Pressure dew point question

[Soundside]

Hi, I'm a newbie and also not a Mech. Engineer. I have several questions about drying air. First, I'm looking for a better understanding of the term "pressure dew point". Is it just another way of stating the dewpoint "at some specified pressure"?

Also, when using refrigerated dryers, is there a practical limit to the dewpoint attainable through this process?

I'm also trying to understand the impact of relative humidity of drying. In my situation, I have a scenario with a stream of air which is at very low pressure (about 1 psig). I'm trying to understand how PDP specifications apply to my scenario. If the incoming air is at 75F, and it goes through a dryer that is specified to obtain 35 degree PDP at that pressure, by my calculations, that comes out to a RH of about 23%. Am I calculating this correctly?

I have another point of confusion on this. Does this PDP specification mean that the airstream would be cooled to some specific temperature?

The next question is, what if I then allow this air to vent to ambient atmosphere. Does the pressure drop then cause the effective RH to drop?

Thanks for any clarifications.

 

[jfriddell]

Soundside:

Your problem sounds like a psychrometrics problem. Anytime you are dealing with air and water, psychrometrics typically apply. If you know the incoming air RH, then with a psychrometric chart, you should be able to predict any other state, such as dew point temperature, enthalphy of the air, energy required to raise/lower the temperature, etc. Check an ASHRAE book or professional society if you do not have a psychrometric chart.

Jeremiah

 

[21121956]

Hello everybody:

Hi soundside.

Search in this site

http://www.delairindia.com,

push on LIBRARY, then select Seminar Material on Compressed Air Drying Systems.

Good luck ¡

 

[quark]

You are right. Pressure dew point is always specified with respect to a pressure. It is always prudent to ask for the value of atmospheric dew point, for you will expand the air, ultimately, to atmospheric pressure in most of the applications.

Refrigerated dryers have a limitation of-25C ADP and you have to for activated alumina or molecular sieves below that. Roughly, A PDP of +3C at 10 barg corresponds to -25C ADP.

PDP is just the temperature to which you lower the air temperature at a given pressure. This is simple to measure (by a thermometer, RTD etc) unlike ADP. When you correspond this to ADP then this gives you a threshold limit below which your air should not be cooled, if you want to eliminate condensation.

When you exhaust air, check what will be the resultant air temperature. If this is below the specified ADP then moisture will condense.