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# Air Dryer sizing - Correction factors for dewpoint

## Air Dryer sizing - Correction factors for dewpoint

(OP)
Greetings,

I'm posting this here, but I may as well have done it in the ChemE forum.

I tumbled into an old Excel calculation worksheet, used for sizing of clean dry air systems.

Our industry is semiconductor, and a pressure dew point of -40 degF is typical.

Basically I'm back checking the calculations and formulas, as the author of the spreadsheet did not think it useful to clarify how the calculations were made, where the formulas came from, etc.

So I'm looking at the "Dryer sizing" section and there is a field for "Flow Capacity corrected for flow @ 100 psig and dew point (if less than -40degF)". The in-cell formula is translated as such:
If dp is lower than -40, then flow x correction factor for pressure x 1.1 /0.8
Else flow x correction factor for pressure x 1.1

I've been looking and searching all over but have not been able yet to find a similar formula. Does it ring a bell to anyone? I get the flow x correction factor part. Just not the 0.8 and 1.1....
What is the impact of a different dewpoint on the flow capacity of the dryer and how do you account for it? Does it matter from a design standpoint? I would think the dryer needs to be able to handle whatever comes out of the compressor.

I design aqueducts in a parallel universe.

### RE: Air Dryer sizing - Correction factors for dewpoint

hello,

I dont fully understand the request here you say Air Dryer but you work in semiconductor.
and you need to calculate air with dew point near -40F right?

So look i got this wonderful website for quick calculation http://www.sugartech.co.za/psychro/index.php

But if you want deep calculation about it, I made my own calulator, essentially its for HVAC purpose but it uses either goff-gratch or arden buck equation i forgot.

here is arden buck equation link
https://en.wikipedia.org/wiki/Arden_Buck_equation
apparently its more precise than goff-gratch.

### RE: Air Dryer sizing - Correction factors for dewpoint

(OP)
Thank you, Marath007., for the links.

Unfortunately, it's not what I'm looking for.

The calculation calculates what the air dryer capacity should be when the dewpoint is lower than -40F.
-40F is a typical value for semiconductor clean dry air systems.

For example, if the dewpoint is -80F, and the design flow 220 SCFM, then the resulting flow capacity corrected for the dewpoint would be 252 CFM. See snip from calculation.

I must edit my statement from my first post:
If dp<-40, then flow / pressure correction factor / 0.8
If dp =-40, then flow / pressure correction factor

Correction factor is pulled out of a dryer table and is based on (pressure x 1.1). Result = 1.09

So my two questions are: where'd the 0.8 come from? Where'd the 1.1 come from?

Again, I'm just trying to validate this spreadsheet calc that has been used in my firm for many years. The engineer who developed it has left the company long time ago.

I design aqueducts in a parallel universe.

### RE: Air Dryer sizing - Correction factors for dewpoint

Well my guess is that the previous engineer added a small correction factor to remedy to the air properties change without going too deep.

From what i observe with a typical 70F dry bulb, 0F Dewpoint its 5% humidity while -40F dew point is 0.5% which is extremely low and the air properties are really essentially the same density/energy density nearly the same for a wide range of low dewpoint.

So my guess is that if the dry bulb is at -40F the density is drasticaly lower which would increase
I don't really understand the problem you need to solve but here is a link to SCFM to CFM with all the variable at play.

http://www.engineeringtoolbox.com/scfm-acfm-icfm-d...

Oh, and in semiconductor, what is the actual dry bulb for a -40F dew point?

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