Steam, Its Generation and Use

Properties of Air

Pure air is a mechanical mixture of oxygen and nitrogen. While different authorities give slightly varying values for the proportion of oxygen and nitrogen contained, the generally accepted values are:

By volume,	oxygen 20.91 per cent,	nitrogen 79.09 per cent.
By weight,	oxygen 23.15 per cent,	nitrogen 76.85 per cent.

Air in nature always contains other constituents in varying amounts, such as dust, carbon dioxide, ozone and water vapor.

Being perfectly elastic, the density or weight per unit of volume decreases in geometric progression with the altitude. This fact has a direct bearing in the proportioning of furnaces, flues and stacks at high altitudes, as will be shown later in the discussion of these subjects. The atmospheric pressures corresponding to various altitudes are given in Table 12.

The weight and volume of air depend upon the pressure and the temperature, as expressed by the formula:

P v = 53.33 T (9)

Where	P	=	the absolute pressure in pounds per square foot,
v		=	the volume in cubic feet of one pound of air,
T		=	the absolute temperature of the air in degrees Fahrenheit,
53.33		=	a constant for air derived from the ratio of pressure, volume and temperature of a perfect gas.

The weight of one cubic foot of air will obviously be the reciprocal of its volume, that is, 1/ v pounds.

TABLE 27

VOLUME AND WEIGHT OF AIR
AT ATMOSPHERIC PRESSURE AT VARIOUS TEMPERATURES

Temperature Degrees Fahrenheit	Volume One Pound in Cubic Feet	Weight One Cubic Foot in Pounds	Temperature Degrees Fahrenheit	Volume One Pound in Cubic Feet	Weight One Cubic Foot in Pounds	Temperature Degrees Fahrenheit	Volume One Pound in Cubic Feet	Weight One Cubic Foot in Pounds
32	12.390	.080710	160	15.615	.064041	340	20.151	.049625
50	12.843	.077863	170	15.867	.063024	360	20.655	.048414
55	12.969	.077107	180	16.119	.062039	380	21.159	.047261
60	13.095	.076365	190	16.371	.061084	400	21.663	.046162
65	13.221	.075637	200	16.623	.060158	425	22.293	.044857
70	13.347	.074923	210	16.875	.059259	450	22.923	.043624
75	13.473	.074223	212	16.925	.059084	475	23.554	.042456
80	13.599	.073535	220	17.127	.058388	500	24.184	.041350
85	13.725	.072860	230	17.379	.057541	525	24.814	.040300
90	13.851	.072197	240	17.631	.056718	550	25.444	.039302
95	13.977	.071546	250	17.883	.055919	575	26.074	.038352
100	14.103	.070907	260	18.135	.055142	600	26.704	.037448
110	14.355	.069662	270	18.387	.054386	650	27.964	.035760
120	14.607	.068460	280	18.639	.053651	700	29.224	.034219
130	14.859	.067299	290	18.891	.052935	750	30.484	.032804
140	15.111	.066177	300	19.143	.052238	800	31.744	.031502
150	15.363	.065092	320	19.647	.050898	850	33.004	.030299

Example: Required the volume of air in cubic feet under 60.3 pounds gauge pressure per square inch at 115 degrees Fahrenheit.

P = 144 (14.7 + 60.3) = 10,800.

T = 115 + 460 = 575 degrees.

Hence v

---

53.33 × 575
––––––––––––––––––––
10,800

---

2.84 cubic feet, and

Weight per cubic foot

---

1
––––
v

---

1
––––––––
2.84

---

0.352 pounds.

Table 27 gives the weights and volumes of air under atmospheric pressure at varying temperatures.

Formula (9) holds good for other gases with the change in the value of the constant as follows:

For oxygen 48.24, nitrogen 54.97, hydrogen 765.71.

The specific heat of air at constant pressure varies with its temperature. A number of determinations of this value have been made and certain of those ordinarily accepted as most authentic are given in Table 28.

TABLE 28

SPECIFIC HEAT OF AIR
AT CONSTANT PRESSURE AND VARIOUS TEMPERATURES

Temperature Range		Specific Heat	Authority
Degrees Centigrade	Degrees Fahrenheit
-30– 10	-22– 50	0.2377	Regnault
0–100	32– 212	0.2374	Regnault
0–200	32– 392	0.2375	Regnault
20–440	68– 824	0.2366	Holborn and Curtis
20–630	68–1166	0.2429	Holborn and Curtis
20–800	68–1472	0.2430	Holborn and Curtis
0–200	32– 392	0.2389	Wiedemann

This value is of particular importance in waste heat work and it is regrettable that there is such a variation in the different experiments. Mallard and Le Chatelier determined values considerably higher than any given in Table 28. All things considered in view of the discrepancy of the values given, there appears to be as much ground for the use of a constant value for the specific heat of air at any temperature as for a variable value. Where this value is used throughout this book, it has been taken as 0.24.

Air may carry a considerable quantity of water vapor, which is frequently 3 per cent of the total weight. This fact is of importance in problems relating to heating drying and the compressing of air. Table 29 gives the amount of vapor required to saturate air at different temperatures, its weight, expansive force, etc., and contains sufficient information for solving practically all problems of this sort that may arise.

TABLE 29

WEIGHTS OF AIR, VAPOR OF WATER, AND SATURATED MIXTURES OF AIR AND VAPOR
AT DIFFERENT TEMPERATURES,
UNDER THE ORDINARY ATMOSPHERIC PRESSURE OF 29.921 INCHES OF MERCURY

Temper- ature Degrees Fahr- enheit	Volume of Dry Air at Different Temper- atures, the Volume at 32 Degrees being 1.000	Weight of Cubic Foot of Dry Air at the Different Temper- atures Pounds	Elastic Force of Vapor in Inches of Mercury (Regnault)	Mixtures of Air Saturated with Vapor						Cubic Feet of Vapor from One Pound of Water at its own Pressure in Column 4
				Elastic Force of the Air in the Mixture of Air and Vapor in Inches of Mercury	Weight of Cubic Foot of the Mixture of Air and Vapor			Weight of Vapor Mixed with One Pound of Air, in Pounds	Weight of Dry Air Mixed with One Pound of Vapor, in Pounds
					Weight of the Air in Pounds	Weight of the Vapor in Pounds	Total Weight of Mixture in Pounds
1	2	3	4	5	6	7	8	9	10	11
0	.935	.0864	.044	29.877	.0863	.000079	.086379	.00092	1092.4	…
12	.960	.0842	.074	29.849	.0840	.000130	.084130	.00155	646.1	…
22	.980	.0824	.118	29.803	.0821	.000202	.082302	.00245	406.4	…
32	1.000	.0807	.181	29.740	.0802	.000304	.080504	.00379	263.81	3289
42	1.020	.0791	.267	29.654	.0784	.000440	.078840	.00561	178.18	2252
52	1.041	.0776	.388	29.533	.0766	.000627	.077227	.00810	122.17	1595
62	1.061	.0761	.556	29.365	.0747	.000881	.075581	.01179	84.79	1135
72	1.082	.0747	.785	29.136	.0727	.001221	.073921	.01680	59.54	819
82	1.102	.0733	1.092	28.829	.0706	.001667	.072267	.02361	42.35	600
92	1.122	.0720	1.501	28.420	.0684	.002250	.070717	.03289	30.40	444
102	1.143	.0707	2.036	27.885	.0659	.002997	.068897	.04547	21.98	334
112	1.163	.0694	2.731	27.190	.0631	.003946	.067046	.06253	15.99	253
122	1.184	.0682	3.621	26.300	.0599	.005142	.065042	.08584	11.65	194
132	1.204	.0671	4.752	25.169	.0564	.006639	.063039	.11771	8.49	151
142	1.224	.0660	6.165	23.756	.0524	.008473	.060873	.16170	6.18	118
152	1.245	.0649	7.930	21.991	.0477	.010716	.058416	.22465	4.45	93.3
162	1.265	.0638	10.099	19.822	.0423	.013415	.055715	.31713	3.15	74.5
172	1.285	.0628	12.758	17.163	.0360	.016682	.052682	.46338	2.16	59.2
182	1.306	.0618	15.960	13.961	.0288	.020536	.049336	.71300	1.402	48.6
192	1.326	.0609	19.828	10.093	.0205	.025142	.045642	1.22643	.815	39.8
202	1.347	.0600	24.450	5.471	.0109	.030545	.041445	2.80230	.357	32.7
212	1.367	.0591	29.921	0.000	.0000	.036820	.036820	Infinite	.000	27.1

Column 5 = barometer pressure of 29.921, minus the proportion of this due to vapor pressure from column 4.