calculation evaporation rate of hot water

[paran]

i need to calculate roughly the hot water evaporation rate from open top vessel (mm/day).
known data:
water temp is 70 degree cel.
aversge wind speed is 2 meter/sec.
aversge air humidity is 50 %.

 

[methaneman]

I am thinking that many of the equations you need are in the example about HCl evaporation which can be found on a website about Aloha.

 

[owg]

Irving Langmuir developed a way to measure vapor pressure
by measuring the evaporation rate, which we will now flip around backwards. His reasoning is that the rate at which molecues are lost due to evaporation to a gas with no partial pressure of the evaporating substance is the same as the rate at which molecules of the substance would hit the surface if it were in equilibrium with the vapor (because in equilibrium the evaporation rate and re-condensation rate cancel each other out). His expression is:

(mass loss rate)/(unit area) = (vapor pressure - ambient partial pressure)*sqrt( (molecular weight)/(2*pi*R*T) )

(from Zemansky and Dittman, Heat and Thermodynamics, McGraw Hill, copyright dates from 1937 to 1981 in my copy).

Here is the link for more.

http://van.hep.uiuc.edu/van/qa/sect...Evaporating_and_Condensing/20020321122324.htm

HAZOP at

http://www.curryhydrocarbons.ca

 

[25362]

Solar radiation, geometry of the open top vessel, and the progressive cooling of the hot water batch also are factors in the estimation of time of evaporation.

 

[mrtangent]

Curry hydrocarbons, hold on a minute.

you forumla for the chaps case is
mass loss rate)/(unit area) = (vapor pressure - ambient partial pressure)*sqrt( (molecular weight)/(2*pi*R*T) )

at 70C P vap for water is 0.3 bara. so working through
(0.3 - 1) * ( 18 / 2* 3.142 * R * 293)
.. gives a negative number. So unless I've miss understood your method here you cannot use this formula.

 

[25362]

To james1030bruce, as I see it, owg's formula (right or wrong) speaks of ambient "partial" pressure (meaning the vol% of moisture in atmospheric air). Thus, there is no negative result.

 

[owg]

Ambient partial pressure of water in 50% relative humidity air is 12.79 mm Hg or about 0.017 bara, so the answer should work out OK.

HAZOP at

http://www.curryhydrocarbons.ca

 

[Nkhurd]

Hi,

http://response.restoration.noaa.gov/cameo/hcl.pdf

This paper documents the procedure to calculate the evaporation rate from a pool of liquid.

NK.

 

[siretb]

check

http://www.cheresources.com

for tables

for no wind the tables give about 1200 Btu/h/ft2 (about 3700W/m2
for 10 mph they say figures could nearly double.