FAN & BLOWERS

To move air in ventilation or exhaust system, energy is required to overcome the system losses. The energy can be in the form of natural convection or buoyancy. Most systems require some powered air-moving device such as a fan or ejectors. Fans are the primary air moving devices used in industrial applications.

Any device that produces a current of air by the movement of a broad surface can be called a fan fans are similar in many respects to pumps and compressors. All three are turbo-machines that transfer energy to a flowing fluid.

It is easy to distinguish between fans and pumps. Pumps handle liquids, while fans handle gases.

Fans and compressors both handle air and other gases but the basic difference is that the function of a fan is to propel, displace or move the air or gas, while the function of a compressor is to increase the pressure, reduce the volume or compress the air or gas.

A fan is a kind of a pump, which is used for pumping or circulation the air. The energy is transmitted to the air but the power driven wheel and a pressure difference is created to produce flow of air. The air may be moved by either creating a pressure; which is above or below the atmospheric pressure. All fans produce both the conditions. The air at inlet to the fan is below atmospheric pressure, while that at the outlet is above atmospheric pressure. The air fed into a fan is called induced draft while the exhausted from a fan is called the forced draft.

The fans, irrespective of their type of construction, may function as either blowers or exhausters. The blowers discharge air against a pressure at their outlet whereas exhausters remove gases from a space by suction.

A wide range of industries chemical, petrochemical, iron, steel, pulp, paper, plastic, glass, hotel, hospital, power plants, refineries, fertilizer units, nuclear power plants, pharmaceuticals, textiles, atomic energy and heavy eater plants utilities the fans for air and pressure application.

For the working medium as air is sucked from atmosphere, due to suction create at the impeller eye as a result of rotating impeller. The air is accelerated in the inlet pipe due to the flow acceleration in the fan’s inlet part the velocity is increase and thus pressure and temperature decreases.

The impeller is rotated and thus each particle of fluid passing through the impeller is accelerated and thus kinetic energy increases. The impeller blades are such that the cross sectional area between two blades increases from inlet to outlet of impeller.

Air moving devices can be divided into two basic classifications: ejectors and fans ejectors have low operating efficiencies and are used only for special material handling applications ejectors sometimes are used when it is not desirable to have contaminated air pass directly through the air-moving device. Ejectors are utilized for air streams containing corrosive, flammable, explosive, hot or sticky materials that might damage a fan ejectors may also be used in pneumatic conveying system.

Fans are primary air moving devices used in industrial application. As a general rule, axial fans are used for higher flow rates at lower resistance and centrifugal fans are used for lower flow rates at higher resistance.

A fan is defined as a rotary, bladed machine maintaining a continuous flow of air continuous because the air flows steadily into, through and out of the fan, a feature distinguishing it from positive displacement machines, which generally produce a pulsating flow. When fan caused air to move, a pressure above that of the atmosphere is set up in the direction in which the air moves.

The range of fans, blowers and exhausters offer high performance units that are both flexible and compact. This heavy duty, high quality units are suitable for a variety of industrial applications which include.

The principle parts of any fan are the impeller and the housing

The impeller is a rotating element that transfers energy to the fluid. The blades are the principle working surfaces of the impeller. Flange/inlet plate may be used to support the blades. Hubs may be used to support the blades directly or through a flange through the shaft.

The housing is the stationary element that guided the air or gas before and after the impeller centrifugal housing components include the side sheets and scroll sheets. The point of closers approach to the wheel is cut off. The area over the cutoff is called the blast area. Axial housing components include the outlet cylinders, the inner cylinder, the belt fairing, the guide vanes and the tailpiece.

The inlet is the opening through which air enters the fan. A stationary inlet piece can be called an inlet cone, an inlet bell, inlet nozzle or a venture. The outlet is the opening through which air leaves the fan.

A diffuser can be provided to transform kinetic energy to pressure energy. When a diffuser is supplied with the fan, the exit opening of the diffuser becomes the outlet of the fan.

An inlet box may be used to provide side entry or a means of keeping the bearing out of the air stream. When an inlet box is provided with the fan, the opening to that box becomes the input of the fan

Stationary vanes may be used to guide the flow. Vanes used upstream of the impeller can be called inlet guide vanes. Vanes used downstream of the impeller can be called discharge guide vanes. The fan may also be equipped with as own shaft and bearings the bearing may be supported on or within the housing.

DIFFERENT WAYS FOR MOUNTING THE IMPELLER

Impeller mounted between the shaft bearings :- This arrangement has a stable, robust design, requires no transmission journal and resists high temperatures. It is designed for the very largest fans and has plenty of space for packing box/cooling plates and auxiliary drives. However, it has an expensive shaft, requires demanding design work, a suction box, two packing boxes and an expensive fan and has a fixed speed (unless a frequency transformer is used).

Impeller mounted on free shaft journal - belt drive :- This arrangement allows the possibility of varying the fan speed by changing the pulley. Plenty of space is supplied for the packing box/cooling plate and it has a good arrangement in a confined space. However, the transmission journal is belt drive and the shaft must be specially designed if the diameter must be increased, if large motors are used and if the belt forces are high. Also, it requires more maintenance in terms of adjustment/tightening the belt drive and is relatively large.

Impeller mounted on free shaft journal - belt drive on oblique pendulum :- This arrangement allows the possibility of varying the fan speed by replacing the pulley and provides plenty of space for the packing box/cooling plate. However, the transmission journal is belt drive and the shaft must be specially designed and the diameter increased, if necessary, by using large motors and high belt forces. Also, it requires more maintenance in terms of adjustment/tightening of belt drive.

Impeller mounted on free shaft journal - direct drive through clutch :- This arrangement features a stable, robust design, no transmission journal, resistance to high temperatures and plenty of spaces for packing box/cooling plate. However, it is relatively large and operates at a fixed speed (unless a frequency transformer is used).