Nature of Heat
Heat is a form of energy contained to some extent in every
substance on earth. All known elements are made up of very
small particles, known as atoms, which, when joined together,
form molecules. These molecules are particular to the form
they represent. For example, carbon and hydrogen in certain
combinations form sugar and in others form alcohol.
 Molecules
are in a constant state of motion. Heat is a form of
molecular energy that results from the motion of these
molecules. The temperature of the molecules dictates
to a degree the molecular activity within a substance.
For this reason, substances exist in three different
states or forms�solid, liquid, and gas. Water, for example,
may exist in any one of these states. As ice, it is
a solid; as water, it is a liquid; and as steam, it
is a gas (vapor).
When heat is added to a substance, the rate of molecular
motion increases, causing the substance to change
from a solid to a liquid, and then to a gas (vapor).
For example, in a cube of ice, molecular motion is
slow, but as heat is added, molecular activity increases,
changing the solid "ice" to a liquid "water". Further
application of heat forces the molecules to greater
separation and speeds up their motion so that the
water changes to steam. The steam formed no longer
has a definite volume, such as a solid or liquid has,
but expands and fills whatever space is provided for
it.
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Heat cannot be destroyed or lost. However, it can be transferred
from one body or substance to another or to another form
of energy. Since heat is not in itself a substance, it can
best be considered in relation to its effect on substances
or bodies. When a body or substance is stated to be cold,
the heat that it contains is less concentrated or less intense
than the heat in some warmer body or substance used for
comparison.
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Units of Heat
In the theory of heat, the speed of the molecules indicates
the temperature or intensity of heat, while the number of
molecules of a substance indicates the quantity of heat.
The intensity and quantity of heat may be explained in
the following simple way. The water in a quart jar and in
a 10-gallon container may have the same intensity or temperature,
but the quantity of heat required to raise these amounts
of water to a higher uniform temperature (from their present
uniform temperature) will differ greatly. The 10 gallons
of water will absorb a greater amount of heat than the quart
jar of water.
The amount of heat added to, or subtracted from, a body
can best be measured by the rise or fall in temperature
of a known weight of a substance. The standard unit of heat
measure is the amount of heat necessary to raise the temperature
of 1 pound of water 1°F at sea level when the water
temperature is between 32°F and 212°F. Conversely,
it is also the amount of heat that must be extracted to
lower by 1 o F the temperature of a pound of water between
the same temperature limits. This unit of heat is called
a British thermal unit (Btu). The Btu's equivalent in the
metric system is the calorie, which is the amount of heat
required to raise one gram of water 1 o Celsius.
Suppose that the temperature of 2 pounds of water was raised
from 35°F to 165°F. To find the number of Btu required
to increase the temperature, subtract 35 from 165. This
equals a 130° temperature rise for 1 pound of water.
Since 2 pounds of water were heated, multiply 130 by 2,
which equals 260 Btu required to raise 2 pounds of water
from 35°F to 165°F.
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Measurement
of Heat
The usual means of measuring temperature is a thermometer.
It measures the degree or intensity of heat and usually consists
of a glass tube with a bulb at the lower portion of the tube
that contains mercury, colored alcohol, or a volatile liquid.
The nature of these liquids causes them to rise or fall uniformly
in the hollow tube with each degree in temperature change.
Thermometers are used to calibrate the controls of refrigeration.
The two most common thermometer scales are the Fahrenheit
and the Celsius.
On the Fahrenheit scale, there is a difference of 180°
between freezing (32°) and the boiling point (212°)
of water. On the Celsius scale, you have only 100° difference
between the same points (0° freezing and 100° boiling
point).
Of course, a Celsius reading can be converted to a Fahrenheit
reading, or vice versa. This can be expressed in terms of
the following formula: F = (C x 1.8) + 32 To change Fahrenheit
to a Celsius reading, the terms of the formula are as follows:
C = (F-32) ÷ 1.8
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Transfer of Heat
Heat flows from a substance of higher temperature to bodies
of lower temperature in the same manner that water flows down
a hill, and like water, it can be raised again to a higher
level so that it may repeat its cycle.
When two substances of different temperatures are brought
in contact with each other, the heat will immediately flow
from the warmer substance to the colder substance. The greater
the difference in temperature between the two substances,
the faster the heat flow. As the temperature of the substances
tends to equalize, the flow of heat slows and stops completely
when the temperatures are equalized. This characteristic
is used in refrigeration. The heat of the air, of the lining
of the refrigerator, and of the food to be preserved is
transferred to a colder substance, called the refrigerant.
Three methods by which heat may be transferred from a warmer
substance to a colder substance are conduction, convection,
and radiation.
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Specific Heat
Specific Heat is the ratio between the quantity of heat required
to change the temperature of 1 pound of any substance 1°F,
as compared to the quantity of heat required to change 1 pound
of water 1°F. Specific heat is equal to the number of
Btu required to raise the temperature of 1 pound of a substance
1°F. For example, the specific heat of milk is .92, which
means that 92 Btu will be needed to raise 100 pounds of milk
1° F. The specific heat of water is 1, by adoption as
a standard, and specific heat of another substance (solid,
liquid, or gas) is determined experimentally by comparing
it to water. Specific heat also expresses the heat-holding
capacity of a substance compared to that of water.
A key rule to remember is that 0.5 Btu of heat is required
to raise 1 pound of ice 1° F when the temperature is
below 32°F; and 0.5 Btu of heat is required to raise
1 pound of steam 1°F above the temperature of 212°F.
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Sensible Heat
Heat that is added to, or subtracted from, a substance that
changes its temperature but not its physical state is called
SENSIBLE HEAT. It is the heat that can be indicated on a thermometer.
This is the heat human senses also can react to, at least
within certain ranges. For example, if a person put their
finger into a cup of water, the senses readily tell that person
whether it is cold, cool, tepid, hot, or very hot. Sensible
heat is applied to a solid, a liquid, or a gas/vapor as indicated
on a thermometer. The term sensible heat does not apply to
the process of conversion from one physical state to another.
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Latent Heat
Latent Heat, or hidden heat, is the term used for the heat
absorbed or given off by a substance while it is changing
its physical state. When this occurs, the heat given off or
absorbed does NOT cause a temperature change in the substance.
In other words, sensible heat is the term for heat that affects
the temperature of things; latent heat is the term for heat
that affects the physical state of things.
To understand the concept of latent heat, you must realize
that many substances may exist as solids, as liquids, or
as gases, depending primarily upon the temperatures and
pressure to which they are subjected.
To change a solid to a liquid or a liquid to a gas, ADD
HEAT; to change a gas to a liquid or a liquid to a solid,
REMOVE HEAT. Suppose you take an uncovered pan of cold water
and put it over a burner. The sensible heat of the water
increases and so does the temperature. As you continue adding
heat to the water in the pan, the temperature of the water
continues to rise until it reaches 212°F. What is happening?
The water is now absorbing its latent heat and is changing
from a liquid to a vapor. The heat required to change a
liquid to a gas (or, the heat that must be removed from
a gas to condense it to a liquid) without any change in
temperature is known as the LATENT HEAT OF VAPORIZATION.
 Now
suppose you take another pan of cold water and put it in a place
where the temperature is below 32°F. The water gradually loses
heat to its surroundings, and the temperature of the water drops
to 32°F until all the water has changed to ice. While the
water is changing to ice, however, it is still losing heat to
its surroundings. The heat that must be removed from a substance
to change it from a liquid to a solid (or, the heat which must
be added to a solid to change it to a liquid) without change in
temperature is called the LATENT HEAT OF FUSION. Note the amount
of heat required to cause a change of state (or the amount of
heat given off when a substance changes its state) varies according
to the pressure under which the process takes place.
The picture shows the relationship between sensible heat
and latent heat for one substance – water at atmospheric
pressure. To raise the temperature of 1 pound of ice from
0°F to 32°F, you must add 16 Btu. To change the
pound of ice at 32°F to a pound of water at 32°F,
you add 144 Btu (latent heat of fusion). There is no change
in temperature while the ice is melting. After the ice is
melted, however, the temperature of the water is raised
when more heat is applied. When 180 Btu are added, the water
boils. To change a pound of water at 212°F to a pound
of steam at 212°F, you must add 970 Btu (latent heat
of vaporization). After the water is converted to steam
at 212°F, the application of additional heat causes
a rise in the temperature of the steam. When you add 44
Btu to the steam at 212°F, the steam is superheated
to 300°F.
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Total Heat
Total Heat is the sum of sensible heat and latent heat. Since
measurements of the total heat in a certain weight of a substance
cannot be started at absolute zero, a temperature is adopted
at which it is assumed that there is no heat; and tables of
data are constructed on that basis for practical use. Data
tables giving the heat content of the most commonly used refrigerants
start at 40°F below zero as the assumed point of no heat;
tables for water and steam start at 32°F above zero. Tables
of data usually contain a notation showing the starting point
for heat content measurement.
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Day-Ton
of Refrigeration
A day-ton of refrigeration (sometimes incorrectly called a
ton of refrigeration) is the amount of refrigeration produced
by melting 1 ton of ice at a temperature of 32°F in 24
hours. A day-ton is often used to express the amount of cooling
produced by a refrigerator or air-conditioner. For example,
a 1-ton air-conditioner can remove as much heat in 24 hours
as 1 ton of 32°F ice that melts and becomes water at 32°F.
It is a rate of removing heat, rather than a quantity of
heat. A rate can be converted to Btu per day, hour, or minute.
To find the rate, proceed as follows:
Per Day: Multiply 2,000 (number of pounds of ice in 1 ton)
by 144 (latent heat of fusion per pound) = 288,000 Btu per
day
Per Hour: 288,000 (Btu per day) ÷ 24 (hours in a
day) = 12,000 So, a "1-ton" air-conditioner would
have a rating of 12,000 Btu per hour.
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Pressure
Pressure is defined as a force per unit area. It is usually
measured in pounds per square inch (psi). Pressure may be
in one direction, several directions, or in all directions,
as shown in picture below. The ice (solid) exerts pressure
downward. The water (fluid) exerts pressure on all wetted
surfaces of the container. Gases exert pressure on al I inside
surfaces of their containers.
Exertion of Pressure
Pressure is usually measured on gauges that have one of
two different scales. One scale is read as so many pounds
per square inch gauge (psig) and indicates the pressure
above atmospheric pressure surrounding the gauge. The other
type of scale is read as so many pounds per square inch
absolute (psia) and indicates the pressure above absolute
zero pressure (a perfect vacuum).
Atmospheric Pressure
Atmospheric pressure is the pressure of the weight of air
above a point on, above, or under the earth. At sea level,
ATMOSPHERIC PRESSURE is 14.7 psia, as shown in picture below.
As one ascends, the atmospheric pressure decreases about
1.0 psi for every 2,343 feet. Below sea level in excavations
and depressions, atmospheric pressure increases. Pressures
under water differ from those under air only because the
weight of the water must be added to the pressure of the
air.
Atmospheric Pressure
Scale Relationships A relationship exists between
the readings of a gauge calibrated in psig and calibrated
in psia. As shown in the picture below, when the psig gauge
reads 0, the psia gauge reads the atmospheric pressure (14.7
psia at sea level). In other words, the psia reading equals
the psig reading plus the atmospheric pressure (7.7 psia
at 16,400 feet), or, a psig reading equals the psia reading
minus the atmospheric pressure.
Scale Relationships
For pressure less than the atmospheric pressure (partial
vacuums), a measuring device with a scale reading in inches
of mercury (Hg) or in inches of water (H2O) is used. A perfect
vacuum is equal to -30 inches of mercury or -408 inches
of water (fig. 6-5). In refrigeration work, pressures above
atmospheric are measured in pounds per square inch, and
pressures below atmospheric are measured in inches of mercury.
Effects of Pressure on Gases
Pressure-Volume Relationship
The exertion of pressure on a substance with a constant
temperature decreases its volume in proportion to the increase
of pressure. For example, suppose that a given amount of
gas is placed in a cylinder that is sealed on one end and
has a movable piston on the other end. When 60 psi of absolute
pressure is exerted on the piston, as shown in view A of
picture above, the volume of the gas is compressed to 3
cubic feet. When 90 psi of absolute pressure is exerted
on the piston, as shown in view B, the volume of the gas
is compressed to 1.5 cubic feet. Finally, when 180 psi of
absolute pressure is exerted on the piston, as shown in
view C, the volume of the gas is compressed to 1 cubic foot.
Thus, if a given amount of gas is confined in a container
and subject to changes of pressure, its volume changes,
so the product of volume multiplied by absolute pressure
is always the same.
 Pressure
has a relationship to the boiling point of a substance
There is a definite temperature at which a liquid
boils for every definite pressure exerted upon it.
For instance, water boils at 212�F at atmospheric
pressure (14.7 psia), shown in view A, figure 1.
The same water boils at 228�F if the pressure
is raised 5.3 psig (20 psia), as shown in view B,
figure 1.
This effect of reduced pressure on the boiling
temperature of refrigerants makes the operation
of a refrigeration system possible. The pressure
temperature relationship chart below gives the pressures
for several different refrigerants.
An increase in the temperature of a refrigerant
results in an increase in pressure, and a decrease
in temperature causes a decrease in pressure. By
the same token, a decrease in pressure results in
a corresponding decrease in temperature.
This means that as the pressure of a refrigerant
is increased, so is the temperature at which the
refrigerant boils. Thus, by regulating the pressure
of the refrigerant, the temperature at which evaporation
takes place and at which the latent heat of evaporation
is used can be controlled. |
Pressure Temperature Chart
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Vaporization
Vaporization is the process of changing a liquid to vapor,
either by evaporation or boiling. When a glass is filled
with water, as shown in picture above, and exposed to the
rays of the sun for a day or two, you should note that the
water level drops gradually. The loss of water is due to
evaporation. Evaporation, in this case, takes place only
at the surface of the liquid. It is gradual, but the evaporation
of the water can be speeded up if additional heat is applied
to it. In this case, the boiling of the water takes place
throughout the interior of the liquid. Thus the absorption
of heat by a liquid causes it to boil and evaporate.
Vaporization can also be increased
by reducing the pressure on the liquid, as shown above Pressure reduction lowers the temperature at which liquid
boils and hastens its evaporation. When a liquid evaporates,
it absorbs heat from warmer surrounding objects and cools
them. Refrigeration by evaporation is based on this method.
The liquid is allowed to expand under reduced pressure,
vaporizing and extracting heat from the container (freezing
compartment), as it changes from a liquid to a gas.
After the gas is expanded (and heated), it is compressed,
cooled, and condensed into a liquid again.
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Condensation
 Condensation
is the process of changing a vapor into a liquid. For example,
a warm atmosphere gives up heat to a cold glass of water,
causing moisture to condense out of the air and form on the
outside surface of the glass. Thus the removal of heat from
a vapor causes the vapor to condense.
An increase in pressure on a confined vapor also
causes the vapor to change to a liquid. This fact
to the right. When the compressor increases the pressure
on the vapor, the condensing vapor changes to a liquid
and gives up heat to the cooler surrounding objects
and atmosphere.
These conditions exist when the vaporized refrigerant
is compressed by the compressor of a refrigeration
system and forced into the condenser. The condenser
removes the superheat, latent heat of vaporization,
and, in some cases, sensible heat from the refrigerant. |
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