Assessment of venturi nozzle for filter bag cleaning in (1).pdf

7/27/2019 Assessment of venturi nozzle for filter bag cleaning in (1).pdf

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ABSTRACT

M. Piskovsk1, J. Hjek1, L. Bbar1, P. Stehlk1, and J. Oral2

1 Institute of Process and Environmental Engineering, Brno University of Technology,Czech Republic

2 EVECO Brno, Ltd., Czech Republic

Design configuration of an industrial device may strongly influence pressure drop,propensity to fouling, uniformity of flow distribution across heat exchange surfaces,intensity of mixing (e.g. in reactors) and so on. Adding, changing or withdrawing a

shape element impacts the spatial distribution of flow field variables.

This contribution presents an application of the CFD approach for assessingsuitability of a Venturi nozzle, placed into the throat of a filter bag in a fabric filter. Thefilter is employed for collection of particulate matter from flue gas in a municipal

waste incineration plant and it is necessary to clean it regularly. The cleaning isperformed periodically by pulse jet method. The analysis examined the influence of

the Venturi nozzle for quality of filter cleaning as well as pressure drop caused by thenozzle during normal filter operation. A commercial CFD software systemFLUENT has been used for all the computations.

Assessment of Venturi nozzle for filter bag cleaning in fabric filtersusing CFD modelling

Keywords: Bag Filter, CFD - Simulation, Gas Cleaning, Pulse Jet Cleaning, Venturi

Nozzle


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1. Introduction

Many publications have investigated filter cleaning methods, but (according to [1])

most of them analysed only rigid filters so far. For the case investigated in thepresent work, the following publications have been found useful:

o Laux [2], [3] inspected pressure profiles along ceramic filter element during thepulse jet cleaning. Experiments where done under operating filter temperatures of

250 and 850 C. Computational model has been constructed in CFD code Fluent3.02., k-e model of turbulence has been used. Only one filter element was simulated(including surrounding area). Pressure drop across the filter element was assumed to

be stationary, permeability was known. Another assumption was that approximatelyone third of surrounding gas is entrained in the compressed air. Flow resistance was

governed by Darcys Law, turbulence in the porous region was not modelled. Theoverpressure in the filter reached its maximum value in 50ms, which was in accordwith experimental measurement. Computational results were in very good

quantitative agreement with the measured data in the middle and on the bottom ofthe filter. However, whereas the computed pressure maximum decreased along the

filter, the measured had an opposite tendency.

o Berbner [4] has discovered by measurements, that pressure wave, generated

during pulse jet cleaning, consist of a dynamic part, when pressure in the filterelement rapidly increases, and a consequent stationary flow into the filter. The results

show that the dust cake is detached during the dynamic part and the length of theconsequent stationary flow has small influence on the cleaning quality. Thisconclusion was later confirmed by others [5].

A commercial CFD software system FLUENT 6.1 has been used for all computationsin this work. The outcome of the analysis is a comparison of cleaning cycleperformed with and without Venturi nozzle. Pressure drop of the Venturi nozzleduring normal filter operation was determined as well.

2. Equipment description

2.1. Arrangement of the filter

Cylindrical filter bags are installed vertically andthey are arranged into a matrix structure (see

figure1). Gas flow through the filter is controlled byflue gas fans, located behind the filter outlet.

2.2. Filter regeneration

Dust cake growths during the standard filteroperation, and the pressure drop is increasing

accordingly. Upon reaching a maximum tolerablepressure drop, the filter has to be cleaned. Above

every row of bags is located a pressure pipe withholes, directed into the filter bags. During cleaningcycle, compressed air is fed into the pipe.

Pressure gradient is higher than critical, but theflow is limited to critical conditions (Mach number

Fig. 1

Arrangement offilter bags in thebaghouse


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M=1). Thus, medium is flowing out of the pipe holes and into the filter bags at the

speed of sound. Under the nozzles, pressure decreases and more gas is entrainedfrom the surrounding space above the cell plate. Shock wave proceeds along the bag

and stretches the walls, shaking down the dust cake. The entrained gas intensifiesthis effect.

It is generally assumed that a Venturi nozzle located in the top end of a filter bag has

a positive impact on the amount of entrained gas. The task of this work has been todetermine how strong is this effect and whether the usage of nozzles is justified.

3. Determination of pressure drop coefficient of the Venturi nozzle

During normal filter operation, Venturi nozzle causes pressure drop. The position andgeometry of the analysed Venturi nozzle is however very specific, so that the

corresponding pressure drop coefficient could not been found in the literature. Anobvious approach to analyse this device is by application of sophisticated CFDmethods, which make possible modelling of the whole device without neglecting any

important influences.

3.1. Model construction

A 2D model of a single filter bag has been created by simplification of geometry ofthe filter bag surroundings, assuming axial symmetry.

On figure 2 are displayed the types of boundary

conditions employed in the model. Conditions at theinletare defined by flue gas mass flow, temperatureand composition, hydraulic diameter of the inlet and

turbulence intensity. For description of the porous

jump (representing fabric of the filter) was usedDarcys law (1), which expresses linear dependence

of flow velocity u through the porous medium on

pressure loss ?Pof the baffle.

1

=h

Pku (1)

The only unknown variable in equation (1) ispermeability of the medium k. In order to find its

value, it was necessary to use operation data (? P=800 Pa); flue gas dynamic viscosity has been

determined by application of the theorem ofcorresponding states; velocity u is given by the ratio of flue gas flow rate and total

filter area; the thickness h of the porous baffle has been arbitrarily set equal to 0.01m.

Conditions at the outlet are defined by relative pressure pg = -1500Pa, which is

related to ambient (atmospheric) pressure equal to 99kPa.

The flowing fluid is flue gas, the composition of which may be determined from

balance of chemical reactions occurring during waste combustion. For individual fluegas species have been introduced the following relationships:

o Densities are calculated from state equation of ideal gaso Specific heats are calculated from polynomials of the FLUENT databaseo Dynamic viscosities are computed from the kinetic theory of gases

porousjump

pressureoutlet

massflowinlet

axis

wall

wall

Fig. 2 Schematic display of the

2D model of normal operation


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3.2. Computational approach

Solution has been sought for a time-independent flow. Turbulence has beenmodelled by the RNG k-e model [6]. The result of the iterative calculation is a

solution representing distribution of physical variables in the modelled domain.

Pressure drop caused by the Venturi nozzle may be determined by measuringpressures before and after installation of the nozzle [7]. If ?p is pressure difference,

measured between two sample points (located in front and beyond the nozzle) beforeinstalling the nozzle and ?p is the pressure difference, measured between the sametwo sample points after installing the nozzle, then pressure drop ?p, caused by the

nozzle, is given by equation (2):ppp = (2)

Considering these facts, a secondmodel has been created, almostidentical with the first, displayed in

figure 2. The only difference in thatthe second model does not contain

the Venturi nozzle. Also this timewas obtained converged solution.

Pressure along filter bag axis in thetwo alternative simulations is shownin figure 3. This way, pressure drop

of the Venturi nozzle during normalfilter operation has been found to be?p =52Pa.

4. Modelling of unsteady flow during pulse cleaning cycle

4.1. Model construction

For the simulation was again constructed

axially symmetrical 2D model, based onthe same simplifying assumption as insection 3.1. Diameter of the outercylindrical wall has been increased 10times, as required by the ratio of volume ofa filter section, cleaned in one cycle, to thevolume of the whole filter (corresponding to

simultaneous cleaning of all filter bags).Geometry of the employed model isschematically displayed in figure 4. The

operating conditions are given by pressurep=99kPa a temperature T=483K (210C).Conditions at the inlet are given by total

pressure of the cleaning air p0=250kPa(i.e. 151kPa gauge pressure), its temperature T=300K (27C), hydraulic diameter of

the inlet and turbulence intensity. For description of the porous jump, representingfilter fabric was used the same equation (1) as in section 3.1, with the same value ofpermeability k.

Flow direction

without Venruriwith Venturiposition of Venturi

Distance in axis direction (m)

? p

? p

? p

Fig. 3 Pressure along filter bag axis

pressureinlet

wall

Monitor1Monitor 2

wall

wall

wall

porousjump

axis

Fig. 4 Schematic display of the 2D model ofthe fabric filter pulse jet clearing


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4.2. Computational approach

As the model includes unsteady effects, its setup is inherently more complicated thanin section 3. The goal here is to predict the course of the pulse jet cleaning cycle,

which is comprised of the following phases:o Fast increase of the inlet pressure to the required valueo Keeping of the required inlet total pressure (250kPa) during 60mso Cutting off the compressed air inputo Equalisation of pressure in the whole filter

The unsteady simulation has been performed with time step size of 1ms. Before startof the computation, the initial conditions in the filter are set to reflect the real state inthe filter.

When the inlet gauge pressure was set directly to the prescribed value of 151kPa

with RNG k-e turbulence model, the simulation diverged. The step change ofpressure, at the domain inlet was too severe. Therefore, the computation wasconditioned in first time step by switching off the turbulence model and decreasing

the inlet gauge pressure down to 25kP. This provided a smooth start-up of thesimulation.

After setting the inlet air pressure back to 151kPa, a large circulation zone has beengenerated in the pressure pipe, reaching back to the domain inlet. This circulation

zone caused backflow on the inlet boundary and the simulation again diverged.Decreasing the inlet pressure has proved efficient in decreasing the size of the

circulation zone. In order to keep the simulation realistic, a critical flow must bemaintained at the orifice of the pressure pipe. This is ensured, when the inletpressure is higher than critical pressure under the given conditions (88kPa).

Therefore, inlet pressure has been set to 100kPa, without compromising accuracy ofthe simulation results.

For the first time step was thus the inlet gauge pressure set to 25kPa and the timestep was solved with laminar solver. In the following 60 time steps the inlet gauge

pressure has been kept at 100kPa with RNG k-e turbulence model active. After thatthe inlet condition has been changed from pressure inlet to wall, thus simulating

interruption of the compressed air supply. In the following time steps, pressure in thesystem has gradually equalised. Hundred and forty more time steps one millisecondlong were necessary for equalising the pressure in the filter.

4.3. Effect of Venturi nozzle on pulse jet cleaningAs mentioned already at the

beginning, it was expected that theVenturi nozzle amplifies gasentrainment from the upper

compartment into the filter bag.The goal was to quantify this effect

and therefore provide justificationof the nozzle usage.

To this purpose, mass flow rates

through reference cross-sectionsmarked Monitor1 and Monitor2

Fig. 5 Mass flow rates at two locations during the cycle


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have been monitored throughout the simulation (see figure 5). Location of the

reference cross-sections is shown in figure 4. In order to provide evaluation ofperformance increase thanks to the Venturi nozzle, a second simulation has been

performed without the Venturi.

The development of mass flow rate during the cleaning cycle is shown at figure 5.The graph shows about three-fold increase of mass flow rate due to the Venturinozzle in the first moments of the cycle. In the alternative without the nozzle is gas

entrainment into the filter bag almost negligible.

After opening of the compressed air inlet, a pressure wave starts to propagate down

through the filter bag. The gauge pressure of the wave is between 1000 and 1500Pa.After fifteen milliseconds the wave reaches bottom of the filter bag and is reflected.

After another fifteen milliseconds the pressure wave returns back to the top part ofthe filter bag. About twenty milliseconds from the start of the cleaning cycle, theamount of gas entrained from the upper compartment into the bag drops down due to

pressure decrease in the compartment. At time t=60ms is shut down the supply ofcompressed air. Its remainder contained in the pressure pipe still expands from the

orifice. Pressure in the compartment above the filter bags is now lower than in thelower part below the cell plate and thus the gas changes at about time t=80ms itsdirection and flows back through the filter bag and above the cell plate (in the

alternative with the nozzle). Pressure is gradually equalised in the whole system.

Pressure was monitored during the whole pulse (60ms) at five locatrions along theaxis of the filter bag. The individual monitoring locations have been marked asMonitor3 to Monitor7. This has been performed for both alternatives (with and

without the Venturi nozzle) so that it would be possible to make a comparison.

Results of the computation are summarised in figure 6. Pressure is the gaugepressure, relative to the operating pressure. From the graphs it may be observed thatthe pressure wave during the cleaning cycle is composed of a dynamic part with fastpressure rise inside of the filter bag, and of a following relatively steady flow into thebag. This observation corresponds well with the description of cleaning cycleprovided in the literature [6] and [7], as noted in part 0. Comparison of the resultsproduced for the alternative with the nozzle and without it shows that the profiles arerather similar, but the values of maximum gauge pressures differ by a factor of 3.

5. Conclusions

From the data obtained from the CFD modelling it may be concluded that the Venturinozzle has a significant influence on quality of the filter cleaning. By influencing gas

entrainment from the area above the cell plate, gas flow into the filter bag increasesthree times. Intensity of pressure wave in the filter bag is significantly increased. The

maximum gauge pressure values increase about three times after installation of theVenturi nozzle, which is an important difference for quality of the cleaning. A negativeconsequence of installation of the Venturi nozzle could be a higher pressure drop of

the filter. However, it has been determined that pressure drop of the nozzle duringnormal operation is about 50Pa, which is a fraction of the total filter pressure drop

(mean value about 800Pa) and does not entail an increase of operating costs.


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It may thus be concluded with confidence that the application of Venturi nozzle is

justified and may be recommended for improving of filter bag cleaning.

Jet cleaning with Venturi nozzle Jet cleaning without Venturi nozzle

Fig. 6 Comparison of the pressure wave along filter length


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References:

[1] Schildermans I., Baeyens J. and Smolders K.: Pulse jet cleaning of rigid filters:

a literature review and introduction to process modelling, Filtration &Separation, Vol. 41, Issue 5, pp. 26-33, June 2004

[2] Laux S.: Drukstossabreinigung keramischer filterelemente zurHeissgasfiltration, Dissertation, RWTH Aachen, Germany, 1993 (in German)

[3] Laux S., Giernoth B., Bulak H. and Renz U.:Aspects of pulse-jet clearing filterof ceramic filter elements, in Gas Cleaning at High Temperatures, R Clift &

J.P.K. Sevile (eds), Blackie Academic & Professional, Glasgow, UK, pp.203-224, 1993

[4] Berbner S.: Zur Drukstobregenierung keramischer filterelemente bei derHeibgasreinigung. Dissertation, Fakultt fr Chemieingenieurwesen der

Universitt Fridericiana zu Karlsruhe, Germany, 1995 (in German)

[5] Kanaoka C., Amornkitbamrung M. and Kishima T.: Cleaning mechanism ofdust from ceramic filter element, in High Temperature Gas Cleaning, Dittler A.,Hemmer G. & Kasper G. (eds) Karlsruhe, Germany, Vol. 2, pp. 142-152, 1999

[6] FLUENT 6.1.22, Users Guide, Fluent Inc., Lebanon, 2003.

[7] CSN ISO 5167-1: Meren prutoku tekutin pomoc snmacu diferencnho tlaku,cst 1: Clony, dzy a Venturiho trubice vloen do zcela vyplnenho potrub

kruhovho prurezu, October 1993. (in Czech)

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