Heat-Conveying Mediums
As mentioned in Chapter 1, several methods are used to classify heating systems. One method is based on the medium that conveys the heat from its source to the point being heated. When the majority of heating systems in use today are examined closely, it can be seen that there are only four basic heat-conveying mediums involved:
1. Air
2. Steam
3. Water
4. Electricity
Air
Air is a gas consisting of a mechanical mixture of 23.2% oxygen (by weight), 75.5% nitrogen, and 1.3% argon with small amounts of other gases. It functions as the heat-conveying medium for warm-air heating systems.
Atmospheric pressure may be defined as the force exerted by the weight of the atmosphere in every point with which it is in contact (Figure 2-8), and is measured in inches of mercury or the corresponding pressure in pounds per square-inch (psi).
The pressure of the atmosphere is approximately 14.7 psi at sea level. The standard atmosphere is 29.921 inches of mercury (in Hg) at 14.696 psi. “Inches of mercury” refers to the height to which the column of mercury in a barometer will remain suspended to balance the pressure caused by the weight of the atmosphere.
Atmospheric pressure varies due to elevation by decreasing approximately 1⁄2 lb for every 1000 ft ascent above sea level. Atmospheric pressure in pounds per square-inch is obtained from a barometer reading by multiplying the barometer reading in inches by 0.49116. Examples are given in Table 2-1.
Gauge pressure is pressure whose scale starts at atmospheric pressure. Absolute pressure, on the other hand, is pressure measured from true zero or the point of no pressure. When the hand of a steam gauge is at zero, the absolute pressure existing in the boiler is approximately 14.7 psi. Thus, by way of example, 5 lb pressure measured by a steam gauge (i.e., gauge pressure) is equal to 5 lb plus 14.7 lb, or 19.7 psi of absolute pressure.
When air is compressed, both its pressure and temperature are changed in accordance with Boyle’s and Charles’ laws. According to Robert Boyle (1627–1691), the English philosopher and founder of
modern chemistry, the absolute pressure of a gas at constant temperature varies inversely as its volume. Jacques Charles (1746–1823) established that the volume of a gas is proportional to its absolute temperature when the volume is kept at constant pressure.
If the cylinder in Figure 2-9 is filled with air at atmospheric pressure (14.7 psi absolute), represented by volume A, and the piston B moved to reduce the volume to, say, 1⁄3 A, as represented by B, then according to Boyle’s law, the pressure will be tripled (14.7 X 3 = 44.1 lb absolute, or 44.1 – 14.7 = 29.4 gauge pressure). According to Charles’ law, a pressure gauge on the cylinder would at this point indicate a higher pressure than 29.4 gauge pressure because of the increase in temperature produced by compressing the air. This is called adiabatic compression if no heat is lost or is received externally.
Steam
Those who design, install, or have charge of steam heating plants certainly should have some knowledge of steam and its formation and behavior under various conditions.
Steam is a colorless, expansive, and invisible gas resulting from the vaporization of water. The white cloud associated with steam is a fog of minute liquid particles formed by condensation, that is to say, finely divided condensation. This white cloud is caused by the exposure of the steam to a temperature lower than that corresponding to its pressure.
If the inside of a steam heating main were visible, it would be filled partway with a white cloud; in traversing the main, the little particles combine, forming drops of condensation too heavy to remain in suspension, which accordingly drop to the bottom of the main and drain off as condensation. This condensation flows into
a drop leg of the system and finally back into the boiler, together with additional condensation draining from the radiators.
Although the word “steam” should be applied only to saturated gas, the five following classes of steam are recognized:
1. Saturated steam
2. Dry steam
3. Wet steam
4. Superheated steam
5. Highly superheated or gaseous steam
Three of these classes of steam (wet, saturated, and super- heated) are shown in the illustration of a safety valve blowing in Figure 2-10. It should be pointed out that neither saturated steam nor superheated steam can be seen by the naked eye.
Saturated steam may be defined as steam of a temperature due to its pressure. Steam containing intermingled moisture, mist, or spray is referred to as wet steam. Dry steam is steam containing no
moisture. It may be either saturated or superheated. Finally, super- heated steam is steam having a temperature higher than that corresponding to its pressure.
The various changes that take place in the making of steam are known as vaporization and are shown in Figure 2-11. For the sake of illustration, only one bubble is shown in each receptacle. In actuality there is a continuous procession upward of a great multiplicity of bubbles.
The amount of heat necessary to cause the generation of steam is the sum of the sensible heat, the internal latent heat, and the external latent heat. As mentioned elsewhere in this chapter, sensi- ble heat is the part of the heat that produces a rise in temperature as indicated by the thermometer. The internal latent heat is the amount of heat that water will absorb at the boiling point without a change in temperature—that is, before vaporization begins. External latent heat is the amount of heat required when vapor- ization begins to push back the atmosphere and make room for the steam.
Another important factor to consider when dealing with steam is the boiling point of liquids. By definition, the boiling point is the 29.74 in.
temperature at which a liquid begins to boil (Figure 2-12), and it depends upon both the pressure and the nature of the liquid. For instance, water boils at 212 F, ether at 9 F, under atmospheric pressure of 14.7 psi.
The relationship between boiling point and pressure is such that there is a definite temperature or boiling point corresponding to each value of pressure. When vaporization occurs in a closed vessel and there is a temperature rise, the pressure will rise until the equilibrium between temperature and pressure is re- established.
One’s knowledge of the fundamentals of steam heating should also include an understanding of the role that condensation plays. By definition, condensation is the change of a substance from the gaseous to the liquid (or condensate) form. This change is caused by a reduction in temperature of the steam below that correspond- ing to its pressure.
The condensation of steam can cause certain problems for steam heating systems unless they are designed to allow for it. The water from which the steam was originally formed contained, mechanically mixed with it, 1⁄20, or 5 percent, of air by volume (at atmospheric pressure). This air is liberated during vaporization and does not recombine with the condensation. As a result, trouble is experienced in heating systems when one attempts to get the air out and keep it out. Suitable air valves are necessary to correct the problem.
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 (ordi- narily expressed as 81⁄3 lb) at a temperature of 62°F. At any other temperature, of course, the weight will be different (Table 2-2).
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 illus- trates the principle of thermal circulation. The weight of the hot
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.
Electricity
Electricity differs from air, steam, or water in that it does not actually convey heat from one point to another; therefore, including it in a list of heat-conveying mediums can be misleading at first glance.
Electricity can best be defined as a quantity of electrons either in motion or in a state of rest. When these electrons are at rest, they are referred to as being static (hence the term static electricity). Electrons in motion move from one atom to another, creating an electrical current and thereby a medium for conveying energy from one point to another. Many different devices have been created to change the energy conveyed by an electric current into heat, light, and other forms of energy. Electric-fired furnaces and boilers are examples of devices used to produce heat.