Integration of Special Purpose Centrifugal Fans into a Process

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Engineering Design Guide: GBHE-EDG-MAC-1024

Integration of Special Purpose Centrifugal Fans into a Process Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



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 Volume Flow Q o

3.2 Definitions

3.3 Estimate of Equivalent Pressure Rise Δ P e 3.4 Choice of Fan Type

3.5 Choice of Control Method

4 GAS DENSITY CONSIDERATIONS 4

4.1 Calculation of Inlet Pressure

4.2 Calculation of Gas Density

4.3 Atmospheric Air Conditions



5 CAPACITY AND PRESSURE RISE RATING 5

5.1 Calculation of Fan Capacity

5.2 Calculation of Fan Pressure Rise

5.3 Multiple Duty Points 5.4 Stability

5.5 Parallel Operation

6 GUIDE TO FAN SELECTION 6

6.1 Effect of Gas Contaminants

6.2 Selection of Blade Type

6.3 Selection of Rotational Speed 6.4 Wind milling and Slowroll

6.5 Estimate of Fan External Dimensions

7 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

8.1 Calculation of Maximum Inlet Pressure ΔP i max

8.2 Calculation of Maximum Pressure Rise Δ P s max

8.3 Calculation of Casing Test Pressure

8.4 Rating for Explosion

9 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 GUIDE TO FAN TYPE 4.5 VARIATION OF AIR DENSITY WITH TEMPERATURE AND ALTITUDE 6.3.1 DUTY BOUNDARY FOR SINGLE - INLET IMPELLERS 6.3.3 RELATIONSHIP BETWEEN HEAD COEFFICIENT AND SPECIFIC SIZE 6.3.6 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

f y 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



Q f Fan volume flowrate at inlet conditions m3/s Tm Mean torque available for acceleration

of fan and motor N m

T Temperature 0C

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 w 1 Liquid droplet concentration in gas g/m3

w s Suspended solids content of gas g/m3

w v 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 ρ v Density of water vapor kg/m3 ρ m Density of impeller material te/m3

ᴽ Isentropic exponent - Ø Relative humidity %



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 ρ = 1.2 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



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:

Q o = W m3/s ρ o where w = normal mass flowrate kg/s This is taken as the largest process flow required for

operation at the rated daily output of the plant. ρ o = gas density at fan inlet conditions kg/m3 3.3 Estimate of Equivalent Pressure Rise ΔPe

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

P e = 1.2 ΔPo ρ o 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 m/s

Ns = impeller shape factor (defined in Clause 6.3). 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 1000

and w s < 5 where w s = suspended solids content of gas g/m3

(for non-sticky crystalline dusts such as milled rock fines)

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 Q min < 0.3 Q o where Q min = minimum volume flowrate m3/s 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. Ws < 0.1 and W1 < 10 where 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: ρ Ň = ρ 1V1 + ρ 2 V2 + ………. + ρ n Vn kg/m3 100 Or

ρ Ň = 0.04464 M where

ρ Ň = density of gas n at normal conditions kg/m3 Vn = percentage volume of gas n in mixture % M = average molecular weight of dry gas 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: ρ I = ρ Ň 273 P I kg/m3 273 + T I 1013



where Ti = Inlet Temperature 0C (c) For gases mixed with water vapor (but free from water droplets), calculate

the wet gas density at fan inlet conditions as: ρ I = ρ Ň 273 P I - 0.01 Ø Ps + 0.01 ρv Ø kg/m3 273 + T I 1013 Where Ø = relative humidity at prevailing dry bulb temperature % P s = saturation pressure at inlet temperature mbar abs ρ v = density of water vapor at inlet temperature kg/m3 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: CAPACITY AND PRESSURE RISE RATING



5 CAPACITY AND PRESSURE RISE RATING 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 ρ Ň m3/s

ρ imin From mass flowrate Q = W m3/s ρ imin 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 P I m3/s 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 Allowance for damper control (a) 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 mbar

rise in the system independent of flow variation.



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 = static pressure at outlet connection mbar abs P td = total pressure at outlet connection mbar abs Psi = static pressure at inlet connection mbar abs Pti = total pressure at inlet connection mbar abs Pvi = velocity pressure at inlet connection mbar abs ρ I x v I 2 mbar 200 Where V i = MEAN FLOW VELOCITY M/S



When the fan inlet dimensions are not available, take v i as 17 m/s. 5.2.3 Fan total pressure rise ΔP t 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: Q f 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 GUIDE TO FAN SELECTION 6 6.1 6.1.1



6 GUIDE TO FAN SELECTION 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 I 1/4 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 Ps ½ m/s Ψ x ρ i Estimate the minimum proof stress required of the impeller material from: f y = U 2 x ρ m MN/m2 28 F Where

f y = 0.2% proof stress ρ m = density of impeller material te/m3 F = 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: Ns = 0.0316 N x Q f

1/2 ρ1 3/4 ΔP s

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 m Π x N Then, for direct drive fans: 1.5 < Y < 2.2 D 1.5 < X < 2.2 D (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 ᶯ. ) (b) 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 ᶯ. )

7.2 Calculation of Absorbed Power

E = 10 Q f x ΔPs x k p

ᶯs Where E = power absorbed kW

ᶯs = fan static efficiency k p = coefficient of compressibility

Obtain from Fig 7.2 for ΔPs > 25 Take as l.0 for ΔPs < 25

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 non-

overloading 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:

Factor Guarantee to BS 848 Class A 1.07 Guarantee to BS 848 Class B 1.10 Guarantee to BS 848 Class C 1.15 Belt drive - vee 1.07

- flat belt 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 5.5 7.5 11 15 18.5 22 30 37 45 55 75 90 110 132 150

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 s E 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 CASING PRESSURE RATING



8 CASING PRESSURE RATING 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 = maximum gas pressure in the inlet vessel taken as the relief valve pressure setting mbar gauge B max = 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 < Q > 1.05 Q bep 9.2 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

L w = 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

L p = 90 dB A Confirm when the project noise specification is defined. 9.4 Assessment of Silencing Requirements 9. 4. 1 Open Inlet or Discharge

Let L 1 = LR - LP - 10 Log10 A And L 2 = LR - LW where A = 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. Q 2 = N 2 ~ D 2 Q 1 N 1 D 1 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 P 2 = N 2 X D 2 2 P 1 N 1 X D 1 The power absorbed varies directly as the cube of the fan speed and approximately as the cube of the impeller diameter. E 2 = N 2 X D 2 3 E 1 N 1 X D 1 D.3 Change of Density (with constant speed and impeller diameter) The differential pressure and power vary directly as the gas density P2 = E 2 = ρ 2 P1 E 1 ρ 1



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).

