Speed of flash steam formation

[Enviro21]

I was wondering what time it takes for hot water condensate to flash in a flash tank from the moment a unit of hot condensate is released into the flash tank to the moment the flash steam is formed?
Has anyone heard of researches that have tried to measure the phenomenon using high speed cameras? If you have please send a link.
Thanks,
Yani.

 

[LittleInch]

How precise do you need to be?

A unit sounds like you're talking a specific quantity which is then somehow exposed to a lower pressure.

Not quite sure why anyone would want or need to find out?

 

[IRstuff]

https://www.google.com/search?q=tim...1GCEU_enUS1161US1162&sourceid=chrome&ie=UTF-8

I would think that allowing for 1-2 seconds should be adequate for most use cases.

 

[LittleInch]

The size of this unit of condensate and how exactly it is "released" will impact the speed in which it turns into steam and water.

The relative level of superheating of the condensate will also impact the speed.

These boys know a lot about steam.... https://www.spiraxsarco.com/learn-about-steam/condensate-recovery/flash-steam?sc_lang=en-GB

And what are you considering as the start and end point? It will start flashing the millisecond it is "released", but the end of a single fixxed volume? of condensate will take longer to stop generating steam until no more is produced.

You need to explain what it is you're looking for and why.

 

[EdStainless]

Yes, this is a real calculus problem.
It will start very fast, and slow as the you reach the endpoint.
For much of the process it will limited by sonic velocity, steam can expand faster than that.

 

[Enviro21]

https://www.google.com/search?q=tim...1GCEU_enUS1161US1162&sourceid=chrome&ie=UTF-8

I would think that allowing for 1-2 seconds should be adequate for most use cases.

Hi, IRstuff,
From what I have read, this phenomenon is called flash steam because it happens in a split second. It would help me to know if it has been researched and quantified relative to a set of standardized test parameters. Why do I want to know this, because it is relative to a project I am working on. If anyone has any clues, let me know.
Cheers.

 

[Enviro21]

How precise do you need to be?

A unit sounds like you're talking a specific quantity which is then somehow exposed to a lower pressure.

Not quite sure why anyone would want or need to find out?

Why do I want to know this, because it is relative to a project I am working on. If anyone has any clues, let me know.

 

[Enviro21]

Why do I want to know this, because it is relative to a project I am working on. If anyone has any clues, let me know.

 

[Enviro21]

The size of this unit of condensate and how exactly it is "released" will impact the speed in which it turns into steam and water.

The relative level of superheating of the condensate will also impact the speed.

These boys know a lot about steam.... https://www.spiraxsarco.com/learn-about-steam/condensate-recovery/flash-steam?sc_lang=en-GB

And what are you considering as the start and end point? It will start flashing the millisecond it is "released", but the end of a single fixxed volume? of condensate will take longer to stop generating steam until no more is produced.

You need to explain what it is you're looking for and why.

Thanks LittleInch,
I was hoping to find an answer to this question in the millisecond range for my research project.

 

[LittleInch]

Thanks LittleInch,
I was hoping to find an answer to this question in the millisecond range for my research project.

Well you haven't let on very much there have you?

There are too many variables for this to be studied or calculated in any meaningful way.
10g of condensate at a high degree of superheat ( say 50 C) would be almost explosive so 10-20 mSec maybe.

Even 100g or 1000g condensate will be in the 500-2000 mSec range. Lower the degree of superheat and it takes even longer.

I'm not sure anyone has really ever seen a reason to find this out....

 

[Latexman]

I'm not sure anyone has really ever seen a reason to find this out....

I’m curious about what kind of process it is that this is even important.

 

[Enviro21]

Well you haven't let on very much there have you?

There are too many variables for this to be studied or calculated in any meaningful way.
10g of condensate at a high degree of superheat ( say 50 C) would be almost explosive so 10-20 mSec maybe.

Even 100g or 1000g condensate will be in the 500-2000 mSec range. Lower the degree of superheat and it takes even longer.

I'm not sure anyone has really ever seen a reason to find this out....

Thanks LittleInch, you have been very helpful again.

I appreciate I haven't given you much to go on, but I really appreciate your expertise and input. May I ask how you came to these estimates. is it from hard earned experience or is there something in the literature I can study further. I'm keen to learn more.

 

[LittleInch]

Basically just experience and thinking about the fundamental issues.

Most issues in the real world are to do with a constant process, whereas I think you're talking about a discrete amount of condensate flashing off as single action?

 

[Enviro21]

Basically just experience and thinking about the fundamental issues.

Most issues in the real world are to do with a constant process, whereas I think you're talking about a discrete amount of condensate flashing off as single action?

Thanks LittleInch, your latter assumption is correct and you've been more helpful that you realize.
I'm getting some new instrumentation in the next two to three weeks which will make things a bit clearer, so till then I'll stay off line.
Cheers for now.

 

[btrueblood]

I've seen multi-phase CFD and cruder combustion simulations (think rockets, jet engines, even motor vehicles) where fuel (or oxidizer, think liquid oxygen) droplet injection is modeled, along with combustion and heat release, and the evaporation of those droplets is tracked over time whilst the surrounding environment temperature and pressure changes from global heat production and transfer. The evaporation rate of the droplets there is driven by the droplet temperature and vapor pressure vs. heat transfer from its environment. The very mention of heat transfer rate on moving droplets involves a whole lot of guesstimates and correlation from sparse data. I'm sure such simulations could be run for the rather less exotic case of flash steam production from a leak/orifice/valve.

 

[Enviro21]

I've seen multi-phase CFD and cruder combustion simulations (think rockets, jet engines, even motor vehicles) where fuel (or oxidizer, think liquid oxygen) droplet injection is modeled, along with combustion and heat release, and the evaporation of those droplets is tracked over time whilst the surrounding environment temperature and pressure changes from global heat production and transfer. The evaporation rate of the droplets there is driven by the droplet temperature and vapor pressure vs. heat transfer from its environment. The very mention of heat transfer rate on moving droplets involves a whole lot of guesstimates and correlation from sparse data. I'm sure such simulations could be run for the rather less exotic case of flash steam production from a leak/orifice/valve.

Hi btrueblood,
I liked the way you put this into simple language. So what information would you need to calculate the evaporation rate for flash steam droplets. Can you give me some examples for other media.
Cheers.

 

[btrueblood]

Take a slug of fluid, and assume: random particles (milli-slugs) across the slug will nucleate to vapor if Tfluid local is > boiling point at local pressure conditions. Note that the local expansion of these nucleate sites will raise the local pressure on surrounding areas of the droplet, inhibiting the boiling and expansion of nearby millislugs. Boiling extracts heat from the fluid, as does expansion of the vapor and this also changes local conditions of both fluid and surrounding gas environment. Modelling this way across several random or quasi-random distributions of the nucleation sites gives approximations of the slug/droplet breakup (shattering) that can then be correlated to high speed video or similar data.

A simpler approach not requiring the CFD and multiphase particle tracking implied in the above - ASSume only the outer layer of the slug/droplet vaporize first, and track the movement of the slug/droplet and evaporation over time/space. This gives an upper bound of the time or distance that the slug will stay liquid, since shattering (internal vapor bubble formation and expansion, with resulting breakup of the slug/droplet/stream/whatever) is not being explicitly modelled.

An even simpler approach - assuming a continuous stream of liquid enters the knockout tank, assume a chunk of it flashes entirely to steam, and then calculate its expansion within the tank and subsequent pressure rise of the tank. Add the next chunk of fluid, but now add a term for removal of some of the vapor in the tank due to venting and condensation...repeat ad infinitum until the flash event is done, or the tank explodes, or the system runs out of water...this could be a spreadsheet model. You could run it multiple times with smaller and smaller time step increments to see if it converges to an answer, and/or changing the assumptions of starting temperature and admittance/exhaust rates. Would fill an otherwise boring afternoon or two.

 

[Enviro21]

Take a slug of fluid, and assume: random particles (milli-slugs) across the slug will nucleate to vapor if Tfluid local is > boiling point at local pressure conditions. Note that the local expansion of these nucleate sites will raise the local pressure on surrounding areas of the droplet, inhibiting the boiling and expansion of nearby millislugs. Boiling extracts heat from the fluid, as does expansion of the vapor and this also changes local conditions of both fluid and surrounding gas environment. Modelling this way across several random or quasi-random distributions of the nucleation sites gives approximations of the slug/droplet breakup (shattering) that can then be correlated to high speed video or similar data.

A simpler approach not requiring the CFD and multiphase particle tracking implied in the above - ASSume only the outer layer of the slug/droplet vaporize first, and track the movement of the slug/droplet and evaporation over time/space. This gives an upper bound of the time or distance that the slug will stay liquid, since shattering (internal vapor bubble formation and expansion, with resulting breakup of the slug/droplet/stream/whatever) is not being explicitly modelled.

An even simpler approach - assuming a continuous stream of liquid enters the knockout tank, assume a chunk of it flashes entirely to steam, and then calculate its expansion within the tank and subsequent pressure rise of the tank. Add the next chunk of fluid, but now add a term for removal of some of the vapor in the tank due to venting and condensation...repeat ad infinitum until the flash event is done, or the tank explodes, or the system runs out of water...this could be a spreadsheet model. You could run it multiple times with smaller and smaller time step increments to see if it converges to an answer, and/or changing the assumptions of starting temperature and admittance/exhaust rates. Would fill an otherwise boring afternoon or two.

Thank you for your interesting analysis, I think the one that would be most useful to me is the third approach. Unfortunately i'm not a boiler engineer, so would you please give me a couple of examples that I could use as models to substitute my values into.
Again, I'm most appreciative of your input here!

 

[btrueblood]

This is Eng-Tips.com, not Eng-domyworkforme.com

 

[Enviro21]

Thank you for pointing that out. My understanding of what Eng-Tips.com is about is the sharing of information, tips and suggestions in an environment of curiosity and learning. My request for examples that I could use as a model was to facilitate my own learning. I'm retired and not a trained professional engineer, but have worked in many engineering fields at senior levels.
I thank you for your valuable input btrueblood and wish you well.