BASIC DESIGN OF CENTRIFUGAL FAN

Centrifugal fan design 

MAJOR CENTRIFUGAL FAN  PARTS

Centrifugal fan design is usually given when the conditions are: the volume flow rate, total pressure, the working medium and its density (or the working medium temperature), and sometimes there are structural requirements and special requirements.

    Design requirements of a large centrifugal fan are: to meet the required flow and pressure conditions at the highest point of efficiency should be near; maximum efficiency value to be as large as some of the efficiency curve flat; pressure curve to a wide range of stability; fan of simple structure , technology is good; convenient choice of materials and accessories; have sufficient strength, stiffness, safe and reliable; running stable, low noise; adjusted performance, work adaptability; fan size as small as possible, light weight; operation and maintenance , disassemble easy transport.

    However, while meeting all the requirements above, is generally not possible. In the aerodynamic performance and structural (strength, process) there are contradictions between the often, usually to seize the main contradiction solve the problem. This requires designers to choose the right design to address the principal contradiction. For example:

    With the use of different fans, not the same requirement, such as public buildings used as a ventilation fan is generally used with, the most important requirement is that the general low-noise, multi-blade centrifugal fan with this feature; and require large flow The centrifugal fan is usually double-suction type; for some of the high-pressure centrifugal fan, low specific speed, the relative proportion of the leakage loss is generally larger

Fan Parts-Impeller Design 
 The centrifugal fans impeller have five basic blade shapes, and a number of impeller configurations (i.e) DWDI (Double width double inlet) or SWSI (Single width single inlet). The impeller design will depend on the aerodynamic duty and the operating conditions.

 Blade Design

TYPES OF BLADE DESIGN

  
Backward inclined, backward curved blades

3D MODEL OF BACKWARD CURVED IMPELLER

A highly efficient design of impeller. Its inherent strength means that in addition to handling air, it can be used with gases containing moderate amounts of erosive particles and at high temperatures. It can carry liners and hard surfacing, if required. 

Backward inclined, flat plate blades
An efficient and strong shape, this design is a cost-effective alternative to the backward curved design but with flat plate blades instead of curved. This results in a slightly lower efficiency, compensated by the easier fitting of liners.

Backward inclined, forward curved blades
Also termed radial tipped blades, this design reduces the potential for dust build-up on the underside of the blades in applications with moderate amounts of gas-borne dust.

Aerofoil blades

3D MODEL OF AEROFOIL IMPELLER

Aerofoil bladed impellers are most commonly used in applications handling large volumes at low pressures. This highly efficient design of fan is used in clean air or with gases containing small amounts of erosive particles.

Radial blades

3D MODEL OF RADIAL TIP IMPELLER

Forming a rotor which is essentially a large paddle wheel, this design results in a relatively inefficient fan with a power consumption higher than that using the much more common backward inclined blade. Its inherent mechanical strength and resistance to wear mean it is generally used when high quantities of abrasive dust are present in the gas stream, or when very high gas temperatures are expected.

Impeller configurations

Single or double inlet
A double inlet impeller consists of two single inlet impellers back to back on the shaft sharing a common backplate, providing almost double the flow of a single inlet fan. The selection of a double inlet fan results in a smaller diameter, faster (and perhaps cheaper) machine compared to the single inlet alternative. Single or multiple stage
A multi-stage fan consists of two or more impellers mounted on the same shaft. The air or gas flow passes through each one in turn, with a consequent increase in pressure

CENTRIFUGAL FAN EFFICIENCY & POWER CONSUMPTION

Fan Power Consumption

The ideal power consumption of a fan (without losses) can be expressed as

P_i = dp q (1)

where

P_i = ideal power consumption (W)

dp = total pressure increase in the fan (Pa)

q = air volume flow delivered by the fan (m³/s)

The ideal power consumptions for fans at different air volumes and pressure increases are expressed in the chart below:

Fan Motors and Starting Torques

A fan motor must be capable not only of driving the fan at operating speed, but also be capable of accelerating the fan wheel to the operating speed

A fan motor must be capable not only of driving the fan at operating conditions, but also be capable of accelerating the fan wheel, drive and shaft to the operating speed. For a fan transporting a large volume of air at low static pressure the motor power required during the continuous operating process may not be enough for starting the fan. Often a correct designed and adjusted motor protection system will stop the fan before the windings are overheated and insulation damaged.

The motor torque should during design be checked against the fan wheel torque up to 90% of the synchronized speed.

CENTRIFUGAL FAN TERMINOLOGY- FAN PARTS

AIR FLOW DIRECTION IN CENTRIFUGAL FAN

Fan Parts
The principal parts of any fan are the impeller and the housing. Various other parts may be necessary or useful in the operation of the fan. 
 The impeller is the rotating element that transfers energy to the fluid. An impeller can also be called a wheel, a rotor, a squirrel cage, a propeller, or a runner. Impeller should probably be preferred as a general name, but wheel and rotor are also commonly used for all types. Squirrel cage is restricted to forward-curve centrifugals, and propeller is restricted to certain simple axials. Runner is used more frequently for pumps than for fans. The blades are the principal working surfaces of the impeller. A blade can also be called a vane, a paddle, a float, or a bucket. Vane should probably be preferred as a general name, but blade is also commonly used for all types. Paddle is usually restricted to an unshrouded type, and float to centrifugals in
general. Bucket is more frequently used for turbines than for fans. Shrouds may be used to support the blades. A shroud can also be called a cover, a disk, a rim, a flange, an inlet plate, a backplate, or a center plate. Shroud should probably be preferred as a general name, but cover and disk are also used. Flange, inlet plate, and rim are restricted to members that shroud the blades on the inlet side of a centrifugal fan. Back plate and center plate are restricted to members that shroud the side opposite the inlet on single-inlet
and double-inlet wheels, respectively. Hubs may be used to support the blades directly or through a shroud to the shaft. A hub can also be called a boss or a disk, but this terminology seems tobe disappearing.

SHROUDED IMPELLER (BACKWARD CURVED BLADE)

The housing is the stationary element that guides the air or gas before andafter the impeller. A housing can also be called a casing, a stator, a scroll, a panel, a ring, or a volute. Housing, casing, and stator are all general. Scroll and volute are both restricted to centrifugal types, whereas panel and ring are restricted to propeller types. Centrifugal housing components include the side sheets and scroll sheets.

FAN CASING (VOLUTE SCROLL)

The point of closest approach to the wheel is the cutoff, also called the tongue.The area over the cutoff is called the blast area.

UNDERSTANDING OF CENTRIFUGAL FAN & BLOWER DESIGN

FEA MODEL OF CENTRIFUGAL FAN IMPELLER

The centrifugal fan is one of the most widely used fans. Centrifugal fans are by far the most prevalent type of fan used in the HVAC industry today. They are usually cheaper than axial fans and simpler in construction. In automotive industries, fans are used for cooling internal combustion engines. It is used in transporting gas or materials and in ventilation system for buildings. They are also used commonly in central heating/cooling systems. They are also well-suited for industrial processes and air pollution control systems.

Centrifugal fans of various descriptions constitute one of several types of turbomachinery, which are used to transfer energy to a flowing fluid. Centrifugal fans are similar in many respects to both centrifugal pumps and
centrifugal compressors. Of course, pumps handle liquids and so can easily be distinguished from fans. But, since fans and compressors both handle gases, their differences are not very distinct. Machines with very low pressure rises are always identified as fans, and those with very high pressure rises as compressors. For intermediate pressure rises, either description can be applied. Even test codes and standards often fail to make a distinction. The flow through centrifugal machines is chiefly radial in the region of energy transfer and is easily distinguished from the flow in axial-flow machines. Axial-flow fans are discussed in the next chapter, but many of the principles of energy transfer given in this chapter are applicable to axial-flow as well as centrifugal fans.