2 Lecture (Refrigerants and Components)

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    Introduction to P-h Charts

    Refrigerants & Components

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    Ideal VC cycle on P-hDiagram

    Introduction to P-h Charts

    Skeleton P-h Diagramfor water

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    Introduction to P-h Charts

    P-h Diagram for R-12

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    Refrigerants

    Refrigerants & Components

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    The R-# numbering system was developed by DuPont and systematically identifies the

    molecular structure of refrigerants made with a single halogenated hydrocarbon. The

    meaning of the codes is as follows:

    Numbering

    Rightmost digit: Number of fluorine atoms per molecule.

    Tens digit: One plus the number of hydrogen atoms per molecule.

    Hundreds digit: The number of carbon atoms minus one. Omitted for methyl halides, which

    have only one carbon atom.

    T

    housands digit: N

    umber of double bonds in the molecule. This is omitted when zero, andin practice is rarely used, since most candidate compounds are unstable.

    A suffix with a capital B and a number indicates the number of bromine atoms, when

    present. This is rarely used.

    Remaining bonds not accounted for are occupied by chlorine atoms.

    A suffix ofa lower-case letter a, b, or c indicates increasingly unbalanced isomers.

    Special cases: R-400 & R-500 series made up of zeotropic & azeotropic blends. R-700series is made up of natural fluids (e.g. CO

    2(R744), Ammonia (R717) etc.)

    e.g., R-134a has 4 fluorine atoms, 2 hydrogen atoms, 2 carbon atoms, with an empirical

    formula of tetrafluoroethane. The "a" suffix indicates that the isomer is unbalanced by one

    atom, giving 1,1,1,2-Tetrafluoroethane. R-134 without the "a" suffix would have a molecular

    structure of1,1,2,2-Tetrafluoroethane a compound not especially effective as a refrigerant.

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    Refrigerants are substances that are used to absorb and transport heat for the purpose of

    cooling. Requirements for a refrigerant are as follows:

    Selecting the Right Refrigerant

    a. High latent heat of vaporization

    b. Non-corrosive, non-toxic and non-flammable (e.g. Ammonia is toxic & flammable)

    c. Critical temperature should be outside working range

    d. Reasonable working pressure

    e. Compatibility with component materials (e.g. Ammonia & copper reaction)

    f. Stability

    g. Low Cost

    h. Availability

    i. Environmental impact

    j. Ease of leak detection

    Examples:

    R-12, R-134a, R-22

    Propane, Ethane

    Ammonia R-717, Water R-718, Carbon dioxide R-744

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    Montreal Protocol (1987): CFC/halon/bromide production phase-out by 1995

    Selecting the Right Refrigerant

    Refrigerant emissions were about 10% of total.

    Kyoto Protocol (1992): At UNCED, established legally binding commitments for the

    reduction of six greenhouse gases (carbon dioxide, methane, nitrous oxide, sulfur

    hexafluoride, hydrofluorocarbons, and perfluorocarbons)

    Q: Each kWh of electricity used in Pakistan produces how much kg of CO2?

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    Selecting the Right Refrigerant

    Overall GWP =Total equivalent

    warming impact

    (TEWI)

    CO2

    emission

    largest

    contributor

    Focus on

    efficiency of Ref.

    system and, thus,

    refrigerant

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    Compressors

    Refrigerants & Components

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    Air CompressorsAir Compressors Gas CompressorsGas Compressors

    (gases other than air)(gases other than air)

    Widely used to supplyWidely used to supply

    compressed air as ancompressed air as an

    energy sourceenergy source

    Used in petroleum,Used in petroleum,

    plastics andplastics and

    chemical industrieschemical industries

    Used in refrigerationUsed in refrigeration

    and air conditioningand air conditioning

    systemssystems

    RefrigerantRefrigerant

    CompressorsCompressors

    General: Types and uses

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    General: Types

    Compressors may be of thepositive displacementordynamic type.PositivePositive

    displacement compressorsdisplacement compressors trap a volume of gas in an enclosed space and increase

    the pressure by reducing the volume of the space while ynamic compressorsDynamic compressorsaccelerate a gas and increase its kinetic energy & this kinetic energy is converted

    into pressure.

    Positive displacement compressorsPositive displacement compressors can be classified into two main

    groups:

    reciprocating compressorsreciprocating compressors - including trunk, crosshead, labyrinth and

    diaphragm types

    rotary compressorsrotary compressors - including vane, screw, liquid ring, and lobe rotor

    types

    Dynamic compressorsDynamic compressors include centrifugal flow and axial flow machines

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    Positive Displacement Compressor: General

    The general form of positive displacement compressor is the piston type, being

    adaptable in size, number of cylinders, speed and method of drive. It works on the

    two-stroke cycle (see Figure).

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    In multicylinder compressors, four, six and eight cylinders are common for large

    systems and, for small systems, it will be of two, three or four cylinders.

    Piston compressors may be generally

    classified by the type of valve.

    Lower loads require reduction in

    capacity and can be achieved in the form

    of bypass ports or by speed control (when

    steam driven). Cold suction gas provides sufficient

    cooling for small machines. Water-cooled

    cylinder heads used where high discharge

    temperatures are involved.

    Lubricant supply is the minimumconsistent with efficient lubrication for

    cylinder lubrication whereas maximum

    viscosity is consistent with efficient

    lubrication forbearing lubrication.

    Positive Displacement Compressors

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    Refrigerator Oils

    Lubrication of the compressor to minimize friction and wear

    Sealing

    between working parts of the compressor

    Cooling

    of the compressor bearings and casing

    Noise reduction

    of noise generated by the moving parts of thecompressor

    Electrical insulation of the motor in hermetically-sealed compressors

    Functions of a refrigerator oil can be as follows:

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    Refrigerator Oils

    What properties should refrigerator oils have?

    ViscosityViscosity

    must be sufficiently viscous to lubricate the compressor effectively andprovide adequate sealing where required

    common refrigerant gases are extremely soluble in oil and may produce asignificant decrease in oil viscosity

    MiscibilityMiscibility

    in sealed systems, the oil must be completely miscible with the refrigerant

    if the oil/refrigerant separates into immiscible layers the efficiency ofrefrigeration and lubrication will be impaired

    there is a risk that the system will become blocked, and the compressor maybecome starved of oil

    Low temperature propertiesLow temperature properties

    the oil must not form waxy deposits in the cold parts of the circuit this would lead to a reduction of heat transfer and could lead to oil starvation

    of the compressor

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    Refrigerator Oils Thermal stabilityThermal stability

    although the temperatures in refrigerator systems are not normally as high as in

    air compression systems, the oil must not break down to form coke-likedeposits, e.g. on compressor chambers and valves

    Chemical stabilityChemical stability

    the oil must not react with the refrigerant, or act as a medium for reaction

    between the refrigerant and other system components

    in the presence of small amounts of air, moisture and other impurities,unsuitable oils may react with refrigerants to form sludges

    CompatibilityCompatibility

    the oil must be compatible with the materials used in the refrigeration system,

    e.g. electrical insulation, varnishes, elastomers, polymers etc.

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    Condensers

    Refrigerants & Components

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    Condenser: Introduction

    The heat-rejection ratio (HRR) is defined as the ratio of the rate of heat rejected

    at the condenser to that absorbed at the evaporator.

    rate of heat rejected at condenser

    rate of heat absorbed at evaporatorHRR=

    The designer and operator of the refrigeration system will usually characterize plant

    size by the refrigeration capacity.

    Ignoring small heat gains and losses,Condenser load = Evaporator load + Compressor power

    Condenser rating is stated as the rate of heat rejection. Some manufacturers give

    ratings in terms of the evaporator load,

    Condenser load = Evaporator load factor

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    Condenser: Types

    The three main types of condensers used in general refrigeration systems are:

    (air-cooled) (water-cooled)

    (evaporative)

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    Condenser Types: Air-cooled

    To provide a comparison, some of the characteristics of each are enumerated:

    Air-cooled condenser: Usually lowest first cost of the three, and least maintenance cost as well, because no

    water circulates or evaporates.

    Example: condenser of the domestic refrigerator. Above this size, the flow of air

    over the condenser surface will be by forced convection, i.e. fans.

    Extended surfaces are almost always used (Ao/A

    i= 5 10).

    Inlet at the top and outlet at the bottom (gravity-assisted flow).

    The flow of air may be vertically upwards or horizontal.

    Where a single fan would be too big, multiple smaller fans give the advantages of

    lower tip speed and noise, and flexibility of operation in winter.

    The low specific heat capacity and high specific volume of air implies a large

    volume to remove the condenser heat. In practice, the temperature rise of the air is kept between 9 and 12 K.

    Materials of construction are aluminium fins on stainless steel tube for ammonia, or

    aluminium orcopperfins on aluminium orcoppertube for the halocarbons.

    Used in desert areas where the supply of cooling water is unreliable.

    Q: On average, approx. whats the air flow rate if 430 kW is rejected by a condenser?

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    Condenser Types: Water-cooled

    Water-cooled condenser with cooling tower:

    Lower condensing temperature than with an air-cooled condenser, because the wet-

    bulb rather than the dry-bulb temperature of the air is the sink toward which thecondensing temperature drives.

    The higher heat capacity and lower specific volume of water make it an ideal

    medium for condenser cooling.

    General form is shell-and-tube having the water in the tubes.

    M

    aterials can be selected for the application and refrigerant, but all mild steel iscommon for fresh water, with cupronickel or aluminium brass tubes for salt water.

    Some condensers have two separate water circuits (double bundle), using the

    warmed water from one circuit as reclaimed heat in another part of the system.

    The supply of water is usually limited and requires the use of a cooling tower.

    Optional sources may include ground water or industrial water.

    Q: By comparison, whats the flow rate now for the condenser?

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    Condenser Types: Evaporative

    Evaporative condenser:

    Compact and provides lower condensing temperatures than the air-cooled condenser

    as well as the water-cooled condenser/cooling tower combination. The mass flow of water over the condenser tubes must be enough to ensure wetting

    of the tube surface, and will be of the order of80160 times the quantity evaporated.

    Evaporative condensers have a higher resistance to air flow than cooling towers.

    Most types use forced draught fans.

    Evaporative condensers may freeze in winter. A common arrangement is to switchoff the fan(s) with a thermostat, to prevent the formation of ice. The water-collection

    tank will have an immersion heater or the tank may be located inside the building

    under the tower structure.

    Materials of construction must be corrosion resistant.

    The atmospheric condenseris a simplified form of evaporative condenser, having

    plain tubes over a collecting tank and relying only on natural air draught. This will belocated on an open roof or large open space to ensure a good flow of air. Space

    required is of the order of 0.2 m2/kW.

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    Evaporators

    Refrigerants & Components

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    Evaporators are classified according to their refrigerant flow pattern and their

    function. The refrigerant flow pattern is usually dependent on the method of ensuring

    oil removal from the evaporator.Floodedevaporators (see Figure) have a body of fluid boiling in a random manner,

    the vapour leaving at the top. In the case of ammonia, any oil present will fall to the

    bottom and be drawn off from the drain pot or oil drain connection. With

    halocarbons, a proportion of the fluid is bled off and rectified. Evaporators which

    keep the oil moving by means of continuous fluid velocity, until it gets back to the

    compressor suction, are termed dry expansion. In these, the refrigerant is totally

    evaporated.

    Evaporators: Classification

    The function of the evaporator will be to cool gas, liquid or

    other product load. In most cases, air or a liquid is first

    cooled, and this is then used to cool the load. For example,

    in a cold-room, air is cooled and this air cools the stored

    produce and carries away heat leaking through the

    structure; in a water chiller system, the water is circulated

    to cool the load, etc.

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    Air cooling evaporators for cold-rooms, air-conditioning, etc., will have finned pipe

    coils.

    In all but very small coolers, there will be fans to blow the air over the coil. Construction materials will be the same as for air-cooled condensers. Aluminium

    fins on copper tube are the most common for the halocarbons, with stainless steel or

    aluminium tube for ammonia.

    Frost or condensed water will form on the fin surface and must be drained away. To

    permit this, fins will be vertical and the air flow horizontal, with a drain tray provided

    under.

    The size of the tube will be such that the velocity of the boiling fluid within it will

    cause turbulence to promote heat transfer.

    Tube diameters will vary from 9 mm to 32 mm, according to the size of coil.

    Fin spacing will depend on factors like compactness and cost and will vary from 2-

    12 mm. The number of air-cooling coils in operation in industrial refrigeration plants farexceeds the number of liquid-chilling evaporators installed.

    Air Cooling Evaporators

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    Liquid cooling is mostly in shell-and-tube or shell-and-coil evaporators.

    Liquid Cooling Evaporators

    In the shell-and-tube type, the liquid is usually in the pipes and the shell is some

    three-quarters full of the liquid, boiling refrigerant. A number of tubes is omitted at

    the top of the shell to give space for the suction gas to escape clear of the surface

    without entraining liquid. Further features such as multiple outlet headers, suction

    trap domes and baffles will help to avoid liquid droplets entering the main suction

    pipe. Gas velocities should not exceed 3 m/s and lower figures are used by some

    designers. The speed of the liquid within the tubes should be about 1 m/s or more, topromote internal turbulence for good heat transfer.

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    Evaporators of this general type with dry expansion circuits will have the refrigerant

    within the tubes, in order to maintain a suitable continuous velocity for oil transport,

    and the liquid in the shell. These can be made as shell-and-tube, with the refrigerantconstrained to a number of passes, or may be shell-and-coil (see Figure). In both these

    configurations, baffles are needed on the water side to improve the turbulence, and

    the tubes may be finned on the outside.

    Liquid Cooling Evaporators

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    Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have

    flooded or dry expansion circuitry. Flooded coils will be connected to a combined

    liquid accumulator and suction separator (usually termed the surge drum), in the formof a horizontal or vertical drum.

    Liquid Cooling Evaporators

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    Some liquids, such as vegetable fats and ice-cream mixes, increase considerably in

    viscosity as they are cooled, sticking to the heat exchanger surface. Evaporators for

    this duty are arranged in the form of a hollow drum surrounded by the refrigerantand having internal rotating blades which scrape the product off as it thickens,

    presenting a clean surface to the flow of product and impelling the cold paste towards

    the outlet.

    Liquid Cooling Evaporators

    Note: There are other methodologies used as well.

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    Plate evaporators are formed by cladding a tubular coil with sheet metal, welding

    together two embossed plates, or from aluminium extrusions.

    The extended flat face may be used for air cooling, for liquid cooling if immersed ina tank.

    The major use for flat plate evaporators is to cool a solid product by conduction, the

    product being in packages and held close between a pair of adjacent plates.

    Has horizontal and vertical types.

    Some of its popularity is attributable to its compactness.

    Plate Evaporators

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    Air cooling evaporators working below 0C will accumulate frost which must be

    removed periodically, since it will obstruct heat transfer.

    Evaporators of suitable and robust construction can be defrosted by brushing,

    scraping or chipping, but these methods are labour-intensive and may lead to damage

    of the plant.

    Where the surrounding air is always at + 4C or higher, it will be sufficient to stop the

    refrigerant for a period and allow the frost to melt off.

    For lower temperatures (< 4C), heat must be applied to melt the frost within a

    reasonable time and ensure that it drains away.Methods include electric resistance

    heaters, hot gas and reverse cycling.

    Defrosting

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    Expansion Valves & Thermostatic Control Devices

    Refrigerants & Components

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    In most room air conditioners, a capillary tube is used as the throttling device. A

    capillary tube, also sometimes called a restrictor tube, is a fixed length of small-

    diameter tubing lying between the outlet of the condenser and the inlet of theevaporator. This has certain limitations, but will give reasonably effective control over

    a wide range of conditions if correctly selected and applied.Mass flow is a strong

    function of pressure difference. Tube bores of0.82 mm with lengths of14 m are

    common. Usually made of copper.

    Capillary Tube

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    In a refrigeration system, the expansion valve is an adjustable throttling device through

    which the refrigerant at condensing pressure is throttled to evaporating pressure or

    interstage pressure. At the same time, the expansion valve regulates its opening to feedthe required amount of refrigerant to the evaporator to meet the refrigeration load at the

    evaporator.

    Thermostatic Expansion Valve

    Thermostatic Expansion Valves (TXVs)

    TXVs are the most widely used expansion devices. A thermostatic expansion valve

    regulates its refrigerant flow rate to the evaporator according to the degree of superheatof vapor refrigerant leaving the evaporator. A thermostatic expansion valve consists of

    a valve body, a valve spring, a diaphragm, and a sensing bulb at the outlet of the

    evaporator.

    When the liquid refrigerant passes through the small opening around the valve pin, its

    pressure is reduced to the evaporating pressure. Liquid refrigerant flows through therefrigerant distributor and tubes, and vaporizes gradually as it flows inside the copper

    tubes. At positionx, as shown in the Figure (next slide), all the liquid has been

    vaporized. By the time the vapor refrigerant reaches the outlet o of the evaporator, it is

    superheated to a few degrees higher than its saturated temperature.

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    If load increases, more refrigerant vaporizes and degree of superheat increases which

    increases temperature of sensing bulb. This exerts higher pressure on diaphram which

    lowers the valve pin.More liquid refrigerant enters to match the increase in load. Thedegree of superheat of the vapor refrigerant at the outlet can be adjusted by varying the

    tension of the spring in the thermostatic expansion valve.

    Thermostatic Expansion Valve

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    Used for dry-expansion circuits as there is no

    liquid level that can be detected.

    Sensing bulb usually contains the samerefrigerant.

    Huntingrefers to a condition that occurs

    when a device continuously undershoots or

    overshoots the control point, with resulting

    fluctuation and loss of control of thecondition to be maintained. Sometimes called

    cycling.May reduce refrigeration capacity.

    Superheat setting of the TXV at full-load

    operation is between 10 and 20 F.

    Thermostatic Expansion Valve

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    Electric, or more truly electronic or microprocessor-controlled, expansion valves can

    provide more sophisticated, effective, and energy-efficient refrigerant flow controls

    than thermostatic expansion valves. Currently, three types of electric expansion valvesare widely available: step motor valves (a), pulse-width-modulated valves (b), and

    analog valves (c).

    Electric Expansion Valve

    (a) (b) (c)

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    Thermal Electric Expansion Valve

    The signal from a suitable thermistor placed at the evaporator outlet will vary,

    depending on whether it senses dry refrigerant gas or traces of liquid. This can be used

    directly to control the current through a thermal element to modulate the expansionvalve. This device usually has no separate adjustable controller and so cannot be

    incorrectly set.

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    The (low-side) float valve controls the liquid refrigerant feed to maintain a constant

    liquid level in the evaporator. As the liquid level in the evaporator drops, the float

    ball moves downward and opens the float valve wider so that more liquid refrigerant isfed to the evaporator. In small refrigeration systems, the float chamber is often placed

    directly inside the evaporator or in an accumulator, instead of a separate low-side float

    chamber. Operation of the low-side float valve may be continuous or intermittent.

    Float Valve