Presentation ppt for Chapter 8 (Part II) PDF

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Chapter 8 (Part II)

Fans and Blowers

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Session Objectives This session is intended to discuss the following: • Types and working principle of fans and blowers •Applications of various fans and blowers • Pressure rise, flow coefficient and efficiency • Velocity triangles • Performance characteristics • Fan laws

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What is a Fan? ➢Any device that produces a current of air by the movement of broad surfaces can be called a fan. ➢Fans fall under the general classification of turbomachinery and have a rotating impeller at least partially encased in a stationary housing. ➢Fans are similar in many respects to pumps. Both are turbomachines that transfer energy to a flowing fluid. It is easy to distinguish between fans and pumps: pumps handle liquids; fans handle gasses. ➢Broadly speaking, the function of a fan is to propel, displace, or move air or gas. 3

Fans, Blowers and Compressors

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Fans, blowers and compressors are differentiated by the method used to move the air, and by the system pressure they must operate against. Difference between Fans, Blower and Compressors Equipment

Specific Ratio

Fans

Up to 1.11

Pressure Rise (mmWC) 1136

Blowers

1.11 to 1.20

1136 to 2066

Compressors

More than 1.20

As per American Society of Mechanical Engineers (ASME): . the specific ratio, the ratio of the discharge pressure to the suction pressure, is used for defining the fans and blowers.

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Components of Fan/Blower System

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Turning Vanes (typically used on short radius elbows) Outlet Diffusers

Provide air for ventilation and industrial process

Heat Exchanger

Baffles

Filter

Inlet Vanes

Motor Controller Centrifugal Fan/Blower Belt Drive

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Motor

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Parts of a Fan / Blower 1. 2. 3. 4. 5. 6. 7. 8.

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Centrifugal housing

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include side plate and scroll sheets. Axial housing includes

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Impeller Blade Shroud Hub Housing Inlet Outlet Guide Vanes

the outer and inner

cylinder, belt tube .

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Fan Types

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Fans are classified according to the direction of flow through the impeller: Flow: Air flows through the impeller parallel to, and at a constant distance from the axis. The pressure rise is provided by the direct action of the blades ❑ Centrifugal or radial flow: Air enters parallel to the axis of the fan and turns through 900 and is discharged radially through the blades. The blade force is tangential causing the air to spin with the blades and the main pressure is attributed to this centrifugal force ❑ Mixed flow: Air enters parallel to the axis of the fan and turns through an angle which may range from 300 to 900. The pressure rise is partially by direct blade action and partially by centrifugal action ❑ Cross Flow: air enters the impeller at one part of the outer periphery flows inward and exits at another part of the outer periphery. 7 . . ❑ Axial

Centrifugal Fans ❑

Rotating impeller increases air velocity

❑

Air speed is converted to pressure

❑

High pressures for harsh conditions • High temperatures • Moist/dirty air streams • Material handling

❑

Categorized by blade shapes • Radial • Forward curved • Backward inclined

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Centrifugal Fan Impeller Types

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Open Type Backward inclined Radial Tip Blades

Backward inclined Radial tip Blades

Airfoil Blades with Higher Efficiency

Backward inclined radial blade .

Open Type Backward inclined Radial Tip Blades

Forward Curved Blades Type

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Centrifugal Fans

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Forward-curved fans are used in clean environments and operate at lower temperatures. Well suited for low tip speed and highairflow at lower pressures

Paddle blade or radial fan Backward curved

Radial fans have high static pressures (up to 1400 mmWC) and well suited for high temperatures and Forward curved or medium blade tip speeds multi-vane radial fan can handle heavily contaminated airstreams. .

Backward-inclined fans are more efficient than forwardcurved fans. Also known as "non-overloading" because changes in static pressure do not overload the motor

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• Augmenting Air

Fan • Scanner Air Fan • Booster Air Fan • Burner Air Fan • Degasser Blower • Combustion Air Fan • Oil Vapour Exhaust Fan • Purge Gas Blowers • Inline Fans • Supply Air Fan • Exhaust Air Fan • Ventilation Fan • Radial Blowers • Turbo Blowers (Centrifugal) • FD Fan • ID Fan • In-series Blowers •. Igniter Air Fan • Seal Air Fan

Applications of Centrifugal Fans .

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Centrifugal Fans Type Radial

Characteristics

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Typical Applications

Various industrial applications, High pressure, medium flow, efficiency close to tube axial fans, suitable for dust laden, moist air/gases power increases continuously

Medium pressure, high flow, dip Low pressure HVAC, packaged Forward curved in pressure curve, efficiency units, suitable for clean and blades higher than radial fans, power rises dust laden air/gases continuously

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Backward curved blades

High pressure, high flow, high efficiency, power reduces as flow increases beyond the point of highest efficiency

HVAC, vaious industrial applications forced draft fans etc

Airfoil type

Same as backward curve type, highest efficiency

Same as backward curved, but for clean air application

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Fan/Blower Blade Types ❑ ❑

❑

❑

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Impeller blades are manufactured either laminar (flat, constant thickness) or aerofoil shape and generally hollow Aerofoil blades have greater efficiencies (up to 90%) compared to constant thickness blades, with the advantages of efficiency spread over the characteristic and lower noise generation However with careful attention to design of blade curvature, inlet eye detail and impeller shrouding, comparable efficiencies can be achieved with constant thickness blades Aerofoil blades are freely used particularly when blade stresses are high and extra stiffening is required

constant thickness blades .

Aerofoil blades

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Axial Flow Fans

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Less efficient, large airflow and low speeds

Vane axial fan Tube axial fan Propeller fan

Higher speeds than propeller fans, highpressures 250 – 400 mm WC and efficiency up to 65%. .

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Pressures upto 500 mmWC and are highly energy-efficient

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Axial Flow Fans – Applications • • • •

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Ventilation Fan Airscrew Fan Wall mounted Supply Fan Wall mounted Exhaust Fan Bifurcated Fan Roof Exhaust Fan

• • • Spark proof Fans • Inline Fans • Fresh Air Unit • Ventilation Unit • Air washer Unit • Smoke Exhaust Unit • Toilet Exhaust Fans • CPU Fans .

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Axial Fans Type

Propeller Tube Axial

Vane Axial

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Characteristics Typical Applications low pressure, high flow, low Air circulation, efficiency, peak efficiency ventilation, exhaust close to point of free air delivery (zero static pressure) Medium pressure, high flow, HVAC, higher efficiency than drying ovens, propeller type, dip in pressure exhaust systems flow curve before peak pressure point High pressure High pressure, medium flow, dip in pressure-flow curve, applications including use of guide vanes improves HVAC systems efficiency exhausts .

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Mixed Flow Fan

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Mixed flow fan with barrel shaped spun housing for small diameters of inlet and outlet ducts. Direct drive, the fan wheel has a conical back plate. Outlet guide vanes prevent excessive air spin at the small outlet diameter. .

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Axial-Centrifugal Fan Types

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Single inlet single width impeller

Double inlet double width impeller

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Single inlet single width fan wheel with six radial blades welded to a back plate .

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Fan Laws

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Operating Point: Fan curve and system curve intersect

Move to flow Q2 by closing damper (increase system resistance) Flow Q1 at pressure P1 and fan speed N1 Move to flow Q2 by reducing fan speed

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Efficiency or BEP (Best Efficiency Point) Type of Fan

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Peak Efficiency Range

Centrifugal fans:

Deviation from BEP results in inefficiency and energy loss Increased system resistance reduces fan efficiency .

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Airfoil, Backward curved/inclined

79-83

Modified radial

72-79

Radial

69-75

Pressure blower

58-68

Forward curved

60-65

Axial fans:

Vane axial

78-85

Tube axial

67-72

Propeller

45-50 30

Performance Characteristics

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The theoretical pressure-quantity curve of an ideal fan (no losses) is a straight line between zero volume and zero pressure .

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System Resistance Sum of static pressure losses in system

Increases with square of air volume

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Configuration of ducts, pickups, elbows

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Pressure drop across equipment

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Long narrow ducts, many bends: more resistance

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Large ducts, few bends: less resistance

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System Resistance Curve

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