or the high head difference. A common design idea in engineering is: Try to maintain a slight positive pressure (such as 0.1 to 0.3 bar) in the pipeline after the trap. This will not only avoid the two-phase flow impact caused by a large amount of flash evaporation of the condensate in the pipeline, but also allow the condensate to be sent back to the recovery main pipe by relying on residual pressure. Therefore, it is often a mixture of two forces, and it is easy to get into trouble if you insist on distinguishing the priorities. If water hammer or pipe vibration often occurs in your system, it is recommended to check the condensate temperature, back pressure behind the trap and whether the pipe diameter matches. These parameters can solve the problem more directly than simply discussing the source of the driving force. Of course, professional debugging still needs to be determined based on the specific parameters of your site.
In most cases, the outlet of the trap is usually a mixture of gas and liquid. Because the outlet pressure is lower than the inlet, and the trap itself will leak, the driving force should be both gas and liquid.
Isn’t the uncondensed vapor pressure the saturated vapor pressure, equilibrium state?
To be precise, it is the pressure of the steam and the gravity of the condensate.