C&G 2079 Version 3 Section 4

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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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