Che10205 Fans

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Fans

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE10205

For additional information on this subject, contact R. A. Al-Husseini on 874-2792

Engineering Encyclopedia

Process Fans

CONTENTS

PAGES

TYPES OF FANS

1

CENTRIFUGAL FANS

2

PERFORMANCE CALCULATIONS FOR CENTRIFUGAL FANS

3

Definitions of Pressure

3

Definitions of Pressure (Cont'd)

4

Total Pressure versus Static Pressure

5

Total Pressure versus Static Pressure (Cont'd)

6

Fan Horsepower

6

Performance Curves

7

CONTROLS FOR CENTRIFUGAL FANS

8

MODIFICATIONS TO CENTRIFUGAL FANS

8

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Process Fans

TYPES OF FANS Only centrifugal fans are discussed in this module. Air fin coolers will be covered in the heat exchanger course. A centrifugal fan is similar to a centrifugal compressor. It contains a rotor inside a housing, which is similar to the casing of a centrifugal compressor. Fans are always single stage and made of light weight materials. Centrifugal fans in Saudi Aramco facilities supply forced air to the burners of boilers and process furnaces. They are also used as induced draft fans; that is, they pull the combustion gases from the firebox of a boiler or furnace to move the gases up through the stack. Induced draft fans are somewhat different from air supply fans because they move hot gases. The bearings are separate from the housing, and the metal plates of the housing are often thicker. Centrifugal fans produce differential pressures of 10 to 40 inH 2O. Axial fans are similar to propellers operating at low speed. Axial fans also handle large volumes of air or gas. Pressure differentials are between 0 and 1 inches of water. Fans are used in three types of services in Saudi Aramco. Air fin coolers are heat exchangers that have bundles of finned tubes across which air is circulated. Large axial fans are used to move the air across the bundles. Cooling towers are large structures that cool recirculating cooling water. Water falls by gravity over the packing of the tower, and air is drawn upward through the structure. A large axial fan at the top of the cooling tower moves air through the packing. Duct fans are axial fans mounted inside ventilation ducts. They are used in the air conditioning systems of buildings.

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CENTRIFUGAL FANS Figure 1 is a schematic diagram of a centrifugal fan. Suction air enters at the center of the rotor and passes radially through blades. A housing contains a volute that leads the air to the discharge duct. The discharge is a rectangular or circular opening at the end of the volute. Rotational speeds are much lower than they are for centrifugal compressors.

Vanes

FIGURE 1. TYPICAL CENTRIFUGAL FAN

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PERFORMANCE CALCULATIONS FOR CENTRIFUGAL FANS Definitions of Pressure Figure 2 illustrates different types of pressure at the discharge of a fan. Static pressure is the pressure in the duct that is independent of velocity head. Velocity pressure is the velocity head, which is measured by a pitot tube facing upstream into the flowing air. Total pressure is the sum of static and velocity pressures.

FIGURE 2. PERFORMANCE CALCULATIONS FOR CENTRIFUGAL FANS DEFINITIONS OF PRESSURE

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Definitions of Pressure (Cont'd) At the discharge of a centrifugal fan, both static pressure and velocity pressure are present. The velocity pressure component is greater at higher flow rates (Figure 3). At maximum flow rate, the total output of the fan is velocity pressure, and static pressure increase is zero. Centrifugal fans normally operate at a point on the curve where velocity pressure is 20 to 40% of the total pressure developed by the fan.

Total Pressure = Static Pressure + Velocity Pressure

FIGURE 3. FAN OUTPUT Velocity pressure can be calculated by the Eqn. (1). Pv = ρ (Vm)2 ___________ 1.203 * 106

Eqn. (1)

where: Pv

=

Velocity pressure, inH20

ρ

=

Gas or air density, lb/ft3 at conditions

Vm

=

Average velocity, ft/min at conditions

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Total Pressure versus Static Pressure When a centrifugal fan discharges into a duct, most of the velocity pressure is efficiently converted to static pressure. Figure 4 shows a typical installation. The fan takes suction from the atmosphere and discharges into a duct. At the other end of the duct is a chamber called a plenum chamber, which conducts the air to several individual burners on a furnace. At the transition from the duct to the plenum, velocity pressure is lost because of the sudden change in velocity. However, the transition from the fan to the duct takes place with essentially no loss because the expansion is gradual. Velocity pressure is efficiently converted to static pressure. Therefore, the total pressure required at the fan discharge in the system shown in Figure 4 is the sum of: The static pressure required at the burner plus friction pressure drop in the duct plus velocity pressure at the velocity in the duct.

FIGURE 4. PRESSURE CONVERSION IN A DUCT

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Total Pressure versus Static Pressure (Cont'd) In most systems, the velocity and the velocity pressure in the duct will be significantly lower than they are at the discharge of the fan. The differential pressure (ÆP) produced by the fan is total pressure at the discharge minus total pressure at the suction. Velocities are usually low at the suction. Therefore, velocity pressure is negligible, and total pressure equals static pressure. Fan Horsepower Fan horsepower can be calculated by the equations below. The fan manufacturer may present the performance data of the fan in one of two ways. The first presents total pressure and total efficiency or mechanical efficiency. The second way presents static pressure and static efficiency. Remember to use mechanical efficiency with total pressure, and static efficiency with static pressure. bhp = ________ Q(Pt) 6350 (Em)

Eqn. (2)

bhp = ________ Q (Ps) 6350 (Es)

Eqn. (3)

or

where: Q

=

Flow, ft3/min at suction conditions

Pt

=

Fan total ÆP, inH20

Em

=

Mechanical Efficiency (Total Efficiency)

Ps

=

Fan static ÆP, inH20

Es

=

Static efficiency

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Performance Curves Figure 5 is a typical performance curve for a centrifugal fan. The upper curve in the figure is total pressure developed by the fan versus air flow rate. Note that this curve has a high point peak at about 40% of maximum capacity. The fan should be operated to the right of this peak. Operation in the region to the left of the peak can be unstable. On this curve both mechanical efficiency and brake horsepower are shown. A manufacturer may show only one of these, since the other value can be calculated if one is known. The basis conditions are also printed on the curve. These are suction pressure, suction temperature, fluid molecular weight, and rotational speed of the fan. In some cases, several curves may be shown for different rotational speeds. The differential pressure produced by a fan is proportional to the density of the gas at suction conditions. You can correct the curve if actual conditions are different from the basis conditions.

Suction Pressure Suction Temperature Fluid Mol. wt. Fan Speed

14.4 psia 120° F 29 850 rpm

FIGURE 5. TYPICAL PERFORMANCE CURVE - CENTRIFUGAL FAN

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CONTROLS FOR CENTRIFUGAL FANS Variable speed is one method of controlling the head and capacity of a centrifugal fan. This is convenient if the driver is a steam turbine, because steam turbines are capable of variable speed. It is also possible to operate a motor driver at variable speed if the appropriate speed control device is installed. For fans that are driven at constant speed, inlet guide vanes can be used to control the capacity. Inlet guide vanes are variable louvers installed in the eye of the impeller. Rotating these louvers changes the direction at which the air strikes the rotating impeller. With inlet guide vanes, some savings in power can be achieved at low flow rates. Simple louvers are also used for throttling fans. They may be installed in the suction duct or in the discharge duct. Inlet guide vanes, since they change the velocity of the gas passing through the fan, are more efficient than louvers used for throttling.

MODIFICATIONS TO CENTRIFUGAL FANS Sometimes you may want to increase the head or the capacity available from an operating fan. This can be done in two ways. The speed of the fan can be changed. If the driver is variable speed this can be done during operation. For a fan operating at fixed speed, the speed can be changed by changing the diameter of the pulleys in the drive belt system. On some fans it is possible to change the impeller diameter. To predict the performance of a fan with a higher speed or a larger diameter impeller, use the fan laws that are presented in the following equations. For speed:

N  Q 2 = Q1  2   N1  H2

N  = H1  2   N1 

bhp2

2

N  = bhp1  2   N1 

3

where: Q H bhp N

= = = =

Suction flow, actual Polytropic head Brake horsepower Speed, rpm

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MODIFICATIONS TO CENTRIFUGAL FANS (CONT'D) Similar relationships exist for impellers of different diameters. Simply replace Speed, N with Diameter, D.

Q1

 D2   D  1

D  H1 2   D1 

2

D  bhp1  2   D1 

3

where: Q H bhp D

= = = =

Flow rate Head Brake horsepower Diameter

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GLOSSARY Air Fin Cooler

A heat exchanger with fluid inside finned tubes, across which air is circulated.

Axial Fan

A fan in which the flow of gas is in the same direction as the axis of rotation. Axial fans resemble slow speed propellers.

Centrifugal Fan

A fan with a rotating impeller and a housing that contains a volute.

Cooling Tower

A structure used to cool recirculating cooling water.

Duct

A thin-walled pipe that may be round or rectangular in cross section.

Fan

A low-head compressor.

Guide Vanes

Variable-position vanes installed in the eye or the suction of a centrifugal fan. They vary the velocity of the gas passing through the fan and can be used to reduce capacity and achieve some savings in power.

Housing

The casing of a centrifugal fan.

Louvers

Variable position slats used to throttle the suction or the discharge of a fan.

Mechanical Efficiency

For a fan, the same as total efficiency. Total pressure developed times flow rate divided by power input and the appropriate conversion factor.

Plenum

A chamber that divides an air or gas stream into several streams.

Static Efficiency

For a fan, static pressure developed by the fan times flow rate divided by horsepower input and the appropriate conversion factor.

Static Pressure

The pressure in a duct that is independent of velocity.

Total Pressure

For a fan, static pressure plus velocity pressure.

Velocity Pressure

The velocity head inside a flowing duct.

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