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formulation of direct contact condensation
6

formulation of direct contact condensation

formulation of direct contact condensation

(OP)
Hi all,
I have some difficulties finding formulation for direct contact condensation process. In our condenser, we trying to fix its vacuum by reducing the exhaust steam temperature, using a sprayed water. Is there any formulation that can explain this wacky idea?
Thanx.

Dwi Handoyo S

RE: formulation of direct contact condensation

Dwi Handoyo S
 Its not clear what you are looking for, are you looking for a formula, if you are what are you trying to calculate.

athomas236

RE: formulation of direct contact condensation

To semarang:

If you are planning something similar to a barometric -direct contact- condenser with pressure to be reduced, there is a complete worked out example in Donald R. Woods' Process Design and Engineering Practice, Prentice Hall, that may of help.

There are no formulas. However, there are a number of ROT and constraints to be taken into account, such as, for example, one cannot condense steam to a pressure of 3 kPa with water at 30oC since that pressure corresponds to 25oC.

It appears that Ludwig E. E. Applied Process Design for Chemical and Petrochemical Plants, Vol 1, also presents a worked example for a barometric condenser.


 

RE: formulation of direct contact condensation

3
What you are seeking is a function of at least three things, maybe more.

One is injection water temperature.  Your approach temperature for the condensing will be governed by that, no matter what you do with the rest of what I state.

The second is your spray patterns.  There are a plethera of styles of spray patterns, some better than others.  Rain tray types tend to be less effictive than cone spray, or multile cone spray types.  Lots of information on the net about that.

You might post back as to some details as to what you have, so we can make suggestions regarding improvements.

The third is duty.  If your condenser is undersized for what you are asking you to do, the best spray patterns and coolest injection water can only do so much.

Give us some conditions, and configurations to work with.

All I say assumes that you mean barometric by direct contact.  If you have a surface condenser, some of the above has to be modified.

rmw

RE: formulation of direct contact condensation

(OP)
Thanks RMW,

My problem is in 30 years old surface condenser. To increase its vacuum performance, I want to use spray water into the hood spray sistem, to decrease the steam temp.

First
Spray water that I want to use is 32oC n 3 bar, from the make up water system. The exhaust steam is 53oC. What is the most relevant type of spray for those configuration? or maybe, those config won't effect the vacuum improvement of the condenser itself?

Second
Is this 32oC water won't be a source of pitting corrosion in the last stage of turbine?

Dwi Handoyo S

RE: formulation of direct contact condensation

We had discussed this concept in an earlier thread, or so it seems to me.  I mentioned a plant where I had knowledge that it had been done, and you asked me for a reference at the plant.  The problem is that the person who related the situation to me had come from that plant, and has since transferred to yet another plant.  It only came up because I was proposing a spray system in a different part of the condenser, for a different reason, and he was part of the plant management that I was proposing it to, and he was favorable to my proposal due to the success he was aware of at the plant he came from.  I have no contacts at the plant in question, so I cannot give you any references.

I will apologize for not answering you in the previous thread.

Now to your current problem.  The spray water is not going to "cool" the steam, it is going to act like a direct contact condenser and condense some of the steam coming out of the turbine exhaust(s), raising the temperature of the spray water in the process.  However, the steam that is condensed by the spray water is now no longer "duty" for the surface condenser.

Look at your condenser curves.  What happens when you reduce the duty for a given CW temperature???  The back pressure drops.  What happens when the back pressure drops??  The turbine gets more efficient, because the pressure drop is higher, and it "wrings" more work out of the steam.  What happens when you get more work out of the steam???  It exhausts from the turbine at a lower temperature, so you haven't "cooled" the steam, but yes, you have "cooled" the steam.

But then, you increase the load, which is what you wanted to accomplish in the first place, and when you do, you increase the duty on the condenser, and raise the back pressure and "heat" your "cooled" steam.  Does this make sense??

What you are doing, if you are able to accomplish the spray thing is to increase your condenser capacity.  You will have added some direct contact, or barometric condenser capacity to your surface condenser capacity.

Now, realize that barometric condensers are animals in their own right.  They are specifically designed to do what they do in a specific way.  Read some of the comments in earlier threads.

Practically speaking, you probably have limited ability to add a lot of "barometric" capability to your surface condenser.  You put too much free water into the only zones of the surface condenser that you have available for your spraying situation, the zone between the turbine exhaust hood and the condenser bundle(s), and at the velocities you have in that area, you will begin to cut things to ribbons, things like your condenser parts.  Look at the zones that get the erosion wear in the winter time when you are at low loads, and the turbine exhaust steam is real "wet".  (assuming you are in a part of the world that has winter)

So, to do what you want to do, you must spray as finely meaning atomize the spray water as much as you possibly can, and do it as close to the turbine exhaust as possible, in order to give enough residence time, for the spray water to absorb heat out of the exhaust.

Look at any information you might have on desuperheaters, because the problems they have to overcome, atomization, velocity, distance, mixing, delta T, are the same ones your sprays will have to overcome.

Based on the comments of the plant manager I referred to earlier, it had been done at a plant where he had worked, and it make enough difference so that he was aware that it was a success, so, that is why I recommended it previously.  Go for it.

I hope this helps.

rmw

RE: formulation of direct contact condensation

(OP)
Thanks RMW,
your comment really inspiring me more.
But there still one obstacle hanging in my head, isn't sure that the spray water won't make an erosion in last stage turbine, if we took it as close as possible to the axhaust steam?
Or maybe, in the other word, if the 30oC droplet water "touching" the last stage turbine, does the erosion happens?

Dwi Handoyo S

RE: formulation of direct contact condensation

The exhaust "hood" of the LP turbine is a high velocity zone, high enough that some turbine OEM's supply a special curve to allow you to determine when the velocity, (and hence exit losses) are too high.  This is more problematic when the CW is cold, and the vacuum is deep, and the specific volume of the steam is high due to the low pressure, or high loads at other operating points.

In my mind, you would want to position the sprays just outside or at the exit of the hoods, to stay away from any problems with eddys and/or swirls in that area.

Mind you, some turbine OEM's put spray nozzles in the hoods, right at, or right by or that is to say right in the anulus of the the last stage buckets for the purpose of hood cooling at very low flows during start up.  So, if what you are worried about would be a problem, they would not do it.

Note, too, however, that these hood sprays are only intended to be used for brief periods during start up.  If your turbine is equipped with such sprays, you could test your theory by running them for a short period of time, and see if it makes any difference in your back pressure.  I can assure you that the velocity leaving your last stage buckets (blades) is much higher than when the sprays are designed to be used at start up, so I don't think the water has any danger of touching your last stage blades.  Some last stage blading has special protective measures, because when CW is cold, or at low loads, the turbine expansion line is already below the expansion line on a Mollier diagram, and the steam in the last stage(s) of the turbine is wet.

Can you give any data as to what your CW conditions are?  And in what part of the world this unit is located??  Is the CW from a river, lake, ocean, cooling tower, etc.

rmw

RE: formulation of direct contact condensation

(OP)
Thanks RMW,

My condenser is in south east asia (ekuatorial), we use sea water as the CW. N here're the annual average data (2003):
CW : in  = 33 oC
     out = 41 oC
cond' vacuum = 668 mmHg


Dwi Handoyo S

RE: formulation of direct contact condensation

2
I have a comment in regards to turbine exhaust sprays.

  When working as a factory field engineer, I was instructed many years ago that the use of cooling sprays was a nesecary evil.  During start up, as the unit approached rated speed, the exhaust rapidlly heats up, but starts to cool back of with initial loading (~2%).  The older turbines use air switch logic that allows sprays while rolling to speed, but shuts them off when valve open to a position corresponding to minimum load. (newer turbines uses a temperture controller to bring in sprays when needed)

The concern was the steam flow was exiting the outer rotor raduis, but the inner radius would draw in the spray droplets and cause the bucket tips to erode.  This became more severe at light load, thus sprays were to be adjusted to cut off as soon as not needed.

RE: formulation of direct contact condensation

byrdj,

Good comment.

Precisely why I recommended locating the sprays at the outlet of the hoods, and sacrificing the mixing and cooling distance between the last stage blades, and the exit of the hoods.

For a clarification, were the sprays you were referring to located in a ring, around the anulus of the exit of the turbine, close, literally inches from the LS Buckets??

So, Semarang, I take it that you don't get to see any "dead of winter" cold water situations.  I seem to have deleted my uconeer conversion program so I have to reload it before I can convert the temperatures to farenheit, so I can think about them.

rmw

RE: formulation of direct contact condensation

Yes, the hood spray ring is located inches away from the outer diameter of the bucket.  

Continuing with the explaination, At No and low load, There is no "work" being performed by the last stages.  The hood steam will be drawn back into the last stages at the root of the bucket and "work" will be added to this recirculating steam to expell it back out at the bucket tips.  The spray is used to cool the exiting flow, but it also is drawn into the last couple stages at the bucket root to provide the bucket cooling effect.

If this no-load cooling is not performed, the buckets will over heat and weaken.  Also the hood heat will distort the bearing supports causing deflection and thus rubbing.  However, there will be erosion of the bucket tips.

There should be a turbine trip to prevent at speed, high stress operating when the exhaust temperature exceeds ~225F

For what I think you are looking at.  "Sprays at high load".  One would think the exhaust flow would be sufficent to prevent the recirculating flow.  The next consideration would be the erosion effects on the internal hood components, especially internal support struts.  Water droplets will cause erosion and this components are overlooked in inspections untill something gives.  Adding shields is sometimes done.

RE: formulation of direct contact condensation

Whoa, whoa,

What I said about getting it as close to the turbine exhaust as possible in a post above, was suppose to convey the idea that the sprays would still be outside of the hoods, based on other comments I also made.

Re-reading my post, I may not have made that crystal clear.  I believe the hood area to be TOO high velocity for any significant spraying, but I can't see getting too far downstream, since the sprays need time to mix, and transfer the heat before the flow stream hits the condenser.

Therefore, I envision such a spray system right at the outlet of the hoods, at the beginning of the transition to the condenser shell.

Sorry for the lack of clarity.

Thanks for your input that made me clarify.

rmw

RE: formulation of direct contact condensation

RMW, I think we were in agreement, saying the same thing from differnet views.  I wanted to emphisize your point to Semarang that the proposed "test" only be runned for a short duration and at a load high enough to prevent recirculating last stage flow, say greater than 50%.   

If the results of the "test" were satisfactry, the turbine exhaust spray location would not be a good location for continous spraying for the reasons you stated 1) high velocity structrual erosion "cutting things to ribons" and 2) recirculating bucket flow at low loads causing bucket erosion.

RE: formulation of direct contact condensation

byrdj,

Well said, or is that written?  Have a star of appreciation.

Now, Dwi Handoyo S, one other precaution on the other end of the process.

No condenser has a lot of wide open spaces with great distances for good mixing, heat transfer from the exhaust steam to the spray water, etc., so you may still have free moisture in the exhaust steam passing through the transition and entering the tube bundle.

There is typically a lot of sturcture, etc., as well as the tubes themselves that can suffer the effects of erosion.

With sea water as your CW, I would guess that you might have soft metallurgy tubes, cu-ni, or the like, so erosion might be a problem.  Where you are located, I doubt that you have the cold water / wet steam problems of other condensers.

But, erosion is a problem even in condensers without spray systems in them.  It is dealt with by using wear shields on the structual members, as well as the tubes in the affected zone.

The only tubes that would need shielding would be the ones in the leading rows, or the rows exposed directly to the steam entering the tube bundle.

There is a trade off.  A tube shield is an impediment to heat transfer to a slight extent, so the benefits of the spraying might be offset by the reduced heat transfer of the shielded tubes.  Probably negligable, unless you got carried away, and put way too many shields.

rmw

RE: formulation of direct contact condensation

(OP)
Wow, great2 comments,
thanks RMW and byrdj, seems that hood spray's application need to be confirmed yet. From the discussion, in no or low load the spray could harm the turbine and condenser's tube itself.
But, before we made a conclusion, there is one information. My hood spary's arrangement is not inches away from the buckets, but right in the condenser's neck.
I think it's position make the spray's application secure enaugh for the turbine and the condenser's tube.
Any comment?

Dwi Handoyo S

RE: formulation of direct contact condensation

Is it down flow??  or axial flow??

What size unit is it, how many MW?

I think that if you are in the neck, or transition, then you are far enough away from the turbine to not have to worry about it.  Even at low loads you would not have to worry.

Understand that the cooling spray nozzles that byrdj and I were discussing, and were concerned about are located right at the exit of the turbine even before the hoods.  They are put there to function as byrdj stated, which is for very short durations.

Yours in the transition shouldn't even be near the turbine blading.

How much make up water do you have available for this cooling water??  And, is the flow going to be continuous??

rmw

RE: formulation of direct contact condensation

(OP)
It's a down flow, with 46 MW full load. The make up is about 1 m3/h, and yes the flow is going to be continuous.

RE: formulation of direct contact condensation

In my experience every STG I have operated, we had Exhaust hood sprays and usually they only come on durning low loads, when the plant is starting up and shutting down.  The power plant that I work at, we cycle daily but hear lately we have ran 1 X 1 durning early morning hours.  When we shut all units down at night We shut steam off to the vacuum skids and open air side valve on the vacuum skids and open spray hood supply and we are able to maintain a very low vacuum for next morning start.. This system is a must and it works very well.  Note are Condenser is an Alstom down ward flow and the nozzels are about 3 feet below the Exit of the LP exhaust and it creates a vacuum like effect at low loads to help pull the steam to and across the condenser tubes.

FLORIDA,
BIGDOG50

RE: formulation of direct contact condensation

(OP)
Thanks all, perhaps i want to refresh my question about this spray modification.
I'm going to use the make up water for the spray with its 3 atm pressure.
Is it enaugh for the droplet water to mix with the exhaust steam (at 45 MW load, steam flow is 180 tonne/hour)?

Dwi Handoyo S

RE: formulation of direct contact condensation

The atomization of the water is a function of the design of the nozzle you select to use.  Does the 3 atm include the fact that the spray nozzles will be located in a ~1 atm vacuum.  In other words is your spray pressure 2 atm, plus 1 atm vacuum, or is it 3 + 1?  If the latter, that will give you 4 atm total pressure drop across the nozzles.

In either case, that does not sound like much differential pressure to work with, unless, of course, you have a very good nozzle selected.  

Search some mechanical desuperheater sites, and see what is the minimum differential pressure they want for mechanical atomization, and use that for your design pressure.

Here is a starting place.

http://www.dezurik.com/MA_desuperheaters.htm

Google will produce others.

rmw

RE: formulation of direct contact condensation

I found another link that will show the differences among nozzles, in the same product.

Look at figure 4 in;http://www.ccivalve.com/pdf/624.pdf

This gives a comparison among different nozzle designs for the same desuperheater.

Select your nozzle(s) carefully.

rmw

RE: formulation of direct contact condensation

(OP)
Thanks RMW,
It will be about 3+1 atm. And this is my design pressure (limited by the available make up pump).
By this differential pressure, I still can't imagine how the water spray can attain and mix with the exhaust steam, with the nozzle that I've choosen. On the other words, if I choose the F nozzle (from figure 4), is it enaugh for the droplet to attain the high mass of exhaust steam? Or maybe, because of the tiny differential pressure, eventhough we use the finest nozzle, won't make any sufficient changes of the condensation process in condensor?

Dwi Handoyo S

RE: formulation of direct contact condensation

I used the desuperheater analogy because it demonstrates the need to break up the droplets as finely as possible in order to expose as much surface area of the spray water as possible to the process for heat transfer purposes.

In the case of the DSH, you are trying to heat the droplet to a point of completely evaporating it, and having no moisture left in the flow stream before you get to your first elbow, etc.

In your case you are trying to do two things.  One is develop a fine enough spray to have good heat transfer in the short distance between your spray nozzle and your tube bundle (unlike a DSH situation, you don't have 10-15 meters to work with), but instead of evaporating, you want the cool water spray droplets to condense some of the steam out of the process, so that the surface condenser doesn't have to do this duty.

Your situation is actually more akin to a cooling tower, where the droplets are constantly splashed over fill material of some type to break them up for good heat transfer.

But, as you condense steam out of the process, by heating the droplet, the droplet grows, because that is what water does when it condenses.  So, if your droplets are too big to start with, your heat transfer is reduced, and the potential for mechanical damage to the condenser parts, especially the tubing is enhanced as the droplet size grows.

If your condenser were located in colder climates, you would see the effects of very wet steam during low load operations in the winter on condensers.  It is very damaging, although fairly localized in the area direcly below the turbine exhausts.  That smaller amount of wetter steam goes straight to the bundle, without spreading out through the complete condenser.

At least in your case, the exhaust steam is still having to spread out throughout the full length of the bundle to find a place to condense.

I mention that to encourage you to locate the spray nozzles in flow areas where the patterns are wider, and not directly below the exhausts in a straight line.

Remember that if you were to take this same amount of water to a side mounted barometric condenser, spray pattern type, and bring some of the exhaust steam over there, your water spray would be a solid cone or curtain of water for the steam to pass through to condense.

You are trying to do the same thing, but you don't have the luxury of being able to do it with a solid curtain of water due to potential condenser tube damage.

I hope this gives you some insight into what is going on in the scenario that you want to create.

rmw

RE: formulation of direct contact condensation

(OP)
Thanks again RMW,
Of course if I re-design the spray I'll do the same thing as you said. But now, my manager just want me to count whether this spray (with its 3 atm supply pressure) have enaugh momentum to attain the exhaust steam. The spray is about 15 cm long. What we affraid of is, this 3 atm supply pressure is not sufficient for the droplet, to reach the whole exhaust steam. What do you think about that?

Dwi Handoyo S

RE: formulation of direct contact condensation

If you have reservations about your ability to get a good spray that will do what you need it to do, let me give you another approach to the problem.

Some condensers are multi-pressure, and the condensate from the lower pressure shells has to be brought into the highest pressure shell and heated up to the hotwell saturation temperature to prevent oxygen saturation due to subcooling.

This is typically done with "rain" trays.  These are wide trough type trays with the bottom of the tray being made of perforated plate.  They are typically located in the lower areas of the condenser, below the bundle, where the "rain" drops directly into the hotwell.  These do the job they are designed to do.

In the process, they absorb heat from the surrounding steam, and hence, contribute to the condensing effort of the high pressure shell.

Not knowing what the geometry looks like in your shell, I can only suggest that you look and see if there is enough open area available for you to build such a tray system.  You will need to calculate the open area required for the water flow you have, and use lots of small holes in your perforated plate, rather than fewer larger ones.

If located directly below the bundle, remember that the condensate falling through the bundle has to flow through the 'rain tray' too.  Be sure to consider this in your hole calculation.

Such a tray will help prevent subcooling in the normal condensate, too.

The higher you can build your tray above the hotwell, giving more distance for the 'rain' to fall through, the better it will heat the make up stream.

Net effect wise, the condenser does not care where you absorb some of the duty that it is required to do, spray, tray or the next one.

One other simple thing that you can do, is put a distribution header, a pipe with an adequate number of holes drilled in it along its length for the fluid to pass without too much pressure drop along the top of the tube bundle, and allow the make up water to dribble down through the bundle, where it will pick up heat from the steam penetrating the bundle.

If you have a two pass condenser, meaning that the cold water flows in one end at the bottom, turns around, and flows back (normally at the upper level of the tube bundle, but not always, I have seen them both ways,) your colder water may just absorb heat from the hotter tubes, and contribute nothing to the duty.  However, if you do most of the distribution at the coldest end of whatever pass you are woking with, single or double, this should be negligable.

The cooler water, as it flows across the tube banks will attract steam, which is attracted to the coldest surface it can find, and will contribute to the duty of the condenser.

So there you have it.  Spray, which is what you initially inquired about, tray, which is commonly done for other reasons, or just spreading the make up over the bundle, which is also quite commonly done (principally to help with the deaeration of the make up.)

rmw

RE: formulation of direct contact condensation

(OP)
Great thanks RMW,
Just to make sure, the condition of my condenser, (also because of my limited ability :)), Do you know what is the phenomena behind the steam flow in the turbine and when entering the condenser? As I know, in the turbine section, there's a drop pressure and velocity of steam, but in the exhaust hood, I still don't know the phenomena in it.

Dwi Handoyo S

RE: formulation of direct contact condensation

I believe the original question was a simple request of "formulation for direct contact condensation process".

I believe the thermodanics would be called a "mixing condensor".  A text book example of how to calculate the spray flow required for a given output turbine can be found at http://www.pocketpe.com/Steamulator/steam_ex6.htm example #3 at the bottom of the page.

If you are familar with thermo, this example could be applied to your situation and allow the MW flow increase that could be achieved with what every flow you are now getting with the esisting nozzle.  (If I get some free time I might try to work it through)

Assuming you can measure the spray flow, possible a flange orifice will need to be added to allow measurement.

RE: formulation of direct contact condensation

I think the way to apply would be
1) determine the delta h for LP exhaust steam from vapor to liquid.  converting the 53C and 668hg into EU i get h of 1115 - 95 = 1020 h
2) determine the total process h.  this will be the mass flow (#/hr) for the turbine times the delta h = total h
3) the spray frow to provide 100% condensing would be total h divided by delta temp.  spray = total h / (127 - 92)
 ratio the spray you have with the calculated flow required for 100% condensing should give an ideal of the MW increase you could expect

RE: formulation of direct contact condensation

Semarang, I am not sure I exactly understand your question regarding the phenomena.  The steam is flowing from high pressue to low pressure.  The turbine is a machine that takes this pressure drop of the steam and gets some work out of it.

At the end of what the turbine can get out of the steam, this steam is still flowing from high pressure to low pressure, so the condenser pressure must be lower than the last stage of the turbine, or no steam would flow.

The condenser operates in a vacuum.  The phenomena is that a unit of steam, take a cubic meter, whatever that weighs at that temperature and pressure, (and I am not going to stop and do math here, just speak in generalities) when it condenses, the volume changes from a cubic meter to a teacup of liquid.  More steam immediately rushes into this void, vacuum, to fill it, and it, too condenses, and now two cubic meters of steam occupies the volume of two teacups of, say a good brand of tea.

So the condenser is creating the lowest pressure in the system, and the steam passing through the control valves of the turbine is flowing toward this low pressure zone.

In the exhaust hood you have some strange things going on, in that there are flow losses there, as well as in other parts of the system.  Vacuum gets too deep, and the flow losses get high.  Vacuum gets too high, and the exhaust doesn't like that either.  Vacuum needs to be maintained in a specific range.  Some turbine OEM's furnish a set of curves with the information kit that shows this range.

Do you know what a Mollier diagram is??  If you do, plot your conditions on a Mollier diagram.  You can see your flow line, plot your turbine efficiency from the slope of the expansion line, and see where it enters the condenser.  Watch out, because the very final part of the expansion line is not straight.  There is a kind of a "j" hook at the end that shows some of the flow losses in the exhaust hoods.  At least in the turbine, the flow is producing work, but alas, in the hood, it is turning in a direction towards adiabatic.  It condenses before it makes much of a turn, however, but it makes the beginning.

One other thing, the flow in this area is very turbulent, because it exits the turbine with a swirl or twisting motion, and has to transition to straight line flow towards the condenser.  In an older condenser, one does not have to ask what direction the turbine is turning, it is obvious from the wear patterns in the condenser.  One side wears more than the other, based on the direction of rotation.

I hope this explains what is going on for you at the outlet of your turbine.

rmw

RE: formulation of direct contact condensation

Sorry to be posting in short segments (as I get time).  But continuing with the equation for condensing turbine exhaust with sprays, the ratio of mass flow (steam) through the LP to the required cooling spray flow will be 30:1 (assuming perfect spray pattens and contact time) 1020/(127-92)~30.  

I am not familar with the exhaust sprays for your turbine, but i know the sprays for GE/LSTG units are only 13,000 #/hr (~3atm pressure)for a unit with 1,100,000 #/hr flow per hood (approx 200MW).  This spray flow rate could only improve LP exhuast flow by 13,000/(1,100,00X30) less than 0.05%

The nozzles for this type unit produce a fine mist and cone pattern with only 3 atm pressure, but there are only 4 per ring.

Therfore the amount of spray flow and its temp would be very important.

I was waiting for someone that would be stronger in thermo and heat transfer to make this point, but any way, there is my 2cents

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