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
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Table 2-2 (continued)
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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
Figure 2-13 The point of maximum density. |
HIGH TEMPERATURE |
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