Superheated steam in Partial Vacuum
Superheated steam in Partial Vacuum
(OP)
I am pretty rusty in Thermodynamics, and I have spent time reading my old texts, but no luck to find a classical text-book solution:
What will the resulting fluid properties be if, 1200kg/h SH steam at 0.8 bar(a) and 340 deg. C is allowed to flow into a exhuast steam duct kept at 0.8 bar(a), 93.5 deg. C (saturated temperature) by means of exhausting to the vacuum pump unit and the condenser.
Will the SH steam 'flash', cool down to 93.5 deg. C? Will the duct heatup to 340 deg. C.
I am unable to formulate the thermodynamic equations. Any help/ hints greatly appreciated.
This condition is encountered during the turbine startup heating.
What will the resulting fluid properties be if, 1200kg/h SH steam at 0.8 bar(a) and 340 deg. C is allowed to flow into a exhuast steam duct kept at 0.8 bar(a), 93.5 deg. C (saturated temperature) by means of exhausting to the vacuum pump unit and the condenser.
Will the SH steam 'flash', cool down to 93.5 deg. C? Will the duct heatup to 340 deg. C.
I am unable to formulate the thermodynamic equations. Any help/ hints greatly appreciated.
This condition is encountered during the turbine startup heating.





RE: Superheated steam in Partial Vacuum
RE: Superheated steam in Partial Vacuum
The exhaust steam appears to be rather hot for warmup. The steam will cool/condense as the components are warmed.
I2I
RE: Superheated steam in Partial Vacuum
However, surprisingly, and unexpectedly, the turbine drains will be SH at 340 deg. C at the end of the initial heating (Steam conditions during running is 410 deg. C, 43 bar). Causing me some design headaches, because the initial mechnical design temperature is 150 deg. C only for the drains system.
This is why I need to refine the problem thermodynamically to see if I can obtain any lower process temperature by means of thermodynamics calculation instead of applying a 360 deg. C Mechnical design temperature. Hence this post to the forum members.
RE: Superheated steam in Partial Vacuum
I2I
RE: Superheated steam in Partial Vacuum
Best of luck!
RE: Superheated steam in Partial Vacuum
RE: Superheated steam in Partial Vacuum
Any consideration on any irrecoverable work done by the SH steam
RE: Superheated steam in Partial Vacuum
RE: Superheated steam in Partial Vacuum
RE: Superheated steam in Partial Vacuum
The superheated steam will heat up the saturated steam to some value above saturation, but the massive area of the condenser can handle that at that point.
It is when the turbine is at full load and there are leaky valves on those drain circuits that bring excessive drains and drips into the turbine that hurt.
For the record, the vacuum equipment doesn't create the vacuum, it just removes the non condensables, mainly air. The condensation of the steam and the change of state of the steam from a vapor to water is what creates the vacuum. As long as there is enough condenser surface area to condense the steam, the vacuum will be created and maintained. The superheated steam, that heated above saturation by the drain inlet flow will have to be cooled to the saturation point for the condenser pressure by sensible heat transfer at which time it will then condense and contribute to the vacuum creation.
So as long as this happens during the warm up/start up part of the cycle, when the steam flows are light, you should be OK. Remember too that depending on lots of factors in your turbine it is possible that the steam from the turbine is entering the condenser with moisture in it. Some of the superheated steam that you are admitting via the drains flow will just bake some of that moisture off. If there is enough moisture present to balance the added heat of the SH drain steam, you won't raise the condenser pressure.
I hope this helps answer what you are looking for.
I have seen leaky drains that were so hot that they warped the walls of the condenser adjacent to the point at which the drain connection was connected. Often drains of this nature are located under the water level of the hotwell and others are located under the condenser bundles so that the SH steam can be quenched by the water droplets falling from the bundle. That helps with the process of raising the condensate temperature to prevent condensate sub cooling.
rmw
RE: Superheated steam in Partial Vacuum
I apologise for my late reply as I had been travelling.
In any case, for closure to this thread, the mech. design temperature of the system is only 150 deg. C and as such, in order to maintain the warranty on the design of the system and for safety, I have decided to go for desuperheating to less than 100 deg. C for the case mentioned above.
The last minute addition for the above is costing top dollars, but necessary, nevertheless.
Thanks, again, all for the many valuable inputs and comments!