Pneumatic conveying

A conveying system of any kind is used to transport a material from one particular place to another. For the chosen method of pneumatic conveying, air is used as the transport medium. It is

Or: VH = where: VH

Equ 21.23

Applicable to most granular materials, be they in that form natu­rally or have to be pulverised or crushed. In this system, energy is needed to accelerate the material from rest, to lift it as re­quired and to overcome the losses due to friction.

In pneumatic conveying installations, the pressure losses due to friction include not only those due to the sliding of the material along horizontal ducts and around the walls of bends, but also those due to the air itself. For the purposes of this Section, it is, of course, assumed that the driving force is supplied by a fan.

The ratio of material flowrate to air flowrate will not normally ex­ceed 6:1 and even this will require a very high pressure fan. So called “dense phase” systems, where the material proceeds es­sentially as a “plug” along the duct to which is applied a pres­sure difference usually incorporate either Rootes blowers or compressors of the rotary or reciprocating type. The theory de­veloped in this Section is not applicable to these.

The basis of a design

There is probably no part of fan engineering where “know how” is more important than pneumatic conveying. It has been claimed to be a “black art” rather than a science. Indeed “trial and error” methods (especially the latter) may be necessary be­fore a satisfactory solution in terms of economy and operating convenience is reached.

Conveying velocities

These are usually based on values obtained through some­times bitter experience. Those given in Table 21.12 are if any­thing conservative, which, as in life, is not a bad thing. However it is necessary to develop values based on elementary fluid mechanics.

Material	Conveying velocity vc m/s
Dry sawdust	15
Pulverised coal	20
Oats	22
Crushed limestone	25
Barley	27
Granulated salt	27
Maize	28
Wheat	27
Cement	35
Sand	35

Table 21.12 Typical conveying velocities

Vp = particle volume (m3)

For spherical particle this can be reduced to:

^gPp^p

3CDpa

Where:

Dp = particle diameter (m)

The coefficient of drag CD for a particle with sharp edges is virtu­ally unaffected by Reynolds number and for most shapes is ap­proximately 1.0. for rounded bodies such as spheres, ellipses, long cylinders etc, the value will vary from 0.5 to 1.0 in the order given.

Horizontal velocity

The particles are seen to bounce along the bottom of the duct and spin with high velocity. The “Magnus Effect” (when any ro­tating object placed in a moving airstream produces circulation and aerodynamic lift) occurs when:

Pf9vp =CLAp-ipFvH2

Ml. Ep A

Ap Pf Cl

Horizontal velocity (m/s)

However, in practice it is found that the velocity calculated for vertical conveying is adequate for horizontal conveying. Much of the work on pneumatic conveying by fan systems has been carried out on cereal grains of various types where the bulk density and specific gravity are known, (see Table 21.13).

Description	Bulk density kg/m3	Specific gravity
Black rape	592	0.865
Oats	416-513	1.25
Barley	592 689	1.3
Maize	592	1.34
Wheat	609	1.3

Table 21.13 Bulk density and specific gravity of grains

Bulk density should not be confused with density. It includes all voids between adjacent grains when in repose and is therefore a lower figure.

The dimensions of certain grains are given in Table 21.14.

Equ21.21

Where Vf G Pp Pa Ap CD

Vertical velocity

For any particular granule, a floating velocity can be derived from the formula:

2gppvp

Cd PaAp

Floating velocity (m/s) acceleration due to gravity (m/s2) particle density (kg/m3) air density (kg/m3) frontal area of particle (m2) coefficient of drag

Description	Dimensions mm x mm
Argentine wheat	6×3
British wheat	6.5 x 3.5
Oats	8 to 13 x 1 to 4
Barley	8 to 14 x 1 to 5

Table 21.14 Typical dimensions of grains

It seems to have become the practice in low pressure fan sys­tems to make an initial design based on a value of R between 1

. _ , weight flow of material, , , r,

And 2, where R = ———————— . Larger values of R

Weight flow of air

May require less power but higher fan pressures (see Section

21.9.4.2).

An alternative semi-empirical approach to the calculation of conveying velocity is obtained from the equation

Vr = k„

Where: Kv SP D

Equ21.15

/Spd

Given in Table 21.15 specific gravity of the particle diameter of the particle (mm) conveying velocity (m/s)

Description	KҐ	CD
Sphere	23	0.47
Rounded	19	0.70
Average	17	0.93
Elongated	16	0.05
Flattened	14	0.04

Table 21.15 Values of k„ for typical particles

Where:

A = duct cross-section area (m2)

G = weight flow of material (kg/s)

V = material velocity (m/s)

Now:

Weight flow of material

I —

Weight flow of air

Thus:

G = RWf = R^Q = Ryf(vr + vm)A = RyfvcA

Or

Pacc

GA

— ^pf ^ m(^r ^ m)

= RK±pvc2

2v„

Pressure losses

The total pressure loss in a pneumatic conveying system may be obtained by summating the pressure losses due to various factors and detailed below:

Pfr = friction of air alone for

Horizontal & vertical straight ducts (Pa)

Pace = acceleration of the particles (Pa)

PL = the lift in vertical ducts (Pa)

Pr = resistance of particles to the air (Pa)

Pc = particle collision and wall friction (Pa)

Pb = loss of forward momentum at bends (Pa)

Ppfd = loss in particle feed device (Pa)

Pps = loss in particle separator (Pa)

Thus:

The fan pressure pF is obtained from

PF=PA + PL + Pr+Pc+Pb+Ppfd+Pps pa Equ21.14

Pressure loss due to air alone

0. 02I 1 2

Pa = ~d X2 pVc

Where:

I = length of duct (m)

D = diameter of duct (m)

P = air density (kg/m3)

Vc = conveying velocity. relative velocity + material

Velocity (m/s)

Bends usually have a large radius i. e. at least 6 times the duct diameter. Their equivalent loss in straight duct may be reason­ably taken.

Pressure loss due to the particles

A) To accelerate the particles:

The force required is equal to the rate of change of momentum

I. e.

PaccA = gJ dv

Where:

K

B) To lift the particles in vertical ducts:

Power supplied by air = Power required for lift or

PLQ = GH=RyfQH

Or

PL=RyfH

Where:

H = total loft of material (m)

Gf = weight/unit volume of conveying air (kg/m3)

C) Resistance of particles to the air:

PrA = CDAn^pvr2

Where:

An = total projected area of particles

Vr = velocity of particles relative to air

I. e.

N drag force

D /Tl 2^

A 2 PVr

G = ypEV

— y pAif^m where:

Ev = total volume/unit time (m3/s)

, _ v

* — — for a single particle

An =

Fypvm

This calculation is virtually impossible to make in practice as f is usually unknown. For horizontal conveying, Gasterstadt found that the pressure loss due to particle resistance

Pr = 1 + kR xloss due to air alone Equ21.16

The factor k is obtained from Table 21.16. Vm + vr = vc m/s K 15.8 0.43 17.8 0.36 26.6 0.30

Table 21.16 Values of k against conveying velocity

D) Particle collision and wall friction:

In collision some particle momentum is lost, but the exact amount may only be found by experiment. H. E. Rose and H. E. Barnach found that

Pc =k x^Pvc2

Figure 21.85 Screw feed system

D) Injector

In the injector system, in order to prevent blowbackat the point of entry of materials, the static pressure at this point must be equal to or below atmospheric pressure, (see Figure 21.87).

Where: K And Sp S,

L SD = 0.00022- I— xR

D VSf

= specific gravity of particle = specific gravity of air

Putting some values to the above

1.3×1000

= 32.9

S,

1.2

Thus:

K =0.0072 — R D

I. e. approximately 15 to 30% of the loss for air alone

E) Loss of forward momentum at bends:

Pb = c xloss due to acceleration

Since bends are gradual, the particles perhaps lose about half of their forward momentum i. e. C = 0.5 and

Throat total pressure = system total pressure + loss in expander or

Pvt + Pst = kPvt + Pl

I. e.

(1-k)Pvt =pL-pst

But

Pst = 0 or — Ap(if below atmospheric pressure)

Pl +V

Pl

Pvt

Or 1-k 1-k

Pb=0.5RK ^pvc2

Equ 21.27

Q

The throat area =

Types of conveying system A) Direct

» to collection device

More information on Venturi injectors was given by Seglerin his book Pneumatic Grain Conveying, this including values of k.

21.7 Bibliography

No ifs or butts, Dr Andrew Geens and Dr Max Graham, Building Services Journal, March 2005.

Figure 21.83 Direct system b) Suction

Proposed amendment of Approved Document F of the Building Regulations for England & Wales: Ventilation, ODPM (Office of the Deputy Prime Minister), Building Regulations Division, UK. Www. odpm. gov. uk/approved documents.

Figure 21.84 Suction system

BRE Digest 398, Continuous mechanical ventilation in dwell­ings: design, installation and operation. Sept 1, 1994, ISBN: 851256414.

CIBSE Guide B5, Noise and vibration control for HVAC (2002).

FRS, the fire division of BRE, Garston, Watford WD25 9XX, UK telephone 01923 664100 fax 01923 664910 e-mail: Frs@bre. co. uk.

World Road Association (PIARC), previously known as Perma­nent International Association of Road Congress, La Grande

Arche, Paroi nord, niveau 8, F-92055 La Defense Cedex France Tel: 01 47 96 81 21, Fax: 01 49 00 02 02, Email: Piarc@wanadoo. fr, Www. piarc. org.

ISO 13350:1999, Industrial fans — Performance testing of jet fans.

Centre for Tunnel Aerodynamics Research, London South Bank University, 103 Borough Road, London SE1 OAAUK.

ISO 5801:1997, Industrial fans — Performance testing using standardized.

BS 6164:2001, Code of practice for safety in tunnelling in the construction industry.

Circular Air Jets, T Djeridane, M Amielh, F Anselmet, and L Fulachier, (1993). Experimental investigation of the near-field region of variable density turbulent jets. Proc. 5th Int. Sympo­sium. on refined flow modelling and turbulence measurement, Paris.

The Efficient Use of Steam, Oliver Lyle (HMSO, London 1947); The Efficient Use of Fuel HMSO, London.

Equipment and Protective systems intended for use in Poten­tially Explosive Atmospheres (ATEX) Directive 94/9/EC. (The

Directive has been mandatory from 1st July 2003; it is named after the French “ATmosphere EXplosible”.)

The Machinery Directive 98/37/EC provides the regulatory ba­sis for the harmonisation of the essential health and safety re­quirements for machinery at European Union level.

PrEN 14986, Design of fans working in potentially explosive at­mospheres.

CEN/TC 305, Standard underdevelopment.

ISO 13349:1999, Industrial fans — Vocabulary and definitions of categories.

Position statement on fans intended for use in Potentially Ex­plosive Atmospheres; Conformity with the ATEX Directive 94/9/EC. UK Fan Manufacturers Association (FMA) the spe­cialist fan group within the HEVAC Association — part of FETA. Tel: +44 0118 940 3416, Www. feta. co. uk.

BS EN 13463-1:2001, Non-electrical equipment for potentially explosive atmospheres. Basic method and requirements.

Pneumatic Grain Conveying with special reference to agricul­tural application, G Segler, published by autho 1951, Braunschweig (Brunnswick).

Posted in Fans Ventilation A Practical Guide