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Chemical Engineering and Processing 49 (2010) 165–176

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

journa l homepage: www.e lsev ier .com/ locate /cep

dentification and prediction of air core diameter in a hydrocyclone by a novelnline sensor based on digital signal processing technique�

enkat Krishnaa, R. Sripriyab, V. Kumara, S. Chakrabortya, B.C. Meikapa,c,∗

Department of Chemical Engineering, Indian Institute of Technology (IIT) Kharaghpur, PO: Kharagpur Technology, Dist: Paschim Medinipur, West Bengal, 721302, IndiaR&D Division, Tata Steel, Jamshedpur, 831003, IndiaSchool of Chemical Engineering, Faculty of Engineering, Howard College Campus, University of Kwazulu-Natal, King George V. Avenue, Durban 4041, South Africa

r t i c l e i n f o

rticle history:eceived 29 October 2009eceived in revised form 5 December 2009ccepted 1 January 2010vailable online 14 January 2010

eywords:ydrocycloneignal processingensor

a b s t r a c t

A hydrocyclone is a particle separation device widely used in chemical and allied process industriesin which a particle-fluid mixture is injected tangentially creating a strong swirling, recirculation flow.The particle separation efficiency increases by suppressing the air core, so online prediction of air coreformation has significant importance in the industrial operations. Performance of hydrocyclone is greatlyinfluenced by shape and size of air core. A novel type of senor technique has been developed to identify andpredict the air core diameter from online live data using data acquisition card. The true signal amplitudeschange as a function of the time was used with noise interruption for random changes in amplitude.Noises are eliminated by using moving average technique. The slope of the curve is continuously trackedto determine sudden or abrupt change and indicates the formation of air core. It has been observed that

ir core diametereparation efficiencyeparation

a strong air core of diameter 0.95 cm to 1.2 cm was formed during experimentation and matched withpredicted values over an entire flow regime. The experimental and finally an alarm is designed whichgives alerts once air core is formed and calculates air core diameter. For calculating air core diameter apolynomial equation is fitted between pressure difference and the pressure transmitter reading. A simplemoving average with a smooth width of 10 was used for prediction of air core. Experimental resultsindicate that the digital signal sensor techniques identify the air core and measure air core diameter very

d in m

accurately and can be use

. Introduction

The air core at the center of the hydrocyclone is an unavoid-ble phenomenon in the rotational flow field. Hydrocyclones formcentral air core which extends over the entire length of hydro-

yclone. Air is sucked in the core at the underflow discharge. Theow split between the products of a hydrocyclone is one of the

east understood aspects of the hydrocyclone operation. This splits greatly influenced by the air core diameter, so an understandingf the air core behaviour would greatly assist in predicting flowplits. The size and shape of the air core is believed to affect the

etallurgical performance of the separator. The geometry and theovement of the air core were identified as being sensitive indica-

ors of the operational state of hydrocyclones. The formation of airore is an indication of vortex stability and the suppression of air

� Patent pending.∗ Corresponding author at: School of Chemical Engineering, Faculty of Engineer-

ng, Howard College Campus, University of Kwazulu-Natal, King George V. Avenue,urban 4041, South Africa. Tel.: +27 31 260 3802.

E-mail addresses: meikap@ukzn.ac.za, bcmeikap@gmail.com (B.C. Meikap).

255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2010.01.003

any mining and mineral based chemical and allied process industries.© 2010 Elsevier B.V. All rights reserved.

core has increased separation efficiency according to some previousresearches which led to a lot of contradictions as few researchersargue suppression of air core results in vortex instability.

During the last five decades a fundamental and applied researchon the design and performance of hydrocyclone has been carriedout extensively [1,2]. One of the most important internal structuresis the generation of air core inside the hydrocyclone. At start up alow pressure region develops causing the formation of an air corealong the central axis.

Literature survey reveals that only limited work have reportedin the literature for formation of air core and measurement of aircore diameter. Binnie and Hookings [3] made probably the earli-est attempt to determine the air core diameter in a swirling flowfrom fundamental of fluid flow principles. They analyzed experi-ments with swirling water discharging through a nozzle in termsof a free vortex (ω ˛ r−1) in inviscid flow. They concluded that theair core diameter adjusts so that the liquid flow is a maximum for

a given total pressure head. Subsequently, Smith [4] applied abovecondition, again assuming free vortex inviscid flow, to a cyclonehaving a single outlet. Svarovsky [5] proposed a model based onexperimental data, for the air core size in terms of both the geo-metrical parameters of hydrocyclone and pressure drop through

1 ring a

tideeibririfesbciawdsceotacheettobttramghrtsotit

mdavawfustwtcdlpm

(p + dp

drdr

)(r d˚ + dr d˚) − p r d˚

= �l

(r + 1

2dr

)d˚ dr

V2

r + 1/2dr(2)

66 V. Krishna et al. / Chemical Enginee

he hydrocyclone. Hsieh and Rajamani [6] reported a mathemat-cal model based on physics of fluid flow. However, such modeloes not allow an analysis of the influence of the physical param-ters of the slurry on performance of the hydrocyclone. Steffenst al. [7] analyzed the air core diameter and the pressure dropn the experiments with a single outlet, cylindrical vortex cham-er over a significant range of operating conditions. They derivedelationships based on a simple model of the flow field and empir-cal correlations for various loss factors. Although their presentesults cannot be expected to predict the air core diameter in typ-cal hydrocyclone, however their approach can be extended forurther application. Barientos et al. [8] have proposed a model forstimating the air core diameter from the force due to surface ten-ion which equals a jump in the normal stress across the air coreoundary. They concluded that an increase in the slurry viscosityauses a decrease in the air core diameter and conversely, increas-ng the feed flow rate increases the air core diameter. Dyakowskind Williams [9] proposed a method to predict the size of the air oreith in a hydrocyclone based on calculating the internal pressureistribution by solving a set of conservation equations. The effect oflurry properties such as bulk viscosity and surface tension and flowonditions are taken into consideration. Prediction of air core diam-ter has given as a function of various hydrocyclone geometries andperating conditions. Davidson [10] developed an expression forhe air core diameter in terms of flow variables at the underflownd the overflow which can be applied iteratively during a hydro-yclone flow calculations. He analyzed the air core diameter in aydrocyclone using the physics of uniform density, inviscid flow atach outlet, modified by an empirical factor to account for viscousffects. Romero and Sampaio [11] reported a numerical methodo predict air in hydrocyclone. Chu et al. [12] studied the separa-ion performance by eliminating the air core with an introductionf solid rod; there analysis revealed that separation efficiency haseen enhanced by eliminating air core. Sripriya et al. [13] studiedhe performance of a hydrocyclone and modeling for flow charac-erization in the presence and absence of air core, and their studiesevealed that the separation efficiency increased on suppression ofir core with insertion of a solid rod. Neese and Dueck [14] deter-ined the air core diameter by balancing the positive pressure

radient and the centrifugal force in the rotational flow field of aydrocyclone. However, in the literature there is method reportedegarding non-intrusive technique using water as primary phase inhe hydrocyclone. Therefore in the present investigation a digitalignal sensor processing technique have been used for predictionf air core diameter without disturbing the flow profiles. Since thisechnique does not alter the flow profile with in a hydrocyclone,t can measure the real phenomena inside the hydrocyclone thanhat of existing intrusive techniques.

The current research study involves detailed study of measure-ent of air core diameter and affect of air core on the velocity

istributions in the hydrocyclone. During experimental study their core diameter at various feed flow rates were measured and theariation in velocity distributions with no air core, with air corend with forceful suppressed air core were studied. A programmeas developed using MATLAB to determine whether air core has

ormed or not in the hydrocyclone in real time by acquiring live datasing a pressure sensor and the national instruments data acqui-ition card to determine the air core diameter. To give an alert athe moment of air core is formed, an alarm system is used withinsound sound card present in the computer. The relevance of

his work in the area of hydrocyclone is for online prediction and

ontrol of air core, so that the separation efficiency will not slowown. This non-intrusive technique is new and not reported in the

iterature. And the outcome of the present study has tremendousotential in multi-phase flow and dense-media hydrocyclone forineral processing.

nd Processing 49 (2010) 165–176

1.1. Theoretical consideration for air core diameter

In dilute flow separations, the air core extends over the entirelength of the hydrocyclone ending as a spray discharge in the under-flow. This discharge shape can be described by a discharge angle˛ which has been computed according to the following relation[15]:

˛ = tan−1(v/u

)= tan−1

(�mDuw/2�m

)(1)

Here u, v, w are the axial, radial and tangential suspension velocities,�m is the density, �m is the effective viscosity of the suspension andDu is the spigot diameter. A spray discharge at the spigot exit canonly exist if at this point the axial velocity is negative that means theflow is directed inside the hydrocyclone towards the overflow. Indense flow separations, as more solids are stored in the conical partof the hydrocyclone and partly forced to the overflow, the air coreformed is not going to oscillate. The discharge assumes the shape ofa rope and is characterized by high solids content. At extremely highsolids content the hydrocyclone operates as it is air sealed in theunderflow. In transition state between spray and rope is an insta-ble state with rapid changes between these two discharge typesmentioned above. Among the velocity components the most usefuland significant components is the tangential velocity. The presenceof outer and inner layers of fluid moving in opposite direction cre-ates a locus of zero vertical velocity in a hydrocyclone. The locus ofzero vertical velocity is cylindrical in the cylindrical section of thecyclone and extends in this form in to conical section until a levelat which the wall radius is 0.7RC. It is then becomes conical in theform with the apex of conical locus probably corresponding to thegeometrical apex of the cone. The diameter of the mantle was foundnot to vary with many changes in cyclone design or operating vari-ables. It is probable that the 0.7RC position varies with volume split.The other component of velocity is the radial flow is a maximumnear to the cyclone wall and it diminishes with decrease in radiusuntil it is zero at the air core interface. The tangential velocity offluid in a cyclone increases as radius decreases starting from thecyclone wall. In a hydrocyclone VRn = constant, the power ‘n’ playsthe key role in finding the air core diameter and in determination ofvortex stability. The ‘n’ is determined using the force balances in ahydrocyclone.

Consider a rotating fluid element as shown in Fig. 1. Let the pres-sure at radius r be p and at the radius (r + dr) be

(p + dp

dr dr)

. Thenet pressure force neglecting the second order terms balances thecentrifugal force of the element.

Fig. 1. Element of fluid in a rotating body.

ring and Processing 49 (2010) 165–176 167

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V

V

a

(

(‘

1

bbs(

w

p

V

V

re

r

1

usgaaeoo2tfittifltpct

trolled by their respective installed valves. A pressure transmitter isconnected to the overflow pipe (Fig. 3) to measure the pressure con-tinuously, and this data is continuously acquired in real time usingthe National Instruments’ data acquisition card (Fig. 4) to analyze

Table 1Dimensions of hydrocyclone used for experimental purpose.

Diameter of cyclone (I.D.) 10 cmLength of cylindrical portion of hydrocyclone 21.6 cm

V. Krishna et al. / Chemical Enginee

Again neglecting the second order terms this becomes

dp

dr= �lV

2

r(3)

ere �l is the liquid density, dpdr is the slope of the pressure radius

urve from which ‘n’ can be evaluated by replacing V in terms of ‘n’.

Rn = k

= k/Rn

Substituting V in Eq. (1) and integrating between limits p1, p2nd r1, r2 we get

p1 − p2) = (�l k2/2n r2n1 ) ((r1/r2)2n−1) (4)

The above is a linear relationship. Plotting (p1 − p2) vs.(r1/r2)2n−1) for different chosen values of ‘n’, the correct value ofn’ is the one which gives a linear plot.

.1.1. Stable air core diameterUnder stable operating conditions the air core is characterized

y constant dimensions. The core diameter can be determinedased on the radial pressure distribution in the hydrocyclone. Con-idering the forces balances in hydrocyclone we arrived at the Eq.4) as in determination of ‘n’ [14].

dp

dr= �lV

2

r(5)

Integrating the above Eq. (5) between the limits pr, pin and r, rc

e get

r = pin −rc∫r

�lV2/rdr (6)

= Vw(rc/r)n (7)

w = 3.7(rin/rc)uin (8)

Substituting V in Eq. (6) and integrating between the limits ra,c and further assuming the pressure at the air core boundary to bequal to pu we get

c/ra = (1 + (2n/�l) (pin − pu)/V2w)

(1/2n)(9)

.2. Experimental set-up and technique

The schematic of the experimental set-up of the hydrocyclonesed for measurement of air core under various flow conditions ishown in Fig. 2. It mainly comprises of a hydrocyclone, centrifu-al pump, rotameter, pressure transmitter, a computer with datacquisition card to acquire live data from pressure transmitter andcollection and supply tank. The hydrocyclone is made of transpar-nt Perspex of cylindro-conical structure having a cyclone diameterf 10 cm, the length of the cylindrical portion is 21.6 cm and thatf conical portion is 36.5 cm. Vortex finder having a diameter of.5 cm fixed at the top in the hydrocyclone body. At the bottom ofhe conical section apex pipe having a diameter of 1.8 cm has beentted through which coarse particles move. The liquid is injectedangentially by an inlet pipe of diameter 2.5 cm fitted tangentiallyo the cylindrical portion of the hydrocyclone. A high capacity tanks provided to collect the recycled liquid from apex pipe and over-

ow pipe. A small tank is provided at bottom section to collecthe underflow liquid and recycled to large storage tank. A net isrovided at exit end of apex pipe and the overflow end point toollect the separated particles from the hydrocyclone body duringhe experiment. The particles were injected in the hydrocyclone

Fig. 2. Schematic representation of experimental set-up.

by a feed inlet line connected with an one-way valve. Due to highvelocity inlet liquid, particles are sucked to the tangential inlet o thehydro-cyclone and collected at the exit of hydrocyclone (underflowand overflow) by screen. The mercury manometers were providedto measure the pressure drop across the hydrocyclone. The detailsof hydrocyclone dimensions are presented in Table 1.The pressuremeasuring test points are provided at the inlet, top (overflow), bot-tom, middle section and various positions on the conical surfaceof the hydrocyclone with mercury manometers. Particle inlet sec-tion is made in the path of inlet pipe called as particle inlet sheath.Central solid rod insertion arrangement is made at the conical sec-tion of the hydrocyclone to eliminate air core formation. Centrifugalpump is provided to feed process liquid to hydrocyclone. Rotameteris installed to measure total inlet volumetric flow rate to the hydro-cyclone. The volumetric flow rate of feed liquid can be maintainedby regulating the flow through a gate valve between centrifugalpump and tank and by regulating the gate valve through the bypassline. The flow through the apex pipe and the overflow pipe is con-

Length of conical portion of hydrocyclone 36.5 cmApex angle 15.6◦

Diameter of vortex finder pipe (I.D.) 2.5 cmDiameter of apex pipe (I.D.) 1.8 cmDiameter of feed inlet pipe (I.D.) 2.5 cm

168 V. Krishna et al. / Chemical Engineering a

Fig. 3. Pressure transmitter connected to overflow pipe.

wtc

a

Fig. 4. Data acquisition card.

hether or not air core is formed inside the hydrocyclone body ando calculate the air core diameter. The details of the data acquisitionard are presented in Table 2.

In actual practice during running the set-up, air core is formedt some particular flow rate range. The pressure transmitter senses

Table 2Details of data acquisition card.

Card NI–PCI 6221 M

Total pins 37Analog inputs 16Analog outputs 10Digital inputs 2

nd Processing 49 (2010) 165–176

the pressure in the overflow which is related to the pressure insidethe hydrocyclone and thus the information about the presence of aircore is conveyed to the computer by the data acquisition card con-nected as shown in Fig. 5. The MATLAB code processes the real timedata and produces a beep sound during the period air core remains.The code also calculates the air core diameter. The experiment isconducted by connecting the pressure transmitter at three differentpositions in the hydrocyclone namely to the overflow pipe, cen-tre of the hydrocyclone and underflow pipe. In all the above threestages the experiments are conducted at four different feed flowrates and for each feed rate at three different underflow rates soas to get the different conditions of no air core, on the verge of aircore formation and fully developed stable air core. An algorithm topredict air core diameter is shown in Fig. 6. To determine air corediameter the pressure difference at different radii of hydrocycloneis determined by using the manometer, thus ‘n’ is determined usingEq. (4). (pin − pu) is determined by the manometer. Fig. 7 shows thepressure difference as a function of voltage obtained by pressuretransmitter. Air core diameter is calculated according to the Eq. (9).

1.3. Measuring length of air core diameter using ‘ImageJ’

The air core diameter was experimentally measured by settingmeasurement scale: a line is drawn between two points of knowndistance such as a ruler on the photograph. Then ‘Set Scale’ is donein ‘Analyze’. In the ‘Set Scale’ window the length of the line, in pix-els, was displayed. Known distance and units of measure in theappropriate boxes were entered. Measurements were then shownusing these settings. (If the pixel/length relationship is known froma previous measurement one may directly type this informationin the ‘Set Scale’ window. It is worth remembering to tick ‘global’to apply this scale to other image frames). To set measurement,we chose parameters to be measured via ‘Analyze’ and then setmeasurements. T measure a distance between two points again aline is drawn between desired points. The status bar showed theangle (from horizontal) and the length. In ‘Analyze’ menu ‘measure’option transferred the values to a data window. The programmingis done using MATLAB-7.4. The data is acquired using the Nationalinstruments data acquisition card, PCI–6221. To acquire data fromour data acquisition card, we need a data source, a data sensor, anda data sink. Here the data source is the voltage input to the dataacquisition card. This voltage comes from the pressure inside thehydrocyclone, the sensor is a pressure transmitter connected to thehydrocyclone and the sink is a channel associated with an analoginput object.

1.4. Data acquisition, processing and analysis

An analog input object associated with PCI-6221 is created witha single channel system. The created analog input object is set-up toacquire data at a sample rate of 100 as soon as the object is started.Basically the samples for air core detection were transported tosensor by a thin poly-propylene tube, which eliminated the othernoises to a computer. This is done by using an ‘Immediate Trigger’with 100 samples per Trigger. To ensure the programme runs con-tinuously until the process shut downs the trigger repeat (no. oftimes the triggers get repeated) is set to be infinite. The ‘Timer Fcn’and ‘Timer Period Properties’ are used to call an M-file function atregular intervals while the acquisition is running. The data process-ing and analysis is done in the M-file function. In many experiments,the true signal amplitudes (y-axis values) change rather smoothly

as a function of the x-axis values, whereas many kinds of noisegets interrupted as a result we see rapid, random changes in ampli-tude from point to point within the signal. Noise is considered tobe any measurement that is not part of the phenomena of inter-est. Noise can be generated within the electrical components of

V. Krishna et al. / Chemical Engineering and Processing 49 (2010) 165–176 169

Fig. 5. Schematic of the online data acquisition system.

Fig. 6. Algorithm of prediction of air

Fig. 7. Pressure difference profile as a function of voltage.

core diameter in a hydroclone.

the input amplifier (internal noise), or it can be added to the sig-nal as it travels down the input wires to the amplifier (externalnoise). The noise data incorporated during collection of data waseliminated by noise-average elimination method by using smooth-ing of the signals. Thus the smoothed data are actually noise freevoltage accrued during experiment. The first step is to eliminatethe noise is by performing smoothing operation. In smoothing, thedata points of a signal are modified so that individual points that arehigher than the immediately adjacent points (presumably becauseof noise) are reduced, and points that are lower than the adjacentpoints are increased. This naturally leads to a smoother signal. Aslong as the true underlying signal is actually smooth, then the truesignal will not be much distorted by smoothing, but the noise willbe reduced. The simplest smoothing algorithm is the simple mov-ing average; it is simply the unweighted mean of the previous ndata points. The simple moving average with a smooth width of 10is given as follows

SMA = pM + pM−1 + . . . + pM−9

10(10)

SMApresent = SMAprevious − pM−n+1 + pM+1 (11)

n n

To find the variations taking place in the system slope and rel-ative difference calculations are performed on the smoothed data.The slope of the smoothed curve is tracked continuously to deter-mine the abrupt or sudden changes. For this purpose a linear fit is

1 ring a

e

Y

s

s

R

w

pwbit

70 V. Krishna et al. / Chemical Enginee

mployed. The general form is:

i = A + BXi (12)

lope = n∑

XY −∑

X∑

Y

n∑

X2 −[∑

X]2

(13)

Relative difference is calculated to determine the changes in theystem with respective to time.

D = X̄2new − X̄2old (14)

here, X̄2new and X̄2 old are means of 2 s.Based on the data values and smoothed voltage we predict the

resence or absence of air core, and if air core exists an alarmill ring to indicate to the operating personnel that air core has

een formed. For alarm purposes the win sound soundcard presentn the system is employed. To give alarm we will output data tohe sound card on our computer. For this purpose an analog out-

Fig. 8. Stages of air core (a) No air core (b) Beginning of air core

nd Processing 49 (2010) 165–176

put object associated with winsound is created with two channelsadded to it. The created analog output object is set-up to outputthe data at a sample rate of 8000 Hz as soon as the object is started.This is done by using an ‘Immediate Trigger’. Air core diameter iscalculated with the help of pressure difference between the inletpipe and the spigot. For this purpose a polynomial equation is fittedbetween the data from pressure transmitter connected to the over-flow pipe and the pressure difference (pin − pu) measured directlyas shown in Fig. 7. This polynomial Eq. (15) is used in the subsequentcalculations.

Thus, the best fit curve gives

p − p = 2E09x3 − 3E07x2 + 79220x + 14606 (15)

in u

rc/ra = (1 + (2n/�l) (pin − pu)/V2w)

(1/2n)(16)

For visualizations of acquired data and variations taking placein it four different graphs were plotted in a single window. The first

formation (c) Partially formed air core (d) Stable air core.

V. Krishna et al. / Chemical Engineering and Processing 49 (2010) 165–176 171

Fm

gdcssi

Fp

ig. 9. Comparison of voltage at different air core conditions with pressure trans-itter connected to overflow.

raph indicates raw data vs. number of samples, second smoothedata vs. number of samples, third slope vs. no of samples, fourth air

ore diameter vs. samples. In order to make more users friendly atart stop button was added so that data logging will take place asoon as the start button is issued and stopped when the stop buttons issued.

ig. 10. Effect of feed velocity y on voltage at different air core conditions withressure transmitter connected at the centre.

Fig. 11. Comparison of voltage at different air core conditions with pressure trans-mitter connected to underflow.

1.5. Online prediction of air core formation and its diameter

The online prediction of air core formation inside the hydro-

cyclone is done based on the data obtained from the pressuretransducer connected to the hydrocyclone. The data that is acquiredusing the pressure transducer and the national instruments dataacquisition card is subjected to various operations simultaneouslywhile the acquisition is running, with the hydrocyclone in the

Fig. 12. Comparison of voltage for no air core condition at different locations in thehyrdocyclone.

172 V. Krishna et al. / Chemical Engineering and Processing 49 (2010) 165–176

Fl

opFa

Fh

ig. 13. Comparison of voltage for on the verge of air core condition at differentocations in the hyrdocyclone.

perating mode. In order to eliminate the unwanted noise contentresent in the acquired data a smoothing operation is done on it.or this purpose the simplest smoothing algorithm simple movingverage is employed. To find any abrupt changes taking place in the

ig. 14. Comparison of voltage for air core condition at different locations in theyrdocyclone.

Fig. 15. Effect of superficial liquid velocity on pressure difference to determine ‘n’.

system the slope of the smoothed data is continuously tracked. Theslope is determined using a simple linear fit. To find the variations

taking place with respective time a relative difference algorithmis employed on the smoothed data. With the hydrocyclone oper-ated at different flow rates and different operating conditions, wehave different data range in the presence and absence of air core.This data range in combination with slope is used to predict the

Fig. 16. Validation of calculated air core diameter with the experimental values.

ring and Processing 49 (2010) 165–176 173

pmtraw

2

aeaiTFttathfarcItrchct

Fc

V. Krishna et al. / Chemical Enginee

resence or absence of air core in the hydrocyclone. Once it deter-ines that air core has formed its diameter is also calculated using

he polynomial equation fitted between the pressure transmittereading and the measured pressure difference. To make the oper-ting personnel alert, when air core is formed, an alarm is designedhich gives a continuous beep sound till the air core is suppressed.

. Results and discussion

Experiments were carried out to see the effect of various flownd geometric parameters on the air core formation and its diam-ter as shown in Fig. 8. It can be clearly seen that the shape of their core and its stability greatly depends on the flow patterns andndirectly gives the generated voltage while signals were collected.he effect of liquid velocity on the developed voltage is shown inig. 9. It shows the range of data values at different airflow condi-ions in the hydrocyclone with the pressure transmitter connectedo the overflow pipe. The no air core lines indicates the completebsence of air inside the hydrocyclone and on the verge indicateshe presence of air bubbles but not the air core. In this state theydrocyclone is in a condition that air core may or may not be

ormed. Aircore-1 and aircore-2 indicates the presence of stableir core in the hydrocyclone. Different feed rates give different dataanges in these three conditions. The decrease in data range as airore develops is due to development of low pressure in central axis.n a typical situation the generated voltage is presented as a func-ion of feed superficial velocity as shown in Fig. 10. It shows the

ange of data values at different airflow conditions in the hydrocy-lone with the pressure transmitter connected at the centre of theydrocyclone. It is interesting to note that the no air core lines indi-ates the complete absence of air inside the hydrocyclone and onhe verge indicates the presence of air bubbles but not the air core

ig. 17. Real time graph showing the air core diameter with pressure transmitteronnected to overflow at a feed velocity of 1.6978 m/s.

Fig. 18. Real time graph showing the air core diameter with pressure transmitterconnected to overflow at a feed velocity of 1.35 m/s.

Fig. 19. Real time graph showing the air core diameter with pressure transmitterconnected to overflow at a feed velocity of 1.191 m/s.

1 ring a

imoasdrcdwbw

vtcn

74 V. Krishna et al. / Chemical Enginee

n this state the hydrocyclone is in a condition that air core may oray not be formed. Aircore-1 and aircore-2 indicates the presence

f stable air core in the hydrocyclone and once such observationsre observed it is crucial to alter the feed velocity so that the particleeparation efficiency remains unchanged. Different feed rates giveifferent data ranges in these three conditions. The decrease in dataange as air core develops is due to development of low pressure inentral axis. Compared to overflow here the data range is closer atifferent conditions. This may be due to the measurements at theall and not at the air core interface and may be due to the distur-

ances created in velocity distributions by port connection at theall.

The effect of superficial liquid velocity on voltage generated atarious conditions of air core formation is shown in Fig. 11. It showshe range of typical data collected at different superficial velocityonditions in the hydrocyclone with the pressure transmitter con-ected to the underflow pipe. It can be seen that there are many

Fig. 20. Air core diameter (a) 0.678 m/s (b) 1

nd Processing 49 (2010) 165–176

situations of no air core or formation of air core. The no air corelines indicates the complete absence of air inside the hydrocycloneand on the verge indicates the presence of air bubbles but not theair core in this state the hydrocyclone is in a condition that air coremay or may not be formed. Aircore-1 and aircore-2 indicates thepresence of stable air core in the hydrocyclone. Different feed ratesgive different data ranges in these three conditions. The decreasein data range as air core develops is due to development of lowpressure in central axis of the hydrocyclone. Similarly experimentswere conducted in underflow of the hydrocyclone which is shownin Fig. 12, where x-axis represents the position at which the pres-sure transmitter is connected and the y-axis represents the voltage

values. Position-1 represents the transmitter connected to overflowpipe, position-2 the centre of the hydrocyclone and the position-3to the underflow pipe. The graph is plotted for no air core condition.It shows for a single flow rate we have different data ranges at dif-ferent positions. This is due to the variation in pressure at different

.0191 m/s (c) 1.35 m/s (d) 1.6478 m/s.

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V. Krishna et al. / Chemical Enginee

ositions, with the overflow having least and highest in the centref the hydrocyclone.

In Fig. 13 x-axis represents the position at which the pressureransmitter is connected and the y-axis represents the voltage val-es. Position 1 represents the transmitter connected to overflowipe, position 2 the centre of the hydrocyclone and position 3 tohe underflow pipe. The graph is plotted for on the verge of air coreormation condition .It shows for a single flow rate we have differ-nt data ranges at different positions. This is due to the variation inressure at different positions, with the overflow having least andighest in the centre of the hydrocyclone. In Fig. 14 x-axis repre-ents the position at which the pressure transmitter is connectednd the y-axis represents the voltage values. Position 1 representshe transmitter connected to overflow pipe, position 2 the centre ofhe hydrocyclone and position 3 to the underflow pipe. The graph islotted for air core condition .It shows for a single flow rate we haveifferent data ranges at different positions. This is due to the vari-tion in pressure at different positions, with the overflow havingeast and highest in the centre of the hydrocyclone. Fig. 15 is plottedetween superficial liquid velocities vs. pressure difference. The n,hich is the characteristic of the hydrocyclone, is found to be 0.6.

his n is constant for the hydrocyclone and it mainly depends on theesign parameters and is almost independent of operating parame-ers. Photographs have been taken from the digital camera and wererocessed in ‘ImageJ’, image processing software. Diameters deter-ined by ‘ImageJ’ are compared with calculated values. The air core

iameter was measured by photographic method with a referenceire. Experiment was performed in a Perspex transparent hydro-

yclone and when air core was formed images were captured bydigital camera and the diameter was measured by image-pro-

lus software with reference to pre-calibrated reference wire. Their core diameter is calculated using n = 0.6. The diameter from thehotos is determined using the ‘ImageJ’. With an increase in feedate we observe an increase in air core diameter because very lowressures develop in the centre of the hydrocyclone. The air coreiameter measured experimentally was compared with that pre-icted from theoretical value and shown in Fig. 16. It is interestingo note that the experimental values and calculated values agreeell.

As shown in Fig. 17 it contains four sub graphs. In all the graphshe x-axis represents the no. of samples. In first and second y-axisefers to voltage where as third refers to slope and fourth refers toelative difference. The first sub graph refers to raw data, second tomoothed data, third to slope, and the fourth refers to the air coreiameter at a feed flow rate of 1.6978 m/s. Fig. 18 also containsour sub graphs. In all the graphs the x-axis represents the no. ofamples. In first and second y-axis refers to voltage where as thirdefers to slope and fourth refers to relative difference. The first subraph refers to raw data, second to smoothed data, third to slope,nd the fourth refers to the air core diameter at a feed velocity of.35 m/s. Similarly, Fig. 19 contains four sub graphs. In all the graphshe x-axis represents the no. of samples. In first and second y-axisefers to voltage where as third refers to slope and fourth refers toelative difference. The first sub graph refers to raw data, second tomoothed data, third to slope, and the fourth refers to the air coreiameter at a feed velocity of 1.191 m/s.

The code successfully identifies the changes in the system,henever there is a change in the system we observe a change

n the voltage and the corresponding change in voltage value islearly displayed in the graph. Whenever an additional phase thats air gets introduced we observe significant changes in the voltage

alue this led us to indicate there took place a change in the systemnd to predict that air core has formed and subsequently its diam-ter is determined. The air core diameter increases with increase ineed flow rate this is clearly depicted using photographs as shownn Fig. 20 as well as by the code developed by digital sensor signal

nd Processing 49 (2010) 165–176 175

processing. To eliminate air core one solid rod is introduced fromside wall and it will affect the flow pattern within the hydrocloneand the problem is serious for large diameter rods. That’s why theobjective of present study is to identify the occurrence of air coreand accordingly take corrective measures to avoid the formation ofit by adjusting flow rates.

3. Conclusions

In this present paper an experimental and analytical methodhave been presented for predict the formation of air core by digitalsignal processing technique. The hydrocyclone along with the pres-sure transmitter and the data acquisition card was used to analyzeonline formation of air core and measurement of air core diame-ter. Data was collected by data acquisition card and a MATLAB codeis written to get the information about air core from the acquireddata. The code successfully predicts the presence or absence of aircore and gives an alarm if air core gets formed when the systemis under operation. The system is designed in such a manner thatonce one comes to know that air core has formed, can take appro-priate actions to suppress air core so that separation efficiency canbe increased. Besides this, the code also calculates the diameterof the air core, if formed in any operating condition. Among thethree positions at which the pressure is connected that is over-flow, centre and the underflow, overflow has been chosen and aircore diameter is determined by connecting pressure transmitterat this position. The n, which is the characteristic of the hydrocy-clone, is found to be 0.6. This n is constant for the hydrocyclone andit mainly depends on the design parameters and is almost indepen-dent of operating parameters. Photographic method was employedby the digital camera and were processed in ‘ImageJ’, image pro-cessing software. As at the centre the data ranges are very closeand moreover by connecting port to the hydrocyclone walls maydisturb the velocity profiles, while coming to the underflow it isfound that some of the industrial hydrocyclones will not have thespigot, so overflow position is fixed taking these data into con-sideration. It has been observed that a strong air core of as highas 0.95 cm to 1.2 cm in diameter was formed during experimenta-tion and matched with predicted values over an entire flow regime.Results were compared with experimental findings and agree wellwith the predicted values. Thus the present studies has significantimportance in many chemical and mineral based process indus-tries.

Acknowledgements

Authors thankfully acknowledges the financial grant receivedfrom M/S Tata Steel, India (No: IIT/SRIC/HDR) to conduct theresearch and National Research Foundation (NRF), South Africa forsupport to prepare research output (UID: 70841).

Appendix A. Nomenclature

(pin − pu) pressure difference between inlet and spigot (Pa).Du spigot diameter (m)pin pressure at the inlet (Pa)pu pressure at the spigot (Pa)ra air core radius (m)rc nominal radius of the hydrocyclone (m)rin inlet pipe radius (m)

u axial velocity (m/s)uin average velocity in inlet pipe (m/s)v radial velocity (m/s)Vw tangential velocity at the hydrocyclone entrance (m/s)w tangential suspension velocities (m/s)

1 ring a

x

G���

R

[

[

[

[mance of a hydrocyclone and modeling for flow characterization in presenceand absence of air core, Chem. Eng. Sci. 62 (2007) 6391–6402.

76 V. Krishna et al. / Chemical Enginee

pressure transmitter data (volts)

reek symbolsm effective viscosity of the suspension (kg/m/s)l liquid density (kg/m3)m suspension density (kg/m3)

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