18 replies to this topic

[abbaschemical]

How can we read psychrometric chart above atmospheric pressure?

I want to calculate the dew point of air at 2.85 KscG in an air compressor at its first discharge.

Suction conditions are:
Suction temperature : 28 C
Relative Humidity : 75 %.

I would like to know what would be the saturation temperature/dew point of air at the air compressor first stage discharge which is at 2.85 KscG ?

How to calculate it? Please help.

[Dacs]

I don't think (not that I know of anyway) of anyway short of using a psychometric chart for that given pressure.

If you have hysys installed, you can use the SATURATE extension.

Unfortunately, I don't have it and I did a very quick (and dirty) calculation and I'm getting around 22.9°C dew point for the given pressure (2.85 kg/cm2G).

Take the value with a grain of salt

[T.S.MURALI]

yOU KNOW THE TOTAL PRESSUE,PRESSURE FRACTION AND MOLE FRRACTION.

FROM CHART (BROWSE ) YOU CAN OBTAIN d.pT VS PPM/MOLE/VOL FRACTION.

ALSO REF. D.P CALCULATOR W.R.T R.H

[PaoloPemi]

I agree with T.S.MURALI,
calculate the water content (at 28C 75% relative humidity) from any Mollier chart or by calc's (very simple, water saturation pressure at 28C), you should obtain a value about 18 g/kg
knowing the water content you can estimate the dew point at 2.85 Kg/cm2 (please do not specify KscG -> Kilogram square cubic centimeter gauge) based on ideal gas law (water vapor pressure) you should obtain a value about 47 C

[Dacs]

yOU KNOW THE TOTAL PRESSUE,PRESSURE FRACTION AND MOLE FRRACTION.

FROM CHART (BROWSE ) YOU CAN OBTAIN d.pT VS PPM/MOLE/VOL FRACTION.

ALSO REF. D.P CALCULATOR W.R.T R.H

I think the question revolves on using the psychometric chart (which is usually based on atmospheric pressure) at the given pressure (which is above atm), in which IMHO can't be used other that the pressure the chart is based from.

I agree with T.S.MURALI,
calculate the water content (at 28C 75% relative humidity) from any Mollier chart or by calc's (very simple, water saturation pressure at 28C), you should obtain a value about 18 g/kg
knowing the water content you can estimate the dew point at 2.85 Kg/cm2 (please do not specify KscG -> Kilogram square cubic centimeter gauge) based on ideal gas law (water vapor pressure) you should obtain a value about 47 C

47°C does not make any sense since it implies that at 28°C your stream is already below dew point!

[PaoloPemi]

Dacs,
the original question

>Suction conditions are:
>Suction temperature : 28 C
>Relative Humidity : 75 %.

the suction pressure should be athmospheric i.e. 0 Kg/cm2 and temperature and relative humidity are specified at suction pressure

>would like to know what would be the saturation temperature/dew point of air at the air compressor first stage discharge which is at 2.85 KscG ?

a dew point about 47 C means that at discharge condition of 2.85 Kg/cm2 condensation begins (with a water content of 18 g/kg as said)

by the way if you compress air with usual polytropic efficiency, discharge temperature will be well above that temperature...

Edited by PaoloPemi, 17 October 2012 - 01:41 AM.

[Dacs]

Oh yes I got it

Yep my bad.

[abbaschemical]

thanks... but still i m not clear... please clarify ...

[Dacs]

Anyway, I did the crunching again and with that, I got around 46.7°C dew point at 2.85 kg/cm2G.

[abbaschemical]

Mr.Dacs , may i know how u calculated it ? formula or method involved?

[Dacs]

I used Hysys to calculate the needed value.

If you must know, I set up a stream of air with saturated water (at ATM/28°C), then obtained the water content (which corresponds to 100% RH), then setting up a new stream with water content corresponding to your desired RH (which is 75%) but at pressure of 2.85 kg/cm2G.

With that, I set the vapor fraction =1 (at dew point), and you can obtain the dew point.

[kkala]

In addition to above useful contributions, a detailed "manual" calculation may be also of interest:
1. Ingoing air is at atmospheric pressure, 28 oC and 75% RH (relative humidity). Steam tables (or other sources) indicate a saturation pressure of 0.037782 Bara at 28 oC. Thus water vapor pressure would be 0.037782 Bara in ingoing air, if it were saturated (RH=100%). But ingoing air has RH=75%, so partial water vapor pressure is actually 75%*0.037782 = 0.028337 Bara (by definition of RH).
2. Total pressure of ingoing air is 1 atma = 1.01325 Bara, of which 0.028337 Bara due to water vapor and 0.984193 Bara due to dry air. For ideal gases molar fraction equals partial pressure fraction, so water vapor in ingoing air is 0.028337/0.984193 = 0.028779 mol H2Og per mol dry air, or 0.028779/(1+0.028779) = 0.027974 mol H2Og per mol humid air.
3. Humid air at first stage discharge has pressure of 2.85 kgf/cm2 g = 3.80815 Bara. Molar composition is same as before (no condensation has occurred). So partial H2Og pressure is 3.80815*0.027974 = 0.10653 Bara (same principle as in para 2). Corresponding saturation steam temperature is 47.07 ~ 47 oC (from steam tables).
4. Therefore dew point at first stage discharge conditions is estimated at 47 oC.
Dacs and PaoloPemi arrived at practical same figure; present calculation seems to be same as PaoloPemi's (probably a bit more "manual"), with additional clarifications.
5. It is noted that ideal dry air / water vapor mixtures were assumed above, justified by low total pressures and small content of water vapor.
Ambient air pressure was assumed as 1 atma = 1.01325 Bara.
Above may have been too detailed, for purpose of clarity. Calculation is quite easy in practice.

Edited by kkala, 22 October 2012 - 04:38 AM.

[S.AHMAD]

1. To determine dew point at higher pressure, psy chart cannot be used.
2. Calculation by kkala is correct.
3. If u understand how to use Dalton's and Roult;s law for ideal gas mixture, you should be able to figure out the method by kkala

[breizh]

Using this resource I got 47.52 C .

http://www.vaisala.c...B210973EN-D.pdf

Hope this helps

Breizh

[PaoloPemi]

kkala,
yes the procedure is the same, I have not posted the details as I would presume that a professional engineer should know Mollier's charts, saturation pressure tables, ideal gas law etc. (differently there is the student's forum),
also I believe that the use of a process simulator can hurt the brain in some cases :-))))

[Dacs]

I have Hysys at hand so I feel more relaxed punching the numbers than looking at charts

Anyway, the important thing is one has to know the basis of the calculation, whether done by hand or by software.

[S.AHMAD]

also I believe that the use of a process simulator can hurt the brain in some cases :-))))

Fully aree with Paolo's comments above. For those engineers that cannot do calculation by hand should not use simulator. In addition hurting brain, it may tannage reputaion as a pro engineer. for any computer program "garbage in = garbage out". The result from simulator may be wrong and it is shameful to present a wrong result to management. Knowing the basis of calculation and be able to calculate by hand normally the engineer can easily detect whether the simulator result makes sense or not.

[abbaschemical]

Dear KKala,
The manual calculation done by me was same as you have reviewed and got the same result. Guidance from experts from you are worthy for all youngsters. This is a great platform....

[T.S.MURALI]

Psych chart gives water content at the standard conditions.Having known the molar volume of water and air , you

can always calculate at other conditions of P & T.

Thanks to Mr.PAOLO , for explaining.