THEORY OF HEAT
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
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
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
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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.
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
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
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
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
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
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
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.
3E1. Ebullition and evaporation. There are
two kinds of vaporization, ebullition and
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
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
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
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
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
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
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
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
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
| 12 |
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
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
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
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
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
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
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
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Copyright (C) 2004 Maritime Park Association
All Rights Reserved
Version 1.20, 22 Oct 04