Desuperheater - Vaporization time of water into superheated steam

[SimonTecnoil]

Hi to all....i've a problem. I have to calculate the position and the time of complete evaporation of a continuous water flow wich is injected trough a nozzle into a continuous flow of steam in superheated condition. The position (and than the time) at wich i'm sure that there aren't water droplets is really important, 'cause i've to choose the position of a thermocouple wich detects the effetc in temperature of the desuperheating stage. Clearly, the measurement of the thermocouple is wrong if is reached by water droplets.

I know all the macroscopic thermodynamic conditions of the system.

In your opinion, have i to consider the water flow according to a continuous or discrete approach? And if discrete....how could i manually calculate the diameter of the droplets of water?

I don't want to use CFD!!!!

Could someone suggest a theoretic approach to the problem???

 

[unclesyd]

You have picked a very complex subject to post a question about. The time factor in such a system as you have is always a big question mark. I have seen temperature measurements that have shown complete vaporization within 5 pipe diameters of the water injection point and some where there was no indication of any loss of superheat at 15 diameters. Her some papers on water droplets and in various and sundry environments.

http://fire.nist.gov/bfrlpubs/fire93/PDF/f93123.pdf

http://rspa.royalsocietypublishing.org/content/462/2074/3115.full.pdf

http://www.uoregon.edu/~linke/papers/Walker_leidenfrost_essay.pdf

http://iufost.edpsciences.org/articles/iufost/pdf/2006/01/iufost06000336.pdf

Addenda:

We have a HRSG system where we inject water mist to cool to cool the 3100°F process gas stream. The initial waters spray heads operated at 150 psig. We changed part of the system to spray heads where we used 1200 psig water and nozzles to give much finer droplets. During the testing we saw evidence of water impinging on the opposite wall 6 ft away. We had to change the nozzles and lower the pressure to around 500 psig to get better gas/water contact. The hot gas system is at 15 inches of water.

 

[athomas236]

I am not sure if there is a satisfactory theoretical method of calculation.

Suggest you talk to suppliers of desuperheaters.

Regards,

athomas236

 

[davefitz]

From memory,

a) older rules of thumb for boiler spray attemporators recommended a stragiht pipe length downstream of the spray nozzle to allow a 0.35 sec steam transit time for "complete " evaporation .

b) the time for the water to evaporate is related to the droplet diameter, the water's degree of subcooling, and the water's surface tension at satrrated conditions ( which is based on operating pressure at the nozzle discharge ). To make a long story short, it is a good idea to use a high quality spray nozzle with wide turndown ( modern nozzles are spring loaded) and source the water from a location that is just slightly cooler than saturated conditions.

c) If you wanted to conduct a theoretical analysis, I believe the process of these droplets evaporating is exactly the same as the non-equilibrium evaporation of droplets inside tubes that are boiling in a furnace waterwall- for that theoretical anlysis, see the tech articles by Siemens KWU engineers Keifer, Kohler and Hein as published in the journal "intl j of multiphase flow" time period 1981-1994.

 

[rmw]

I was just reading on Copes Vulcan's site today that their VO-76 desuperheater completes evaporation before the steam leaves the DSH body.

rmw

 

[unclesyd]

Komax gives essentially the same information on distance.

http://www.komax.com/products/desuperheater.html

 

[davefitz]

I think you are going to have to carefully believe or interpret a spray vendor's claim with a few grains of salt. The cases for which the spray may be completely evoporated before the mixtrue leaves the spray body mgith in fact be limited to a very small subset fo all the process conditions the spray will be subjected to. For example, it may only be for cases where the available DP across teh spray nozzle is greater than 200 psid .AND. the spray water is not more than 5 F subcooled.

There are a lot of recnet cases of HRSG spray attemporators causing un-evaporated slugs of water accumulating on the donwstrem elbow and then flowig to the next downstream bank of tubes, causing large tube to tube temperature unabalances, high thermal stresses,tube overheates, tube to header weld failures, and steam turbine bypass pipe water hammer failures.

I guess a lot depends on what your specific responsibility is in the design process. If you are working for an EPC or boiler OEM and have the freedom to blame all field failures on "operator error" , then you can pretend the spray vendors' claims are valid for all possible operating scenarios and get the paperwork off your desk pretty quick. On the other hand, if you actually need to make the process work, you will need to look at the process a little closer.

 

[rmw]

Gee, Dave, how is it that I get the feeling you have suffered greatly in this regard? (Said with the empathy of a fellow sufferer.)

rmw

 

[MJCronin]

unclesyd...

"3100°F process gas stream"..!!!!

WOW !!!!

-MJC

 

[unclesyd]

That was just luke compared to when it gets really hot.

In reality we like to operate around 2900°F and when it gets to 3100°F we have to cut back on the fuels along with increasing the water spray volume.

This is system that is called Thermal Reduction Unit where in reality it is an Oxidizer. Our process where we use HNO3 to oxidize Cyclohexanol to Adipic Acid generates a tremendous ammount of non=reoverable NOX. NOX is a very bad actor in more ways than you can think of so we have this unit were we add the NOX to swirling natural gas flame. Uncontrolled this would raise the temperature to around 3300°F. We add some control in the plenums by adding water. This reaction is relatively slow we gain residence time by adding two forty foot towers in series. Its at the base of tower number one where the two burners enter that we see these temperatures regularly.

Anecdotal:
My wife has a couple of earrings made from sapphires recovered from overheated insulation in this area.