patrickraj:
You request us to clarify the differences you get and ask what is wrong in your calculation. As a professional engineer, I'm going to assume you mean what you ask and I'll take it from there.
1) Reason for the difference:
This could be, as TD2K indicates, that you trying to dot the i's and cross the t's on accuracy when there is no justification to do so. Your data sheet was filled in by another engineer that may have been using other basic data. Additionally, assuming you are basing yourself on John Dalton's famous Law, you assume that the partial pressure fraction is equal to the mole fraction of a component in a mixture (water, in this case). Water vapor is regarded as and ideal gas for the purpose of Dalton's theory, i.e. water molecules act independently from each other in the air mixture. In reality there is a small interaction between molecules which leads to a small increase of saturation vapor in air. As the system pressure increases, the effect of the compressibility factor also increases. Other factors to consider are found below, in 2).
2) Contrary to what TD2K states, I find your calculations flawed and poorly documented - resulting, as usually is the case in such work, in errors. Assuming you'll accept well-intentioned, constructive criticism, lets look at the flaws:
a) You state that you are checking the air moisture content in the air compressor SECOND stage intercooler; yet, you take basic data related to the FIRST stage. Obviously, something is wrong here. I'll assume you just got your stages crossed-up for now.
b) You cite something called "LN (VP)" without identifying what it is or where you obtained (or based) the relationship. This is bad engineering. No responsible supervisor will accept this type of documentation. Sorry, but that is the basic law in engineering: document, document.
c) Your vapor pressure value for water at 42 oC is off by 4.5% (1.1907 psia) according to the NIST website information - which is my documented source. NIST = National Institute of Standards & Technology (USA).
d) You say that the partial pressure of water vapor = 33.6 psia; I believe this is wrong. The partial pressure of the water vapor is the same as its vapor pressure - 1.1907 psia. I think the partial pressure of the air is 33.6 psia
e) I believe Dalton's Law states that the partial pressures in a mixture are cummulative to the total system pressure. That means the partial pressure fraction of water = 1.1907/34.84 = 0.03417 and not 0.03702 as you note. The important point here is applying Dalton's Law and differentiating between the different values and why.
f) If you really wanted accuracy (since you note the answer down to the first decimal place), you would use the molecular weights of air and water as 29.964 and 18.015. Otherwise, you'll get an estimated answer -- which is OK.
On a more practical and less theoretical level, your answer is close enough for engineering purposes - in my opinion. What I object to is the wrong employment of Dalton's great work and not documenting your calculations the way they should be. Engineering answers are worthless if they are not backed by credible references or sources. I don't care a hoot what the calculator or the computer spit out. The answer is only as valid as the logic and engineering prowess of the engineer. And if this isn't clearly denoted in the calculations, I can't tell if it exists. I didn't want to go into details, but you asked what was wrong with your calculation and I did the best I could.
Regards,
Art Montemayor
Spring, TX
