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    Lecture 3:

    September 21, 2014 1

    Prepared by:

    Engr. AN Tabish

    Lecturer

    Department of

    Chemical Engineering

    University of Engineering

    & Technology, Lahore -Pakistan

    Fluid Dynamics

    Fluid Dynamics

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    September 21, 2014 2

    Course Outline:

    Fluid Dynamics

    1. Introduction to fluid dynamics; Transportation of fluids;

    2. Storage tanks, pipes, fittings and valves;

    - Selection, design, procedures and cost

    3. Pumps, compressors, blowers, fans, turbines and expanders;

    - Selection, cavitation, chocking, surging, corrosion

    prevention

    4. Metering of fluids;

    5. Agitation and mixing of fluids; static fluid phenomenon in

    static mixers; dynamic mixers;

    6. Flow passed immersed bodies; fluidization;

    7. Chemical engineering CFD;

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    September 21, 2014 3

    Compressor

    Fluid Dynamics

    Compressor is a device used to increase the pressure of

    compressible fluid.

    Modes of Compression:

    1. Intermittent (+ve displacement)

    2. Continuous (Dynamic)

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    Compressor

    Fluid Dynamics

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    September 21, 2014 5

    Compressor

    Fluid Dynamics

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    September 21, 2014 6

    Compressible flow:

    Fluid Dynamics

    Compressor Cooling System:

    Inter-stage coolers

    Mack Number:

    As gas moves in the constant diameter suction line of

    compressor: It experiences

    Pressure drops

    Gas volume increases as mass flow rate remains constant

    Velocity increase =>in sever cases it approaches sonic

    velocity

    )(CVelocityAcoustic

    )(VVelocityGasNumberMack

    s

    g

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    Compressible flow:

    Fluid Dynamics

    )(CVelocityAcoustic

    )(VVelocityGas

    NumberMacks

    g

    Keep Mack Number as low as practical

    Temperature at compressor

    suction must be considered.

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

    Compressor surges when the differential pressure-imposed by

    process-exceeds the differential pressure which it can develop.

    The surging occurs when the compressor back pressure ishigh and the compressor can not pump against this high head,

    causing the flow to separate and reverse its direction.

    Surging results in changes in direction of the thrust forces on therotor, creating damage to the blades and the thrust bearings.

    lower limit of stable operation in a compressor

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

    If Pd

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

    Consequences of Surging

    Consequences of surging can include:

    1. Rapid flow and pressure oscillations cause process instabilities

    2. Rising temperatures inside the compressor

    3. Tripping of the compressor

    4. Mechanical damage

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

    Mechanical damage can include:

    1. Radial bearing load during the initial phase of

    surging. A side load is placed on the rotor which acts

    perpendicular to the axis.

    2. Thrust bearing load due to loading and unloading.

    3. Seal rubbing

    4. Stationary and rotating part contact if thrust bearing isoverloaded.

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    Compressible flow:

    Fluid Dynamics

    Surging and Surge Control:

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    Compressible flow:

    Fluid Dynamics

    Surge Control:

    The surge of centrifugal compressor is controlled by two methods:

    1. Discharge flow.

    2. Varying the drive speed.

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    Thank You

    [email protected]