Gas spray cooling temperature control 2

[kpar]

I am cooling gas in a bypass around a CO Boiler from 1200 deg F to about 600 deg F using water sprays. I am looking for a way to calculate the evaporation time in order to locate the downstream temperature sensor for the temperature control which will adjust the water rate. Any rules of thumb would be helpful too.

 

[Fizzhead]

kpar:

Biggest variable here is the type of nozzle you and use, the pressure drop across it and the nozzle arrangement (i.e., whether or not you have a hose stuck into the side of the duct, or a bank of fog nozzles. However at those temperatures, it won't take long to flash off the water, assuming you have even a halfway decent spray setup. I would suggest you do a quick heat transfer calculation for a representative droplet in your gas stream. Bet you find total flash time is about 1.5 - 2.5 seconds.

 

[25362]

kpar: Fizzhead is correct.

There is one formula I found for the approximate time, t, a droplet takes to evaporate in stagnant surroundings:

t=([λ])([ρ][sub]L[/sub])(D[sup]2[/sup])/[8 (k[sub]g[/sub])(LMTD)]​

[λ]: latent heat of evaporation
[ρ][sub]L[/sub]: liquid density
D: original drop diameter
k[sub]g[/sub]: gas thermal conductivity
LMTD: logarithmic temperature difference


Let's work out an example with rough figures:

[λ]: 2,400 kJ/kg
[ρ][sub]L[/sub]: 1000 kg/m[sup]3[/sup]
D: 300 microns = 0.0003 m
k[sub]g[/sub]: 0.05 W/(m.K)
LMTD: 700 K

t=(2,400,000)(1000)(0.0003[sup]2[/sup])/[8(0.05)(700)]~0.8 s

Estimates as this, may enable one to see whether at the prevailing gas velocities there is enough residence time to accomplish full evaporation and the desired gas quenching.