formulation of direct contact condensation 6

[semarang]

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

 

[athomas236]

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

 

[25362]

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 30^oC since that pressure corresponds to 25^oC.

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.

 

[rmw]

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

 

[semarang]

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

 

[rmw]

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

 

[semarang]

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

 

[rmw]

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

 

[semarang]

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

 

[byrdj]

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.

 

[rmw]

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

 

[byrdj]

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.

 

[rmw]

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

 

[byrdj]

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.

 

[rmw]

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

 

[semarang]

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

 

[rmw]

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

 

[semarang]

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.

 

[bigdog50]

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

 

[semarang]

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