moisture removal from compressed air

[honche725]

good day to everyone! i would like to ask you guys for help in removing moisture from our compressed air.

our compressed air is used for conveying pellet resins through the pipelines and into silos. it is a must that moisture is virtually eliminated from the compressed air. air is discharged from the compressor at 3.5 barg and at 220 deg C. compressed air goes through a shell-and-tube heat exchanger where its temperature is decreased to 165 deg C after which condensed water is removed in a moisture separation unit.

here are my queries:

1. i understand that in order to remove water, you have to condense it out by bringing it to a temperature equal or lower to its dew point at that pressure. how can i know the dew points of water vapor at various pressures? what really is pressure dew point temperature? is it the outlet temperature in the heat exchanger?

2. what are parameters that i need to know in order to see which part of the system is undersized or underperforming? do i need to know the original moisture content of ambient air and/or its relative humidity?

3. how do i know if the moisture separator is not functioning efficiently?

4. if one heat exchanger is not enough, will another heat exchanger help solve the problem?

thank you so much.

 

[ash9144]

Get psychrometric chart first. That will give you TP diagrams to know how much moisture you should have. It doesn't sound like your system is designed to provide dry air, just free of entrained liquid. If your mist eliminator is not performing you will have water droplets in the outlet stream. Check your low points for water.

 

[zdas04]

The psychrometric chart is good advice. What it will tell you is that as you raise the temperature and pressure of the gas in the compressor, the relative humidity will drop at every step. The mass of water vapor stays about the same, but the air's ability to hold the water vapor increases dramatically. It is pretty common for the air at the compressor discharge (before the aftercooler) to be 10-15% RH when it started at 60-70%RH.

You can cool the gas to ambient (at constant pressure) and it will probably go to 100% RH and have some condensation (that can be removed in a mechanical separator with a mist extractor). At that point you can add refrigeration to drop the temp to 0C and get some more condensation. Dropping the temperature farther will certainly cause the process to freeze.

Typically, acheiving dew points much below ambient temperature is done with a chemical process--either a mole seive, adsorbtion contactor, or a deliquiscent dryer. Trying to get really low dew points with mechanical/thermal means is very energy ineffecient.

David

 

[JLSeagull]

Define the moisture and other requirements. For instrument air we typically need clean, oil free air with a dew point around -40 degrees, lower in the arctic. Plant utility air can be much higher. Some equipment requires pneumatic lubricators. Different air requirements apply for different applications.

 

[toothless]

For every 20 F drop in air temp, you will cut your moisture in half (approx.). So the first and easiest thing to do is to drop your temperature even further. You should easily be able to get to 100 F using cooling tower water. That will give you 100% relative humidity air, though it will contain much less moister than your 329 F air. Since this air will likely be travelling in a pipeline at a temperature less than 100 F, and certainly less than 329 F, then more moisture will drop out in the line and be carried to your product.

A refrigerated air dryer can then drop the air temp to just above freezing removing a great deal more moisture. Then when you reheat the air to 80-90 F, generally via a precooler/reheater exchanger, the air will be superheated and any remaining moisture will be in the vapor form.