Desuperheater - Vaporization time of water into superheated steam
Desuperheater - Vaporization time of water into superheated steam
(OP)
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???
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???





RE: Desuperheater - Vaporization time of water into superheated steam
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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.
RE: Desuperheater - Vaporization time of water into superheated steam
Suggest you talk to suppliers of desuperheaters.
Regards,
athomas236
RE: Desuperheater - Vaporization time of water into superheated steam
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.
RE: Desuperheater - Vaporization time of water into superheated steam
rmw
RE: Desuperheater - Vaporization time of water into superheated steam
http://www.komax.com/products/desuperheater.html
RE: Desuperheater - Vaporization time of water into superheated steam
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.
RE: Desuperheater - Vaporization time of water into superheated steam
rmw
RE: Desuperheater - Vaporization time of water into superheated steam
"3100°F process gas stream"..!!!!
WOW !!!!
-MJC
RE: Desuperheater - Vaporization time of water into superheated steam
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.