dew point at higher pressures

[honche725]

i want to get the dew point of compressed air. how does pressure (e.g. compressed air is at 3 barg) affect the dew point of the original air?

is there a formula or chart that i can use? the only data i have are dry bulb temperature and %RH. the charts and tables at perry's is only up to about 1.5 bars.

thanks.

 

[quark]

Case 1 - Refrigerated Air Dryer

You already know the outlet temperature of the compressed air(this is what is called as pressure dew point). Go to the water TD property table and get the vapor pressure value corresponding to this temperature. The pressure dew point is a useless value if your process ultimately expands air to atmospheric pressure (as in most of the applications). Now devide the vapor pressure by the ratio of absolute pressures (i.e absolute pressure of the compressed air to atmospheric pressure) and read corresponding dew point for this vapor pressure. This is the atmospheric dew point of air, which means there will be condensation if the ambient temperature(or air temperature) falls below this temperature.

Case2 - Desiccant Dryers

Get the humidity ratio corresponding to your DBT and RH. This remains same throughout compression process. Now you know the capacity of your dryer. Deduct the moisture content that was separated from the initial condition and you will get the new humidity ratio. Read the corresponding dew point temperature from TD properties of water at saturation. If you want accurate results, you should deduct the moisture separated by intercooler, after cooler and air receiver.

 

[hacksaw]

Lets drop back a bit. Unless you are using a dessicant dryer, the dew point(temperature) of compressed air(air+water), at equilibrium, is its temperature.

So at the exit of a large reciever the air is saturated. The water collected at the bottom insures this.

As soon as you depressurize the air, the absolute humiditiy (the amount of water)stays the same, but the relative humidity and the dew point drop (apart from carry over).

 

[Mr2jz]

A simple calculator can be downloaded at

http://www.us.michell-instruments.com/support/downloads/

that will be easy to play with some numbers to see the pressure effect is - the higher you pressurize a gas with water vapor the higher the dew point.