Download Principles of Refrigeration Chapter 5 PPT

Internal Properties of Matter
Topics for Discussion
•Law of Conservation of Energy
•Types of Internal Energy
•Physical States of Matter
•Temperature
•Heat and the convertibility of heat and work
Conservation of Energy
•The law of conservation of energy states that
energy cannot be created or destroyed, it can only
be converted from one form to another.
•Work done on an object changes its levels of
external kinetic and potential energy.
•Therefore the amount of work done on an object
changes its energy level. Energy can be described as
the amount of work stored within a substance.
Conservation of Energy
•When work is done on an object, some of the work
is converted into energy that is stored within its
molecular structure for use at a later time.
•In addition to these external or mechanical forms of
energy, an object also possesses energy in a number
of internal forms.
•This internal energy is stored within the molecular
structure of substances, and can be released as work
Conservation of Energy
•Work, external energy, and internal energy are all
interchangeable. No matter what types of energy are
stored within an object.
•Electrical energy can be converted to thermal
energy in an electric heater. Electrical energy can be
converted to mechanical energy in an electric motor.
Chemical energy can be converted to electrical
energy in batteries to power our portable devices.
Thermodynamics
•Thermodynamics is the study of a substance’s
energy-related properties and processes.
•Thermodynamics quantifies the changes in energy
that occur as heat is transferred from one object to
another.
•In the HVAC-R field the processes that we utilize
generate changes in the internal and external energy
levels in substances within the field.
Types of Internal Energy
•Internal energy is the energy contained within a
substance by virtue of the motions and forces
between its individual atoms and molecules.
•Internal kinetic energy is the energy of molecular
motion. Molecules are always in a state of rapid and
constant vibration.
•The rate of these vibrations depends on the amount
of internal kinetic energy that the substance has.
Types of Internal Energy
•One method of altering the internal kinetic energy
of a substance is to transfer thermal energy to it.
•Whenever thermal energy transfers into a
substance, it increases the intensity of the molecular
vibrations. And when ever thermal energy is
removed from a substance it reduces a substances
kinetic energy, and slows the vibrations.
Types of Internal Energy
•A relationship exists between the kinetic energy
content of a substance, its level of molecular
vibrations, and its temperature.
•The temperature of a substance is an index of the
average velocity of molecular vibrations, and
therefore the amount of kinetic energy stored in its
molecules.
•At absolute zero internal kinetic energy is zero.
Internal Potential Energy
•The molecules of a substance are held together by
cohesion. Cohesion is the tendency of matter to hold
itself together be attractive forces between
molecules.
•As two molecules or atoms approach each other,
their internal potential energies decrease towards a
minimum value.
•This min value is called the equilibrium distance.
Internal Potential Energy
•Increasing or decreasing the equilibrium distance
requires the performance of work.
•Work can push the molecules closer together or pull
them farther apart.
•Whenever work is done to move molecules closer
together, the applied force must overcome a rapidly
increasing force reactive force called compressive
elasticity.
Internal Potential Energy
•Similarly any work done to increase the distance
between the molecules is opposed by cohesive
forces.
•Cohesive forces are greatest in solid, much weaker
in liquids and nonexistent in gases.
•Since molecules are bound to one another by
cohesive forces, internal work must be done in order
to further separate them.
Internal Potential Energy
•When a material expands, contracts or changes its
physical state, a rearrangement of its molecules
takes place in response to the work or change in
energy.
•These changes alter the average distance between
molecules within a substance. Therefore the work
done, or energy transfer generates changes in the
quantity of stored internal potential energy.
Internal Potential Energy
•This change in the stored energy of the substance
has no influence on the intensity of the molecular
vibrations. Therefore the kinetic energy stays
constant during potential energy altering processes.
•Kinetic energy = energy in motion = molecular
movement = temperature = sensible heat.
•Potential energy = stored energy = physical state =
state change = latent heat.
Physical States of Matter
•Many chemical compositions, under the proper
conditions of pressure and temperature, can exist in
three distinct phases, or states of matter.
•A material in its solid phase has relatively small
amounts of internal potential and kinetic energies.
•Its molecules are generally positioned at their
equilibrium distance, arranged in well formed
crystalline structures.
Physical States of Matter
•The molecules of a substance in its liquid phase
have more kinetic and potential energy than they
have when they are in their solid phase.
•With an increase in kinetic energy absorbed by a
solid substance, increases the molecular vibration
increasing the temperature of the substance.
•An additional increase in potential energy absorbed
causes the molecules to break their cohesive bonds.
Physical States of Matter
•When a solid breaks its crystalline structure bonds,
the material begins to flow.
•The ability for a substance to flow is a characteristic
of all fluids, no matter if they are liquids, vapors or
gases.
•Consequently liquids cannot retain their shape and
are forced to assume the shape of the vessel in
which they are contained.
Physical States of Matter
•Liquids are also considered incompressible because
their density and molecular spacing are nearly the
same as they are in the solid phase.
•The incompressibility and flow characteristics of a
liquid permit forces to be transmitted equally in all
directions.
•Pascal’s law demonstrates this characteristic,
utilized in hydraulic systems.
Physical States of Matter
•As a liquid substance absorbs more heat, its
molecules vibrate with greater forces.
•This additional energy overcomes all restraining
forces between its molecules, eliminating the
molecules cohesive bonds.
•The gas molecules are free to fly about at high
velocities, for this reason a gas cannot retain its size
or shape.
Physical States of Matter
•The lack of cohesive structure means that gases are
readily compressible.
•The also move to maintain the greatest possible
distance between the molecules, completely filling
the vessel in which they are contained.
•If a gas is not contained in sealed contained, it will
escape and diffuse into the surrounding ambient.
Physical States of Matter
•A vapor is a gas that exists at conditions that permit
it be easily returned to its liquid state.
•As a liquid absorbs sufficient potential energy, some
of the molecules will break free of their cohesive
bonds and turn to a vapor.
•The molecules that have sufficient energy to break
their bonds and remain free of the liquid surface are
classified as a vapor.
Physical States of Matter
•As more energy is added to a vapor, its internal
kinetic energy increases, this raises the temperature
of the vapor making the molecules move faster.
•In this condition it becomes more difficult to return
the molecules to their liquid state.
•When the temperature of a vapor is raised to a level
much higher than the temp at which it changed state
from a liquid, the molecules are classified a gas.
Temperature
•The temperature of a substance is a measure of the
amount of kinetic energy it contains.
•Temperature is a property of a substance that
measures its thermal intensity. Thermal intensity is
an indication of the average molecular velocity
within a substance.
•Temperature is also used to indicate the direction of
thermal energy transfer.
Temperature
•Thermometers are used to measure temperature,
their operation depends on the characteristic of
liquids to expand when their temperature is
increased.
•The coefficient of expansion is the rate at which a
substance expands when its temperature changes.
•Mercury thermometers are more accurate than
alcohol because their coefficient of expansion is
more consistent through a greater temp range.
Heat
•Heat is the transfer of energy produced by a
temperature change.
•Heat is the thermal counterpart of energy transfer
by work in mechanical systems, where work is
transfer of energy by a force that moves a mass
through a distance.
•Heat is the transfer of energy from one object to
another caused by a thermal force created by a
temperature difference between the objects.
Heat
•Heat transfers internal kinetic energy from the
molecules of the warmer object to those of the
cooler object.
•This energy transfer reduces the kinetic energy level
of the molecules in the warmer object, producing a
corresponding decrease in its temperature.
•Simultaneously the cooler objects kinetic energy is
increasing.
Heat
•Energy transfers that affect the temperature of
objects are called heat.
•Although heat transfer occurs primarily in response
to temperature difference, it can also be brought
about by friction forces.
•Friction forces are conflicts and interactions that
occur on the molecular level as objects move relative
to one another.
Heat
•These interactions generate changes in the kinetic
energy levels of the molecules located at the
interface between two surfaces.
•The elastic stretching and snapping of the molecules
sliding past each other changes their velocities and
the temperatures of the objects.
•This change in thermal energy is a conversion
process where some of the work done is stored
energy in the substance.
Heat
•There is a technical difference between the terms
thermal energy and heat.
•Thermal energy is the ability if an object to do work
that results from energy contained within its
molecule structure.
•Heat is the actual condition of energy being
transferred, not a property of a substance.
Heat Transfer
•The phrase heat transfer is often used to describe
the energy transfer, although the term heat
incorporates the transfer aspect of the energy
movement.
•Thermal energy will be transmitted from one object
to another when they are at different temperatures.
•The following three relationships govern the flow of
thermal energy between objects.
Heat Transfer
1. Heat flows from a higher temperature to a lower
temperature
2. Whenever an object is in thermal equilibrium
with its surroundings, both will have the same
temperature. Therefore no thermal energy will
transfer between the objects.
3. Heat transfer cannot spontaneously occur from
cool to warm, without work done on the system.
Heat Transfer
• A mechanical refrigeration system is an example
of a system that moves heat from a cooler
location to a warmer one.
• Since heat is energy, and cannot be consumed or
destroyed in any process, all of the thermal
energy that leaves one object must be absorbed
by other objects, or substances.
Conduction Heat Transfer
• Energy transfer by conduction requires physical
contact exist between the objects transferring
heat.
• Therefore, this mode is only applicable between
solids and motionless liquids.
• The contact allows kinetic energy to physically
transfer from molecule to molecule.
Conduction Heat Transfer
• As the higher energy level molecules of the
warmer object collide with the slower moving
molecules of the cooler object, an energy transfer
occurs between the objects.
• This energy transfer occurs along the interface
where the objects are touching, as the faster
molecules collide with the slower molecules the
faster molecules slow down as the slower
molecules now speed up.
Conduction Heat Transfer
• Some materials are better at conducting heat than
others, Iron, Copper, Silver, Gold and similar
metals are good conductors of heat.
• The increase in the rate of heat transfer in good
thermal conductors occurs because these
materials employ two methods to transfer kinetic
energy.
• In addition to energy transfer due to collisions,
energy can also transfer due to free electrons.
Conduction Heat Transfer
• In addition to energy transfer due to collisions,
energy can also transfer due to free electrons.
• Free electrons are valance electrons within
material that break free from the orbits of their
parent atoms when they absorb sufficient thermal
or electrical energy.
• These electrons are then free to move through the
material, transferring their excess kinetic energy.
Conduction Heat Transfer
• Conversely, materials that are insulators can only
conduct energy by molecular vibrations to
adjoining molecules.
• The relative capacity of a material to conduct heat
is known as its thermal conductivity (k).
• Thermal conductivity is a measure of the amount
of energy that can pass through one square foot
unit of material, one inch thick, in one hour, with
1°F temperature difference. Btu/ft2/hr/°F
Convection Heat Transfer
• Heat transfer by convection can only occur in
moving fluids. Thermal energy is moved by
currents that form within a fluid.
• Natural convection currents are generated by
changes in a fluid’s density brought about by the
expansion of the portion of the fluid being
heated.
• As a fluid is heated it expands increasing its
specific volume, becoming more buoyant.
Convection Heat Transfer
• Heat transfer by convection can only occur in
moving fluids. Thermal energy is moved by
currents that form within a fluid.
• Natural convection currents are generated by
changes in a fluid’s density brought about by the
expansion of the portion of the fluid being
heated.
• As a fluid is heated it expands increasing its
specific volume, becoming more buoyant.
Radiation Heat Transfer
• Radiation heat transfer occurs between materials
at different temperatures within sight of each
other.
• By its nature radiation heat transfer does not rely
on physical contact, moving fluids, or any other
medium to exchange energy.
• Therefore heat transfer across a vacuum is only
possible by radiation.
Radiation Heat Transfer
• The term radiation indicates that the energy is
transferred between objects in the form of
electromagnetic waves.
• Electromagnetic waves of heat are very similar to
electromagnetic waves of light. They only differ in
their energy level, frequency and therefore
wavelength.
• Thermal energy waves are found in the infrared
portion on the electromagnetic spectrum.
Radiation Heat Transfer
• Whether heat waves are visible or invisible
depends on the temperature of the radiating
object.
• When electromagnetic energy waves produced by
an object are intercepted by another object, they
can be either absorbed, reflected and/or
transmitted through the intercepting material.
Radiation Heat Transfer
• When the radiant energy is absorbed the energy
in the wave increases the kinetic energy of the
molecules within the intercepting object.
• This action causes the temperature of the
absorbing material to increase, indicating heat
transfer has occurred.
• The amount of radiant energy that is absorbed,
reflected or transmitted depends on the materials
surface, texture and its colour.
Units of Heat
• Heat (Q) is the transfer of energy that produces a
change in phase or temperature of an object or
material.
• Heat can be converted into work, and work can be
converted into heat.
• Imperial Units Heat = BTU & Work = FT-LBF
• Metric Units Heat = Joule & Work = Joule
Specific Heat
• Specific heat is a property of a substance that
indicates the quantity of energy that is needed to
change one unit of mass one degree.
• The specific heat of any substance varies with its
temperature, generally the variance is small and
can be assumed to be constant for most calcs.
• Their specific heat changes significantly when the
substance experiences a phase change.
Sensible Heat of the Solid
• Understanding internal kinetic energy starts from
the solid state at absolute zero starting point.
• As thermal energy is transferred the molecules
begin to vibrate, as more energy is transferred the
molecule vibration increases raising the
temperature of the solid.
• The total temperature required to raise the solid
from absolute zero to its fusion temperature is
known as the sensible heat of the solid.
Latent Heat of Fusion
• As the temperature of the solid reaches its fusion
temperature, the molecules are vibrating at the
maximum intensity possible within the limits of
the rigid crystalline structure.
• At the fusion temperature any additional energy
supplied to the solid causes the molecular
vibrations to be so intense that the crystalline
bonds fracture.
Sensible Heat of the Liquid
• As a solid substance begins converting into its
liquid phase, the resulting fluid remains at its
fusion temperature through the process.
• Once the state change is finished kinetic energy
can be then absorbed increasing its temperature.
• When sufficient energy is absorbed a point is
reached where the molecules have reached the
maximum velocity possible within the limits of
the liquid phase.
Sensible Heat of the Liquid
• At this point the liquid will be at it the maximum
temperature it can have, and still remain in the
liquid phase at the current pressure.
• Any additional energy will begin to produce vapor.
• The total quantity of energy supplied to a liquid to
increase its temperature from its fusion
temperature to its vaporizing temperature, is
knows as the sensible heat of the liquid.
Saturation Temperature
• The temperature at which a fluid changes from its
liquid phase to its vapor phase or from its vapor
phase to its liquid phase, is called saturation
temperature.
• For any given ambient or pressure, the saturation
temperature is the maximum temperature at
which the substance can stay in its liquid phase, or
the minimum temperature it can stay in its vapor
phase.
Latent heat of Vaporization
• As a substance changes phase from a liquid to a
vapor, its molecules acquire sufficient energy to
substantially overcome all restraining forces.
• The amount of energy required to do the internal
work necessary to overcome these restraining
forces is very great.
• Therefore the latent heat capacity of a liquid is
very high compared to a solid.
Superheat
• Once a liquid has been vaporized, any additional
energy or heat transfer will raise its internal
kinetic energy and its temperature.
• This heat is called the sensible of the vapor or
superheat.
• Therefore any time the temperature of a vapor is
raised above its saturation temperature, the vapor
is said to be “Superheated”
The Convertibility of Heat and Work
• Work can be converted into thermal energy and
thermal energy can be converted into work.
• Work can be performed on a system to increase
its thermal energy, or thermal energy can be used
to perform work.
• When a refrigeration compressor performs work
on the refrigerant vapor during compression, its
molecular vibrations increase raising temperature.