What Happened When Pressurized Steam At Low Temperature Is Heated?

this is happening in conventional super heated boilers. that is also the same isobaric process. the temperature is varying since its volume is not fixed. if you keep the volume as a fixed one, then the pressure and the temp. will change.

Your both questions are different.

Pressure of the steam at 190C will be higher, if we assume that the volume of the steam remained constant (simple gas eqn).

"But, what will happen if the vapor is kept at comparatively lower pressure and is gradually heated in the Boiler? Does in that case too its pressure will reach the pressure level of the conventional thermal power plant level or the pressure will remain the same?" The answer is NO, because volume is not constant. Pressure is generated at boiler feed pump only which is maximum in the cycle. More pressure in downstream of pump (at boiler) will not allow feed water to flow and go through boiler.

Thank for answering my question. I want to know, if a convergent nozzle can be fitted at the outlet of pipes of Boiler in the abovementioned process, then does this nozzle can help in increasing the apparent pressure of the steam.

How? We all know that convergent nozzles can convert enthalpy of gas/steam into velocity in a particular direction. As sufficient enthalpy is added to the the gas/steam as heat, then whether the nozzle can convert that enthalpy into velocity in the direction to turbine and the effect can be same as high-pressure Boilers of conventional thermal power plants?

Don't we do something like this in gas Turbines ?

BTW- Steam Turbine is reaction (rankine's cycle) against impulse : Brayton (for Gas Turbine)

UD15

Do you really assumed that at 27 deg C and 12 bar it will be steam and not water?

BFP handles water at about 100 deg C and not ambient temperature

The other part (PV/RT) has already been mentioned above.

If you keep the vapour or water at low pressureand then try to heat it and increase its pressure -then you lose in volume at least partially- what stops the resultant high pressure steam not to flow back through BFP and back to turbine outlet?

UD15

In my opinion, it's the siphon effect. In a siphon, water at the highest point of the tube always flows in the direction of lower level. By the same manner, gas/steam in the Boiler always tends to move towards the Condenser, where temperature and pressure is lower than that of the FWP.

Instead, such flow creates vacuum and draws more water to the Boiler to be heated.

PV = RT; Kinetic energy of molecules which is also you know as temperature (average velocity = 3/2KT or 1/2KT for each degree of freedom in 3D motion) becomes pressure at constant volume.

Well, my field of query is when the gas is in motion. We all know that pressure of gas reduced perpendicular to the direction of its motion. I am curious whether its pressure will increase in the direction of velocity or not.

My little knowledge of physics says "yes". And as a convergent nozzle can convert enthalpy into velocity in a particular direction, therefore the pressure of the gas too.

Wikipedia steam injector description.

In the description of steam injector in wikipedia (above mentioned link), it has been clearly written that the purpose of the injectors is to use the latent heat embedded in steam to reinject it into high pressure. I curious whether this can be done with steam at comparatively low pressure but with sufficient enthalpy. In that case, that means increasing the exergy level of the steam at low pressure.

Pressure difference makes flow and flow must increase when you heat up pressurized gas. If availability of gases come down then again pressure will come down to make no flow to settle down.

Thanks, that's the main answer I am searching for. Kindly just tell me that by heating up the gas/steam, pressure can not remain in the same level.

This sounds like a thermodynamics lab experiment to me. I grew up with steam tables and mollier diagrams; so for you "youngsters" who do everything on the computer I may not be helping much.

So this is a fixed volume process with "steam" starting at a 27C and 4.24 KPa. on the saturation line. (right on the constant entropy coordinate line of my old Mollier diagram; how coincidental!) Then heat is added to the constant volume to raise the temperature to 190C. So the condition point goes right up the constant entropy line to the intersection of about 30KPA pressure line and the 190C temperature line. (about 122C of superheat.)

Doesn't matter if the heating takes place in a boiler designed to operate at some higher pressure or temperature or somewhere else given two conditions at the first point and two at the second point other conditions remain the same.

Now you take the second question and this involves a new set of conditions at the second point. Instead of entropy and temperature at the second point we are given a new pressure (1254K Pa) and saturation (which defines the temperature of 190C). In order to put the given quantity of water (liquid or vapor or a mix) it must have a combined mechanical energy (pressure head and velocity head) greater than that of the water in the boiler. This applies whether you have a boiler feed pump or an injector. The latter is often deemed to be more reliable in small or portable installations. An effective boiled feed injector has multiple stages to raise pressure to a sufficient level to enter the boiler. The wikipedia article you cited explains the apparent paradox of how steam at pressure the same or slightly lower than the boiler can raise the pressure of water high enough to overcome the pressure in the boiler as it enters.

With respect to the injector with its convergent-divergent elements the guiding principle is the Bernoulli principal which essentially means that energy is conserved. Putting aside second order effects such as friction losses in a well designed nozzle any change in kinetic energy (velocity) in the flow has an opposite and equivalent change in the potential energy pressure (pressure). A diffuser increases pressure while reducing velocity to conserve energy.

Ed Weldon

Actually I am more interested in whether convergent nozzles can convert entropy into velocity so that by fitting a convergent nozzle to a compressor, we can reduce the energy consumption by the compressor so that the enthalpy of gas/steam can be used to compress the gas/steam at higher pressure.

You have said velocity at the expense of pressure. But, pressure itself is an expression of motion of molecules, and we say "velocity", that means it is pointed towards a specific direction. We all know that pressure of fluids (both liquid, gas) is exerted in all direction the same way when it's static. But, what happened when some motion is added to it, specially when it's a gas. The Bernoulli's law states that pressure of a moving fluid decreases perpendicular to the direction of motion, but the question is whether it increased towards the direction of motion.

A convergent nozzle increased the velocity of gas/steam along the direction at the expense of the enthalpy (not pressure) based very simple equation of nozzles which A1/A2=V2/V1. That means velocity increased with decrease in cross-sectional area and the velocity is maximum at the throat. Just google with "convergent nozzle" and you can see phrases that nozzles convert gas enthalpy into velocity and that's why it's used in jets and rockets.

pranabjyoti - Like cwarner said in reply#10 I am somewhat "stumped". In my case it is because I think we are not assigning the same meaning to "pressure". To me the meaning is well described albeit with perhaps too much rigor (and too many words) by the following Wikipedia entry:

http://en.wikipedia.org/wiki/Pressure

All succesful communications are based on the sender and recipient interpreting each element of the communication the same way. When we have differing meanings for a word like pressure it gets difficult to discuss it. So here I fear I am like the father trying to help his child with algebra. Father learned algebra through traditional teaching methods of 2 generations past. Child is being taught using "new math" techniques that employ vastly different terminologies. They are confronted by a communication problem that must be solved before the algebra problem can be solved.

I have never had any formal teaching experience so am hampered by a very amateur level of explaining things to others. Takes me a good bit of effort to explain things effectively. So while I try to come up with an explanation of my position, that pressure only increases with flow direction in a diverging nozzle, please go take a look at what seems to be accepted as a proper technical English definition of "pressure".

Thanks for your patience. ............ Ed Weldon

I am really stumped on this one. How does one generate steam at 12 bar at 300 ºK? What is the fluid we are talking about?

"I am really stumped on this one. How does one generate steam at 12 bar at 300 ºK? What is the fluid we are talking about?"

cwarner -- It depends on what you mean by steam. Colloquially we refer to the vapor phase of water at temperatures over 100C (400K) as "steam". But below that temperature water can exist as a vapor that we refer to as humidity when it is mixed with air.

Let's say we take a vacuum chamber and evacuate the air down to a few torr pressure. Next we introduce pure water with insignificant amount of air mixed or dissolved into the chamber. Initially the water entering the vacuum chamber flashes to water vapor which fills the chamber to a pressure determined by it's temperature. We watch the water inlet through a window in the chamber and when we see liquid water collecting on the bottom of the chamber and no more evaporation taking place. We shut off the water flow in and give the chamber an hour or two for the temperatures of everything to even out. Then we read the temperature and absolute pressure inside the chamber. We will find that pressure is exactly related to the temperature. A graph of that relationship is known as the "saturation line". At 27 degrees C the pressure will be 3.5 KPa. Whatever part of the chamber does not contain water will be filled with water vapor at those conditions.

Now let's say we continue filling the chamber with water while carefully regulating the temperature to maintain 27degrees C. (OK, this is a pricey vacuum chamber with pretty good auxiliary heating and cooling controls). Throughout this process the pressure will remain unchanged as long as the temperature is maintained up to the point that the last space is filled. Where did the water vapor go? It changed to liquid form and joined the rest of the liquid water. In doing so it added some heat of vaporizatio0n back to the system. An almost negligible amount given the heat capacity of the water already in the chamber. Note that while filling the pressure never changed in the chamber and water was sucked in rather than having to be pumped in. As soon as the chamber fills the sucking stops. Note we were smart enough to maintain the level of the vessel supplying the water so there would be no gravity effects on the flow.

Now the only way to get more pressure in the chamber to continue our test is to pump water in under pressure of an external pump. Up goes the pressure as the pump pushes a small amount of extra water into and elastically stretching the metal chamber .

But note that as soon as the absolute pressure in the chamber went above 3.5 KPa the water vapor bubble disappeared and only liquid water remained. this demonstrates what theory predicts. That water vapor cannot exist above the pressure that is related to its temperature on the saturation line and there is no other gaseous substance for it to mix with or other solid or liquid material in which water can dissolve. If, on the other hand, there is some air in the chamber for the water vapor to mix with then it can exist as a vapor and the relationships having to do with humidity come into play. This is about partial pressures and absolute and relative humidity and pretty much OT for this discussion.

Remember we talked about the definition of "steam"? Very likely the OP's training in this subject treated the word "steam" as any vapor of water regardless of temperature unlike our colloquial definition in the USA.

Ed Weldon

Probably the problem is I am a student of physics, not engineering and you are an engineer. As per my little knowledge of physics, pressure and motion i.e. velocity aren't two different things but pressure itself is an expression of motion. Therefore, increase in temperature means increase in random motion of molecules and therefore increase in pressure.

Same physics is applicable for engineering too, its not a different one. I will try to explain you in following very simple lines:

1. Pressure can not be more than BFP discharge at any point of cycle. Static pressure readings will show a down ward trend only.

2. Increase in temperature will cause increase in pressure too, if we restrict the volume, which we are not doing as steam is flowing towards turbine. Instantaneous increase in pressure at boiler provide further resistance to BFP, increasing its discharge pressure, so that condition 1 above is always fulfilled.

3. Convergent nozzles can increase velocity (increasing velocity head in total energy), and the same is adopted before turbines. I think, if pressure increase (due to more temp) instantaneously converts in to velocity then static pressure may remain below BFP discharge. But I doubt whether it is practicable due to length of piping between boiler & turbine inlet.

Wow, I'm surprised there's such an in depth discussion on this. I'm also surprised that you know what isobaric means but you didn't consult steam tables.

First off, you don't have steam at 1200 kPa and 27C. Off hand, I don't know what the saturation temperature is at 1200 kPa but until you reach it you don't have steam of any kind. If you want to know if the pressure of the steam is the same at 190C then check the steam tables.

Well, you need habit of in depth thinking. My query is when steam under pressure is heated and the temperature of the steam passed the point, where its pressure will surpass the pressure in which it has been kept, whether the pressure of steam will still remain the same or will increase with increase in temperature. So, better leave the 27C factor and think it that way.

I'm not sure if this is an insult or not so I'll try not to be insulting back.

Definition of partial pressure from Wikipedia:

"In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.^[1] The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture."

In a controlled volume, pressure and temperature will go up together. If you are at 27C at 1200 kPa then you have 0% steam. Therefore, your partial pressure for steam is 0 kPa.

You have 3 phases of "water" that need to be discussed here: liquid, saturated steam, and superheated steam. Partial pressure of steam is 0 until you heat up enough to reach the saturated steam line. At that point, the partial pressure of steam is based on the Quality of the mixture. This is true until you've evaporated all of the liquid. At that point, you are superheated steam so the partial pressure of the steam is the same as the pressure of the system.

I don't know if 190C at 1200 kPa is on the saturated steam line or not. The only time in a controlled volume that you should have an increase in steam pressure without increasing the temperature is in a saturated state.

Are you talking about something else here?

Have you read my post at 19 and understood? Have you referred 'steam tables' as suggested by cingold before asking same thing again & again?

I have read your post, but still I must clarify to cingold what I am asking for.

<...Steam is kept at around 12 bars or 1200 kPa of pressure at atmospheric temperature (27C or 300K)....>

According to Mayhew & Rogers, "Thermodynamic Properties of Fluids", at that temperature and pressure it is water, and not steam.

<...to know that whether the pressure of the steam will be the same at 190C or not...>

If the same material is constrained at 12bar and then heated, then the container may rupture befroe it gets there, depending on the differential expansion of the liquid, its container and the mechanical strength of the latter.

Saturated steam at 12 bar gauge pressure will have a temperature 0f 191.6 ºC. It cannot be at room temperature.

This is a single component system and so thermodynamically only one parameter can be varied: either pressure or temperature for saturated steam.

Bioramani