FAN SELECTION

5-92 FIGURE 6-3

6.3 FAN SELECTION

Fan selection involves not on ly finding a fan to match the required flow and pressure considerations but all aspects of

TYPE IMPELLER DESIGN

Highest efficiency of all centrifugal fan

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designs, 9 to 16 blades of airfoil contour

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curved away from the direction of rotation (5 Air leaves the impeller at a velocity less a: ~ than its tip speed and relatively deep

;;;: blades provide for efficient expansion within the blade passages. For given duty, this will be the highest speed of the centrifugal fan designs.

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Efficiency is only slightly less than that of :!!:>

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.... a: airfoil fans. Backward-inclined or back- e,.):::1

:Z:e,.) ward-curved blades are single thickness.

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' 0 ^, 9 to 16 blades curved or inclined away 0a: a:< from the direction of rotation Efficient for

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Simplest of all centrifugal fans and least

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and the wheel is easily repaired, For a

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< given point of rating, this fan requires

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a: includes radial blades (R) and modifi- ed radial blades (M), usually 6 to 10 in number.

Efficiency is less than airfoil and back- 0 ward-curved bladed fans, Usually fab- w

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ricated of lightweight and low cost con-

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a: struction, Has 24 to 64 shallow blades :::I

e,.) with both the heel and tip curved forward,

6 Air leaves wheel at velocity greater than a: wheel. Tip speed and primary energy

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3: transferred to the air is by use of high a: velocity in the wheeL For given duty, 0 ~ wheel is the smallest of all centrifugal

types and operates at lowest speed,

FIGURE 6-4. Types of fans: impeller and housing designs (see facing page)

an installation including the air stream characteristics, oper- ating temperature, drive arrangement, and mounting. Section 6.2 discussed the various fan types and why they might be selected. This section offers guidelines to fan selection; how- ever, the exact performance and operating limitations of a particular fan should be obtained from the original equipment manufacturer.

6.3.1 Considerations for Fan Selection:

CAPACITY

Flow Rate (Q): Based on system requirements and ex- pressed as actual cubic feet per minute (acfm) at the fan inlet.

Pressure Requirements: Based on system pressure require- ments which normally are expressed as Fan Static Pressure (FSP) or Fan Total Pressure (FTP) in inches of water gauge

HOUSING DESIGN

Scroll-type, usually designed to permit efficient conversion of velocity pressure

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to static pressure, thus permitting a high

-~ static efficiency; essential that clearance and alignment between wheel and inlet bell be very close in order to reach the maxiumum efficiency capability, Con- centric housings can also be used as in power roof ventilators, since there is effi- cient pressure conversion in the wheel

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^-~ Utilizes the same housing configuration as the airfoil design,

Scroll·type, usually the narrowest design

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of all centrifugal fan designs described here because of required high velocity discharge. Dimensional requirements of this housing are more critical than for air- foil and backward-inclined blades,

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Scroll is similar to other centrifugal-fan designs, The fit between the wheel and inlet is not as critical as on airfoil and backward-inclinded bladed fans Uses large cut-off sheet in housing,

at standard conditions (0.075 Ibm/ftl). If the required pressure is known only at non standard conditions, a density correction (see Section 6.3.8) must be made.

stream). Conform to the standards of the National Board of Fire Underwriters, the National Fire Protection Association and governmental regulations (see Section 6.3.9).

AIR STREAM Corrosive Applications: May require a protective coating

or special materials of construction (stainless, fiberglass, etc.) Material handled through the fan: When the exhaust air

contains a small amount of smoke or dust, a backward inclined centrifugal or axial fan should be selected. With light dust, fume or moisture, a backward inclined or radial centrifugal fan would be the preferred selection. If the particulate loading is high, or when material is handled, the normal selection would be a radial centrifugal fan.

Elevated Air Stream Temperatures: Maximum operating temperature affects strength of materials and therefore must be known for selection of correct materials of construction, arrangement, and bearing types.

PHYSICAL LIMITATIONS Explosive or Flammable Material: Use spark resistant con-

struction (explosion-proof motor if the motor is in the air

Fan size should be determined by performance require- ments. Inlet size and location, fan weight, and ease of main-

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PERFORMANCE CHARACTERISTICS *

Highest efficiencies occur 50 to 65% of wide open volume. This is also the area of good pressure characteristics; the horse- power curve reaches a maximum near the peak efficiency area and becomes lower toward free delivery, a self-limiting power characteristic as shown.

Operating characteristics of this fan are similar to the airfoil fan mentioned above Peak efficiency for this fan is slightly lower than the airfoil fan. Normally unstable left of peak pressure.

Higher pressure characteristics than the above mentioned fans. Power rises con- tinually to free delivery.

Pressure curve is less steep than that of backward-curved bladed fans. There is a dip in the pressure curve left of the peak pressure point and highest efficiency occurs to the right of peak pressure, 40 to 50% of wide open volume. Fan should be rated to the right of peak pressure. Power curve rises continually toward free delivery and this must be taken into account when motor is selected.

APPLICATIONS

General heating, ventilating and air-con- ditioning systems. Used in large sizes for clean air industrial applications where power savings are significant.

Same heating, ventilating, and air-con- ditioning applications as the airfoil fan. Also used in some industrial applications where the airfoil blade is not acceptable because of corrosive and/or erosion environment.

Used primarily for material-handing applications in industrial plants. Wheel can be of rugged construction and is simple to repair in the field. Wheel is sometimes coated with special material. This design also used for high-pressure industrial requirements. Not commonly found in HVAC applications.

Used primarily in low-pressure heating, ventilating, and air-conditioning applica- tions such as domestic furnaces, central station units, and packaged air-con- ditioning equipment from room air-con- ditioning units to roof top units

Types of fans: Performance characteristics and applications. (*These performance curves reflect the general characteristics of various fans as commonly employed. They are not intended to provide complete selection criteria for application purposes, since other parameters, such as diameter and speed, are not defined.)

TYPE IMPELLER DESIGN HOUSING DESIGN

a: Efficiency is low. Impellers are usually of

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Simple circular ring, orifice plate, or ven- LU ~ inexpensive construction and limited to

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low pressure applications. Impeller is of 2

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or more blades, usually of single thick-

LU design is reasonably close to the blade

Q. ness attached to relatively small hub.

0 Energy transfer is primarily in form of tips and forms a smooth inlet flow contour

a: to the wheel.

0. velocity pressure

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fan design and is capable of developing

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Cylindrical tube formed so that the run-

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a more useful static pressure range.

ning clearance between the wheel tip

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and tube is close. This results in signifi-

<C ^LU m hub is usually less than 50% of fan tip cant improvement over propeller fans.

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^::J diameter. Blades can be of airfoil or single

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Good design of blades permits medium-

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Cylindrical tube closely fitted to the outer

<C to high-pressure capability at good effi- diameter of blade tips and fitted with a set

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ciency. The most efficient fans of this type of guide vanes. Upstream or downstream c( have airfoil blades. Blades are fixed or from the impeller, guide vanes convert the LU

Z adjustable pitch types and hub is usually rotary energy imparted to the air and

~ greater than 50% of fan tip diameter. increase pressure and efficiency of fan.

...I This fan usually has a wheel similar to the Cylindrical shell similar to a vaneaxial fan

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airfoil backward-inclined or backward- housing, except the outer diameter of the

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efficiency when used in fan of type.) and must change direction by 90

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LU used. section.

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sense since the air is simply discharged w ::J u.. above to produce a low-pressure, high- from the impeller in a 360 degree pattern C

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I- volume flow rate characteristic. In addi-tion, many special centrifugal impeller and usually does not include a configura-tion to recover the velocity pressure com-

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a: Essentially a propeller fan mounted in a

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supporting structure with a cover for

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Q" <C weather protection and safety considera-

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employed with the objective of high-vol- tions. The air is discharged through the ume flow rate at low pressure. I I annular space around the bottom of the

weather hood.

FIGURE 6-4 (continued). Types of fans: impeller and housing design

PERFORMANCE CURVES

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PERFORMANCE CHARACTERISTICS·

High flow rate but very low-pressure capa- bilities and maxiumum efficiency is reached near free delivery. The discharge pattern of the air is circular in shape and the air stream swirls because of the action of the blades and the lack of straightening facilities.

High flow-rate characteristics with medium- pressure capabilities. Performance curve includes a dip to the left of peak pressure which should be avoided. The discharge air pattern is circular and is rotating or whirl- ing because of the propeller rotation and lack of guide vanes.

High-pressure characteristics with medium volume flow rate capabilities. Performance curve includes a dip caused by aero- dynamic stall to the left of peak pressure, which should be avoided. Guide vanes cor- rect the circular motion imparted to the air by the wheel and improve pressure charac- teristics and efficiency of the fan.

Performance is similar to backward-curved fan, except lower capacity and pressure because of the 90 degree change in direc- tion of the air flow in the housing. The effi- ciency will be lower than the backward- curved fan. Some designs may have a dip in the curve similar to the axial-flow fan.

Usually intended to operate without attached ductwork and therefore to operate against a very low-pressure head. It is usu- ally intended to have a rather high-vclume flow rate characteristic. Only static pres- sure and static efficiency are shown for this type of product

Usually intended to operate without attached ductwork and therefore to operate against very low-pressure head. It is usually intended to have a high-volume flow rate characteristic. Only static pressure and static efHciency are shown for this type of product.

Types offans: performance characteristics and applications

APPLICATIONS

For low-pressure, high-volume air moving applications such as air circulation within a space or ventilation through a wall without attached duct work. Used for replacement air applications

Low- and medium-pressure ducted heat- ing, ventilating, and air-conditioning applications where air distribution on the downstream side is not critical. Also used in some industrial applications such as dry- ing ovens, paint spray booths, and fume exhaust systems.

General heating, ventilating, and air-con- ditioning systems in low-, medium-, and high-pressure applications is of advantage where straight-through flow and compact instalation are required; air distribution on downstream side is good. Also used in industrial application similar to the tubeax- ial fan. Relatively more compact than com- parable centrifugal-type fans for same duty.

Used primarily for low-pressure return air systems in heating, ventilating, and air-con- ditioning applications. Has straight- through flow configuration.

For low-pressure exhaust systems such as general factory, kitchen, warehouse, and commercial installations where the low- pressure rise limitation can be tolerated Unit is low in first cost and low in operating cost and provides positive exhaust ventila- tion in the space which is a decided advan- tage over gravity-type exhaust units. The centrifugal unit is somewhat quieter than the axial unit desribed below.

For low-pressure exhaust systems such as general factory, kitchen, warehouse, and some commercial installations where the low-pressure rise limitations can be toler- ated. Unit is low in first cost and low in oper- ating cost and provides positive exhaust ventilation in the space which is a decided advantage over gravity-type exhaust units.

tenance also must be considered. The most efficient fan size may not fit the physical space available.

DRIVE ARRANGEMENTS

All fans must have some type of power source - usually an electric motor. On packaged fans, the motor is furnished and mounted by the manufacturer. On larger units, the motor is mounted separately and coupled directly to the fan or indirectly by a belt drive. A number of standard drive arrange- ments are shown in Figures 6-5a, 6-5b, and 6-5c.

Direct Drive offers a more compact assembly and assures constant fan speed. Fan speeds are limited to available motor speeds (except in the case of variable frequency controllers).

Capacity is set during construction by variations in impeller geometry and motor speed.

Belt Drive offers flexibility in that fan speed can be changed by altering the drive ratio. This may be important in some applications to provide for changes in system capacity or pressure requirements due to changes in process, hood design, equipment location or air cleaning equipment. V-belt drives must be maintained and have some power losses which can be estimated from the chart in Figure 6-6.

NOISE

Fan noise is generated by turbulence within the fan housing and will vary by fan type, flow rate, pressure, and fan effi- ciency. Because each design is different, noise ratings must be obtained from the fan manufacturer. Most fans produce a

"white" noise which is a mixture of all frequencies. In addition to white noise, radial blade fans also produce a pure tone at a frequency equal to the blade passage frequency (BPF):

BPF = RPMxNxCF where:

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blade passage frequency, Hz RPM

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rotational rate, rpm

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number of blades

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conversion coefficient, \/60

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This tone can be very noticeable in some installations and should be considered in the system design.

Because of its higher efficiency, the backward inclined impeller design is generally the quietest. However, for al\ fan types, non-uniform air flow at the fan inlet or outlet can increase the fan noise level. This is another problem related to "system effect" (see Section 6.4.1).

Most fan manufacturers publish sound ratings for their products. There are a variety of ways to present the ratings.

One popular way is to list sound power levels for eight ANSI standard octave bands. The sound power levels are typically in units called "decibels" (dB). The sound power level is a characteristic of a fan that varies with the fan speed and point of operation.

For an installed fan, the surrounding environment affects the sound level that is measured or heard. Walls, floors, and other equipment reflect and absorb sound to varying degrees.

The sound that reaches the listener will be different than the fan's rated sound power level. Typical sound measuring de- vices detect sound with a microphone and display sound pressure level in decibels. This sound pressure is an environ- ment-dependent measurement that changes with listener lo- cation andlor environment changes.

While the decibel unit is used for sound power and sound pressure, the two measures are not interchangeable. For in- stance, 70 dB sound power is not 70 dB sound pressure. The decibel is not an absolute unit of measure. It is a ratio between a measured quantity and an agreed reference level. Both dB scales are logarithmic. The sound power is the log ofthe ratio of two power levels. The sound pressure is the log ofthe ratio of two pressure levels. The sound power scale uses a reference of 10-¹² watts. The sound pressure scale uses a reference of20

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10-⁶ N/M2.

For an installed fan, the sound pressure levels are usually measured in dB using the "A" weighting scale. The A-weight- ing is used to measure environmental noise as it most closely reflects the human auditory response to noise of various frequencies. A sound level meter set on the "A" scale auto- matically integrates the noise of all frequencies to give a single dBA noise measurement. Expanded detail can be obtained by taking noise measurements with a meter capable of measuring the sound pressure level in each octave band. Such detail can help indicate the predominant source of a noise.

The topic of sound is quite broad and there are many reference texts available to cover it. For a concise introduction, the ASHRAE Fundamentals HandbooH6~) is a good starting point.

SAFETY AND ACCESSORIES

Safety Guards are required. Consider all danger points such as inlet, outlet, shaft, drive and cleanout doors. Construction should comply with applicable governmental safety require- ments, and attachment must be secure.

Accessories can help in the installation and in future main- tenance requirements. Examples might include drains, c1eanout doors, split housings, and shaft seals.

FLOW CONTROL

There are various accessories that can be used to change fan performance. Such changes may be required on systems that vary throughout the day or for reduction in flow rate in anticipation of some future requirement. Dampers, variable pitch blades, and speed control are three common accessories used with fans.

Dampers are installed directly on the fan inlet or outlet.

Because they are in the air stream, dampers can build up with

SW - Single Width SI - Single Inlet

ow -Double Width 01 - Double Inlet

Arrangements 1. 3 7 and 8 are also available with bearings mounted on pedestals or base set Independent of the fan housing

ARR. 2 SWSI For belt drive or di- rect connection. Impeller overhung Bearings in bracket supported by fan housing.

ARR_ 4 SWSI For direct drive. Im- peller overhung on prime mover shaft. No bearings on fan. Prime mover base mounted or integrally directfy connected

ARR. 8 SWSI For belt drIVe or di- rect connection Arrangement 1 plus extended base for prime mover

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ARR. 3 SWSI For belt drive or di- rect connection One bearing on each side and supported by fan housing

ARR. 7 SWSI For belt drive or di- rect connection Arrangement 3 plus base for prime mover

ARR. 9 SWSI For belt drive Im- peller overhung. two bearings. with prime mover oulslde base

Reprinted from AMCA Publication 99-86, STANDARDS HAND- BOOK, by permission of the Air Movement and Control Association, Inc.(6.1)

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ARR. 1 SWSI For belt drive or di- rect connection Impeller overhung Two bearings on base

ARR.3 DWDI For belt drive or di- rect connection One bearing on each side and supported by fan housing

ARR. 7 OWOI For belt drive or di- rect connection. Arrangement 3 plus base for prime mover

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ARR_ 10 SWSI For belt drive Im- peller overhung. two bearings. with prime mover Inside base

AMERICAN CONFERENCE OF GOVERNMENTAL INDUSTRIAL HYGIENISTS

DRIVE ARRANGEMENTS