Integration of Special Purpose Centrifugal Fans into a Process

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-1024

Integration of Special Purpose
Centrifugal Fans into a Process

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Engineering Design Guide:

Integration of Special
Purpose Centrifugal Fans
into a Process

CONTENTS

SECTION

0

INTRODUCTION

1

SCOPE

1

2

NOTATION

2

3

PRELIMINARY CHOICE OF NUMBER OF FANS

3

3.1
3.2

Definitions

3.3

Estimate of Equivalent Pressure Rise Δ P e

3.4

Choice of Fan Type

3.5

4

Volume Flow Q o

Choice of Control Method

GAS DENSITY CONSIDERATIONS
4.1

Calculation of Inlet Pressure

4.2

Calculation of Gas Density

4.3

4

Atmospheric Air Conditions


5

CAPACITY AND PRESSURE RISE RATING
5.1

Calculation of Fan Pressure Rise

5.3

Multiple Duty Points

5.4

Stability

5.5

6

Calculation of Fan Capacity

5.2

Parallel Operation

GUIDE TO FAN SELECTION
6.1

Selection of Blade Type

6.3

Selection of Rotational Speed

6.4

Wind milling and Slowroll

6.5

6

Effect of Gas Contaminants

6.2

7

5

Estimate of Fan External Dimensions

POWER RATING

7

7.1

Estimate of Fan Efficiency

7.2

Calculation of Absorbed Power

7.3

Calculation of Driver Power Rating

7.4

Motor Power Ratings

7.5

Starting Conditions for Electric Motors


8

CASING PRESSURE RATING
8.1

Calculation of Maximum Pressure Rise Δ P s max

8.3

Calculation of Casing Test Pressure

8.4

9

Calculation of Maximum Inlet Pressure ΔP i max

8.2

8

Rating for Explosion

NOISE RATING

9

9.1

Estimate of Fan Sound Power Rating LR

9.2

Acceptable Sound Power Level LW

9.3

Acceptable Sound Pressure Level L p

9.4

Assessment of Silencing Requirements


APPENDICES
A

RELIABILITY CLASSIFICATION

B

FAN LAWS

FIGURES
3.4
4.5
6.3.1
6.3.3
6.3.6

GUIDE TO FAN TYPE
VARIATION OF AIR DENSITY WITH TEMPERATURE AND ALTITUDE
DUTY BOUNDARY FOR SINGLE - INLET IMPELLERS
RELATIONSHIP BETWEEN HEAD COEFFICIENT AND SPECIFIC SIZE
ROTATIONAL SPEEDS FOR FAN IMPELLERS WITH BACK SWEPT
VANES
6.3.7 ROTATIONAL SPEED FOR FAN IMPELLERS WITH RADIAL VANES
6.3.8 RELATIONSHIP OF IMPELLER TIP SPEED TO SHAPE
6.3.9 BOUNDARY DEFINING ARDUOUS DUTY
7.1
NOMOGRAPH FOR ESTIMATING THE EFFICIENCY OF A SINGLE
STAGE FAN
7.2
GRAPH: COEFFICIENT OF COMPRESSIBILITY vs PRESSURE RATIO
7.5
GRAPH: MOMENT OF INERTIA OF FAN AND MOTOR (wR2) vs kW

DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE


0

INTRODUCTION

Although the traditional terms of fan, blower and compressor are extensively
used they have no universally accepted definitions.
In these Engineering Design Guides, a fan is arbitrarily defined by a duty envelop
appropriate to the safe use of simple design methods and constructions where
low production cost is the dominant consideration.

1

SCOPE

This Engineering Design Guide integrates a fan into a process, giving:
(a)

The specification of the fan duty for enquiries to be sent to selected
vendors

(b)

The estimation of the characteristics and requirements of the fan in order
to provide preliminary information for design work by others.

It applies to fans in Groups 2 and 3 and is an essential preliminary step for a fan
in Group 1 whose final duty is negotiated with the chosen fan vendor.
It may be used for general-purpose fans in Group 4 but such fans are more
usually specified by reference to the manufacturer's data for a fan satisfactorily
fulfilling the same process need in an existing plant.
Fans for heating and ventilation duties are not covered by this Design Guide.


2

NOTATION

A

Area of duct

m2

B

Barometric pressure at fan location

mbar abs

D

Impeller diameter

m

E

Power absorbed by fan

kW

fy

Material 0.2% proof stress

MN/m2

J

Polar moment of inertia of rotating assembly

kg m2

Kp

Coefficient of compressibility of gas

L

Overall length of fan and motor unit

m

Lp

Acceptable sound pressure level

dB A

LR

Fan sound power rating

-

LW

Acceptable sound power level for fan system

dB A

M

Molecular weight

kg/kg mole

N

Rotational speed of impeller

rev/s

Ns

Impeller Shape Factor

-

P

Absolute pressure

mbar

Pc

Friction loss across damper

mbar

PI

Pressure in inlet vessel

mbar gauge

Pr

Friction loss in system (excluding damper)

mbar

ΔP

Fan pressure rise

mbar

Ps

Saturation pressure at inlet temperature

mbar abs

Q

Volume flow rate

m3/s


Qf

Fan volume flowrate at inlet conditions

m3/s

Tm

Mean torque available for acceleration
of fan and motor

Nm

T

Temperature

0

t

Time for motor run-up

s

u

Impeller tip speed

m/s

V

Percentage volume of gas in mixture

%

v

Mean flow velocity

m/s

w

Mass flowrate

kg/s

w1

Liquid droplet concentration in gas

g/m3

ws

Suspended solids content of gas

g/m3

wv

Mass of water vapor per unit volume of dry gas

kg/m3

X

Overall width of fan casing

m

Y

Over all height of fan casing

m

Z

Specific size parameter

-

ᵑ

Fan efficiency

%

ρ

Gas density

kg/m3

ρv

Density of water vapor

kg/m3

ρm

Density of impeller material

te/m3

ᴽ

Isentropic exponent

-

Ø

Relative humidity

%

C


Ψ

Head coefficient

-

Subscripts
max

refers to maximum value of

min

refers to minimum value of

norm

refers to normal value of

0

refers to preliminary estimate of quantity

e

refers to equivalent value for

i

refers to fan inlet quantity

d

refers to fan discharge quantity

N

refers to quantity at Normal conditions (1013 mbar, 0°C)

s

refers to quantity related to fan static pressure

t

refers to quantity related to fan total pressure

ρ = 1.2


CHECKLIST 3:

PRELIMINARY CHOICE OF FAN AND CONTROL


3

PRELIMINARY CHOICE OF FAN

3.1

Preliminary Choice of Number of Fans

Special-purpose centrifugal fans are of high intrinsic reliability, although the
performance may deteriorate due to corrosion, or particle-induced wear or
deposition. Generally, they fall into reliability class 1, 2 or 3 as defined in
Appendix A. Consequently the standard arrangement is a single unspared unit.

3.2

Volume Flow Q o

First estimate the normal volume flowrate at the fan inlet as:
m3/s

Qo

=

W

=

normal mass flowrate

ρo

where

w

kg/s

This is taken as the largest process flow required for
operation at the rated daily output of the plant.
ρo

=

3.3

Estimate of Equivalent Pressure Rise ΔPe

gas density at fan inlet conditions

kg/m3

First obtain the fan pressure rise from:
ΔPe

=

Pd - Pi

mbar

Where
Pd

=

pressure at fan discharge for ρ o

mbar abs

Pi

=

pressure at fan inlet for ρ o

mbar abs


Then calculate equivalent fan pressure rise

Pe

=

1.2 ΔPo
ρo

3.4.1 Centrifugal Type

3.3

The Electrical Area Classification [3]

Confirm preliminary choice of centrifugal fan when the equivalent duty falls within
the prescribed zone of Fig. 3.4.

3.4.2 Other types of fan
When the duty falls within zone A of Fig.3.4 and gas is clean ( < 0.07 g solids/m3
gas), then consider the peripheral fan type as well as a high-speed centrifugal
fan.
When the duty falls within zone B of Fig.3.4 consider an axial fan as well as a
double-entry centrifugal fan.

3.4.3 Blowers
The upper limit for a special purpose centrifugal fan is defined by:
U Ns = 33

and

U = 250

where

U

=

impeller tip speed

Ns

=

impeller shape factor (defined in Clause 6.3).

m/s

Where this limit is exceeded the design method and equipment specification
should be those appropriate to blowers.


FIGURE 3.4 GUIDE TO FAN TYPES


CHECKLIST 3.5 - SELECTION OF CONTROL METHOD


3.5

Choice of Control Method
Preliminary selection criteria are:

3.5.1 Damper
This should be the first choice when:
Q o Δ Po
and

ws < 5

ws

=

1000

where

suspended solids content of gas
(for non-sticky crystalline dusts such as
milled rock fines)

g/m3

The damper should be located on the inlet side, remote from the fan so that the
fan inlet flow pattern is not distorted. Where this is not practicable, position the
damper downstream from the fan.
If
where

Q min < 0.3 Q o
m3/s

Q min = minimum volume flowrate

and the fan may run for long periods at this rate, then a supplementary bleed-off
or bypass system should be considered.

3.5.2 Inlet Guide Vane (ICV) Unit
This should be the first choice for large fans where power saving is important,
typically when:
Q o Δ Po > 1000
and the gas is clean, i.e.
where

Ws < 0.1

and

W1 < 10

w1 = liquid droplet concentration in gas

g/m3


If prolonged operation at Q < 0.7 Q o is anticipated then consider the use of a
two-speed electric motor with ICV control.
3.5.3 Variable Speed
This should be the first choice when:
( a)

Ws > 5

and the suspended solid particles are known to be
highly abrasive.

(b)

Q o • .Δ Po

> 1000

and gas is not clean
(c)

Large variations in gas composition or inlet conditions are expected.

Variable speed control needs special investigation for high turndowns greater
than 3 (at substantially constant pressure rise). A supplementary control system
such as a damper may be needed.

3.5.4 Multiple Fan Installations
Two or three damper-controlled fans, running in parallel but arranged for
sequential autostop and autostart action on long term change in flow, form an
arrangement which should be considered for systems requiring substantially
constant head over a wide range of flowrate.


CHECKLIST 4

DENSITY CONSIDERATIONS


4

GAS DENSITY CONSIDERATIONS

Intake air density is affected by barometric pressure, ambient temperature and
relative humidity. These are not independent variables and their combination
yields a minimum and maximum density.
For both air and process gas duties:
(a)

Calculating the fan capacity and pressure rise requires the minimum gas
density.

(b)

Calculating the driver power rating and the casing pressure rating requires
the maximum gas density.

4.1

Calculation of Inlet Pressure

The absolute pressure at fan inlet (Pi) is given by:
P i = PI + KB - P ri

mbar abs

where
P ri

=

friction loss inlet system including exit losses from inlet vessel
and losses in transition piece between ducting and fan inlet
connection
mbar

PI

=

gas pressure in inlet vessel

mbar gauge

B

=

mean barometric pressure at fan location

mbar abs

K

=

factor to allow for extreme values of barometric pressure
over a ten-year period of weather changes

=
=

0.965 for conditions of minimum density
1.035 for conditions of maximum density


4.2

Calculation of Gas Density

The gas density is usually given on the Process Data Sheet. A useful check is
as follows:
(a)

Calculate the dry density at Normal conditions (1013.2 mbar abs and 0° C)
ρ Ň as:

ρŇ

=

ρ 1V1

+

ρ 2 V2 + ………. + ρ n Vn

kg/m3

100
Or

ρŇ

=

ρŇ

=

density of gas n at normal conditions

kg/m3

Vn

=

percentage volume of gas n in mixture

%

M

=

average molecular weight of dry gas

0.04464 M

where

The densities of some common gases at Normal conditions are given in
Appendix B.

(b)

For dry gases, calculate the density at fan inlet conditions, P I , as:

ρI

=

ρŇ

273
273 + T I

PI

kg/m3

1013


where

(c)

Ti

=

0

Inlet Temperature

C

For gases mixed with water vapor (but free from water droplets), calculate
the wet gas density at fan inlet conditions as:

ρI

=

ρŇ

Ø

=

relative humidity at prevailing dry bulb temperature

%

Ps

=

saturation pressure at inlet temperature

mbar abs

ρv

=

density of water vapor at inlet temperature

kg/m3

273
273 + T I

PI

- 0.01 Ø Ps
1013

+ 0.01 ρv Ø

kg/m3

Where

Values of P s, P v and weight of water vapor at saturation are given in Appendix C.


4.3

Atmospheric Air Conditions

For an illustrative example, appropriate values are:

For other sites, meteorological data should be obtained. For sites at a known
altitude Fig 4.5 may be used to obtain preliminary estimates of air density.
Remember to correct the density for pressure and temperature changes for fans
drawing atmospheric air through inlet filters, heaters and ducting.


FIG 4.5 -

VARIATION OF AIR DENSITY WITH TEMPERATURE
AND ALTITUDE


CHECKLIST 5:



5


5.1

Calculation of Fan Capacity

5.1.1

Basic Capacity

The capacity requirement is usually for a mass flowrate of dry gas.
Take the normal capacity as the largest process flow required for operation at the
rated plant daily output. This may include a process margin to cover uncertainties
in process calculations.
The normal capacity may be expressed as mass flowrate, W, in kg/s or volume
flowrate Q in m3/s at prescribed conditions (usually Normal, N, conditions of 1013
mbar a and O°C).
Let the basic capacity, Q, be the normal capacity converted to actual volume flow
at the fan inlet conditions.
From volume flowrate at Ň conditions
Q = QN

ρŇ
ρ imin

m3/s

From mass flowrate
Q =

W
ρ imin

m3/s

NOTES:
(a)

If W is for wet gas flowrate, use the wet gas density

(b)

If W or QN is for dry gas flowrate, use the dry gas density and correct for
water vapor content to obtain:


Q

m3/s

PI
P I - Ø Ps
100

5. I. 2 Corrections to basic capacity
Apply the appropriate cumulative corrections to the basic capacity to determine
the required fan capacity Q f :
(a)

Design and manufacturing tolerances
To ensure that the fan capacity (on test) will not be less than the required
capacity, add the following margins on flowrate and specify testing to the
appropriate Class.

If the fan performance is specified and tested to VDI 2044, do NOT add a margin.
(b)

Deterioration

For fans handling dirty gases, a significant margin is required to allow for
performance deterioration due to deposition or erosion
As a guide, add the following margins on flowrate:


The speed N is estimated using the method given in Clause 6.

5.2

Calculation of Fan Pressure Rise

5.2.1
(a)

Allowance for damper control
Automatic control

For automatic control systems where the flow will vary over a wide range, take:

Pc

=

0.3 (P r + P j)

mbar

At the basic capacity, Q.
where
Pc

=

friction loss across damper

mbar

Pr

=

total friction loss in rest of system

mbar

Δ Pj

=

virtual pressure loss in fan

mbar


The value of ΔP j can be obtained from the fans estimated performance curve
using the method shown in Fig 5.2.1.

If the performance curve is rot available, assume the virtual pressure loss to be
35% of the fan pressure rise and take

Pc (0.45 P r + 0.12 ΔP ss)
Where

ΔP ss =

maximum static pressure
rise in the system independent of
flow variation.

mbar


This procedure limits the maximum flow with the damper fully open to about
110% of the normal flow. When a greater maximum flow is required, calculate the
damper friction loss at the specified maximum flow, P cmax as:

P cmax (0.13 P r max + 0.04 ΔP ss)
where

P cmax =

frictional loss in system for maximum flow
specified

mbar

The procedure is strictly valid only for dampers with an equal percentage
characteristic.
(b)

Manual control

For local manual control, the. pressure loss across the damper at the basic
capacity (assuming ΔP j as 35% of fan pressure rise) should be:

Pc >> (0.22 P r + 0.06 ΔP ss)
This procedure limits the maximum flow to about 105% of normal flow. When a
greater maximum flow is required take the damper friction loss at the specified
maximum flow
as:-

Pc >> (0.07 P rmax + 0.02 ΔP ss)


5.2.2 Fan static pressure rise Δ Ps
The fan static pressure rise is defined as the static pressure at the fan outlet
minus the total pressure at the fan inlet.
The definition is peculiar to the fan industry and is not consistent with the
conventional meaning. The term is derived from methods of testing the
performance of fans and is defined in BS 848 Part I.
For a free intake (where P i = P si = P ti)

Δ Ps

= Psd - Pi

mbar

For a free outlet (where Pd = P sd = P td):

Δ Ps

= Psd - Pti

mbar

= Psd - Psi - Pvi

mbar

For ducted inlet and outlet:

Δ Ps

= Psd - Pti

mbar

= Psd - Psi - Pvi

mbar

where

Psd

P td
Psi
Pti
Pvi

=
=
=
=
=

static pressure at outlet connection
total pressure at outlet connection
static pressure at inlet connection
total pressure at inlet connection
velocity pressure at inlet connection
ρ I x vI 2
200

Where

V i = MEAN FLOW VELOCITY

mbar abs
mbar abs
mbar abs
mbar abs
mbar abs
mbar

M/S


When the fan inlet dimensions are not available, take v i as 17 m/s.
Fan total pressure rise ΔP t

5.2.3

The fan total pressure is defined as the total pressure at the fan outlet minus the
total pressure at the fan inlet.
This is seldom used for centrifugal fan duties encountered in chemical plants,
and is covered in guides for axial-flow type fans.
5. 2.4 Duty Point
The duty is conventionally given as the fan static pressure ΔP s required at the
minimum inlet gas density, with the corresponding volume flow at the fan inlet
conditions.

5.3

Multiple Duty Points

Fan manufacturers guarantee only one duty point. Specify all others in terms of
minimum or maximum quantities.
Where the fan has multiple duties, check the following information and include at
least two duty points on the Enquiry.
(a)

Maximum fan capacity Q f
Give the corresponding pressure rise ΔP s as a 'minimum' value.

(b)

Maximum pressure rise ΔP s
Give the corresponding fan capacity Q f as a 'minimum' value.

(c)

Pressure rise, density and capacity corresponding to the maximum value
of the product:

Qf

ΔP s
ρ1


5.4

Stability

An absolutely stable characteristic is defined as one where the fan pressure
decreases continuously with increase of flow over the full operating range from
zero flow. Commercial fans rarely have this characteristic.
Conditional stability is sufficient for most duties and should be specified in the
Enquiry. It is defined by a characteristic
Where:(a)

There is an increase of at least 15% in the fan pressure rise ΔP s from the
rated point to the highest point on the curve.

(b)

the highest point on the curve occurs at a flow less than 90% of any
predicted process minimum flow requirement and not more than 50% of
the flow at the best efficiency point.

(c)

no system characteristic cuts the Q - ΔP s characteristic at more than one
point.

5.5

Parallel Operation

When two or more fans run in parallel specify in the Enquiry that the duty range
lies wholly within the stable part of the COMBINED Q/ Δ P characteristic and
specify that all the fans shall have:
(a)

Conditional stability

(b)

A non-overloading power characteristic with the driver rated for the
maximum power required.

(c)

a shut-off pressure exceeding the maximum pressure obtained with all
remaining fans running. This requirement is most economically met when
individual dampers are used for control, NOT a common damper in the
process system.


CHECKLIST 6


6
6.1
6.1.1


6


6.1

Effect of Gas Contaminants

6.1.1 Dry Non-sticky Solids
The degree of erosion due to dry non-sticky particles in the gas stream is related
to the following properties:
(a)

Hardness of Particles
Particles of greater intrinsic hardness than the impeller material will cause
erosion damage of metals on impact.

(b)

Particle Size
Particles larger than 20 µm are very likely to impact the impeller. Smaller
particles will rarely penetrate the boundary layer.
A typical contaminant source is airborne dust containing sand or milled
rock product. At ground level where fan intakes are commonly located t
the predominant particle size lies in the 70 to 140 µm band. At higher
elevation, at least 2 m above ground in unobstructed areas, the airborne
dust size lies in the 0.1 to 5 µm band.
Check the nominal cut-off size for gas cyclones preceding the fan.

(c)

Particle Shape
Freshly formed crystals possess sharp edges; those that have suffered
attrition are rounded and consequently not so damaging. Caution is
necessary when judging the abrasive nature of particles. There is a
marked change in the 'feel' of hard sharp grains rubbed between the
fingers when the particle size falls below about 70 µm; the material then
seems smooth, not gritty.

(d)

Gas Velocities and Particle Concentration
Above a certain threshold velocity, erosion increases rapidly with increase
in gas velocity relative to the impeller. For a given velocity, the wear is
roughly proportional to the total mass of the particles.


Abrasive wear is minimized by operating at close to the fans best
efficiency point. Flow regulation by variable speed should be used.

6. l. 2 Solids in Presence of Liquid
The presence of a small quantity of liquid encourages agglomeration. The
particles adhere to fan surfaces and build up a layer which destabilizes the flow
pattern and encourages further local deposits.
Liquid wash facilities should be provided for routine washing if the solid is
deliquescent or sticky. The wash liquid system should provide a flow of 20 - 40
liter of liquid per 1000 m3 of gas, in addition to the flow required to saturate the
gas.
High liquid concentrations (typically W I > 40) combine with solid contaminants
to form erosive slurry streams attached to the metal surfaces. These cause
severe gouging at blade to backplate junctions. Special construction features are
required for backward inclined bladed impellers.

6.2

Selection of Blade Type

Aerofoil blades are the first choice when Q f x ΔP s > 400 and the gas is
clean, i.e. WI <: 40 and Ws < 0.1 for dry non-sticky dusts such as milling fines
and coal fired boiler exhausts where the predominant particle size is 20 – 200 µm
Select laminar blades when Q f x ΔP s < 400 or the gas is moderately
contaminated with erosive solids, i.e.
Ws > 0.1
Select radial blades when the gas is very dirty,
Ws > 1, or very wet,

Wi > 40


CHECKLIST 6.2

SELECTION OF BLADE TYPE


6.3

Selection of Rotational Speed

Enter Fig 6.3.1 to select single or double entry configuration. For double entry
impellers take Q f as half of total flow. For radial-bladed impellers specify only the
single-entry configuration.
Calculate Specific Size Z from:

Z = 0.316 Q f ½

1/4

I
ΔP s

With this value of Z, enter Fig 6.3.3 to obtain the Head Coefficient Ψ.
Calculate the impeller tip speed U from:

U = 14.3

½

Ps

m/s

Ψ x ρi

Estimate the minimum proof stress required of the impeller material from:

fy =

U
28 F

2

x ρm

MN/m2

Where
fy =

0.2% proof stress

ρm =

density of impeller material

F

=

te/m3

blade factor = 0.8 for laminar blades
= 0.7 for aerofoil blades
= 2.1 for radial blades


Compare this value of fy with the value for the proposed material. If the latter
value is the lower, then consider a 2- stage fan besides re-considering the choice
of material.
Enter Fig 6.3.6 or 6.3.7 to select normal rotational speed N

rev/s

FIG 6.3.1 - DUTY BOUNDARY FOR SINGLE - INLET IMPELLERS


FIG 6.3.3 -

RELATIONSHIP BETWEEN HEAD COEFFICIENT AND
SPECIFIC SIZE


FIG 6.3.6 -

ROTATIONAL SPEEDS FOR FAN IMPELLERS WITH
BACKSWEPT VANES


FIG 6.3.7 -

ROTATIONAL SPEED FOR FAN IMPELLERS WITH
RADIAL VANES


FIG 6.3.8 - RELATIONSHIP OF IMPELLER TIP SPEED TO SHAPE


FIG 6.3.9 - BOUNDARY DEFINING ARDUOUS DUTY


Calculate Impeller Shape Factor Ns from:
1/2

Ns = 0.0316 N x Q f

ρ1
ΔP s

3/4

Enter Fig 6.3.8. If the point falls in:Zone A -

consider higher rotational speed and check that a single
entry configuration has been chosen.

Zone B -

the fan speed is confirmed and should be inserted on the
Fan Data Sheet

Zone C -

consider lower rotational speed

Zone D -

opt for blower or use lower rotational speed

Enter Fig 6.3.9 to establish if the fan should be designed and constructed for
arduous duty to Specification GBHE-EDS-MAC-1809.

6.4

Windmilling and Slowroll

To maintain the mechanical balance of the rotor during a plant shutdown. a
slowroll auxiliary drive may be required when:
(a)

the rated gas temperature exceeds 2000C

(b)

process gas contaminants will deposit a metastable layer of material
which can creep along the impeller surface (exemplified by kiln exhauster
duties on sulfuric acid plants using the anhydrite process).

Begin by including in the Enquiry provision of a geared electric motor driving the
rotor through an auto-release clutch at roughly 10% of the rated speed.
Windmilling is usually beneficial and obviates the need for an auxiliary slowroll
driver.

Consider need for an emergency brake. to prevent rotation due to gas being
drawn through the fan, after trip of the driver.

6.5

Estimate of Fan External Dimensions

The following estimate of the fan external dimensions is for provisional layout
work. Check after receipt of vendor offers.

6.5.1 Casing/Motor Size
Consider a vertical-shaft fan when the process gas contains a high concentration
of liquid droplets ( > 100 g/m3).
For horizontal-shaft fans, first calculate the impeller diameter D from:

D =

U
ΠxN

m

Then, for direct drive fans:
1.5 < Y
D

< 2.2

1.5 < X
D

< 2.2

(2.5 + k x Ns) <

L < (4 + 1.5 K x Ns)
D

Where
Y

=

overall height of fan casing

X

=

overall width of fan casing

L

=

overall length of fan and motor unit

K

=
=

2 for single-inlet impeller
4 for double-inlet impeller


6.5.2 Discharge Flow Distribution
Fans directly discharging into plenum chambers may create process problems
due to non-uniform distribution of flow to the process especially for fans where
Ns > 0.3.
Where such problems are likely, provide a minimum length of straight discharge
duct, including any inline silencers, more than:
7 x (cross sectional area)1/2
If this is not practicable, then specify that the fan shall have a uniform distribution
of the discharge flow. This may be aided by locating multivane dampers at the
fan discharge.

6.5.3 Inlet Duct Layout
Fans with ducted inlets should have a minimum length of straight duct of:
3 x (duct cross sectional area)1/2
If not investigate provision of flow straighteners. These may take the form of
vaned bends.
Fans with open inlet should be provided with a bell mouth entry. Such an entry
should be provided with a gust shield for fans with a pressure rise less than 6
mbar. This will lessen but not eliminate transient flow variations.


CHECKLIST 7

POWER RATING


7

POWER RATING

Use this power rating only for preliminary estimates. Check required power rating
after selection of fan supplier.
7.1

Estimate of Fan Efficiency

First obtain the peak efficiency at the maximum efficiency point using nomograph
Fig 7.1.
Adjust this efficiency for fan type as follows:
(a)

For laminar bladed impellers multiply by the factor
(1 - 0.0006

(b)

ᶯ. )

For radial bladed impellers, multiply by the factor 0.6.

Then obtain the average efficiency, for the preliminary estimation of power
consumption, by multiplying the adjusted peak value by the factor:
(0.85 + 0.001

7.2

ᶯ. )

Calculation of Absorbed Power

E

= 10 Q f x ΔPs x k p

ᶯs

=

power absorbed

=

kp

=

fan static efficiency
coefficient of compressibility
Obtain from Fig 7.2 for ΔPs > 25
Take as l.0 for ΔPs < 25

ᶯs

Where
E

kW

The power absorbed by a fan varies with gas density at the fan inlet. For
calculating the maximum absorbed power, take the fan pressure rise at the
maximum density, viz

ρ imax
ρ imin
times the pressure rise used for fan capacity and pressure rating in Clause 5.

FIG 7.1 - NOMOGRAPH FOR ESTIMATING THE EFFICIENCY OF A SINGLE
STAGE FAN


FIG 7.2 GRAPH: COEFFICIENT OF COMPRESSIBILITY vs PRESSURE
RATIO

NOTES
(a)

For hot gas fans drawing cold air at start up, check that the rating chosen
is suitable for a cold start with the maximum air density during the most
adverse meteorological conditions. This is particularly important where no
means are provided for operating at reduced load. Where capacity control
is provided cold start can be at reduced load but note that process
requirements usually limit the reduction in mass flow.

(b)

Backward swept aerofoil and laminar bladed fans will exhibit a nonoverloading power characteristic. Enquiries should request vendors to
state the peak power in addition to the powers at the duty points.


(c)

Radial/paddle bladed fans will exhibit an overloading power characteristic
and the maximum absorbed power used for motor rating should be based
on the power at the maximum foreseeable rate recognizing probable
inaccuracies in pressure loss estimates. Remember to verify the operation
of one fan when all companion fans normally running in parallel are strut
down.

(d)

During liquid washing there will be an increase in power demand for the
driver. As a preliminary estimate multiply the absorbed power by the factor
(1 + 2.10-6 U2).

7.3

Calculation of Driver Power Rating

Estimate the maximum power requirement by multiplying the maximum absorbed
power from Clause 7.2 by the following factors as appropriate:

Guarantee to BS 848
Guarantee to BS 848
Guarantee to BS 848
Belt drive

-

Factor
1.07
1.10
1.15

Class A
Class B
Class C

vee
flat belt

1.07
1.04

Gearbox drive (transmitting E KW)

1.02 + 0.2
E

Variable speed coupling

1.02 + 0.06
E1/2

For an electric motor drive select the next highest standard motor power rating
from Clause 7.4.


7.4

Motor Power Ratings

7.4. 1 Ratings below 150 kW
For fixed speed motors specify the induction type.
Standard Motor Rating, kW (for 50 Hz supply)
4
37

5.5
45

7.5
55

11
75

15
90

18.5
110

22
132

30
150

7.4.2 Ratings above 150 kW
Consult Electrical Section for appropriate motor rating and type.

7.5

Starting Conditions for Electric Motors

Standard motors with direct on-line starting have a torque which is adequate for
most fan applications. However, for other starting methods and for large, low
speed fans, where E > 50 and N < 16, check that the run-up time from rest does
not exceed the permissible run-up time for the motor.
For provisional estimation of the run-up time, t, take:

t

=

k x N2 x J
E

s

where
J =

polar moment of inertia of fan and motor
rotating assemblies (WR2)

kg m2

See Fig 7.5.3 for preliminary estimate
K =

empirical constant taken as 0.025

Take the permissible run-up time for standard motors with DOL starting as 12
secs.


For E > 600 or N < 16 consult Electrical Section, usually up to 40 secs can be
allowed.
FIG 7.5 GRAPH:

MOMENTOF INERTIA OF FAN AND MOTOR
(WR2) VS KW
REV/S


CHECKLIST 8



8


8. 1

Calculation of Maximum Inlet Pressure ΔPi max

Calculate the maximum inlet pressure P i max
as

P i max = P I max + B i max

mbar abs

Where
P Imax

B max

=

=

maximum gas pressure in the inlet vessel
taken as the relief valve pressure setting

mbar gauge

maximum barometric pressure for the
site location

mbar abs

Any pressure loss through the inlet system is ignored. Remember that the fan
may be run on air with open inlet for test purposes.

8.2

Calculation of Maximum Pressure Rise ΔPs max

As a first estimate take the peak pressure rise as 115% of the static pressure
head used for pressure rise rating in Clause 5 and adjusted for the maximum
inlet density,
When the maximum density of the process gas is less than that of atmospheric
air, take the latter value. Re-determine the maximum pressure rise when the
actual fan characteristic is available.

8.3

Calculation of Casing Test Pressure

Take the maximum casing pressure as the greater of
P imax + P smax


or the delivery system relief system setting.
The test pressure of the casing should exceed 150% of this maximum pressure
but should not be less than 70 mbar.
Fans handling hot gases (>200°C) require individual consideration taking into
account the reduction in material strength with increase in temperature and the
ducting loads.
Note that the criteria for the design of casings for small and low pressure fans are
usually to restrict drumming and noise transmission rather than the maximum
operating pressure.

8.4

Rating for Explosion

For processes handling explosive dust concentrations, specify design to an
appropriate design code (e.g. National Fire Protection Association (USA) Code
91).
Obtain a value from Process Design Section for the maximum pressure which
may be generated in an explosion. Specify the casing to withstand that pressure,
with a pressure test of not less than twice this pressure.
Where escape of hot gases may cause damage external to the fan casing or
injury to personnel, mechanical shaft seals or other approved positive seals
should be specified.
Rating for detonation lies outside the scope of this Design Guide.


CHECKLIST 9

NOISE RATING


9

NOISE RATING

9.1

Estimate of Fan Sound Power Rating LR

Estimate the in-duct sound power rating of the fan from:

LR = 10 log10 E + 30 log10 U - 15 log10 Ti + k1 + k2
Where
k1

=
=

70 for backward swept bladed impeller
80 for radial bladed impeller

k2

=

0 for fans operating close to their best efficiency
point (bep)

=

10 for fans operating away from the best efficiency point
i.e., 0.9 <

9.2

Q
> 1.05
Q bep

Acceptable Sound Power Level L w

Obtain the acceptable sound power level in dB A re 10-12W, for fan system from
the project noise specification.
For provisional estimates, when the project noise specification is not established
and fan system is not installed close to a site boundary, take
Lw

=

100

dB A

Confirm when the project noise specification is defined.


9.3

Acceptable Sound Pressure Level Lp

Obtain the acceptable sound pressure level in dB A re 2 x 10-5 N/m2, for fan and
system from the project noise specification.
For provisional estimates when the project noise specification is
not established, for on-plant systems take
Lp

=

90

dB A

Confirm when the project noise specification is defined.

9.4

Assessment of Silencing Requirements

9. 4. 1

Open Inlet or Discharge

And

L 1 = LR - LP - 10 Log10 A
L 2 = LR - LW

where

A =

Let

area of duct open end

m2

If ΔL 1 > 0 or ΔL2 > 0
install in-duct silencer between fan and open end and as close as practicable to
fan.
Note: intermediate equipment such as fluid beds or rotary heaters may provide
sufficient attenuation to avoid need for silencer, but should be ignored for the
preliminary evaluation.

9.4.2 Fan Casing
Let

ΔL 1 = LR - LP - 35

and

ΔL 2 = LR - LW - 25


(a)

If ΔL 1 and ΔL 2 lie between 0 and 10
Provide acoustic insulation of fan casing.

(b)

If either ΔL 1 or ΔL 2 lies between 10 and 20
Provide suspended acoustic insulation of fan casing.

(b)

If ΔL 1 > 20 or ΔL 2 > 20
Provide separate noise hood or consider variable speed drive to reduce L.

9.4.3 Ducting
Let

ΔL1 =

LR - LP - 30

and

ΔL2 =

LR - LW - 20

(a)

If ΔL1 and ΔL2 lie between 0 and 10
provide acoustic insulation of ducting (as far as silencer when fitted).

(b)

If either ΔL1 or ΔL2 lies between 10 and 2O
Provide suspended acoustic insulation of ducting.
(as far as silencer when fitted).

(c)

If ΔL1 > 20 or ΔL2 > 20
Install in-duct silencers at both inlet and discharge of fan.

Confirm silencing requirements when fan supplier is selected, including the
economics of installing in-duct silencers against the provision of acoustic
insulation on long ducting lengths.


APPENDIX A
RELIABILITY CLASSIFICATION (abstracted from GBHE-EDG-MAC-5100)
Installations having high availability are conveniently classified as follows:
Class 1
A Class 1 installation achieves high availability by having units of high intrinsic
reliability, and is characterized by:(a)

The machine being a single unspared unit upon which the process stream
is wholly dependent.

(b)

The plant section having a single process stream with a long process
recovery time after a shutdown so that the loss of product owing to a
machine stoppage is large even though the shutdown is for a short time.

(c)

A capability of a continuous operation within given process performance
tolerances over a period of more than three years, without enforced halts
for inspection or adjustment.

(d)

Component life expectancies exceeding 100,000 hours operation.

Class 2
As for Class 1 but where infrequent plant shutdowns of short duration are
acceptable because the process recovery time is short. Consequently the period
of continuous operation capability can be reduced and is taken as 4,000 hours
for this classification.
Class 3
As for Class 1 or 2 but with a machine performance deterioration during the
operating period accepted, or countered by adjustment of process conditions
or by other action on the part of the plant operators.


Class 4
A Class 4 installation achieves high availability by redundancy, and is
characterized by having:
(a)

One or more machines operating with one or more standby machines
at instant readiness at all times to take over automatically upon
malfunction of a running machine.

(b)

Operating and standby machines designed specifically for their functions
so that they are not necessarily identical.

(c)

Component life expectancies exceeding 25,000 hours operation.

Class 5
A Class 5 installation follows the Class 4 redundancy concept. and is
characterized by:
(a)

One or more machines operating with one or more identical machines
installed as spares to take over the process duty at the discretion of the
plant operators.

(b)

One or more machines operating in Plant sections where product
storage is sufficient to give the plant operators adequate time to assess
the malfunction and take remedial action. Alternatively, plant sections
where a single machine stoppage does not cause a disproportionately
large process upset.

Class 6
Machines intended for batch or intermittent duty.
Where high demand availability is essential such machines lie within Class 4.


APPENDIX B
FAN LAWS
B.1

Limitations to Use of Fan Laws

The following fan laws apply to the specified changes in a given fan and not to
geometrically similar fans of different size.
The pressure and power relations are limited to conversions where the values of
the compressibility coefficient (Kp) do not differ by more than 0.01.
B.2

Change of Speed or Impeller Diameter

The inlet volume varies directly as fan speed and approximately as impeller
diameter.

Q2 = N 2 ~ D2
Q1
N 1 D1
The pressure rise varies directly as the square of fan speed and approximately
as the square of the impeller diameter. For a constant inlet gas density

P2 =
P1

N2 X D2
N1 X D1

2

The power absorbed varies directly as the cube of the fan speed and
approximately as the cube of the impeller diameter.

E2 =
E1
D.3

N2 X D2
N1 X D1

3

Change of Density (with constant speed and impeller diameter)

The differential pressure and power vary directly as the gas density
P2
P1

= E2
E1

= ρ2
ρ1


DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE
This Engineering Design Guide makes reference to the following documents:
NATIONAL FIRE PROTECTION ASSOCIATION (USA)
Code 91

Standard for the Installation of Blower and Exhaust Systems 1or
Dust, Stock and Vapor Removal or Conveying (referred to
in Clause 8.4).

GERMAN STANDARDS
VDI 2044

Acceptance & Performance Testing of Fans (referred to in Clause
5.l.2).

BRITISH STANDARDS
BS 848

Fans for General Purposes (referred to in Clause 7.3
Part 1 Methods of Testing Performance (referred to in Clauses
5. l • 2, 5. 2. 2 and 7. 3) •

ENGINEERING DESIGN GUIDE

GBHE-EDG-MAC-5100

Reliability Analysis - The Wei bull Method (referred to
in Appendix A).

ENGINEERING SPECIFICATION

GBHE-EDS-MAC-1809

The Design and Construction of Steel Centrifugal
Fans Operating on Arduous Duties (referred to in
Clause 6.3).


Integration of Special Purpose Centrifugal Fans into a Process

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