Pressure change in a fixed volume as steam condenses
Pressure change in a fixed volume as steam condenses
(OP)
In a fixed volume, such as a steam coil with a now closed steam valve on one end and a closed trap on the other end, and no vacuum breaker, how do I quantify the the pressure change in the coil as the steam condenses? Let's say I have 10 psig steam. I want to calculate the pressure after it condenses. I know that it goes into a vacuum, but how do I quantify it?
Same question if the steam is already in a vacuum, say 15" Hg. When it condenses, it will no doubt go into a deeper vacuum. I'm troubleshooting a vacuum steam system and think I may need a "vacuum breaker" between the steam side and the condensate side. They are both already in vacuum, but I an suspecting the steam side falls into a deeper vacuum when the steam condenses, thus preventing gravity drainage from the coil.
Same question if the steam is already in a vacuum, say 15" Hg. When it condenses, it will no doubt go into a deeper vacuum. I'm troubleshooting a vacuum steam system and think I may need a "vacuum breaker" between the steam side and the condensate side. They are both already in vacuum, but I an suspecting the steam side falls into a deeper vacuum when the steam condenses, thus preventing gravity drainage from the coil.





RE: Pressure change in a fixed volume as steam condenses
If you assume that all the steam is gone and calculate a final pressure less than 0 psia, then your assumption is wrong and when you run out of pressure there is still some steam in the head space.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pressure change in a fixed volume as steam condenses
RE: Pressure change in a fixed volume as steam condenses
If non-condensables are present in the steam, the actual pressure will be higher than that from the table. How much higher, depends on the amount (fraction) of non-condensables in steam.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Pressure change in a fixed volume as steam condenses
I am not simply talking about the pressure-temperature relationship for saturated steam. I am looking at a fixed vessel containing steam, so my mass is also fixed. if the vessel is at 10 psig and the steam condenses, how do I calculate the resulting pressure? My vessel volume doesn't change, so I am not sure how the ideal gas law applies.
RE: Pressure change in a fixed volume as steam condenses
A reverse example is LPG cylinder. It doesn't matter if you bleed off LPG to consumers, pressure inside the cylinder will always remain the same as long as there is liquid phase. The mass inside the vessel decreases, but pressure does not. It is vapor pressure of LPG at storage temperature. The same goes for steam inside closed system.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Pressure change in a fixed volume as steam condenses
P1/P2 = T1/T2 IFF volume is constant
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RE: Pressure change in a fixed volume as steam condenses
RE: Pressure change in a fixed volume as steam condenses
So the pressure will "ride the saturation curve" and be at whatever pressure corresponds to the current cooled-off temperature, correct?
If that is right, then isn't my initial pressure irreverent? So no matter if I start with a vessel at 100 psig steam or 5 psig steam, if it is completely sealed, the equilibrium pressure will correspond to the saturation pressure at the cooler temperature?
RE: Pressure change in a fixed volume as steam condenses
Your first statement about mass being constant is incorrect. The total mass in the vessel will be constant, but you don't care about the mass of liquid at any time, and your mass of steam is changing.
Your last paragraph is pretty close (assuming you meant "irrelevant" instead of "irreverent"). Problem is that your heat sink is probably colder than the saturation temperature at a near total vacuum (1 psia saturation temp is 102F).
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pressure change in a fixed volume as steam condenses
Steam (vapor) must always be in equilibrium with condensate (liquid) at any temperature. Looking at the steam T-P tables, you will find the equilibrium pressure for any temperature of interest. If there are other, non-condensible components in the steam, adjustments need to be made based on Dalton law.
Steam can also be superheated. If you start with steam at 100 psig and 500 degF, you can apply the ideal gas low till you reach the condensation point. Beyond this point, you need to follow steam tables.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Pressure change in a fixed volume as steam condenses
I'm moving towards his final conclusion. The amount of superheat, or even the original T&P do not matter in a real system. Eventually the trapped volume will reach heat sink temperature and pressure will be a function of that heat sink temp. If the heat sink is well below the freezing point of the condensate at 0 psia, then the answer is a little tough since the pressure before the onset of freezing will be close to absolute zero, but then the condensate volume will increase about 4% and raise the pressure a bit.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pressure change in a fixed volume as steam condenses
Till the saturation point is reached (before that we have the superheated region), one can follow ideal gas law but with caution.
If heat sink temperature is below triple point of water (0 degc = 32 degF), water inside the coil will freeze.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Pressure change in a fixed volume as steam condenses
So the pressure will "ride the saturation curve" and be at whatever pressure corresponds to the current cooled-off temperature, correct?
If that is right, then isn't my initial pressure irreverent? So no matter if I start with a vessel at 100 psig steam or 5 psig steam, if it is completely sealed, the equilibrium pressure will correspond to the saturation pressure at the cooler temperature?"
Absolutely correct. Your final state is a saturated mixture (vapor and liquid). The pressure is uniquely defined by the temperature. The only thing that will change depending on the initial state will be the "quality" (ratio of vapor to liquid)
je suis charlie
RE: Pressure change in a fixed volume as steam condenses
RE: Pressure change in a fixed volume as steam condenses
I think he is talking about a strength of materials issue, not a current operational issue. With hundreds or thousands of tubes in a condenser, your solution would result in a lot of needle valves.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pressure change in a fixed volume as steam condenses
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RE: Pressure change in a fixed volume as steam condenses
The last thing you'd want to do is to introduce air into this coil to break the vacuum - the coil wont work after that with all the noncondensibles floating about.
RE: Pressure change in a fixed volume as steam condenses
RE: Pressure change in a fixed volume as steam condenses
Silly question time- why do you want to get the condensate out if the steam supply is switched off? Why do you care if the heating coil stays full of condy?
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Pressure change in a fixed volume as steam condenses
Maybe to prevent it from freezing and breaking the coil? Or to prevent CIWH when steam is reintroduced?
RE: Pressure change in a fixed volume as steam condenses
Freezing- good point. Not an issue in my location so it didn't spring to mind (if it gets realllly cold- we might hit -2Celsius, for a couple of hours before dawn in winter).
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"