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

 

[rmw]

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

 

[rmw]

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

 

[semarang]

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

 

[rmw]

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

 

[semarang]

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

 

[rmw]

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

 

[semarang]

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

 

[byrdj]

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.

 

[byrdj]

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

 

[rmw]

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

 

[byrdj]

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