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THEORY OF HEAT
 
A. INTRODUCTION
 
3A1. General. When a substance is cooled, something called coldness is not added to it, but rather heat is taken out of it. In order to understand and operate refrigerating machinery, it is necessary to know a few simple facts about heat.

3A2. Three states of matter. Matter is any thing that occupies space and has weight. Matter can exist in three different physical states: solid, liquid, and gaseous. A common example is water, which can assume all three states: as a solid-ice; as a liquid-water; and as a gas-steam.

Theoretically, all substances can be converted from one to another of the three states by the addition or withdrawal of heat. However,

  chemical compounds differ in the ease or difficulty with which they may be changed from one to another of the three physical states. Some, like water, can very readily be converted into each of the three states; others, like paper, oxidize, or burn, at high temperatures and cannot be converted into all three. Before paper burns, it changes to a gas, but never to a liquid. The science of refrigeration depends upon changes in physical state through heating or cooling.

3A3. Definition of heat. Heat is a form of energy. It cannot be seen, shaped, or touched, nor can it be created or destroyed. It is known only through its effects on the human body, on the air, and on other matter.

 
B. MEASUREMENT OF HEAT
 
3B1. Intensity and quantity. Heat is measured 1) by its intensity, and 2) by the quantity of it possessed by a substance. This is readily understood by comparing a spoonful of hot water with a pailful of warm water. The hot water in the spoon has a greater intensity of heat, but the warm water in the pail possesses a larger quantity of heat, though at a lower intensity.

3B2. Thermometer. Intensity of heat is measured by an ordinary thermometer, with which everyone is familiar. Two methods of dividing and numbering the thermometer scale are in common use: the Fahrenheit and the centigrade. The conditions discussed in the following paragraphs are for pure water under sea-level barometric pressure.

3B3. Fahrenheit scale. Thermometers calibrated in the Fahrenheit scale register the freezing point of water at 32 degrees, and its boiling point at 212 degrees. Such thermometers are used in civil life and in most engineering practice, including refrigeration, in the United States and the British Empire.

3B4. Centigrade scale. The centigrade thermometer

  is calibrated to register the freezing point of water at 0 degrees, and its boiling point at 100 degrees. It is used in most countries except the United States and the British Empire, and is used universally in scientific work.

3B5. Reading the thermometer. In recording thermometer readings, the general practice is to use a small superior circle instead of the term degree, and to indicate the type of thermometer by the initial letter of its name. Thus 212 degrees F means two hundred twelve degrees on the Fahrenheit scale, and 100 degrees C means one hundred degrees on the centigrade scale.

Temperatures below zero degrees are recorded with a minus (-) sign before the number, thus -50 degrees F. In speech, such temperatures are said to be one degree below zero, two degrees below zero, and so forth.

3B6. Absolute, or Kelvin scale. Still another system of indicating temperatures has been found useful in certain scientific and engineering work. This scale begins at absolute zero, a temperature at which a substance possesses no heat. Obviously, if the temperature

 
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of a substance is lowered by removing heat, there must be some point at which no more heat remains to be removed. Absolute zero has been approached very closely in physical laboratories, but has not yet been reached.

The absolute scale is customarily calibrated in centigrade divisions. Absolute zero is 273.16 degrees below zero centigrade. This is equal to 459.69 degrees Fahrenheit below zero, the lowest temperature that can exist. It is impossible for anything to become colder than this. On the absolute, or Kelvin, scale there are no minus degrees, and absolute temperatures are marked with a K for Lord Kelvin who devised the system. The freezing point of water is 273.16 degrees K, and the boiling point 373.16 degrees K.

3B7. Relations between various thermometer scales. A temperature read on one type of thermometer can be stated in terms of any other type by using conversion formulas.

To change a reading on the Fahrenheit

  scale to centigrade, use the following conversion formula:

C = 5/9 (F - 32)

To change a reading on the centigrade scale to Fahrenheit, use this conversion formula:

F= 9/5 C + 32

3B8. The British thermal unit. The quantity of heat possessed by a substance is measured in terms of the British thermal unit, abbreviated Btu. A Btu is the quantity of heat required to raise the temperature of 1 pound of pure water 1 degree Fahrenheit at or ear 39.10 degrees F. This is the temperature at which water is at maximum density. For example, to raise the temperature of 5 pounds of water from 39 degrees to 49 degrees F, or from 160 degrees to 170 degrees F requires 5 x 10 = 50 Btu. For all practical purposes, the Btu is considered constant between 32 degrees and 212 degrees F, though it does vary a slight amount.

 
C. DIFFERENT KINDS OF HEAT
 
3C1. Definitions of terms. It is convenient to have special terms by which to refer to heat in different substances and in various operations.

3C2. Specific heat. Specific heat is the number of Btu that must be added to a unit weight of a substance to raise the temperature of that substance one degree Fahrenheit. Since most substances held to a constant weight vary in volume, varying numbers of additional Btu are required to result in a change of temperature of 1 degree Fahrenheit per pound.

Technically, the specific heat of a substance is the ratio of the amount of heat required to change the temperature of a unit weight of that substance 1 degree to the amount of heat required to change the temperature of the same weight of water one degree. Since the specific heat of water is, by definition, equal to 1, the specific heats of other substances are expressed as decimals. A few examples are: ice, 0.504; cast iron, 0.119; alcohol, 0.70; machine oil, 0.40. Thus, it takes only about half as much heat to change the temperature of a pound of ice 1 degree as it does to change the temperature of a pound of water.

3C3. Thermal capacity. Thermal capacity is

  closely related to specific heat. The specific heat of a substance is the heat necessary to raise the temperature of 1 pound of the substance 1 degree; the thermal capacity of a substance is the amount of heat necessary to raise the temperature of its whole mass 1 degree. Hence, thermal capacity equals the specific heat of a substance multiplied by its mass. Thermal capacity may be said to express the total capacity of a given quantity of a substance for absorbing and storing heat. Thermal capacity is stated, not as a ratio, but as a certain number of Btu.

3C4. Sensible heat. When the heat that is applied to a substance merely raises its temperature, but does not change its physical state, such heat is called sensible heat. It is the heat which, added to or subtracted from a substance, produces the changes in temperature indicated on a thermometer. It is the heat concerning which human senses also can give some information, at least within certain ranges. For example, if a person puts his finger into a cup of water, his senses readily tell him whether it is cold, cool, tepid, hot, or very hot.

Human senses are not sufficiently discriminating to give precise information about the

 
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extreme temperatures of ice and steam and other substances having temperatures beyond the range of human sensory mechanisms. Ice merely seems cold and steam seems hot, what ever their temperatures may be. The term sensible heat is applied to the various temperatures of a solid (as ice), or a vapor (as steam), or a gaseous state (as air), indicated on a thermometer. The term sensible heat does not apply to the process of conversion from one physical state to another.

3C5. Latent heat. For heat during the conversion from one physical state to another, a different term is used. This term is latent heat and it is used in two forms: 1) latent heat of fusion in the conversion of a liquid to a solid, or vice versa; and 2) latent heat of vaporization in the conversion of a liquid to a vapor, or vice versa.

3C6. Latent heat of fusion. If heat is applied to a piece of ice at a low temperature, say 0 degrees F, the temperature of the ice gradually rises. This change in temperature, which is indicated by a thermometer placed on the ice, is caused by sensible heat, as stated previously. No change of state occurs during this rise in temperature-the ice remains a solid. But as more heat is added to the ice, a temperature is finally reached at which the ice begins to melt, or turn into a liquid. This added heat now changes the physical state of the water from the solid state to the liquid state. The thermometer on the ice stops rising and remains throughout the melting period (at sea level barometric pressure) at 32 degrees F, In other words, the heat added does not cause any rise in the temperature, but is used entirely in converting the solid to the liquid state.

Heat, when used in the conversion of a solid to a liquid, is called latent heat of fusion, the word latent meaning hidden or not indicated on a thermometer as a temperature change.

But note that at the instant the last bit of ice melts, if we continue to apply heat, the temperature immediately begins to rise. The heat is now again called sensible heat.

The reverse process also takes place. If liquid water at 32 degrees F, and at sea-level atmospheric pressure, is cooled, it is converted (frozen) to ice. All during this freezing process, no change of temperature occurs-all the

  heat removed is latent heat of fusion, and it is used entirely in converting the liquid to a solid, that is, in changing its physical state.

3C7. Value of latent heat of fusion. For the same amount of substance, exactly the same quantity of latent heat of fusion must be added in converting from a solid to a liquid, or must be removed in converting from a liquid to a solid. All substances differ in the quantity of latent heat required per unit amount. The latent heat of fusion for pure water at 32 degrees F, in liquid form or as ice, at sea-level pressure, is 143.33 Btu per pound.

3C8. Latent heat of vaporization. Similarly, if heat is applied to a container filled with cold water, the temperature of the liquid water gradually rises, as seen on a thermometer placed in it. The heat causing the rise in temperature is sensible heat. No physical change of state takes place in the water-it remains a liquid-until the temperature rises to 212 degrees F. At this point, the liquid begins to boil, then turns into steam (vapor) and the temperature stops rising. Throughout the boiling, or vaporization, of the liquid water, its temperature remains unchanged at 212 degrees F. All the heat that is added to it during boiling is latent heat of vaporization, which acts entirely to change the physical state of water from a liquid to a vapor state.

The reverse process also takes place. If steam at 212 degrees F and at sea-level pressure, is cooled, it converts (condenses) to liquid water. Throughout this condensation process, no change in temperature occurs. All the heat removed is latent heat of vaporization, which is used entirely in condensing the vapor to a liquid, that is, in changing its physical state

3C9. Value of latent heat of vaporization. For the same amount of substance, exactly the same quantity of latent heat of vaporization must be added in changing it from a liquid to a vapor as must be removed in changing it from a vapor to a liquid. All substances differ in the quantity of latent heat required per unit amount. For pure water at 212 degrees F, it liquid form or as vapor, at sea-level pressure the latent heat of vaporization is 970.4 Btu per pound. This value varies, of course, for different pressures and temperatures of the same substance.

 
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Figure 3-1. Simple heat diagram.
Figure 3-1. Simple heat diagram.
3C10. Total heat. The term total heat is used with two different meanings and care must be used in reading any text in order that the meaning intended is properly understood. These two usages are as follows:

Strictly speaking, the total heat of a substance is the total heat energy calculated from absolute zero in Btu. It is the specific heat x mass x absolute temperature. However, since there is no instrument for measuring heat directly on this absolute scale, and since it would also require high numbers, other starting points are arbitrarily chosen. For liquid water and steam, the arbitrary starting point is 32 degrees F. For the refrigerant Freon 12, it is -40 degrees F. For example, in a table of data for Freon 12, a column is headed "Heat Content

  From -40 degrees F." The figures in the column represent the number of Btu per pound of liquid or vapor Freon 12 at various temperatures. For practical purposes, we are interested only in differences in total heat at the start and end of the, process. Consequently the choice of the point on which to base the measurement is relatively unimportant.

In refrigeration and air-conditioning, the total heat of a substance or of the air in a room is all the heat present, that is:

Total heat = Sensible heat + Latent heat

3C11. Heat content. The term heat content is sometimes used in discussion. It means the total heat present in a substance.

3C12. Simple heat diagram. In Figure 3-1,

 
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the data on the changes of state with variation of temperature, and the number of Btu required in such changes for a pound of water,   are gathered in a simple graph through a range from 0 degrees to 300 degrees F. This graph is schematic only and is not drawn to scale.
 
D. PRESSURE
 
3D1. Atmospheric pressure. Everything on or in the earth is subject to pressures of various sorts. For example, everything open to the air is under what is called atmospheric pressure. This pressure is caused by the weight of the air above us. With the air near sea level at 32 degrees F, the weight, or pressure, of a column of air 1 inch square in cross-sectional area at the base and reaching from sea level to the upper limit of the earth's atmosphere is 14.696 pounds. This value varies slightly from day to day because of changing conditions in the atmosphere. For practical engineering purposes, standard sea level pressure is considered as 14.7 pounds per square inch.

3D2. Mercury barometer. A mercury barometer is an instrument for measuring atmospheric pressure. It is a vertical glass tube a little over 30 inches long. The upper end is closed, and the lower end is inserted in a small open dish. Both tube and dish contain mercury. The weight of the mercury column in the vertical tube exactly balances the atmospheric pressure on the mercury in the open dish. At sea level pressure of 14.7 pounds per square inch, the mercury column stands at a height of 29.921 inches above the surface of the mercury in the dish, regardless of the size of the cross-section of the mercury column or of the area of the surface of the mercury in the dish. Any variation in atmospheric pressure is indicated by a change in the height of this mercury column. The scale alongside the tube is usually divided into inches or some other unit of length. The space above the top of the mercury in the closed end of the tube is a nearly perfect vacuum. Since it contains no air or other substance, the pressure is practically zero.

3D3. Aneroid barometer. An aneroid barometer is another instrument for measuring atmospheric pressure. It is mechanical in nature, much smaller than the mercury barometer, and less liable to derangement. It consists of

  a small airtight metal box, with a partial vacuum inside, and a flexible side that can move slightly under varying outside (atmospheric) pressures. This motion is communicated by a delicate lever system to a pointer which indicates the atmospheric pressure on a circular scale.

3D4. Converting barometer readings to pressure in pounds per square inch. Aneroid as well as mercury barometers are calibrated in inches. At mean sea level and air temperature of 32 degrees F, the mercury column stands at 29.921 inches, corresponding to an air pressure of 14.696 pounds per square inch. Since 14.696/29.921 is equal to 0.491, to convert a barometer reading in inches to pressure in pounds per square inch, multiply the height of the mercury column in inches by 0.491.

3D5. Variation of pressure and boiling point with altitude. If an uncovered container filled with fresh water at mean sea level is heated until the water boils, a thermometer inserted in the water shows that its temperature is 212 degrees F, and a barometer shows that the atmospheric pressure is approximately 14.7 pounds per square inch.

However, if the pot of boiling water is on a hilltop 1,000 feet above sea level, the thermometer shows that the water boils at 210 degrees F when the barometer reads approximately 14.14 pounds' pressure.

Similar variations in boiling point and barometric pressure are observed at different heights, as indicated in the following table:

Altitude Above Sea Level in Feet Pressure in Pounds per Square Inch Boiling Point of Water in Degrees Fahrenheit
Sea level 14.70 212
2,000 13.57 208
4,000 12.49 204
6,000 11.54 200
8,000 10.62 196

3D6. Pressure-temperature relationship for change of state. It is not these variations of pressure and temperature at different altitudes to which special attention is here

 
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directed, but the relationship between the temperature of vaporization and the corresponding pressure. For it is not necessary to go to different heights to obtain different pressures. Different pressures may be obtained by mechanical means at any location.

For example, a boiling liquid and its vapor may be contained in an airtight metal cylinder with a piston. By pushing in or pulling out the piston, the pressure within may be increased or decreased. If the piston is pushed in, thus increasing the pressure inside, a thermometer shows that the change of state from liquid to vapor requires a temperature higher than 212 degrees F. If the piston is pulled out, thus decreasing the pressure within, the thermometer shows that the change of state from liquid to vapor takes place at a temperature lower than 212 degrees F. Many types of such mechanical arrangements are in common use.

This relationship of vaporization temperature and pressure, which varies for different substances, follows an exact law, and may be tabulated accurately for each substance.

3D7. Pressure gage. Pressures within an air tight system of pipes, tanks, and cylinders, are usually measured by a type of gage known as the Bourbon-tube pressure gage. In this gage there is a small tube, flattened (not round) in cross-section, and curved to about three-quarters of a circle. One end of this curved tube is firmly fixed to the mounting, or case; the other end is free and slightly movable. A delicate lever system which turns a pointer on a circular scale is attached to the free end. The fixed end of this tube is joined by its connections to the vapor system and made part of that system. Increases in vapor pressure tend to straighten the curved tube, thus rotating the pointer. The scale is marked to indicate the pressure values in units of pounds per square inch.

3D8. Reading the pressure gage. The scale on the Bourbon-tube pressure gage is marked with zero to correspond to standard atmospheric pressure. Consequently, zero gage pressure equals 14.7 pounds per square inch. When the pressure of the vapor inside the curved tube is 14.7 pounds per square inch, it is equal to the atmospheric pressure outside the tube, and there is no tendency for the curved tube

  to straighten. Hence this pressure is taken as the zero point of the gage.

3D9. Gage pressure. The pressure indicated by a pressure gage of this type is in reality the difference between the vapor pressure in side and the air pressure outside the curved tube. Readings from such a gage are always designated as gage pressure.

Pounds per square inch. For convenience, this term is indicated by its abbreviated form psi. Often, where the meaning is unmistakable, the word pounds alone is used; for example, 20 pounds' pressure, but 20 pounds per square inch pressure is meant.

3D10. Absolute pressure. The term absolute pressure is used to designate the true total pressure inside the enclosed vapor system. Suppose the pressure gage stands at 6 pounds. Then, since zero gage pressure means 14.7 pounds inside (to balance 14.7 pounds air pressure outside the tube), the total, or absolute pressure of the vapor is 14.7 pounds plus 6 pounds, or 20.7 pounds. If an accurate knowledge of the pressure is required, the atmospheric pressure, converted from a barometer reading, is used instead of the 14.7-pound standard.

3D11. Vacuum, or negative, gage pressure. As stated, the standard atmospheric pressure of 14.7 pounds per square inch is taken as the zero point on the pressure gage. A gage dealing only with increases in that pressure has a single scale marked from 0 to 300 pounds or some other upper limit, and is read in psi gage pressure.

But pressures may decrease below atmospheric pressure as well as increase. Pressures below 14.7 pounds per square inch are known as partial vacuums. This term is used merely for convenience in referring to pressures below ordinary atmospheric pressure, since such a pressure is far from approaching a vacuum or even a partial vacuum.

A gage that registers pressures lower than standard atmospheric pressure is called a vacuum gage. Such gages are graduated to read in inches of vacuum. Approximately 30 inches of vacuum equal zero pounds' absolute pressure.

3D12. Compound gage. A compound gage is

 
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sometimes called a compound pressure and vacuum gage. It also is in frequent use. The gage has an extended range, covering pressures both below and above atmospheric pressure. The scale is graduated to the left and   right of zero (atmospheric pressure). Above atmospheric pressure, readings are in psi; below atmospheric pressure, readings are in inches of vacuum. Freon gages are normally of this type.
 
E. VAPORIZATION
 
3E1. Ebullition and evaporation. There are two kinds of vaporization, ebullition and evaporation.

3E2. Ebullition. Ebullition is the technical term for ordinary boiling. It is a rapid and visible process. By looking into an uncovered container of boiling water, one can see that ebullition (bubbling) is taking place. Starting from the bottom and sides, large and small bubbles rise to the surface and break out of the liquid.

3E3. Evaporation. Evaporation is a slow and invisible process which takes place only from the surface of a liquid. Under ordinary conditions, evaporation cannot be seen. Any liquid in an uncovered container will gradually evaporate, its level falling very slowly until all the liquid is gone. Water vapor continually evaporates from the surfaces of all open bodies of water, rivers, lakes, and seas. Wet clothing or washed articles hung on a line dry by evaporation.

Since evaporation is a form of vaporization, it results in the removal of latent heat. Therefore, it is a cooling process, though a slow one. When a person goes in bathing on a cool day with a wind blowing, it is the evaporation process that makes him feel uncomfortable, rather than the temperature itself. The human body gets rid of excess heat and moisture naturally and continually by evaporation. Some liquids evaporate much faster than water; for example, alcohol.

3E4. Sublimation. There is a third method of converting from one physical state to another.

  This process is called sublimation and consists of changing from a solid directly to the vapor state, without passing through the intermediate liquid state. Ice and snow, even when much below the freezing point, slowly disappear without melting; Washed clothing, hung outdoors in a temperature below 32 degrees F, first freezes stiff, and then dries soft. Both these phenomena are caused by sublimation. Sublimation has little application to refrigeration engineering. It has, however, considerable use in the small-scale cooling of bottled goods, ice cream, and other foodstuffs by the use of solid carbon dioxide (dry ice) which sublimes to a vapor under atmospheric pressure.

3E5. Vapor and gas. The terms vapor and gas both refer to matter in the physical state that is neither solid nor liquid. There is, how ever, a definite distinction between them.

A vapor condenses very readily to the liquid state under small changes of temperature or pressure or both, and constantly does so under ordinary conditions in nature. It may be said to be very close to the liquid state, although it is a vapor. A gas, on the other hand, exists under ordinary conditions in the gaseous state. To change it to the liquid state, special laboratory apparatus with extreme changes of pressure and temperature is required. A gas may be said to be far removed from the liquid state, and cannot change to it under ordinary natural conditions.

In refrigeration, the word gas is frequently used instead of the more correct term vapor.

 
F. PHYSICAL CONDITIONS OF VAPORS AND LIQUIDS
 
3F1. State and condition. The term state is used to refer to the three forms of matter: solid, liquid, and gas or vapor. However, a substance in any one of these three states may be found in different conditions, and hence the term condition is also used.   3F2. The two conditions of vapor. A vapor ordinarily exists in either of two conditions, either as saturated vapor or as superheated vapor.

3F3. Saturated vapor. A saturated vapor is one that is at the temperature corresponding

 
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to the boiling point of the substance at any given pressure. The boiling liquid and its saturated vapor are always at the same temperature. Saturated vapors may be either wet or dry.

a. Wet saturated vapor. When a vapor contains some liquid particles, in the form of fine mist or tiny droplets, it is called wet saturated vapor.

b. Dry saturated vapor. When no liquid particles are present, the vapor is said to be dry saturated vapor. In practice, vapors are usually wet. It is not easy to produce a completely dry vapor, because boiling, by its agitation of the liquid and the rising bubbles of vapor, always throws a number of liquid particles out beyond the surface of the liquid. Some of these liquid particles remain suspended and are carried by the vapor. Also, in any long piping system a small loss of heat through the pipes themselves is probable. This causes some condensation, with the resulting appearance of liquid mist in the vapor.

3F4. Superheated vapor. Saturated vapor and the boiling liquid with which it is in contact have only one temperature, and that temperature is the result of the existing pressure.

  However, if a vapor is not in contact with a boiling liquid, either because the liquid has all been converted into vapor, or because the vapor has been separated from contact with the boiling liquid, further application of heat produces a rise in the temperature of the vapor under the same given pressure. Such a vapor is called superheated vapor.

The quantity of superheat in such a vapor is equal to the difference between its temperature and the temperature of its saturated vapor at the same pressure. For example, superheated vapor at 20 degrees F above its saturated temperature is said to contain 20 degrees of superheat.

3F5. Saturation temperature. If a liquid is heated, it finally boils at a temperature that is the result of the pressure present. Such a temperature is called the saturation temperature corresponding to the given pressure. This term is frequently used in air-conditioning and means merely the boiling point or the condensation point at the given pressure.

3F6. Saturated liquid. A liquid that is at the saturation temperature corresponding to a given pressure, and is under that pressure, is termed a saturated liquid.

 
G. EXPANSION AND CONTRACTION OF SUBSTANCES
 
3G1. Variation of size with change in temperature. In general, all substances-solids, liquids, and gases-decrease in volume when cooled and increase in volume when heated. In gases and vapors, the amount of change is large; in liquids and solids it is small. In all cases, great forces are produced and it is necessary in all engineering construction to allow for the operation of these forces. Different substances vary in the amount of change in volume they undergo for the same difference in temperature.

3G2. Expansion and contraction of water. Water contracts as it is cooled, until the temperature 39.2 degrees F is reached. At this point, the change in volume reverses, and if the water is further cooled, the volume increases instead of continuing to decrease. When water freezes into ice, an enormous force is brought into play. This force is sufficient to split large rocks, burst iron pipes, and even steel tanks,

  unless provision is made to allow for the expansion.

3G3. Expansion and contraction of the change of state. At their melting points, substances follow no general rule regarding expansion and contraction. Some metals, like iron, bismuth, and antimony, contract on melting and expand on solidifying; but most others, like gold, silver, and copper, expand on melting and contract on solidifying. All liquids, however, expand greatly when changing into vapor, unless constrained mechanically, as in a closed container. An example of this expansion is the large clouds of "steam" continually rising from a container of boiling water.

3G4. Specific volume. The specific volume of a substance is a number that indicates the number of cubic feet occupied by 1 pound of that substance at a given temperature and pressure. Specific volume varies greatly for

 
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different substances and for the same substance at different temperatures and pressures. For example, the specific volume of liquid boiling water at atmospheric pressure is 0.0167 cubic feet per pound, and of steam   at the same pressure it is 26.79 cubic feet per pound. Thus, water in changing its state from liquid to vapor at ordinary atmospheric pressure increases in volume 1604 times (26.79 / 0.0167).
 
H. HEAT TRANSFER
 
3H1. Heat travels. Heat travels, and its flow can be definitely felt as it comes from the sun or from a fire. Heat moves from one place to another in one of three ways: 1) by radiation, 2) by convection, and 3) by conduction. These three processes may take place singly or in combination.

3H2. Radiation of heat. In radiation, heat is transmitted through empty space (a vacuum), as from the sun to the earth's atmosphere. Heat, light, electricity, radio, x-rays, are all known to, be energy in the form of transverse vibrations. Physically, they differ only in their wave lengths, but their physical effects are quite different, as is evident by comparing heat with radio waves. In radiation, nothing but energy really travels. Radiation is the propagation of energy of vibration. Radiation also takes place through air and transparent substances. Radiation does not heat the air through which it passes; it heats only the object upon which it falls. Not only the sun, but all other objects such as flames, stoves, electric light bulbs, our bodies, machines, foods, streets, buildings, walls, and the earth itself radiate heat to some extent.

3H3. Convection of heat. In convection, heat actually does travel. Convection is the movement through space of heat-containing particles of a substance in the form of a cur rent of heat-containing particles. This current may be small or large. Examples include: a current of warm air in a room; a current of hot water, steam, or other fluid in a pipe; a current of warmer water flowing in the ocean, such as the Gulf Stream. The human body gives off excess heat not only by radiation, but also by conduction and convection every

  time a breath is taken. The air breathed, after having picked up some heat in the lungs, passes out again in a current, carrying heat with it.

3H4. Conduction of heat. The transfer of heat energy from one molecule to another, either of the same substance or of different substances, is called conduction. A molecule of a substance is the smallest particle of a substance that retains the special qualities of that substance. Any further subdivision of a molecule separates it into the atoms of which it is composed. Physical contact is necessary for the conduction of heat, and the conduction takes place from the region of higher temperature to the region of lower temperature. For example, if a person holds a bar of iron in his hand with one end of the bar in a fire, the heat passes by conduction from the fire into the end of the bar, then by conduction along the bar, and finally by conduction to the hand. In each case, the energy moves from a region of higher temperature to a region of lower temperature.

3H5. Thermal conductance. Suppose that two bars are held, one of iron and one of copper, of exactly the same size and temperature. If an end of each bar is placed in a fire at the same time, it will be noticed that heat reaches the hand through the copper bar much more quickly than it does through the iron bar. It is thus evident that some substances conduct heat more-readily than others. This characteristic of a substance is called its thermal conductance or heat conductance. The low thermal conductance of some substances is of great value in both heating and refrigerating, in preventing a flow of heat.

 
I. INSULATION OF HEAT
 
3I1. Need for insulation. It is comparatively easy to heat or cool articles or enclosed spaces. However, it is not so easy to keep them hot or   cold very long. Heat constantly tends to flow from higher to lower temperature levels.

If it is desired that a substance or an

 
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enclosed space be kept hot, it is necessary to prevent the heat already present from flowing out. If it is desired that a substance be kept cold, it is necessary to prevent heat from flowing in. Fortunately, this can be done, to a fairly successful extent, by making use of the low thermal conductance of certain substances.

3I2. Good conductors and poor conductors. Different substances vary greatly in heat conductance. In general, metals are good conductors, natural liquids are poor conductors, and gases very poor conductors. Nonmetallic solids are usually poor conductors. Poor conductors are also called heat insulators.

3I3. Insulators. Poor conductors include such substances as cork, wood, sawdust, paper, brick, fur, feathers, felt, animal wool, asbestos, glass, rubber, plastics, cotton, water, and dead air spaces.

Most solids that are poor conductors are also porous in nature (with important exceptions like glass, rubber, and the plastics), and the air pores, or air cells, are small in size. Much of the insulating quality of these substances results from the presence of in numerable tiny pockets of enclosed air, and from the fact that air is a poor conductor of heat. The air cells must be small; if they are not, the insulating quality is diminished, because the larger spaces of air permit heat to pass through by radiation and convection.

3I4. Low-temperature insulation. The requirements for low-temperature insulation are somewhat different from those for high-temperature insulation. Any water vapor present in the air tends to condense into liquid drops or film on a cold surface. This is commonly called sweating. This sweated water penetrates a porous insulating material and fills the air cells, thus greatly lessening its insulating ability. It may even freeze there, and ice is a poor insulator of heat. Insulating materials for use with refrigerating systems are therefore manufactured especially to resist the penetration of moisture and to be durable in the presence of conditions of high moisture.

  3I5. Insulation of cold pipes. Low-temperature pipe lines must be thoroughly insulated to prevent heat from entering the refrigerant contained therein. The usual insulation is a cork composition molded into sections that fit snugly around the pipes and fittings. Other materials, such as rock wool and mineral wool, are also molded in the same way. Fittings in elude bends, elbows, and tees.

Before applying the cork covering, all pipe lines should be carefully cleaned and all rust scrubbed away to a clean metal surface with a stiff wire brush.

If possible, the hangers or braces that support the pipes should be placed around the outside of the covering. If the hangers are attached directly to the pipes, heat travels by conduction through them to the pipe. Moisture may also enter along such hangers and freeze, causing the covering to burst. When molded sections of covering are placed on pipes, the sections should be staggered and all end joints thoroughly coated with waterproof cement. Longitudinal joints should come at the top and bottom of pipes, and not at the sides. After the covering is placed properly, all seams should be rubbed flush and smooth with brine putty and the whole surface of the covering painted with asphalt paint.

Cold-water pipes are frequently insulated with fibrous materials, such as felted hair or various vegetable fibers. Such fibrous materials, when used for insulating, must be completely covered with canvas or similar fabric, and painted to make the covering waterproof.

3I6. Repair of cold pipe insulation. In the event of damage to the insulation covering a pipe, if molded sections are not available, use whatever materials may be at hand to prevent 1) the entrance of heat, and 2) the entrance of moisture. In general, this requires water proofing by whatever means may be available. Particular attention must be given to the seams.

 
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