If the system has a total Air Flow = 23,300 cfm @ 55o F
with Min. Outdoor Air = 3,725 cfm, And design = 10o F @ 60% R.H.
with return Air = 19,575 cfm, And design = 70o F @ 45% R.H.
With Max. Outdoor air design = 23,300 cfm at 10oF
From standard humidity tables we find that:
70o F @ 45% R.H. = 3.10 lbs. H2O per 100 cfm per Hr.
10o F @ 60% R.H. = 0.40 lbs H2O per 100 cfm per Hr.
Net difference to be supplied = 2.70 lbs H2O per 100 cfm per Hr.
Under Min. 3,725 Outdoor cfm/100 x 2.70 = 100.6 lbs. H2O per Hr. required.
For 55o F supply air @ 90% R.H. (maximum upper limit), at this condition the air
will hold 3.76 lbs. H2O per 100 cfm per Hr. We therefore can not attempt
to have the air hold more then this amount.
To do so, will cause moisture to fall out of the air stream.
19,575 cfm (return air) x 3.1 lbs. H2O/Hr./100 cfm = 606.8 lbs. H2O/Hr.
3,725 cfm (outside air) x 0.40 lbs. H2O/Hr/100 cfm = 0.40 lbs. H2O/Hr.
Moisture added by the Humdifier (load) = 100.6 lbs. H2O/Hr.
"See The National Resource Centers publications #29, #31and #37."
Total moisture which will be contained by the air = 722.4 lbs. H2O/Hr.
Which for the 23,300 cfm (total supply air) at 55o F = 722.4/23,300/100 = 3.1 lbs H20/100 cfm,
a value below the maximum the air could hold which was indicated above
as 3.76 lbs. H2O per 100 cfm. Therefore moisture fall out should not occur.
It is still advisable however to not install any filters, controls or interior duct insulation
within 15 feet of the duct humidifier.