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Liquids and Vapours
Refrigeration depends on a working fluid that has to
constantly change from a liquid into a vapour and
from a vapour into a liquid
Whats the science of liquids and Vapours got to do
with Refrigeration ?
What knowledge of Liquids and vapours do I need toknow?
A knowledge and understanding of the way in which
temperature, pressure and volume change in relation
to each is needed The GAS LAWS
Boyles Law & Charles Law are two of the Laws
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Boyles Law
the volume of a given mass of gas is inversely
proportional to the pressure applied to the gas,provided that the temperature remains constant.
What does this mean?
If the pressure is doubled the volume will be halved
Start volume = 4m3 at 100kPa
Pressure increased to 200kPa, volume = 2m3
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Charles Law
At constant pressure, the volume of a given mass of a
gas increases or decreases by the same factor as itstemperature on the absolute temperature scale
What does this mean?
The gas expands as the temperature increases.
http://upload.wikimedia.org/wikipedia/commons/e/e4/Charles_and_Gay-Lussac's_Law_animated.gif -
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Combined Gas LawsBoyles and Charles Laws can be combined to provide
the following gas law equation.
Temperature & pressure must be in absolute units!
Lets look at an
example:what would the
pressure be?
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Combined Gas Laws
P1 = 6 bar G
V1 = 64m3
T1 = 27oC
Initial Condition Final Condition Convert to absolute units:
P1 = 7 bar A P2 = ?
V1 = 64m3 V2 = 8m3
T1 = 300K T2 = 313K
P2 = ?
V2 = 8m3
T2 = 40oC
7 x 64 = P2 x 8
300 313
P2 = 7 x 64 x 313 = 58.4 bar A
300 x 8
P2 = 58.4 1 = 57.4 bar G
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Daltons LawThe total pressure of a combined mixture of gases is
the sum of the pressures of each of the gases in the
mixture
=
Nitrogen
N2
10 bar
Carbon
DioxideCO2
10 bar
Mixture
20 bar
What does this mean ?
In a gas mixture, the pressure exerted will be the sum
of the pressures that the individual gases would exert
in the system if they solely occupied the space
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Isothermal and adiabatic changes
Ideal gases are theoretical in that they undergoAdiabaticexpansion and contraction.
This would occur if a gas was placed in a perfectlyinsulated cylinder with a frictionless piston.
The work required to compress the gas would create
heat which would increase the temperature of the gasand therefore the pressure.
No heat would escape nor enter the imaginary piston
chamber.
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Isothermal and adiabatic changes
If the change in pressure occurs so quickly that
there is a change in temperature of the gas it issaid to be adiabatic
Where does this happen in a system?
In the compressor and expansion device
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Isothermal and adiabatic changesWith an adiabatic compression there is always an
increase in temperature.
For an adiabatic expansion there is a decrease in
temperature.
There is more than one form of adiabatic change.
One form occurs with no change of enthalpy, and
other with no change of entropy.
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Isothermal and adiabatic changes
Do you know where each of these occur in the cycle?
Expansion valve there is no change in enthalpy
Compressor where there is no change in entropy
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If a volume of gas is subjected to a slow increase inpressure, with no change in temperature, then thechange is said to be isothermal.
Boyles law applies to isothermal changes.
Isothermal and adiabatic changes
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Saturated vapours and liquids
Vapour which is above the surface of the liquid is called a
saturated vapour.Molecules from the liquid are passing through the surface and
entering the vapour the whole time, while molecules from thevapour re-enter the liquid.
The liquid is called a saturated liquid.
The point at which a liquid changes into a vapour or a vapour
changes into a liquid is called the
saturation temperature.
The saturation temperature depends upon
the pressure above the surface of the liquid.
The saturation temperature is equal to the
boil ing point of a liquid.
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Saturated vapours and liquidsAs pressure changes, so will the saturation temperature.
For example, at atmospheric pressure water boils at 100oC
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Saturated vapours and liquidsAs the pressure falls the boiling point falls
If the pressure falls to 50kPa the boiling point will drop to
80oC
If the pressure increases the boiling point will increase
It is the pressure above the
surface of the liquid which isimportant and this can be at
atmospheric or any other
pressure. The pressures
inside the boilers of modern
power stations producesaturation temperatures of
several hundred degrees
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Saturated vapours and liquids
Vapour which is above the surface of the liquid is called a
saturated vapour.Molecules from the liquid are passing through the surface and
entering the vapour the whole time, while molecules from thevapour re-enter the liquid.
The liquid is called a saturated liquid.
The point at which a liquid changes into a vapour or a vapour
changes into a liquid is called the
saturation temperature.
The saturation temperature depends upon
the pressure above the surface of the liquid.
The saturation temperature is equal to the
boil ing point of a liquid.
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If a refrigerant cylinder filled only with vapour. What
will be the pressure inside the cylinder?Depends on the refrigerant and also the temperature of
the refrigerant.
An increase in temperature will cause the pressure to
increase.
The molecules become more energized and bounceabout more violently which increases the pressure they
exert against the inside walls of the cylinder.
The pressure inside the tank is the sum total of the
pressure that all the individual molecules exert on the
inside walls of the jug. The force caused by a single
molecule is unmentionably small.
However there is an even more unmentionable quantity of
molecules contained in the cylinder so it adds up to
something measurable.
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A refrigerant at a particular temperature will
have a known pressure. This is always true
when there are Saturated Conditions.
Saturated Conditions occur when liquid andvapour are in contact with each other.
The same pressure exists everywhere inside
the cylinder and is exerted in all directions.
The gas is pressing down on the surface ofthe liquid with the same pressure that the
liquid is pressing up.
There is something special about a system containing
both liquid and vapour, known as the
Pressure Temperature Relationship.
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All liquids have different rates of pressure change for a given
temperature change.
For a chemical stable liquid, the rate of change of pressurewith temperature is consistent. This enables charts or tables
to be created showing values of pressure against
temperature. These are the scales shown on pressure gauges
and on refrigerant comparators.
Pressure Temperature Relationship.
As seen when the properties of R134a are
examined the saturation or boiling temperature
of R134a at atmospheric pressure is -26oC
and at 15 bar a the saturation temperature is55oC.
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The refrigeration system relies on providing a continuous
source of low temperature refrigerant boiling inside a heatexchanger inside the space being kept cool, the evaporator.
Another fact that governs this process is that the latent heat of
a liquid is massively greater than the specific heat of liquid.
Pressure Temperature Relationship.
The latent heat of R134a at -26oC is220.2kJ/kg while the specific heat capacity at -
26oC is 1.05kJ/kgK
During a latent heat process the temperatureof the boiling liquid remains the same making
the provision of a constant low temperature
heat sink possible
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A mixture of liquid and vapour at the
boiling temperature, the proportion of
vapour present compared to the
proportion of liquid is described by thedryness fraction i.e. O.5 dry indicates that
50% of the refrigeration has become
vapour, 0.2 dry indicates that 20% of
refrigeration has become vapour
Two Phase Mixture
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If there is a single droplet of liquid in contact with
vapour the PT relationship remains true.
There is an interaction between the liquid and
vapour which causes an equilibrium.If the pressure of the vapour is lower than it
should be then some of the liquid immediately
flashes into a gas which increases the pressure
of the gas and therefore makes it difficult for moreliquid to evaporate.
We know that a liquid evaporates when the
pressure above it's surface is reduced.
When saturated conditions exist an equilibrium
will always develop between the vapour and
liquid and this equilibrium will always create a
predictable pressure at any given temperature.
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A cylinder is only filled to 80% level for a
specific reason.
Everything expands and contracts withtemperature.
When the liquid expands due to an increase in
temperature the vapour portion is compressed in
the process allowing the liquid to take up morevolume.
A cylinder of refrigerant must NEVER be
completely filled.
If the temperature of was increased, the liquid
would try to expand and having nowhere to go
hydrostat icpressure could develop and the
container could rupture.
S h t d
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Superheated vapour
A vapour heated above its saturationtemperature is said to be
superheated.The temperature can only be raised by
the addition of heat energy knownas superheat.
Superheating can only occur oncea liquid has been completelyvaporised, or the vapour isremoved completely from thepresence of the liquid.
S b l d li id
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Subcooled liquids
Critical temperature
A liquid at any temperature below its saturation temperature is
called a subcooled liquid.
A vapour can be condensed, to form a liquid, by cooling it.
A vapour can also be condensed by increasing the pressure
without changing the temperature.As the pressure is further increased more and more vapour
will condense into the liquid phase.
The temperature of a vapour can be raised to a point at
which it cannot be liquefied (or saturated) by pressure alone.
The temperature at which this occurs is thecritical temperature.
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Enthalpy
The enthalpy of a body is the total amount of energy supplied
to it, relative to an energy taken as zero point of enthalpy.
Enthalpy is defined by the formula:
H= U+pV
Where H is the enthalpy in joules (J),U is the internal energy, also in joules,
p is the pressure in pascals and
V is the volume in m3.
Enthalpy is sometimes known as total heat.
ifi th l
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Is the total enthalpy per unit mass: H
h = m (J/kg)where h is the specific enthalpy
m is the mass.
The internal energy (U) of a body depends upon the potentialand kinetic energies of its molecules:
U= P.E. + K.E.
P.E. is the potential energy of attraction between the molecules.
The forces of (chemical) attraction are electrical in nature, sothe potential energy is electrical.
The reference point for enthalpy is at a temperature of 0oC,with a specific enthalpy of 100kJ/kgK.
In theory all enthalpies and entropies could be measured fromabsolute zero (creates practical difficulties)
specific enthalpy
E t
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EntropyEntropy measures the energy dispersion in a system divided by
temperature. This ratio represents the tendency of energy to
spread out, to diffuse, to become less concentrated in onephysical location or one energetic state.
That spreading out is often done by molecules because
molecules above absolute zero always have energy inside of
them.That's why they are incessantly speeding through space and
hitting each other and rotating and vibrating in a gas or liquid.
Entropy is measured in joules per kelvin change for a
substance.
E t
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EntropyThe entropy of a body depends on two factors, heat flow (or
transfer) into the body and absolute temperature in Kelvin:
Heat transferred
Change of entropy = absolute temperature (J/K)
Specific entropy is the entropy per unit mass and itis measured in joules per kilogramme Kelvin
(J/kgK).
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