1. PRINCIPLES OF HEATING
Learning Objective: Understand the basic principles and theory of heat, heat measurement, and heat transfer.
Long after people had advanced to the stage of house building, heating methods had not improved much. For centuries fires for heating and lighting were contained in braziers or confined to an unused corner of a room. The smoke was supposed to escape through a hole left in the roof of the building during construction. Of course, a considerable amount of rain and snow entered the room during bad weather. During the twelfth century, however, the people in the northern part of Europe started using crude fireplaces and flues to replace the brazier and hole-in-the-roof method of heating. Some of these rudimentary heating systems still exist in France.
In the thirteenth and fourteenth centuries, the round, hollow stone chimneys began to be used. At the end of the fourteenth century, people were using a number of fireplaces in their homes and grouping the chimneys together in a vertical, rectangular mass of masonry with decorative effect. By the end of the Italian renaissance period, chimneys were in common use.
During colonial days in America, the fireplace chimneys were a large masonry mass projected through the center of the roof or were an important feature of the gable end walls. This general trend is often followed in architecture today because central heating, required in places where fires are required 5 or 6 months of the year, makes the chimney an important feature of a heating plant. There are heating installations, however, that do not make use of the masonry chimney and have substituted an inconspicuous metal smoke pipe. Other types of heating, such as electrical heating, require no chimney. Methods and equipment used for heating the places we live and work have progressed quickly in the last 100 years. This quick advance is due to our understanding of the principles and theory of heat, which in earlier times was not yet understood.
THEORY OF HEAT
Heat is a form of energy that is known for its effect. Heat can be produced or generated by the combustion of fuels, by friction, by chemical action, and by the resistance offered to the flow of electricity in a circuit. However, the particular form of generated heat with which the heating specialist will be dealing is produced by combustion. Generated heat is obtained by burning common types of fuels, such as coal, oil, and gas.
To operate a heating plant efficiently, you must be familiar with the measurement of heat and how this heat is transferred from the plant to the space being heated. The first part of this section is devoted to measuring temperature; the second part is concerned with the transfer of heat from the plant to the space being heated.
Measurements of temperature and pressure, which are obtained continuously, are very important factors in the operation of a heating plant. The degree of correctness of these measurements directly affects the safety, the efficiency, and the reliability of the operation of the heating plant. Although heat and temperature have a direct relationship, there is also a distinction between them. For example, a burning match develops a much higher temperature than a steam radiator, but the match does not give off enough heat to warm a room. Another example tells us that 10 pounds of water at 80°F will melt more ice in a given length of time than 1 pound of water at 100oF. The former has more heat, but the latter has a higher temperature. Temperature is the measurement of heat intensity in degrees Fahrenheit or Celsius. Therefore, temperature measurements can be made by using a glass thermometer calibrated either in degrees Fahrenheit or Celsius. The generally accepted way of stating measurements of temperature in English-speaking countries is in degrees Fahrenheit.
The thermometer measures the degree of sensible heat of different bodies. The thermometer can make a comparison only between the temperature of a body and some definitely known temperature such as the melting point of ice or the boiling point of water. Figure 4-1 shows a comparison of the scales of Fahrenheit and Celsius thermometers. It also shows the marking of the freezing and boiling points of pure water at sea level. The range of the Fahrenheit thermometer between the freezing point and the boiling point is 180° (32° to 212° = 180°).
Figure 4-1.—Comparison of Fahrenheit and Celsius thermometers.
On the Celsius thermometer, the range is 100° (0° to 100° = 100°) from the freezing point to the boiling point.
To convert Fahrenheit readings to Celsius:
(°F - 32°) ÷ 1.8 = °C
To convert Celsius readings to Fahrenheit:
(°C x 1.8) + 32° = °F
The heat that can be measured by a thermometer and sensed or felt is referred to as sensible heat. An example of sensible heat is presented by placing a small vessel of cold water over a gas flame and putting a thermometer in the water. Upon observation, you note that the thermometer indicates a rise in temperature. Also, if you place your finger in the water several times, you will feel (or sense) the change in temperature that has taken place.
The unit of measurement for a given quantity of heat is the British thermal unit, abbreviated and commonly known as Btu. One Btu is the amount of heat needed to change the temperature of 1 pound of water 1° Fahrenheit at sea level. If one Btu is added to 1 pound at 50°F, the temperature of that pound of water will be raised to 51°F.
All substances above absolute zero contain heat. There is heat even in ice, and its melting point is fixed at 32°F. Because of a fundamental law of nature, when ice at 32°F melts into water at 32°F, a change of state takes place. The ice (solid) has turned into water (liquid). A certain amount of heat is required during this change of state. This heat is known as latent heat. Latent heat is the amount of heat required to change the state of a substance without a measurable change in temperature.
There are other types of heat that you will encounter in heating. These are as follows:
- Specific heat–The ratio between the quantity of heat required to raise 1 pound of any substance 1oF and the amount of heat required to raise the temperature of 1 pound of water 1oF.
- Superheat–The amount of heat added to a substance above its boiling point.
- Total heat–Is the sum of sensible heat plus latent heat.
We previously mentioned absolute zero. But, what is absolute zero? Scientists have determined that when the temperature of a substance has been reduced to -460°F that all the heat has been removed from a substance. At this point all the molecules cease to have motion. Absolute zero is the lowest temperature obtainable. Heat is present in all substances when the temperature is above absolute zero.
The transfer of heat is the next problem to consider after the heat has been produced. It must be moved to the space where it is to be used. Heat always flows from a warmer to a cooler substance; consequently, there must be a temperature difference before heat can flow. Naturally, the greater the temperature difference, the faster the heat flow. Two objects that have different temperatures, when placed together, tend to equalize their temperature. Heat travels in heating systems from one place to another by three different methods. All three of these methods are used in most heating systems. They are discussed in the paragraphs that follow
Conduction is the flow of heat from one part of a substance to another part of the same substance or from one substance to another when they are in direct contact.
When one end of a stove poker is held in a flame, the other end will soon be too hot to hold. This indicates that the heat is being conducted, or transferred, from one end of the poker to the other end. Such a transfer of heat is called conduction. Conduction is used to transfer heat through the walls of a stove, furnace, or radiator so that the warmth can be used for heating. Some materials do not conduct heat as well as others. For example, if a piece of wood had been used instead of the poker, the end of the wood away from the fire would have remained cool. Those materials that offer considerable resistance to heat flow are referred to as insulators or poor conductors.
Convection is the transfer of heat by means of mediums, such as water, air, and steam. When air is heated, it expands, becomes lighter in weight, and rises. The cooler air, which is heavier, then flows in to replace the warm air. Thus a convection current is set up. Water, when heated, acts in the same way as air. The water next to the heating surface becomes warmer, lighter, and rises. This action allows the cooler water to flow in next to the heating surface and become heated. Convection is a very important factor in a heating system. It is this force, developed by heating the medium, which circulates that medium to the space to be heated.
Radiation is the transfer of heat through space. When a hand is held in front of a stove, it is quickly warmed by means of radiation. In this same manner, the earth receives its heat from the sun.
Radiated heat is transferred by heat waves, similar to radio waves. Heat waves do not warm the air through which they pass, but they must be absorbed by some substance to produce heat. For example, when you stand in the shade of a tree, you feel cool because the leaves and limbs are absorbing the heat waves before they reach you.
When heat waves strike an object, some are reflected, some may pass through, and the object absorbs the rest. Polished metals are the best reflectors known; therefore, they are a poor absorber of heat. A poor absorber is also a good radiator. Rough metal absorbs heat more readily than a highly polished metal, and it also loses heat faster by radiation.
The color of a substance also affects its absorbing power. A black surface absorbs heat faster than a white one. That is why light-colored clothes are cooler in summer than are dark-colored clothes.
Questions for Lesson 1
- Heat can be produced or generated by what methods?
- What two types of measurements directly affect the safety, efficiency, and reliability of heat plant operations?
- Temperature is the measurement of what?
- Convert 82 degrees Fahrenheit to degrees Celsius.
- Heat travels in heating systems by what three methods?