Dew point temp in CDA

You can not control what you do not measure.

Dryers, seperators and everything are not magic. They all have constraints and limitations. If you put enough moisture in you will eventually cease to get CDA out.

Yes, an increase in dewpoint can indicate a problem with a compressor and/or aftercooler systems.

Liquid water is responsible for billions of dollars damage to mechanical and electronic systems each year. Therefore, determining the temperature at which liquid water condenses out of your compressed air can be an important indicator of potential damage. The ability to measure the dew point (water concentration) of a gas is the first step toward adjusing and controlling it.

If you have limits of moisture that are acceptable in your system requirements. The dew point temp is a quick way to tell if you are meeting the requirements. It will also let you know if your drier is working properly.

The amount of moisture the air will hold is relative to its temperature. Air that saturated with moisture (100% relative humidity) is a lot drier at 45 degrees than at 100 degrees. The air coming from the compressor may reach 250 degrees so will take more water to reach its saturation point.

At any given temperature we know approximately how much water, air can hold at saturation. Other factors like pressure and elevation change the amount.

Air that saturated with moisture (100% relative humidity) is a lot drier at 45 degrees than at 100 degrees

It is SATURATED, not drier!!

It contains les water at 45 deg than at 100 deg

I don't know if the other folks have answered your question, but let me expand.

The water content of compressed air is important, depending on the process. In some processes such as semiconductor manufacture, the air has to be extremely dry or it will ruin the wafers. The specification in this application is a dewpoint of -100C or drier, and it is critically maintained to be within spec. In other places where air is simply used to activate valves etc., the water content is not as critical, as long as it does not condense in the pipes etc. Typical specifications here are for a dewpoint of -40C or better, but it is often not controlled that well.

Perhaps you are asking why is dewpoint measured? (the point at which water condenses). In fact, dewpoint is very rarely measured directly. Often electrolytic cells are used to measure the water content, and Dewpoint is calculated and reported, as a common unit. Just as feet, meters and furlongs are all units of length and can be used interchangeably, water content can be reported as dewpoint, ppm, mg/m3 etc.

Finally, dewpoint has nothing to do with the compressor. The compressor simply compresses whatever air it is taking in. The dewpoint is controlled by driers after the compressor. High dewpoints indicate that these items are not working correctly.

Tad

Tad,

A blown gasket on a water cooled compressor can have an enormous impact on the amount of moisture in the system.

Because___

If water condenses on cooling of the air, then this condensate will cause clogging up Nozzles in the pneumatic controllers. Or even freeze up at flapper/nozzles.

"....can anyone here share why it is important to measure the dew point temperature in compressors which provide CDA to factories (utility air to drive pneumatics, etc.)?"

From a practical stand point the air leaving the compressor after cooler should have as low a dew point as possible to minimize the load on the air drying units. Sizing of the driers was contingent on a specified d.pt. If that is exceeded the driers will be unable to provide the dry air expected in the plant distribution system.

Thanks for all the responses. It is now clearer to me. I guess I have to whip out the old psychometric chart (did I spell that right) to find out the actual moisture content. Thanks!

As I recall from my old oilfield days, condensation in a compressed pipeline requires horsepower to move, that horsepower comes from the compressor which makes it less efficient and eventually the pipeline pressure will go down (lack of horsepower) which means less differential at the outlet end, which translates to less flow. Thats why you run "PIGS" through pipelines to flush (push) the condensation out of the line.

While we are on the topic...

I can use a psychrometric chart to determine %RH at the dry bulb temperature, but I don't know how to convert %RH at one temp to %RH at a different temp (when the air is heated of cooled). I have found sites on the web that will make the conversion for me, but is there a chart for this? I would like to be able to make the conversions myself.

Bill Morrow

In given temp (50dgr) you know the water amount of saturated air (100%RH) (1kg)

If you have 50%RH you have the half. (0.5kg)

If you heat up this air (80dgr) with 0.5 kg of water, again you know the water amount of saturated air (100%) (2kg)

You divide 0.5/2=0.25%RH

Ah, yes.

Using psycrometric chart;

87 deg. F and 34 %RH --> humidity ratio of 64 grams H20/pound dry air

then,

64 gm./Lb and 70 deg. F --> 58 %RM

Thanks!

Bill Morrow

Oops! Make that grains, not grams.

Bill Morrow

you ara welcom

hope i was a help

apologizing for my poor english