Complete vaporization time of water into superheated steam - DESUPERHEATER

[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 which is injected trough a nozzle into a continuous flow of steam in superheated condition. The position (and than the time) at which I'm sure that there aren't water droplets is really important, 'cause I've to choose the position of a thermocouple which 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, i have to consider the water flow according to a continuous or discrete approach? And if discrete...how i could 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?

 

[anorlunda]

ping @Chestermiller , @jrmichler This is a spring cleaning post from 2010. It sounds like an interesting question. Thermo, but not thermo because it would have to do with drop size distribution and velocity.

Perhaps he could estimate the worst case with the largest drop moving at the highest velocity.

 

[anorlunda]

Whoops, ping @russ_watters also.

 

[Chestermiller]

It seems to me that there is not enough information provided to suggest a reasonable approach to this. There isn't even an indication of whether the flow is horizontal or vertical, and whether the drops are large enough for gravity to play a role. There is also no indication of the ratio of the inlet flow rates or the degree of superheating and/or subcooling.

 

[jrmichler]

A desuperheater sprays water into a steam line to reduce the steam temperature down to somewhat above the saturation temperature. Typically (and ideally), the water is condensate at 90 to 100 deg C. Normally the steam velocity is high enough, and the droplet size small enough, to carry the water droplets in suspension. The water flow rate is controlled by the temperature of the desuperheated steam, hence the need to measure the steam temperature. The temperature sensor needs to be far enough downstream that the water is all evaporated, but not so far as to create a time lag in the control system. The desuperheater systems that I worked with had water flow rates from near zero with the steam turbine running, to quite high when the steam turbine was down.

If you really want to calculate droplet evaporation time, the spray nozzle manufacturer can supply droplet size information. The temperature difference will decrease as the droplets get smaller. The droplet size will change with water flow rate. Attempting to calculate the evaporation distance can easily turn into an exercise of paralysis by analysis because of the number of variables and unknowns.

A much easier approach is to find a desuperheater system of similar size and capacity, then ask how far downstream they put their temperature sensor. That approach will almost certainly give better results than a theoretical calculation.