Air Humidifying with Steam - Imperial Units

The amount of vapor (lb/h in 100 cfm) in humid air

The table below can be used to calculate the amount of steam needed to humidify 100 cfm (cubic feet per minute) air.

Vapor in 100 cfm air (lb/h)
Dry Temperature (oF)	Humidity (%)
	100	85	80	75	70	65	60	55	50	45	40	35	30
90	12.84	10.92	10.26	9.6	9	8.34	7.68	7.08	6.42	5.78	5.14	4.49	3.85
88	12.06	10.26	9.66	9.06	8.46	7.87	7.26	6.66	6.06	5.41	4.82	4.22	3.62
86	11.4	9.66	9.12	8.52	7.98	7.38	6.85	6.3	5.7	5.13	4.56	3.99	3.42
85	11.04	9.36	8.82	8.28	7.74	7.2	6.6	6.06	5.52	4.97	4.42	3.86	3.31
84	10.68	9.06	8.52	7.98	7.5	6.96	6.42	5.87	5.34	4.81	4.27	3.74	3.2
83	10.38	8.82	8.34	7.8	7.26	6.72	6.24	5.71	5.19	4.67	4.15	3.63	3.11
82	10.08	8.58	8.04	7.56	7.08	6.54	6.06	5.54	5.04	4.54	4.03	3.53	3.02
81	9.78	8.34	7.8	7.32	6.84	6.36	5.87	5.38	4.89	4.4	3.91	3.42	2.93
80	9.48	8.04	7.56	7.08	6.66	6.18	5.69	5.21	4.74	4.27	3.79	3.32	2.84
79	9.18	7.8	7.32	6.9	6.42	5.96	5.51	5.05	4.59	4.13	3.67	3.21	2.75
78	8.88	7.56	7.08	6.66	6.24	5.77	5.33	4.88	4.44	3.99	3.55	3.11	2.66
77	8.64	7.32	6.9	6.48	6.06	5.62	5.18	4.75	4.32	3.89	3.46	3.02	2.59
76	8.4	7.14	6.72	6.3	5.88	5.46	5.04	4.62	4.2	3.78	3.36	2.94	2.52
75	8.1	6.9	6.48	0.06	5.67	5.26	4.86	4.48	4.05	3.64	3.24	2.83	2.43
74	7.86	6.66	6.3	5.89	5.5	5.11	4.72	4.32	3.93	3.53	3.14	2.75	2.36
73	7.62	6.48	6.12	5.71	5.33	4.95	4.57	4.12	3.81	3.43	3.05	2.66	2.29
72	7.38	6.3	5.9	5.53	5.17	4.79	4.43	4.06	3.69	3.32	2.95	2.58	2.21
71	7.14	6.06	5.71	5.35	4.99	4.64	4.29	3.93	3.57	3.21	2.86	2.5	2.14
70	6.9	5.86	5.52	5.17	4.84	4.48	4.14	3.79	3.45	3.1	2.76	2.41	2.07
69	6.72	5.71	5.38	5.04	4.7	4.37	4.03	3.7	3.36	3.02	2.67	2.35	2.02
68	6.48	5.51	5.18	4.86	4.54	4.21	3.89	3.55	3.24	2.92	2.59	2.27	1.94
67	6.24	5.3	4.99	4.68	4.37	4.06	3.74	3.43	3.12	2.81	2.5	2.18	1.87
66	6.06	5.15	4.85	4.54	4.24	3.94	3.64	3.33	3.03	2.72	2.42	2.12	1.82
65	5.87	4.99	4.7	4.4	4.11	3.82	3.52	3.23	2.93	2.65	2.35	2.06	1.76
60	4.97	4.23	3.98	3.73	3.48	3.23	2.98	2.74	2.48	2.24	1.99	1.74	1.49
55	4.19	3.56	3.35	3.14	2.93	2.72	2.51	2.31	2.09	1.89	1.68	1.47	1.26
50	3.52	2.99	2.82	2.64	2.47	2.29	2.11	1.94	1.76	1.58	1.41	1.23	1.06
45	2.95	2.51	2.36	2.21	2.06	1.92	1.77	1.62	1.47	1.33	1.18	1.03	0.89
40	2.45	2.09	1.96	1.84	1.72	1.6	1.47	1.35	1.22	1.1	0.98	0.86	0.74
30	1.67	1.42	1.33	1.25	1.17	1.09	1	0.92	0.83	0.75	0.67	0.58	0.5
20	1.07	0.91	0.85	0.8	0.75	0.7	0.64	0.59	0.53	0.48	0.43	0.37	0.32
10	0.67	0.56	0.53	0.5	0.47	0.43	0.4	0.37	0.33	0.3	0.26	0.23	0.2
0	0.41	0.35	0.32	0.31	0.29	0.26	0.25	0.22	0.2	0.19	0.16	0.13	0.12
-10	0.25	0.21	0.2	0.19	0.17	0.16	0.15	0.14	0.12	0.11	0.1	0.08	0.08
-20	0.14	0.12	0.11	0.11	0.1	0.1	0.08	0.08	0.72	0.06	0.05	0.05	0.04

Example - Humidifying Air with Steam

How much steam is needed to transform air at 30^oF with 100% humidity to air at 70^oF and 40% humidity?

In air at 30^oF and 100% humidity, the steam content is 1.67 lb/h (100 cfm/h air flow).

At air 70^oF and 40% humidity, the steam content is 2.76 lb/h (100 cfm/h air flow).

The difference (2.76 - 1.67) = 1.09 lb/h is the amount of steam necessary for 100 cfm/h air flow.

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Air Composition

Dry air is a mechanical mixture of nitrogen, oxygen, carbon dioxide and more

Air is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components. We usually model air as a uniform (no variation or fluctuation) gas with properties that are averaged from all the individual components.

Gas	Ratio compared to Dry Air (%)		Molecular Mass - M - (kg/kmol)	Chemical Symbol	Boiling Point
	By volume	By weight			K	(oC)
Oxygen	20.95	23.20	32.00	O2	90.2	-182.95
Nitrogen	78.09	75.47	28.02	N2	77.4	-195.79
Carbon Dioxide	0.03	0.046	44.01	CO2	194.7	-78.5
Hydrogen	0.00005	~ 0	2.02	H2	20.3	-252.87
Argon	0.933	1.28	39.94	Ar	84.2	-186
Neon	0.0018	0.0012	20.18	Ne	27.2	-246
Helium	0.0005	0.00007	4.00	He	4.2	-269
Krypton	0.0001	0.0003	83.8	Kr	119.8	-153.4
Xenon	9 10-6	0.00004	131.29	Xe	165.1	-108.1

The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on the temperature.

The composition of air is unchanged until the height of approximately 10.000 m.

The average air temperature diminishes at the rate of about 0.6^oC for each 100 m vertical height

"One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0^oC sea level and at standard gravity (32.174 ft/sec²).

Common Pressure Units frequently used as alternative to "one Atmosphere"

• 76 Centimeters (760 mm) of Mercury
• 29.921 Inches of Mercury
• 10.332 Meters of Water
• 406.78 Inches of Water
• 33.899 Feet of Water
• 14.696 Pound-Force per Square Inch
• 2116.2 Pounds-Force per Square Foot
• 1.033 Kilograms-Force per Square Centimeter
• 101.33 Kilopascal

More about Temperature, Density, Specific heat, Thermal Conductivity - the properties of air.

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The density and the specific volume of air varies with the elevation above sea level

The density of air will vary with the pressure (the Ideal Gas Law) and the altitude above sea level. Mean absolute pressure at sea level is approximately 760 mmHg (101.325 kPa, kN/m²) with a variation is about +/- 5%.

Altitude and Air Pressure

The air pressure varies with altitude as shown in the table below:

Altitude (m)	Barometer (mmHg)
0	749
75	743
150	735
250	726
300	723
450	709
600	695
750	681
900	668
1,000	658
1,200	643
1,350	632
1,500	620
1,800	598
2,100	577
2,400	555

Altitude and Air Density

Air density and specific volume as functions of altitude above sea level are indicated in the diagrams below:

STP - Standard Temperature and Pressure

• STP - Standard Temperature and Pressure is 0^oC and 101.325 kN/m².

NTP - Normal Temperature and Pressure

• NTP - Normal Temperature and Pressure, commonly used for testing and documentation of fan capacities, is 20^oC and 101.6 kN/m² (60^oF , 30 in Hg)

Altitude and Specific Volume Correction

The specific volume of standard air at a certain altitude can be calculated by multiplying with the volume correction factor below

Altitude		Volume Correction Factor
(m)	(ft)
0	0	1.00
480	1600	1.05
990	3300	1.11
1500	5000	1.17
1980	6600	1.24
2460	8200	1.31
2970	9900	1.39

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