Steam

Mike T., Swampeast MO

When it comes to steam I feel as dumb as a rock. Just when I think I fairly well understand something throws me a curve and it's back to step one.

1) Superheated steam is steam with a greater latent heat than required to produce steam at that pressure, right?

2) Superheated steam cannot co-exist with water, right?

3) Humidity in the air is superheated steam at extremely low pressure?????

How on earth can #3 be true if #1 and #2 are also true???

Is it that steam in the form of humidity is somehow steam as it would be existing at sub-admospheric pressure? If that is the case then it would seem there is even more than 970 btus per pound of water in the condensate from flue gas...

Unknown

My understanding

of humidity is that it is droplets of water, not steam, suspended in the air.

Not too sure how it happens, though.

Noel

Mike T., Swampeast MO

That's what I was taught in earth science...

...or at least I believe that is how we were taught to think of it, but...

http://www.w-dhave.inet.co.th/humidity.html

http://www.fphcare.com/humidification/humidity.asp

http://members.aol.com/_ht_a/rogluthier/humidity.html

many, many more....

I can't tell if it's something simple that I'm just not getting or something incredibly complicated that just "works that way" to spite logic.

jerry scharf_2

let me give this a try

Since this is physics/chemistry and not steam heat, I can take a shot at this.

#3: superheated steam at low temperature makes no sense to me. Relative humidity is not water drops in the air, it is gaseous H2O. There is a thing called vapor pressure, which is the percentage of H2O in the gas times the pressure of the gas. A saturated gas is one that if the temperature drops at all, some of the gas will condese into liquid or solid. The ratio of the vapor pressure of the H2O to the saturation vapor pressure at that temperature is the Relative Humidity. Note that H2O makes up a small amount of normal air whether at saturation or below.

#1: Steam is a gas that is 100% H2O. Superheated steam can have different meanings. To some it's steam at a temperature over the atmospheric boiling point. Technically, it is more accurately defines as steam that is below saturation point. Said another way it's steam that is lower in pressure than the pressure that would cause it to change back to water or ice.

#2: By the sloppy definition, it would be possible. By the strict definition, it is impossible.

From a practical point of view, it's not what people want. The water is uncompressable relative to the steam and even small amounts can cause serious problems with the steam equipment. So you superheat the steam, thus keep it at a pressure below the saturation point and it's guaranteed to be dry.

did this help at all?

jerry

Mike T., Swampeast MO

Superheated Steam

"Technically, it is more accurately defines as steam that is below saturation point. Said another way it's steam that is lower in pressure than the pressure that would cause it to change back to water or ice."

Doesn't that define humidity as "superheated steam at extremely low pressure"?

Dale

Superheat

I think of it like an AC system, for any given pressure there is a corresponding temperature, like r22 at 69 psi is 40 degrees saturation, if the suction line is 50 degrees there's 10 degees superheat. Same with steam, if the sat temp shown on the steam table at a given pressure is say 600 degrees and the actual outlet temp of the header is 700 degrees you have 100 degrees superheat. Large generating plants have desuperheaters that capture this heat sometimes for a lower pressure turbine, rather than having the condenser run at a higher temp.

jerry scharf_2

I don't think so

Mike,

you missed the comment that steam is a gas that is 100% H2O. Not all water vapor is steam. So the relative humidity in air isn't steam, since it's not 100% H2O. It's just water vapor with a partial pressure. I think the superheated steam is a distraction and leads to confusion.

As for how they can coexist, with RH you're looking at a bulk air measurement, and everything that has exposed water will evaporate to reach 100% RH within a few microns of the surface. Think like sweat. The more the breeze blows, the more effective the sweat is at cooling you.

jerry

Mike T., Swampeast MO

THANKS for your patience Jerry!

I "got" the thing regarding saturated steam, but it seemed like the "technical" definition of superheated steam prevented such from being considered saturated steam because there technically must be more steam at that temperature to be considered fully saturated...

Too many names for similar things and too many "simple" and "technical" explanations that [appear] to conflict one another!

I thought I had a decent grip on steam, saturated steam, superheated steam, vapor pressure and vapor until I found, studied and posted that "saturated steam at sub-atmospheric pressure" chart in another active post.

It (and another when I verified) shows steam increasing in latent heat as pressure decreases towards vacuum. That seems REALLY goofy when you consider that atmospheric pressure is approaching vapor pressure (logic would tell you that it takes less energy to change state).

Is this perhaps the latent heat you get from the sub-atmospheric steam when released into normal atmospheric pressure? If so, it wouldn't seem that you could use this number for a "Vari-vac" or similar system as ideally you've kept pressure sub-atmospheric in the actual heating device (or at least I believe you have) during moderate outdoor weather...

Unknown

Yes, it does stay negative

The king valves only open (modulate) enough to maintain a level of vacuum that is higher in pressure than the vacuum that the pump produces with no steam present.

In other words, they maintain a vacuum, by injecting steam into the main, that is closer to zero than the vacuum pump vacuum reading. The pump pulls the off system down to, let's say, 15" hg vacuum. The south zone king valve, on a given outdoor temp (reset schedule), would open to maintain 10" hg vacuum. This would deliver steam at a lowered temperature. The north zone could be delivered steam at a different pressuse, say 3" hg vacuum. That would be hotter steam. The radiators still would all be at negative pressures, with a vacuum pump on the return end, and steam in a vacuum on the supply end.

Pretty cool steam.

Noel

Ken_8

There is a distinct difference between

moisture in the air called humidity - and moisture created by steam.

Steam is truly a gas - and behaves as such. Humidity is not steam and is not a gas. It is water vapor entraned in air. When it comes out of the vaop state, the water molecules join to form droplets too large to "float" weightlessly in the "atmospheric solution" - thereby precipitating into rain droplets. Gravity then takes over.

Boyle's law tells us how steam acts - because it is a gas. Boyles Law also tells us how air acts as well because it too is a gas - ALMOST! Air is "contaminated" by humidity (except of course in Arizona)(:-o)

No wait. That's not right.

Or is it?

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Christian Egli

My shot at the humidity riddle


Here is one topic that has been keeping me awake.

All three statements are correct and can happily stand side by side, and it all hinges on what humid air is.

In statements 1 and 2, we consider only steam, that is a straight shot of nothing but hard water vapor. (and properties table for steam talk about this stuff when giving you values for saturated water, saturated steam and superheated steam, and, within reason, all these things can exist for any temperature and related pressure).

But humid air is a mixture of dry air and water vapor, like a cocktail. For the low atmospheric mixture pressure at which we breathe air there corresponds two separate partial pressures for each the dry air and the water vapor. The partial pressure each ingredient exerts is related to its mass (and other physics things too). In humid air we really have only a little bit of water mass compared to the weight of the air, and so that little bit of water mass showing up as superheated steam is spreading itself to make up its part of the partial pressure.

So indeed, in humid air we have superheated steam (or superheated water vapor, I think the terms are interchangeable, steam being the special name for water vapor) appearing at a low mixture pressure.

Now, that's only if there is no condensation going on. Make the air more humid, reach the dew point, and the partial pressure of the water vapor no longer corresponds to superheated vapor but to the saturation pressure of water (where vapor and liquid exist together) for a given temperature.

This last case, when there is dew, makes statement 3 false because you no longer have corresponding superheated steam but only saturated steam as an ingredient for our lovely humid summer air mixture.

Boy, this makes me thirsty for a drink. It'll make all this stuff more clear too, I hope.

Thanks to all of you who contributed to my vacuumized steam post. Thanks for the encouragement. I am still digesting the information and it is making me think. I am happy.

Christian Egli

jerry scharf_2

follow the bouncing molecules

Mike,

So you want to get some gaseous H2O to condense. You have got to get the H2O molecules to group together. Pressure helps this and heat resists this. So when you lower the pressure, the amount of energy you need to pull out of the system (latent heat) to get condensation goes up.

You still need to remember that relative humidity isn't designed as a measure for pure H2O. It's looking at H2O as a component of a gas mix. The key is that the other gases still act like gas while the water boils, condenses, solidifies or melts. Even though the volume of the H2O changes radically, since it's a small part of the overall gas mix, volume is little changed.

I think part of your problem is not seeing this difference between the gas mix and pure H2O. They have radically different actions at phase change temperatures.

jerry

Mike T., Swampeast MO

"Dalton's Law of Partial Pressures -- I hate to get too technical here but I feel this is important. The law states that: When a mixture of gases is confined in an area, each gas exerts a partial pressure equal to the pressure it exerts if occupying the space alone. So, if you have dry air with a partial pressure of 14.5 lbs. per sq. in. and you add water vapor with a partial pressure of .2 lbs./sq. in. the combined air mixture will have a pressure of: 14.5 + .2 = 14.7 lbs./sq. in."

Quoted from "A PRIMER ON HUMIDITY by William C Schreiner" (link in previous message this thread)

-----------------------------

I think part of your problem is not seeing this difference between the gas mix and pure H2O.

How is there such a thing as a "gas mix" when their properties (at least pressure in some form) are additive as if they exist exclusively?

Mike T., Swampeast MO

Bless you Christian

[Sorry I couldn't resist...]

Those conflicting properties of water never cease to amaze and produce avenues for "new" understanding.

Mike T., Swampeast MO

BTW

I just checked again. It's still atomic theory--not atomic law...

Christian Egli

Steam church?


Blessed are those who come in the name of God... Thanks Mike.

Christian Egli

I didn't reply earlier, but the fog is getting thick over here. I looked back at Dalton's law and here is how I look at it.

A. Mixing the properties of gas

We've all heard of how, if you take a human being and imagine that you squeeze all the molecules and atoms together, removing all the vacant space between them, you've only got pin head sized amount of matter left, still weighing the same number of pounds. This makes it clear that there is lots of space out there (between the ears notably!), and to account for the fact that we are solid beings, we have intermolecular force (care to check out my hard as rock muscle?).

This intermolecular force is exactly the one we break up in a boiler. Apply lots and lots of energy to water and the intermolecular bond is broken, you can even melt ice! Now, you have happy and carefree water molecules floating in space with no bonds tying them to their neighboring friends anymore. Getting back to the pin headed being with the abs of steel, well, beat him up and break his bonds and he's turned into Casper the ghost. You can walk straight across him and yet have none of your molecules bump into any of his because there is no network of bonds to keep you out like a chain link fence does, and the molecules fly through the holes because the space they take up is so small (a pin sized speck is all that we are for all our inflated ego).

Put two ghostly gaseous substances into a confined space, let's say air and water vapor, both will float together without noticing the presence of the other, free to ghostly roam the entire space, behaving just like gases do. That is, following the ideal gas law and the universal gas constant. Since steam is a gas, and steam heat is the ideal way to go, it must be that we can assume the ideal gas law is the way to go...

The ideal gas law gives us a way to determine the pressure (p) exerted by a gas if we know (n) whatever quantity of gas we have in moles (which can be determined from the weight and molecular composition of the gas), (R) the ideal gas constant, (T) the temperature of the gas, and (V) the volume in which we have confined our gas.

p = (nRT) / V which we can reshape to

p = n ((RT) / V)

This will give you that pressure for the first gas in our mixture. Now assume we add more of the very same gas, an additional number of moles (n2) to the same volume and we make sure the temperature stays the same. We'll see a bigger pressure (P), like so

P = (n1 + n2) ((RT) / V) which we can factor out, like so

P = n1 ((RT) / V) + n2 ((RT) / V) which resembles what we had 2 steps ago

P = p1 + p2

Lieutenant Columbo would take a puff at his cigar, the partial pressures (p1 and p2) are additive just as we added more gas (n1 and n2) into our volume. And in fact, the moles each in (n1 and n2) don't even have to be of the same gas since we know molecules of any gas keep their distance and do not visit each other. There doesn't need to be anything in common between them, we only need to know many there are of each type within our volume.

B. The air we breathe makes us smart

For your earlier example for room air taken at P atmospheric pressure, and let's assume 55% humidity measured by an hygrometer and a room temperature of 70 F. (I have predetermined those figures to match the example). Let's determine the partial pressures we have.

Since we have only 55% humidity, we are dealing with superheated water vapor. And the number 55 tells us what percentage discount we have on maximum humidity we can have at the full condensing value of water vapor for our temperature of 70 F

So, from a table, we can get the pressure at which water condenses and boils according to a determined temperature. At 70 F this happens in a vacuum, as we know, and according to the table, this is at a pressure of 0.3632 PSI. This is the point we have boiling water or saturated steam and thus, maximum humidity.

Taking our 55% relative humidity discount on that value will give us the partial pressure due to the water vapor

p(vapor) = 0.55 x 0.3632 PSI

p(vapor) = 0.2 PSI ooh

Since we can't change the atmospheric pressure (in my earlier example we had fixed the volume and temperature but not the pressure, here we have a fixed pressure and temperature, the rest changes)

Since partial pressures are additive, it follows that the partial pressure of the air would be

p(air) = p(atmosphere) - p(vapor)

p(air) = 14.7 PSI - 0.2 PSI = 14.2 PSI aah


But after all this, I am not sure I can still breathe. I did not find a good book that laid it out as plainly as this, so I worked it out in a way I hope you'll find logical, understandable and fun.

Christian Egli

The Vari-Vac info I have requested from Dunham following your post hasn't arrived yet, but I am anxious about it.

Thanks Mike

40616.2209

Thanks for all the contributions.

I had not thought at all about the Vari-Vac and about resetting the steam pressure to follow the weather.

In fact, I am not sure I see the value of outdoor reset for steam heat since steam heat is so quick to adapt even when the weather is moody. But I see the necessity of outdoor reset with, for instance, floor heat, where the reaction time is slow and anticipating the heat loss according to the weather is a good way to go.

Thanks Noel, for pointing out that the thermostatic traps are just as affected by the vacuum as the boiling water. Vari-Vac has a control that seems to ensure that the pressure difference between boiler and return lines stays at around 2 PSI. I am guessing that if you let the difference go to 10 PSI or more, the thermostatic valves will either not be able to reopen, or will cycle very quickly.

Actually, with the traps working according to general pressure, this would mean that in a standard vacuum hook-up the vacuum should already go back to the boiler, but I have not seen that. I do think that the vacuum pump, even at 1CFM per 1000 EDR are undersized to keep up. Has anybody supersized a vacuum pump on a standard vacuumized job?

My idea of boiler control in this case has revolved around under-sizing the fire, that way no head is generated through steam pressure but only by the vacuum pump. I would not worry about steam not reaching all the radiators because the way for it to flow is cleared by the vacuum pump, there is no air to push out. I do think I would not fully fill the radiators with steam, but so what? The idea was to save fuel, and a half hot radiator here means a half bill, it is not the ordinary air bound radiator case which is wasteful.

Does the Vari-Vac work this way:

1) measure outdoor temperature

2) according to outdoor temperature set value for vacuum and control vacuum pump with vacuum gauge

3) measure pressure difference between vacuum line and boiler

4) control fire to maintain pressure difference at say 1/2, 1, or 2 PSI

or

3) set boiler pressure at say 2 PSI

4) control fire like any boiler, with a pressure control

5) use special reducing valve to bring steam pressure down to the desired radiator pressure

I am anxiously waiting for the technical info which I just ordered from MEPCO.

I don't like the idea of using either throttling valve or pressure reducer, their operation is totally wasteful of developed pressure head, plus its another gadget that breaks.

Lastly, I fully agree with you about gravity. It works and it is free. But if I have 10 PSI difference from the vacuum to the boiler I'll either need a tall A dimension or a Differential loop or an alternating receiver. As far as pipe size, I am sure it is cheaper in the long run to buy big pipes than it is to pay for the pumping (without even considering pump cost).

Outdoor reset, Vari-Vac, pressure reducing vavles, differential loop, I hear the Dead Men laughing and banging away at my pipes. What a party to have, they knew how to have fun.

Best regards to all

Christian Egli

Christian Egli

And who's Dalton anyway?


I didn't reply earlier, but the fog is getting thick over here. I looked back at Dalton's law and here is how I look at it.

A. Mixing the properties of gas

We've all heard of how, if you take a human being and imagine that you squeeze all the molecules and atoms together, removing all the vacant space between them, you've only got pin head sized amount of matter left, still weighing the same number of pounds. This makes it clear that there is lots of space out there (between the ears notably!), and to account for the fact that we are solid beings, we have intermolecular force (care to check out my hard as rock muscle?).

This intermolecular force is exactly the one we break up in a boiler. Apply lots and lots of energy to water and the intermolecular bond is broken, you can even melt ice! Now, you have happy and carefree water molecules floating in space with no bonds tying them to their neighboring friends anymore. Getting back to the pin headed guy with the abs of steel, well, beat him up and break his bonds and he's turned into Casper the ghost. You can walk straight across him and yet have none of your molecules bump into any of his because there is no network of bonds to keep you out like a chain link fence does, and the molecules fly through the holes because the space they take up is so small (a pin sized speck is all that we are for all our inflated ego).

Put two ghostly gaseous substances into a confined space, let's say air and water vapor, both will float together without noticing the presence of the other, free to ghostly roam the entire space, behaving just like gases do. That is, following the ideal gas law and the universal gas constant. Since steam is a gas, and steam heat is the ideal way to go, it must be that we can assume the ideal gas law is the way to go...

The ideal gas law gives us a way to determine the pressure (p) exerted by a gas if we know (n) whatever quantity of gas we have in moles (which can be determined from the weight and molecular composition of the gas), (R) the ideal gas constant, (T) the temperature of the gas, and (V) the volume in which we have confined our gas.

p = (nRT) / V which we can reshape to

p = n ((RT) / V)

This will give you that pressure for the first gas in our mixture. Now assume we add more of the very same gas, an additional number of moles (n2) to the same volume and we make sure the temperature stays the same. We'll see a bigger pressure (P), like so

P = (n1 + n2) ((RT) / V) which we can factor out, like so

P = n1 ((RT) / V) + n2 ((RT) / V) which resembles what we had 2 steps ago

P = p1 + p2

Lieutenant Columbo would take a puff at his cigar, the partial pressures (p1 and p2) are additive just as we added more gas (n1 and n2) into our volume. And in fact, the moles each in (n1 and n2) don't even have to be of the same gas since we know molecules of any gas keep their distance and do not visit each other. There doesn't need to be anything in common between them, we only need to know many there are of each type within our volume.

B. The air we breathe makes us smart

For your earlier example for room air taken at P atmospheric pressure, and let's assume 55% humidity measured by an hygrometer and a room temperature of 70 F. (I have predetermined those figures to match the example). Let's determine the partial pressures we have.

Since we have only 55% humidity, we are dealing with superheated water vapor. And the number 55 tells us what percentage discount we have on maximum humidity we can have at the full condensing value of water vapor for our temperature of 70 F

So, from a table, we can get the pressure at which water condenses and boils according to a determined temperature. At 70 F this happens in a vacuum, as we know, and according to the table, this is at a pressure of 0.3632 PSI. This is the point we have boiling water or saturated steam and thus, maximum humidity.

Taking our 55% relative humidity discount on that value will give us the partial pressure due to the water vapor

p(vapor) = 0.55 x 0.3632 PSI

p(vapor) = 0.2 PSI ooh

Since we can't change the atmospheric pressure (in my earlier example we had fixed the volume and temperature but not the pressure, here we have a fixed pressure and temperature, the rest changes)

Since partial pressures are additive, it follows that the partial pressure of the air would be

p(air) = p(atmosphere) - p(vapor)

p(air) = 14.7 PSI - 0.2 PSI = 14.2 PSI aah


But after all this, I am not sure I can still breathe. I did not find a good book that laid it out as plainly as this, so I worked it out in a way I hope you'll find logical, understandable and fun.

Christian Egli

The Vari-Vac info I have requested from Dunham following the other post hasn't arrived yet, but I am anxious about it.

Thanks Mike

Mark Hunt

look up


fugacity.

Water is whacky stuff.

Mark H

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Mike T., Swampeast MO

Fugacity

From the Merriam-Webster unabridged dictionary:

1 : lack of enduring qualities : TRANSIENCE

2 a : the vapor pressure of a vapor assumed to be an ideal gas obtained by correcting the determined vapor pressure and useful as a measure of the escaping tendency of a substance from a heterogeneous system b : a correction for the deviation in the behavior of an actual solution from that of an ideal solution -- compare ACTIVITY

--------------------------------------

Am still digesting 2) but it almost sounds like, "The difference between what something should be and what it appears to be is equal to its desire to escape."

Is that vapor or humans?

Christian Egli

Fugacity? does it smell good?


Good Gracious, where do you guys come up with this stuff? In boiler room talk, I've never overheard anyone say: "Man, I wish my steam had more fugacity, let's install a vacuumizer!" Have you?

I couldn't resist looking it up either. I found the word came from the Latin fuga like fugitive in English which means apt to flee. I have heard fugacity used to describe a perfume, which seems to make sense, but sorry, I don't understand a thing about either your definitions 1) and 2).

What's next? Perfumed boiler water

Mind you, we once worked on a one pipe steam system in an old house owned by an airline pilot. Interestingly, he maintained everything real well and to keep things from rusting ever so slightly, he sprayed everything with some kind of airplane juice (WD-40 deluxe). He got all the stuff he wanted from the airline. The juice smelled like strawberries, it was pink and it was approved by the FAA. I don't know whether that boiler has ever flown away, not enough fugacity, I suppose, but it smelled so good it was a boiler you wanted to lick like a spoon. Pilots must love their planes.

Christian Egli

bob young

SERIOUS STEAMFITTERS

WOW NOW WE ARE LICKING THE BOILERS--TALK ABOUT LOVIN' YOUR WORK. FUGACITY ? MAYBE FUGITABOTIT. LOL

Mark Hunt

HAHAHAHAHAHAHAHAHAHAHA!!!!!!!!!


Scented water? Maybe use the boiler as a room freshener?!?!?!?!?!?!?

Man I love doing this stuff!!!!!!

Mark H

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GaryDidier

Stram/ vapor /Water

After reading this thread I need a couple of aspirin.

Gary from Granville