The centrifugal fan is a radial flow machine which produces the necessary pressure to move gas by the centrifugal force built up inside the fan casing. The design of the fan blade has a primary influence on performance.

These types of fans are usually employed for ventilating duties requiring a somewhat higher delivery pressure than that available from axial fans.

The centrifugal fans can efficiently move large or small quantity of air over a greater range of operating pressures. All centrifugal fans have an impeller or wheel mounted in a scroll type of housing, as shown. The impeller is turned either by the direct drive or more frequently by an electric motor employing pulleys and belts. The centrifugal force created by the rotating impeller moves the air outward along the blade channels. The outward moving air stream is combined by the scroll into a single large air stream. These air streams leave the fan through the discharge outlet.

Due to the enormous variance in fan applications, fans from a vast array of materials such as:

• Mild steel. • Stainless steel. • High nickel alloys. • PVC. • Specialized coatings are also available to suit client specifications. An extensive list of accessories is available for all centrifugal fans • Noise attenuators. • Filters. • Multi-leaf dampers. • Variable inlet vanes. • Actuators and positioners. • Anti-vibration mounts.

For smaller applications, forward curve multivane fans and pressed construction high pressure fans in both mild and stainless steel. As most parts are off-the-shelf, speedy deliveries can be affected for these economical fans. On the basis of the pressure, we can classify centrifugal fans as follows:

1. High pressure centrifugal fan 2. Medium pressure centrifugal fan 3. Low pressure centrifugal fan

High-Pressure centrifugal fans: High-pressure centrifugal fans are ideal for situations involving temporary wet grain holding, deep storage, or wherever high static pressures are required.

Ten gauge or heavier steel housings and non-overloading, vibration-proof fan wheels make fans tough. And when these features are combined with low power consumption and ample air delivery, the result is low cost, efficient aeration for almost any application.

FEATURES:

 Fan wheels are assembled with chuck bolts and / or welds to prevent loosening, and are statically and dynamically balanced for smooth, trouble-free operation.  Fan wheels are direct connected to motor shafts to eliminate belt fatigue and drive failure problems  Taper-lock bushings are used to secure fan wheels to motor shafts - so fan wheel removal is simplified.  Flanged outlets and extended inlet collars provide strong connections and simple installation.  Specially-designed inlet cones improve fan performance by providing smoother inlet air flow for steady, even air delivery.

In C-2700 and C-3000 series, centrifugal fans are designed with large diameter fan wheels and low speed motors to provide high volumes of air at lower static pressures, while keeping operating noise to a minimum. These fans feature the same rugged construction and exacting specifications as the high-pressure units described above.

The C-2700 series fans are available with 7-1/2, 10, 15, and 20 horsepower motors. The C-3000 series fans are available with 25, 30, and 40 horsepower motors; in addition, two medium-to-low pressure fans (the C-2003 and C-2205) are available for smaller aeration jobs. All fans operate at 1750 RPM.

Similarly to close-coupled pumping units, the fan impeller is fitted directly on the shaft of the motor mounted on a base which is cast in iron or fabricated as a welded steel structure. As a whole the design is very compact.

1. Forward curved blades 2. Radial or straight blades. 3. Backward curved blades.

Forward curved blades, which curve toward the direction of rotation. These fans have low space requirements, low tip speeds, and quiet in operation.

They are used against low to moderate static pressure such as those encountered in heating and air conditioning work and replacement air systems. These types of fans are not recommended for dust or particulate that would adhere to the short curved blades and cause imbalance. Since the blades are very shallow in depth, therefore the diameter of housing air inlet opening more nearly approaches to that of the impeller. The ample inlet opening, together with stream lined hub of the wheel promotes a smooth flow of air in to the rotating blades.

This increases the efficiency of the fan and reduces its noise. The forward curved blades are more capable of overcoming the attached duct system resistance when their operation is at low speeds.

The centrifugal fan with radial blades has simple impeller construction. The blades run straight out from a central hub. Some fans of this type have heavy steel blades with high structural strength. These fans provide very high pressure at high speeds. The housings are designed with their inlets and outlets sized to produce material conveying velocities.

There is a variety of impeller type available ranging from “high efficiency minimum material to “heavy impact resistance” design. The radial blade shape will resist material build up. This fan design is used for most exhaust systems applicable when particular wheel pass through the fan. These fans usually have medium tip speed and are used fro wide variety of exhaust systems which handle either clean or dirty air.

The backward curve blades must be operated at much higher speed of rotation than the forward curve blades, if the same static pressure is to be produced in each case in some cases, the higher speed may be an advantage because of a possible direct connection t the driving motor. The fan impeller having backward curve blades operate at higher efficiency and have no overloading power characteristic. They also offer the advantage of wide range of capacity at constant speed with small changes in the power requirement.

These impeller blades are inclined opposite to the direction of fan rotation. This type usually has high tip speeds and provides high fan efficiency and relatively low noise levels. In non-overloading fan, the maximum power occurs near the optimum operating point. So any variation from that point due to the change in system resistance will result in reduction in operating power.

The blade shape is conducive to material built up so, fan in this group should be limited as follows. Single thickness blade, solid blades allow the unit to handle light dust loading or moisture. It should not be used with particulate that would build up on the underside of the blade surfaces.

Airfoil blades offer higher efficiencies and low noise characteristics. Hollow blades erode more quickly with material and can fill with liquid in high humidity applications and should be limited to clean air service.

Two types turbine wheel are used:

Type A - with forward blades - designed for transportation of fresh air without essential impurity of the dust and steams. Type B - with backward blades - appropriated for work with polluted air and adhesive fumes.

The turbine wheels are made from PUNKER GmbH & Co - Germany. The static and dynamic balance is made from the manufacturing plant. This is made the fans low-noised and without vibrations.

The fan casing and steps are produced from galvanized metal sheet.

 The motors are three phase electrical supplies 380 V/50 Hz  Protection class IP 54 Din 40050  Class 'F' insulation VDE 0530  Maximal temperature of the environment is + 40 C  Working temperatures: from -30 C to + 50 C

The small type (dimensions) (up to Ф 250) can be completed with single phase motors on individual order. The fans are not appropriated for transportation of explosion-risk mixtures and corrosive gas.

The positions of the fans are defined from the exhaustion flange side:

- Right (R) - on the right-handed direction - Left (L) - on the left-handed direction

Options and accessories In addition to the fan can be ordered the following:

1. Protective net 2. Inlet connector 3. Inlet flexible connector 4. Outlet flexible connector 5. Outlet flange 6. Rubber pads

At work with humid air and conditions for condensation, on the bottom of the fan can be mounted drainage outlet. Instructional Capabilities

Measurement of inherent-speed machine performance in terms of static and total pressures, rotor speed and motor input power, as a function of inlet flow. Measurement of overall efficiency and estimation of impeller power efficiency. Measurement of performance at constant speeds. Introduction to similarity laws for scale-up.

Ordering Specification :-  Small-scale centrifugal fan demonstration unit  Equipped with electronic measurement sensors for fan differential pressure, orifice differential pressure (flow-rate), temperature and rotational speed.  Capable of being linked to a PC (not supplied) via the IFD6 interface console (available as an essential accessory).  Powered via the SWA1 power measurement unit (available as an essential accessory).  Self-contained, bench-top machine.  Supplied with interchangeable forward and backward curved blade impellers.  Adjustable aperture device for varying the air flow rate.  Transparent ducting and fan cover.  Supplied with software providing full instructions for setting up, operating, calibrating and performing the teaching exercises.

By running a fan at constant speed and measuring the pressure developed and the power input at different flow rates, the manufacturer obtains data on the fan’s performance. These measurements, together withy the computed efficiency, when graphed are known as fan characteristics.

Backward curved impeller with 6-16 blades has power characteristics, which rises to a maximum at the middle of its flow rate range and then falls at its highest flow rates. This is known as non-overloading power characteristics. These fans have efficiency up to 90% where airfoil section blades are used. They are most commonly used when continuous high efficiency operation is required.

Radial blade impeller, with 6-16 blades, has a continuously rising, or overloading, power characteristic. With flat blades, these fan types have efficiency up to 60% with blades slightly curved at the heel up to 75%. It is most often used where corrosion and blade wear may be a problem, as in industrial exhausts systems, the blades being self cleaning and often replaceable.

Forwarded curved impeller fans have 40-50 blades with an overloading power characteristics and efficiency up to 75%. It is compact in size and because of this is the most commonly used centrifugal fans for A/C and mechanical ventilation systems.