Moisture Content in Compressed Air

Good luck....

http://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html

That set of equations and graphs may tell you the water vapor density, so I guess you are supposed to figure out the mass content from the ideal gas law or other equation of state for water vapor, where the partial pressure is known in a control volume?

Somewhere, I saw tables that reveal the saturation moisture content in g/m³. Try:

http://www.fstweb.de/Water-vapour-content-table.170.0.html?&L=1 and then use the relative humidity to get an idea of how much mass is present.

Example: at 30°C, 30.078 g/m³ represents 100% humidity. If the ambient humidity is 45%, then moisture content I estimate to be 13.5 g/m³ . Seems rather straightforward to me, until another "guru" corrects me on that.

Since the air dryers discharge some condensed water, I suppose you could try to collect this, but I expect the air behind it will spray it out of the container. There are moisture detectors for compressed air. Get one.

I would think it should be measured, not just calculated.

Well, it would rather depend upon the atmospheric conditions, the pressure and the temperature of the air upstream of the compressor, which details haven't been stated in the original post.

Thanks , the atmospheric conditions are 30 Deg C, pressure at upstream is atmospheric

Oh, good. Follow #8 below for a Job Done Sticker.

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It will depend on how wet the air going into the compressor is.

Is that desiccant dryer on the suction or HP side of the compressor? How is that desiccant drier reactivated?

Calculating...not sure. Measurement would be the better bet.

The weight of the water that you put into the compressor will be equal to the weight of the water that you get out or the compressor. Take the tables for relative humidity, temperature, volume of air and calculate it.

The performance of the desiccant dryer is dependent on several things. There should be a performance curve by the manufacturer that will show you its efficiency. Also put the refrigerated dryer before the desiccant dryer. It will extend the cycle of the desiccant dryer so that it doesn't have to be regenerated as often. This will also reduce the moisture in the compressed air.

Combine the two and you have your answer. Better answer could be obtained if more information was given in original posting. Such as what is the volume of air being handled?

Good Luck, Old Salt

Concur with you on the order of placement for the 2 drying processes. That's why I asked where the desiccant doohickey was actually located.

Where's that OP?

Probably drying out somewhere...

Wal-

Good questions! Better answers if more information was given.

The placement of the desiccant dryer should be in the discharge line as the final treatment after a filter, a automatic condensate drain, the refrigeration dryer and then finally the desiccant dryer. If the drying is done on the inlet you have to use a lot of drying power to get the water vapors out of the air.

When the drying is done after the compressor, the compression of the air/water vapor makes much of the water vapor turn to liquid because of the increase in pressure. That pressurized water condensate is easy to reduce by simply using an automatic condensate drain, a simple mechanic means. It is a unit that uses no energy and will do a better job than the desiccant dryer on the inlet and need no regeneration.

It would also be prudent to locate other condensate drains within the system piping at appropriate locations. With this, if there was a dryer failure the moisture that got into the system could be removed automatically instead of having to do it manually or left in the piping where it could possibly do damage to other equipment.

Good Luck, Old Salt

Do it the same way as any Process Engineer would: carry out a mass balance between all the inlet streams and all the outlet streams.

Simples!

As the water vapour in the atmosphere is not likely to be below 12.5%RH you can assume the air when compressed to 7 bar and over will be fully saturated at 100%RH

You need to look up water density v temperature tables for the exact amounts.

When leaving the compressor at 40°C, the air will hold 51 g/m³ plus any surplus water as a liquid. 40°C is hot - you need an after-cooler to protect the fridge dryer.

(Borrowed from Wiki) T = temp in K. M = molar mass = 18 for water.

ppm = mg/m³ x 0.08205 x T/M

Therefore 51 g/m³ is 73,000 ppm at 40°C (in round figures)(watch the base units)

Generally speaking, ppm is not much use in everyday practical terms.

Dropping the temperature to 10°C reduces the vapour to 9 g/m³.

Leaving 1 g/m³ at -20°C when the desiccant does it's job.

Since this is a calculation it's on paper. The lowest temperature is -20C which is the ideal working specs of the desiccant dryer. If you look up the % of moisture in tables of dew points. You will find that the moisture content is 0.1%. Percent is just parts per hundred so an easy conversion to ppm. 1000 ppm

This figure is only good on paper. It has no real value in a working system. To many variables and nothing works ideally. If you want to an accurate measure you have to test the air.