FANS General

The fan is the heart of a ventilation system. The industrial engineer requires a comprehensive working knowledge of the subject of fan engineering for the following reasons:

1. The designer must fully appreciate the correct fan selection, by ensuring that its duty meets the requirements of the task for which it is intended.

2. In many cases an existing fan system has to be adapted or modified to meet new demand requirements.

3. The owning and operating costs must be fully considered.

A fan is a rotodynamic device and is the driving part of all mechanical ventilating systems. The energy of rotation applied to the fan shaft is con­verted into a pressure difference, causing the air, gas, or particulate matter to flow through the ductwork or discharge into a free space.

To be classed as a fan, the work per unit mass on the gas must be less than 25 kj kg’1; above this value it is called a turbo compressor.

Hence, the fan pressure must not exceed the standard air density of

1.2 kg nr5 x25 kj kg-1, giving 30 kPa, with the pressure ratio not exceed­ing 1.30, taking the atmospheric pressure as 100 kPa.

ISO gives the classifications for fans shown in Table 9.6.

TABLE 9.6 Category According to Work per Unit Mass

Description

Code

Work per unit mass, kj kg-1

Maximum fan pressure, kPa

Class

Low pressure

LB

0 to 0.6

0 to 0.7

0

0.6 to 0.833

0.7 to 1.0

1

0.833 to 1.333

1.0 to 1.6

?

1.333 to 1.667

1.6 to 2.0

3

Medium pressure

MP

] .667 ro 3.00

3.0 to 3.6

4

3.0 to 5.25

3.6 to 6.3

5

5.25 to 8.33

6.3 to 10

6

High pressure

HA

8.33 to 13.33

10.0 to 16.0

7

13.33 to 18.67

16.0 to 22.4

8

18.67 to 25.00

22.4 to 30

9

Tu rbo compressor

25+

30+

Fans can be divided into four general categories: propeller, axial, centrifugal, and special purpose. These are defined according to the direction of gas flow.

Propeller

This type consists of a propeller or disk-type wheel within a mounting- ring panel or cage. The wheel or housing is constructed from either sheet metal, cast aluminium, plastic, or plastic-coated material. It may be a direct drive with the wheel on the motor shaft or belt driven.

Advantages and Typical Uses

• Wide range of volumes

• A minimum operating cost per m3 s-1

• Minimum space and weight per m3 s~l

• Blast or man cooling for hot processes

• Dilution ventilation for toxic and odor removal

Disadvantages/Limitations

• Limited to resistances of 250 kPa

• Sound-level problems with high speeds

• Not suited for corrosive or abrasive applications, bearings to be protected

• Direct-drive fans should not be used in area in which explosive or flammable gases or vapors are handled by the fan

• Operating temperature limitations

Axial Fan

A tube axial fan (see Fig. 9.32) is essentially a propeller fan located in a short cylinder housing, the gas flowing in an axial direction. A vane axial fan incorporates specially designed vanes, which are positioned either upstream or downstream of the fan.

The axial fan consists of an impeller fitted with airfoil blades mounted on a rotating hub. The hub is positioned in a cylindrical casing in line with the gas flow direction. If safe gases are being handled, the motor is located in the airflow. If, however, explosive, abrasive, flammable, or corrosive gases are conveyed, a bifurcated fan is used, with the motor positioned outside the gas stream. A motor located outside the casing allows the fan to be belt driven, providing easy speed changes if necessary. Due to advances in motor electronic speed control, the use of belts for speed control is on the decline.

An airfoil fan may have an efficiency as high as 80%. It has the advan­tages of being compact and able to fit in line with the ductwork. Its disadvan­tage is that it may not be capable of developing the high pressures required for many industrial ventilation applications.

The purpose of the blades is to reduce the degree of flow spiraling and to convert some of the velocity component into useful static pressure.

Vane axial fans develop a greater static pressure than tube axial fans. They are constructed from a variety of materials, depending on the applica­tion. They may use either a direct drive or a belt drive. More expensive models

FANS General

Impeller

I in _ ————————— ►

Guide vanes

FIGURE 9.32 Centrifugal and axial fans.1

Axial fan

подпись: 
axial fan
FANS General

Spiral casing

подпись: spiral casing

Shaft

подпись: shaft

Centrifugal fan

подпись: centrifugal fan FANS General

/

подпись: /

Impeller

/

подпись: impeller
/
Are fitted with adjustable pitch blades, allowing a direct-driven fan to cover the same capacities as belt-driven fans of the same diameter.

Advantages and Typical Uses

• Operates from low — to high-volume flow rates.

• The actual pressure range of some vane axials is similar to high­efficiency backward-curved centrifugal fans. By fitting the fans in series, the operating pressure can be increased.

• Compact, low-space environment and low weight per unit volume handled make it second only to the propeller fan.

• Applications include comfort, dilution, man cooling, paint-spray-booth extract, etc.

Dis ad vantages/Limitations

• Inherently higher sound levels than most efficient centrifugal fans for the same duty

• Unsuitable in abrasive or corrosive atmospheres

• Problems in protecting bearings

• Unsuitable for flammable and explosive gases and vapors unless a bifurcated fan is used

• Fan curve problems with damper closing

Centrifugal Fan

This is the most common type of fan encountered in industrial ventilation systems. These fans are similar to a water wheel, with blades mounted on

Back plates. The impeller is positioned in a volute or scroll casing. The air en­ters with the line of the driving shaft through the eye and discharges at 90° to the entering air.

The centrifugal fan may have one of four impeller designs.

1. Airfoil

2. Backward inclined-backward curved

3. Radial (paddle)

4. Forward curved

Airfoil, Backward Curved

These low-cost fans are of simple construction and have static efficiencies up to 80%.

This fan design is capable of the highest efficiency with the lowest sound level of all the centrifugal fans for a given duty, particularly those with airfoil­shaped double blades.

They are normally fitted with 10-12 blades, the blade tips inclined away from the angle of rotation. They have the advantage of being very efficient, with fan static efficiencies in excess of 80% for airfoil blades. This type of fan has a nonoverloading characteristic curve.

Advantages and Typical Use

• Handles moderate to high airflow rates

• Static pressure up to the 7.5 kPa range

• Highest efficiency of any fan type

• Lowest noise level of any fan type for a given duty

• Self-limiting power characteristics, nonoverloading characteristic

• Variable air volume systems

Dis ad vantages/Limitations

• For equal pressure and volume, axial fans’ weight and size are greater for the backward-curved fan

• Unsuitable for high dust loading, due to particulate buildup on the impeller causing imbalance and vibration

• Wheels are difficult to clean and paint

Radial Blade, Straight Paddle Blade

This fan group, sometimes referred to as industrial exhaust fans’, is char­acterized by its simple, rugged construction.

They may or may not have flat, radial blades. The wheel configuration may be from simple paddle blades to the semi-open and enclosed types. They may be belt or direct driven.

Advantages and Typical Use

• Ease of maintenance and cleaning

• Ease of repair due to simple construction

• Can handle any type of gas or dust

• Used for pneumatic conveying

• Suitable for high-temperature operation

• Suitable for corrosive and abrasive material if correctly designed

Disadvantages/Limitations

• Lowest efficiency of all centrifugal fans.

• Highest sound level of all centrifugal fans for a given duty.

• Shaft power increases as the fan approaches the maximum volume.

Forward-Curved Blade, Centrifugal

These are made up of a large number of wide, shallow blades with a very large inlet area relative to the wheel diameter. For equal duty the speeds are lower than other centrifugal fans. They are sometimes called multivane; the operating efficiencies are in the 65-75% range. They consist of a large number of relatively small blades mounted on the impeller. The blade tips are inclined toward the direction of rotation. The actual flow rate can be 2.5 times as high as the same size backward-curved fan.

Advantages and Typical Uses

• Ideal for any volume at low to medium static pressures.

• Due to low speeds they are quiet in operation.

• The space and weight requirements are about the same as backward — inclined blade fans.

Disadvantages/Limitations

• Lower efficiency than backward-curved fan.

• Greater space requirement for a given duty than an axial flow fan.

• Unsuitable for high dust loading.

• Shaft power increases as fan approaches maximum volume, unlike the backward-curved fan, where it decreases.

The types of fans are given in Fig. 9.32.

As the gas density handled by a fan is relatively constant, the pressure in­crease is small, and the pressure ratio is less than 1.1.

Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK


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