8 replies to this topic

[Venkat @89]

Hello everyone,

 

 

Generally any liquid that converts into vapor will happen when it reach its boiling point corresponding to its external pressure. the latent heat of vaporization for vaporization  depends on the saturation pressure or saturation temperature at which it is boiling.

 

But in the case of water evaporating in to air, which happens for water below its saturation temperature at the atmospheric pressure.

 

I would like to know, with respect to what pressure the Latent heat of vaporization of water is to be taken when it evaporates into air below its boiling point of 100 degree centigrade?

 

 Latent heat of vaporization is 2257 KJ/kg at atmospheric pressure. Thank you.

[StealthProg]

Its the difference between the enthalpy of vapour and the enthalpy of liquid at the conditions of interest, typically steam tables should have that information.

[MrShorty]

Almost all the correlations for heat of vaporization that I have seen give latent heat as a function of temperature. Naturally one can also talk about the vapor pressure equation which relates temperature and saturation pressure, but I think it is easiest to think of these things as being functions of temperature first. Given the temperature, you can calculate the partial pressure of water in air, and the corresponding heat of vaporization.

Edited by MrShorty, 29 December 2014 - 10:39 AM.

[shan]

Contrast to your statement, generally, vapor and liquid always co-exist toward VLE (Vapor Liquid Equilibrium).  The chemical component volatility is measured by vapor/liquid distribution ratio Ki = yi/xi (yi = i component mole fraction in vapor and xi = I component mole fraction in liquid.

 

You may figure out water latent heat at the different air temperature by the enthalpy difference of wet air and dry air from Psychrometric Chart.

[StealthProg]

Almost all the correlations for heat of vaporization that I have seen give latent heat as a function of pressure. Naturally one can also talk about the vapor pressure equation which relates temperature and saturation pressure, but I think it is easiest to think of these things as being functions of temperature first. Given the temperature, you can calculate the partial pressure of water in air, and the corresponding heat of vaporization.

Its actually a function of both reduced pressure and reduced temperature. The closer you get to supercritical conditions the smaller the heat of vaporisation gets.

[StealthProg]

Contrast to your statement, generally, vapor and liquid always co-exist toward VLE (Vapor Liquid Equilibrium).  The chemical component volatility is measured by vapor/liquid distribution ratio Ki = yi/xi (yi = i component mole fraction in vapor and xi = I component mole fraction in liquid.
 
You may figure out water latent heat at the different air temperature by the enthalpy difference of wet air and dry air from Psychrometric Chart.

No offence but that has to be the most complicated way ever invented to avoid using some basic steam tables...

[shan]

Steam Table just covers properties of saturated and superheated steam.  I have no way to read the enthalpy for 0 psig, 80 F water vapor from 1967 ASME Steam Table, which is widely referred by the industry.  Probably, there are some extensive "Steam Table" that may be out of my knowledge.

 

If you have access to process simulator such Hysys, you may obtain the steam latent heat you want easily.  For example, you want to 0 psig, 80 F water/vapor latent heat.  (1)  Specify an air stream 1000 lb/hr at 0 psig, 80F  (2) Specify water stream 1 lb/hr at 0 psig, 80F.  (3) Combine the air stream and water stream together.  You should have a wet air stream at the temperature less than 80 F.  (4) Heat the combined wet air stream temperature back to 80 F.  The heater duty (1057 btu/hr) is the water/vapor latent heat (but/hr) at 0 psig, 80F.

 

No offence, but facts. 

[Profe]

Hi Chalapathi

 

Attached you find an Excel file that gives the thermodinamic water properties.

Study the file and you can obtain the Latent heat of vaporization at diferent conditions, pressure, temperature.

The first tab "Hoja 1" gives a brief explanation of the workbook

 

In the tab Properties you find 4 tables:

1. Saturation properties given temperature
2. Saturation properties given pressure
3. Properties given pressure and temperature
4. Properties given pressure and enthalpy

Study each sections and go to the tab Functions, this tab gives the functions used for calculates the properties

The last tab y prepared for your question about" Latent heat of vaporization" at diferent temperatures at atmospheric pressure.

Study this spreadsheet for the properties

If you are excelent with Excel the job will be easy.

 

Good luck

Fausto

Attached Files

•  XSteam_Excel_v2.4.xls   438KB   133 downloads

[Venkat @89]

Thank you all for the best responses.