Water

Water is a chemical compound of two gases, oxygen and hydrogen, in the proportion of two parts by weight of hydrogen to 16 parts by weight of oxygen, having mixed with it about 5 percent of air by volume at 14.7 lb absolute pressure. It may exist as ice, water, or steam due to changes in temperature (water freezes at 32°F and boils at 212°F when the barometer reads 29.921 in).

One cubic foot of water weighs 62.41 lb at 32°F and 59.82 lb at 212°F. One U. S. gallon of water (231 in3) weighs 8.33111 lb (ordinarily expressed as 8V3 lb) at a temperature of 62°F. At any other temperature, of course, the weight will be different (Table 2-2).

Table 2-2 Weight of Water per Cubic Foot at Different Temperatures

Temp., °F	Lb per ft3	Temp., °F	Lb per ft3	Temp., °F	Lb per ft3	Temp., °F	Lb per ft3
32	62.41	55	62.38	78	62.23	101	61.98
33	62.41	56	62.38	79	62.22	102	61.96
34	62.42	57	62.38	80	62.21	103	61.95
35	62.42	58	62.37	81	62.20	104	61.94
36	62.42	59	62.37	82	62.19	105	61.93
37	62.42	60	32.36	83	62.18	106	61.91
38	62.42	61	62.35	84	62.17	107	61.90
39	62.42	62	62.35	85	62.16	108	61.89
40	62.42	63	62.34	86	62.15	109	61.87
41	62.42	64	62.34	87	62.14	110	61.86
42	62.42	65	62.33	88	62.13	111	61.84
43	62.42	66	62.32	89	62.12	112	61.83
44	62.42	67	62.32	90	62.11	113	61.81
45	62.42	68	62.31	91	62.10	114	61.80
46	62.41	69	62.30	92	62.08	115	61.78
47	62.41	70	62.30	93	62.07	116	61.77
48	62.41	71	62.29	94	62.06	117	61.75
49	62.41	72	62.28	95	62.05	118	61.74
50	62.40	73	62.27	96	62.04	119	61.72
51	62.40	74	62.26	97	62.02	120	61.71
52	62.40	75	62.25	98	62.01	121	61.69
53	62.39	76	62.25	99	62.00	122	61.68
54	62.39	77	62.24	100	61.99	123	61.66

Table 2-2 (continued)

Temp., °F	Lb per ft3	Temp., °F	Lb per ft3	Temp., °F	Lb per ft3	Temp., °F	Lb per ft3
124	61.64	160	60.99	196	60.21	380	54.47
125	61.63	161	60.97	197	60.19	390	54.05
126	61.61	162	60.95	198	60.16	400	53.62
127	61.60	163	60.93	199	60.14	410	53.19
128	61.58	164	60.91	200	60.11	420	52.74
129	61.56	165	60.89	201	60.09	430	52.33
130	61.55	166	60.87	202	60.07	440	51.87
131	61.53	167	60.85	203	60.04	450	51.28
132	61.51	168	60.83	204	60.02	460	51.02
133	61.50	169	60.81	205	59.99	470	50.51
134	61.48	170	60.79	206	59.97	480	50.00
135	61.46	171	60.77	207	59.95	490	49.50
136	61.44	172	60.75	208	59.92	500	48.78
137	61.43	173	60.73	209	59.90	510	48.31
138	61.41	174	60.71	210	59.87	520	47.62
139	61.39	175	60.68	211	59.85	530	46.95
140	61.37	176	60.66	212	59.82	540	46.30
141	61.36	177	60.64	214	59.81	550	45.66
142	61.34	178	60.62	216	59.77	560	44.84
143	61.32	179	60.60	218	59.70	570	44.05
144	61.30	180	60.57	220	59.67	580	43.29
145	61.28	181	60.55	230	59.42	590	42.37
146	61.26	182	60.53	240	59.17	600	41.49
147	61.25	183	60.51	250	58.89	610	40.49
148	61.23	184	60.49	260	58.62	620	39.37
149	61.21	185	60.46	270	58.34	630	38.31
150	61.19	186	60.44	280	58.04	640	37.17
151	61.17	187	60.42	290	57.74	650	35.97
152	61.15	188	60.40	300	57.41	660	34.48
153	61.13	189	60.37	310	57.08	670	32.89
154	61.11	190	60.35	320	56.75	680	31.06
155	61.09	191	60.33	330	56.40	690	28.82
156	61.07	192	60.30	340	56.02	700	25.38
157	61.05	193	60.28	350	55.65
158	61.03	194	60.26	360	55.25
159	61.01	195	60.23	370	54.85

Water changes in weight with changes of temperature. That is, the higher the temperature of the water, the less it weighs. It is this property of water that causes circulation in boilers and in hot-water heating systems. The change in weight is due to expansion and a reduction in water volume. As the temperature rises, the water expands, resulting in a unit volume of water containing less water at higher temperature than lower temperature.

Fill a vessel with cold water and heat it to the boiling point. Note that boiling causes it to overflow due to expansion. Now let the water cool. You will note that when the water is cold, the vessel will not be as full because the water will have contracted.

The point of maximum density of water is 39.1°F. The most remarkable characteristic of water is its expansion below and above its point of maximum density. Imagine 1 lb of water at 39.1°F placed in a cylinder having a cross-sectional area of 1 in2 (Figure 2-13). The water having a volume of 27.68 in3 will fill the cylinder to a height of 27.68 in. If the water is cooled, it will expand, and at, say, 32°F (the freezing point) will rise in the tube to a height of 27.7 in before freezing. If the water is heated, it will also expand and rise in the tube; and at the boiling point (for atmospheric pressure 212°F) it will occupy the tube to a height of 28.88 in.

The elementary hot-water heating system in Figure 2-14 illustrates the principle of thermal circulation. The weight of the hot

Water

Figure 2-13 The point of maximum density.

HIGH TEMPERATURE

Water

And expanded water in the upflow column C, being less than that of the cold and contracted water in the downflow column C’, upsets the equilibrium of the system and results in a continuous circulation of water as indicated by the arrows. In other words, the heavy, low-temperature water sinks to the lowest point in the boiler (or system) and displaces the light, high-temperature water, thus causing continuous circulation as long as there is a temperature difference in different parts of the boiler (or system). This is referred to as thermal circulation.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers

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