5 replies to this topic

[rameshbeemani]

generally there are lot of online caliculators for PDP to ADP conversion but i want to know the manual conversion formula to convert PDP to ADP can any one help me out with this.

[MrShorty]

Can I safely assume that we are talking about water dew point in air? Even if not, the same basic procedure applies, though some of the assumptions I make will need to be revisited.

 

In essence, this problem is a VLE/thermodynamics problem. I don't know how much thermo/phase equilibria you've studied (obviously I can't teach the whole course here). The simplest mathematical expression for VLE calculations is Raoult's law P(i)=x(i)*P0(i) at a given T where P(i) is the partial pressure of i, x is the liquid mole fraction of i, and P0(i) is the pure component vapor pressure of i.

 

If we assume Raoult's law, here's how the equation goes for water in clean air (no other "condensables" present). Starting with the initial Dew point T0

 

PH2O(T0)=xH2O*P0H2O(T0). For water in air at its dew point, we'll assume that xH2O=1. Then we can compute the partial pressure of water at the initial dew point T from whatever vapor pressure equation or table or chart we are using.

 

With the partial pressure of water, we can now compute the vapor mole fraction of water y as y=PH2O(T0)/Ptotal(0). At this point, if your initial pressure has been given as a gauge pressure, be sure to convert it to absolute pressure before calculating y.

 

When we change the pressure on the gas, y stays the same. So we can now compute the partial pressure of water at the new condition from Dalton's law again. y=PH2O/Pnew -> PH2O(T1)=Pnew*y

 

Back to Raoult's law PH2O(T1)=xH2O*P0H2O(T1). Again assume xH2O=1 (at the dew point), then you can solve the vapor pressure equation (or use the table/chart) for water for the given partial pressure to find T1.

Edited by MrShorty, 30 January 2013 - 11:30 AM.

[narendrasony]

 Dew Point Calculation.xlsm   31.34KB   233 downloadsFind attached the calculation sheet for converting atmospheric RH to corresponding atmospheric dew point  and dew point at some other pressure. (Macros to be enabled).

 

Moles of H2O/mol of dry air remains same.Partial pressure of both water vapor and for dry air increase in proportion to the total pressure (absolute). So the new dew point is equal to the temperature at which vapor pressure is equal to the new partial pressure.

 

Regards

Narendra

   

Edited by narendrasony, 31 January 2013 - 06:50 AM.

[rameshbeemani]

thans for the replys but i want direct formula ie.  1 deg of pressure dew point is equal to how much deg of atmospheric dew point and vice versa at a standard pressure say 7 bar .

[MrShorty]

Using the procedure I gave (I expect narendrasony's formula reduces to the same thing), you should be able to derive an expression for T2=f(T1) using basic high school algebra. I'm not sure it is fair to ask us to do the algebra for you. We would be happy to review your work after to make sure you ended up with the right answer, or help with specific questions along the way if you get stuck. But we aren't fond of doing someone's "homework" for him/her.

 

The only thing I would recommend in doing this derivation is to pick a vapor pressure equation (like the Antoine equation) that can be solved explicitly for T. Vapor pressure equations like the Wagner equation cannot be solved explicitly for T, which will make it impossible to find an explicit function T2=f(T1).

[breizh]

http://www.uigi.com/WebPsycH.html

 

Let you consider this resource to validate your hand calculation.

Pierre