Duct Relative Humidity

Room Temp	Duct Temperature °F  °C		Room Relative Humidity - %R.H.
			25	30	35	40	45	50	55	60	65	70	75	80
68° F 19° C	50	10	46	57	65	75	83	93	85	94	87	91	98
	55	12.8	39	48	54	62	70	79	71	79	73	79	82
	60	15.6	31	40	46	51	59	66	60	66	61	65	70
	65	18.3	28	33	39	44	50	55	51	56				89
	70	21.1	22	29	31	38	41	48						75
70° F 21° C	50	10	50	61	72	81	92	84	94	85	92
	55	12.8	40	51	60	68	77	70	77	71	78
	60	15.6	36	42	50	56	63	60	66	60	65
	65	18.3	30	36	41	50	53	50	56			82	89	96
	70	21.1	25	30	35	40	45					70	75	80
72° F 22° C	50	10	50	65	75	85	96	90	97	90	98
	55	12.8	42	54	62	70	80	74	81	76	81
	60	15.6	37	46	52	60	68	61	70	54	70
	65	18.3	30	39	44	50	56	52	59			88	94
	70	21.1	27	31	38	41	48					73	80	85
74° F 23° C	50	10	59	69	80	92	87	97	89	98
	55	12.8	49	58	67	78	71	80	73	81
	60	15.6	40	48	56	65	60	69	62	69	89
	65	18.3	33	39	48	54	51	58			73	95
	70	21.1	29	34	40	47						80	86	91

Some upstream clearance is required for the nozzle discharge into the airstream. This varies depending on temperature, the obstruction, velocity and nozzle sizing. The chart below is for duct temperature above 50 degrees F. Distances shown are from the center of the steam distributor tube in inches. 

Air	Steam Intensity, lbm/hr/ft
Velocity, fpm	5.0	5.0	10	15	20	25	30	50
400	2.5	2.5	3.0	3.0	3.0	4.0	4.0	6.0
500	2.0	2.0	3.5	2.5	2.5	3.5	3.5	5.0
750	1.75	1.75	2.25	2.25	.25	3.0	3.0	4.0
1000	1.5	1.5	2.0	2.0	2.0	2.5	2.5	3.0
>1000	<1.5	<1.5	<2.0	<2.0	<2.0	<2.5	<2.5	<3.0

We recommend a minimum of 6" downstream of an object for service and installation concerns. The only time 6" is insufficient would be on a VAV system which combines manifolds in the air handler and a very high steam intensity - which is a rare condition. The upstream clearance chart is shown for confirmation that the 6" number is adequate. 
For example: We have a total of 30" to evaporate the steam at 60 degrees F, 80% RH and 1500 fpm. The chart shows we only need between <1.5' and <3.0" at the velocities over 1000 fpm. The 6' recommended clearance is more than sufficient. Therefore, our net available evaporation distance is 24". 

Duct temperature and downstream duct relative humidity have a profound effect on the evaporative distance of any humidifier. Together with air velocity which will be addressed in step #4, these are the factors that can make or break an otherwise good application. All three must be addressed for a good application. 

The correction factor will take the conditions present in the subject duct/air handler and translate those conditions to the standard conditions of 55 degrees F @ 70% RH. The evaporation distances at standard conditions are detailed on the Steam Intensity Chart. The correction factor will make the conditions in the duct/air handler you are working on equal to the standard conditions of 55 degrees F @ 70% RH. For example, if the conditions in the duct are less conducive to evaporation, say 60 F @ 80% RH. To use the Steam Intensity Chart, we must solve for a shorter evaporation distance. 

Correction Factor

DUCT TEMP, F	RELATIVE HUMIDITY
	30%	40%	50%	60%	70%	80%	90%
46	0.75	0.88	1.06	1.32	1.76	2.64	5.28
45	0.62	0.73	0.87	1.09	1.45	2.18	4.35
50	0.52	0.60	0.72	0.90	1.20	1.80	3.61
55	0.43	0.50	0.60	0.75	1.10	1.50	3.00
60	0.36	0.42	0.50	0.63	0.84	1.25	2.51
65	0.30	0.35	0.42	0.53	0.70	1.05	2.10
70	0.25	0.30	0.35	0.44	0.59	0.88	1.77
75	0.21	0.25	0.30	0.37	0.50	0.75	1.49
80	0.18	0.21	0.25	0.32	0.42	0.63	1.27
85	0.15	0.18	0.22	0.27	0.36	0.54	1.08
90	0.13	0.15	0.18	0.23	0.31	0.46	0.92
95	0.11	0.13	0.16	0.20	0.26	0.39	0.79
100	0.10	0.11	0.14	0.17	0.23	0.34	0.68

Notice that the correction factor is 1.25 and we want the bulk of the steam evaporated in 24". The corrected evaporation distance is: 

Ec= 24" / 1.25 = 19.2" or 1.6'

The steam intensity is defined as the amount of steam in lbs/hr. to be distributed by a foot of evaporative tube. The lower the steam intensity, the quicker the evaporation. Using the velocity of the duct or air handler to be humidified, together with the corrected evaporative distance in feet, Ec, the necessary steam intensity can be determined. 

Steam Intensity, #/hr/ft
Air Velocity, fpm	5 Bulk	5 Last Wisp	7.5 Bulk	7.5 Last Wisp	10 Bulk	10 Last Wisp	15 Bulk	15 Last Wisp
500	0.6	7.8	0.9	9.8	1.4	12.4	2.0	15.7
750	0.6	6.9	0.9	8.8	1.3	11.1	2.0	14.1
1000	0.5	6.5	0.8	8.1	1.2	10.1	1.9	12.6
1250	0.5	5.6	0.7	7.0	1.2	8.7	1.9	10.9
1500	0.4	4.8	0.7	6.0	1.1	7.6	1.9	9.4
Air Velocity, fpm	20 Bulk	20 Last Wisp	30 Bulk	30 Last Wisp	50 Bulk	50 Last Wisp
500	2.9	19.8	4.3	25.1	6.3	31.8
750	3.0	17.8	4.5	22.6	6.9	28.6
1000	3.0	15.8	4.8	19.8	7.5	24.8
1250	3.1	13.7	5.1	17.1	8.1	21.4
1500	3.2	11.8	5.3	14.8	8.7	18.5

"Steam Intensity" is the amount of steam discharged from a steam manifold in lbs of steam/ft of manifold 
"Bulk" evaporation is the point beyond which no condensation will occur on objects at the duct temperature 
"Last Wisp" evaporation is the point beyond which there is no visible steam and it is safe to install high efficiency filters 

From our example in step #3, the corrected evaporative distance, Ec, was equal to 19.2" or 1.6'. Our duct velocity was 1500 fpm, the required steam intensity would be 10 lbs/hr/ft. The highlighted spot shows an evaporation distance of 1.1' at 1500 fpm velocity. Steam which would evaporate in 1.1' at 55 degrees F @ 70% would evaporate in 1.1' x 1.25 correction factor or 1.38' at 60 degrees F, 80% RH. 

WARNING: Utilizing the three charts above will allow you to accurately predict the evaporative distance based on the design parameters you select. Any deviation from the parameters you've selected (temperature, downstream RH, velocity, steam intensity) or turbulent airflow will dramatically affect the evaporative distance. Consult the factory prior to running a system that has been modified from the design parameters to insure a trouble-free installation.