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Page 30
2008-10-01
B. CALCULATING THE DOWN STREAM HUMIDITY LEVEL
(1) Table 10 indicates the amount of water that each 1000 CFM of duct air holds in pounds
of water per hour at various combinations or air temperature and relative humidity.
Subtract the water vapour in the duct air before the steam distributor from the
maximum allowed after the distributor. The chart indicates how many lbs/hr of water
can be added to 1000 CFM of airflow.
Table 10. Water (lbs/hr) Contained in 1000 CFM of Air
(2) The following are two examples:
Example 1:
3000 CFM air handler with 60°F and 30% rh air before the steam distributor.
Calculate the maximum rate of steam addition without exceeding 80% rh in the duct.
60° & 80% = 39.76 lbs/hr
60° & 30% = 14.91 lbs/hr
____________________
24.85 lbs/hr can be added to 1000 CFM
x 3 (3000CFM)
____________
74.55 lbs/hr can be added to 3000 CFM
If the load calculation of this system requires a 100 lbs/hr unit, then due to the above
possible conditions, the project requires a modulating high limit humidistat to limit the
humidifier output to a maximum of 74 lbs/hr and maintain 80% rh.
Example 2:
Humidification load is 30 lbs/hr. To avoid exceeding 80% rh in the duct, calculate the
minimum allowable CFM at 55°F. Duct conditions are 55°F and 50% rh.
55° & 80% = 33.53 lbs/hr
55° & 50% = 20.95 lbs/hr
____________________
12.57 lbs/hr can be added to 1000 CFM
(30 lbs/hr ÷ 12.57) x 1000 = 2386 CFM minimum
°F 10% 20% 30% 40% 50% 60% 70% 80% 85%
50 3.52 7.04 10.56 14.06 17.60 21.12 24.65 26.16 29.92
55 4.19 8.38 12.57 16.76 20.95 25.14 29.33 33.52 35.62
60 4.97 9.94 14.91 19.88 24.85 29.82 34.79 39.76 42.25
65 5.85 11.74 17.61 23.48 29.35 35.22 41.09 46.96 49.73
70 6.94 13.80 20.82 27.78 34.70 41.64 48.58 55.52 58.99
75 8.10 16.20 24.30 32.40 40.50 48.60 56.70 64.80 68.85
80 9.46 18.92 28.36 37.84 47.30 56.76 66.22 75.68 80.41
85 11.03 22.08 33.09 44.12 55.15 66.18 77.21 88.24 93.75
90 12.80 25.60 38.40 51.20 64.00 76.80 89.60 102.4 108.80
95 14.81 29.62 44.43 59.24 74.05 88.86 103.67 118.48 125.89
100 17.10 34.20 51.20 68.40 85.50 102.60 119.60 136.80 145.54