Psychrometric ratio formula and units
Psychrometric ratio formula and units
(OP)
What is the usual formula for the "psychrometric ratio"?
I find two versions in on-line documents:
h ttp://www. geocities. com/drying _guru/chap ter1fundam entals.pdf
r = h / (M_air k_y)
h ttp://www. probec.org /fileuploa ds/fl12033 6971099294 500CHAP12_ Dryers.pdf
page 12-3: r = h_c /( k c_s)
The latter seems more usual to me, but the units for k seem to be
"mass / (area * time)".
But these units differ from what I find in Wikipedia here:
http ://en.wiki pedia.org/ wiki/Mass_ transfer_c oefficient
What is the "correct" formula for "psychrometric ratio" and what are the units in its factors?
Thanks,
Yittri
I find two versions in on-line documents:
h
r = h / (M_air k_y)
h
page 12-3: r = h_c /( k c_s)
The latter seems more usual to me, but the units for k seem to be
"mass / (area * time)".
But these units differ from what I find in Wikipedia here:
http
What is the "correct" formula for "psychrometric ratio" and what are the units in its factors?
Thanks,
Yittri





RE: Psychrometric ratio formula and units
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Psychrometric ratio formula and units
http://en.wikipedia.org/wiki/Psychrometrics
is like the latter of the two in the OP.
And it also has k with units of "mass / (area * time)".
This document
h
Gives the units of k_y as (kg /( m^2 s)).
The Wiki article on Mass_transfer_coefficient is pretty lean. It gives the units for k_c, not k_y. From what I read in an old chemical engineering reference book on "unit operations", 'c' is for concentration and 'y' is for mole-fraction.
Perhaps more pedantic units for k_y might be "mass / (area * time * mole_fraction)", where mole_fraction is a dimensionless ratio?
This page
http://www
defines k_y in terms of humidity ratio.
So it seems that pedantic units for k_y would include some dimensionless ratio, but they reduce to "mass / (area * time)".
The units for the first formula in the OP seem odd:
r = (Power / (area * temp) / ((mass/mole) * (mass / (area * time))
which seems to be, in metric units:
r= mole-m^2 / (kg s^2 K)
There seems to be some controversy about what label applies to which formula:
h
"Keey (1992) takes great care to distinguish between the psychrometric coefficient ?, which relates overall heat and mass transfer and hence the adiabatic saturation and wet-bulb temperatures, and the psychrometric ratio ? as defined by Wilke and Wasan (1965)"
The Wikipedia statement suggests that a dimensionless number is in view there:
http://en.wikipedia.org/wiki/Psychrometrics
"The psychrometric ratio of air-water vapor mixtures is approximately unity"
That would seem to exclude the first version in the OP, with the weird units noted above.
So perhaps the formula here
http://en.wikipedia.org/wiki/Psychrometrics
is the most accepted one?
Thanks,
Yittri
RE: Psychrometric ratio formula and units
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Psychrometric ratio formula and units
Which of the two formulae in the OP do you count as defining the psychrometric ratio? Why that one? Or do you use some other definition?
Thanks,
Yittri