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Biosystems Engineering 2005 91 4 441 453 doi 10 1016 j biosystemseng 2005 05 009 a D 14469 potsdam de form itions of different nozzles and their washing effects Single droplets formed in the spray were submitted to an the washing performance Scott et al 1981 Investiga tions of the fundamental aspects of spray performance process The washing effect of the nozzles is primarily based on the total applied impulse of the spray which is ARTICLE IN PRESS engineering and high pressure topics and little of the information reported can be applied to vegetable cleaning situations Mo nicke 1971 Sandler 1976 Scott pressure Geyer 1999 Momber 1993 The main inputs for the variation of the spray parameters and thus the spray effects are the operating and the nozzle forcleaning have mostly been focused on chemical composed of the amount of waterused and the water 1537 5110 30 00 441 r 2005 Silsoe Research Institute All rights reserved energetic assessment The nozzles were evaluated with regard to their area washing performance Z as a ratio of the effective erosion area to spray area and effectiveness E s Ve The agricultural nozzle for plant protection with a ow rate Q lowerthan 3 l min C01 at pressure p of 3barand a spray angle a h 0 of 901 was found to be ineffective considering the determined area washing performance area ratio Z 0C110 as its spray parameters proved to be inappropriate concerning the droplet size spectrum volume intensity per unit area and mean impulse distribution Conversely the at fan nozzle with a ow rate Q of 6C12 l min C01 at pressure p of 3barand a spray angle a h 0 of 901 produces a spray with a satisfactory area washing performance area ratio Z 0C191 but a smallerarea washing effectiveness E s Ve forthe spray conditions used in this experiment r 2005 Silsoe Research Institute All rights reserved Published by ElsevierLtd 1 Introduction Several washing machines with different principles are used forvegetable washing with varying sensitivities Geyer 1999 Nozzle washing machines are predomi nantly used forleafy and bunched vegetables such as lettuce leek bunched carrots and radishes They clean the vegetables hydraulically by means of spray nozzles Vegetable washing with nozzles must be performed carefully in a short time without damaging the tissue using as little fresh water and energy as possible Although spray cleaning has been used in the dairy and other food industries for many years there has been very little research effort directed towards an analysis of et al 1981 Schikorr Spillman 1984 Krautter Wu Kru ger 1998 Ludewig 1998 Meng et al 1998 Louis et al 1999 Liu 2000 Sivakumar accepted in revised Experiments were conducted to investigate the interdepend the spray washing process under low pressure cond distance spray pressure and nozzle diameter is derive washing mechanism Four measuring systems were used of Vegetables and Potatoes M Geyer Potsdam Bornim Germany e mail of corresponding author 12 May 2005 published online 7 July 2005 ence between the different in uencing factors on The washing effect as a function of standoff d by considering the spray structure and the spray to determine the relevant spray structure parameters Published by ElsevierLtd ARTICLE IN PRESS E MULUGETA M GEYER442 Notation A e total eroded area cm 2 A e eff effective eroded area cm 2 A I Tekscan spray impact area on the sensor surface Tekscan Inc cm 2 2 parameters The output and thus the direct results of the spray structure are the size and velocity spectra of droplets as well as the spray geometry These physical spray characteristics are relevant for the emerging droplet impulses as well as for the spray impact pressure during washing The following application technical demand 1 A s spray area cm a p mean area of the detection volume mm 2 C m spec mean speci c volume intensity of spray perunit area mm 3 s C01 mm C02 d d droplet diameter mm C01 d 0 nozzle diameter mm d 32 Sautermean diameter mm C01 d pen depth of penetration mm d pen eff depth of effective penetration mm E s Ve area washing effectiveness mm 3 Nm C01 F max Tekscan maximal impact force recorded using a sensor Tekscan Inc N Operating parameters spray pressure nozzle distance standoff distance nozzle number nozzle arrangement impact loading time traverse speed Nozzle parameters nozzle geometry nozzle type Spray parameters droplet size droplet velocity number of droplets spray geometry droplet impulse spray impact pressure Product characteristics shape surface properties susceptibility Dirt parameters adhesive forces of dirt composition of the adhering soil Spray angle a Stand off distance Fig 1 Influencing factors on the washing process with spray nozzles after Geyer and 2 optimal macrostructure meaning that the washing wateris uniformly distributed on the vegetable surface with an optimal circular spray width and From cerning Roches 1983 of pressur direct wave crack spray cracks coati durati the surfa impac The the impac optimal microstructure meaning that the transfor mation of spray into droplets with optimal size and velocity spectra leads to an optimal mean droplet during 2003 s on the spray structure should be ful lled washing after Kru ger 1998 Freudig et al h standoff distance cm I d mean impulse of droplet size group mgm s C01 I d spec speci c impulse rate of effective dro plets perunit area gms C02 mm C02 m droplet mass mg P max Tekscan maximal impact pressure using a sensor Tekscan Inc kPa p spray pressure bar q 3 x volume density distribution mm C01 Q ow rate l min C01 n droplet velocity ms C01 y impact angle of droplet deg Z area ratio a h x spray angle at the standoff distance h of x cm deg spray effectiveness the numerous experimental investigations con spray coating removal Adler 1979 Brunton Hammitt et al 1974 Lesser Obara et al 1995 the perpendicular component droplet impulse generates an extremely high impact e as a result of the water hammer effect The deformation and the propagation of mechanical s caused by the impact pressure are responsible for initiation in the erosion process Lateral out ow and hydraulic penetration extend the existing which lead to erosion by the separation of ng material from the substrate The intensity and on of surface loading is based on the kinematics of droplet impact and is greater if the impacting ce is in contact with the compressed zone of the ted droplet relation between the different factors in uencing spray structure and the spray washing effect on the t surface were analysed in order to show the possibilities of optimising the nozzles and theiroperat ing conditions as well as the washing process A standard testing method was developed to express the material removal as a function of a set of spray conditions nozzle and operating parameters and other system parameters based on the spray structure In this paper the relations are described exemplarily for two washing nozzles 2 Materials and methods Nozzle selection was primarily based on the ow rate representing the broad range of nozzle diameters d 0 as found in the present nozzle washing machines In the following article the results are described for the investigation of sprays produced by two 901 at jet nozzles out of 11 examined spray nozzles industrial nozzle 632C1726 I 90 and agricultural nozzle LU 90 04 A 90 both manufactured by Lechler GmbH Metzin gen Germany Table 1 The nozzle angle a h 0 of 90 o mgms C01 impacting on a surface is described as I d m n sin y 1 in which m is the droplet mass in mg n the vertical velocity of the droplets in ms C01 and y theirangle of impact on the surface The speci c impulse rate of effective droplets d d 4300mm perunit area I d spec in gms C02 mm C02 was simulated by calculating the speci c mass ow rate of spray per unit area and relating that to the correspond ing measurements of droplet size and velocity spectra An energetic analysis of the droplets formed in the spray was obtained A standardised procedure for the evalua tion of the spray parameters of the nozzles with regard to their area washing performance and effectiveness was developed this was accomplished by records of the droplet impulse distributions in the spray and the erosion effect along the radial spray dispersion With regard to this the following parameters were deter mined 1 Fluid volume distribution and spray geometry The spatial distribution of the spray was measured ARTICLE IN PRESS the I 90 3 7C1219C128 59C1921C169 812C1622C149 WASHING PROCESSES OF VEGETABLES AND POTATOES 443 Standoff distance h 20 cm A 90 3 1C1541C169 52C1148C10 82C1649C16 I 90 3 7C1235C148 59C1937C168 812C1639C128 determined in contrast to the spray angle a h x immedi ately after spray discharge through the nozzle exit is recommended by the nozzle manufacturer The water pressure used in the experiments was 3 5 and 8bar The investigations were conducted for a perpendicular arrangement of the nozzles at xed standoff distances h of 10 and 20cm and without travelling The spray structures and their impact effects were examined In this context the mean impulse potential I d of individual droplet size groups in Table Values of spray geometries obtained at two distances h from Nozzle Spray pressure Flow rate Mean values p bar Q l min C01 Width cm Standoff distance h 10 cm A 90 3 1C1522C149 52C1125C16 82C1627C12 through horizontal swivelling of a row of test tubes internal diameter of 16mm arranged next to each otherovera time period of 7s Gebhardt 1958 Scott et al 1981 In this way the spray area A s and the intensity of the spray throughout the area were determined Records made with less than 5mm of the lling height in the test tubes were ignored The information was presented in the form of a sheet diagram The entire spray area was divided into small areas of 256mm 2 showing the corresponding speci c 1 nozzle exit for two nozzle sizes and varied spray pressure p of the spray geometries derived based on measured data Angle deg Depth cm Area A s cm 2 6C144C1871C19 104C101C164C10 107C131C1 3C16 7C161C1630C14 4C141C1 3C14 6C1 C1 5C13 2C128C10 314C19 100C134C18 216C16 102C124C18 223C18 3C103C1294C17 6C143C12 101C13 8C183C12 109C17 ignored This mean threshold value was obtained from The into 3 Results and discussion ARTICLE IN PRESS E MULUGETA M GEYER444 volume intensity of spray per unit area C m spec in mm 3 s C01 mm C02 It was computed by assuming the distribution of the obtained spray volume to be constant overthe catchment area of 256mm 2 2 Size and velocity spectra of the droplets Simultaneous measurements of droplet sizes and velocities in the spray which were meant as a basis for an energetic viewing of individual droplets were accomplished by means of a phase double particle analyser PDA Tropea 1999 These investigations were carried out in the laboratory of the nozzle manufacturer Lechler Ltd Metzingen Germany The numberof droplets within given upperand lower limits of size for contiguous intervals was determined The raw data were processed using area and volume weighing factors to compute the droplet size number and velocity distribution for the entire spray assuming a mean size and velocity foreach chosen droplet size group From the computed distribution the Sauter mean diameterSMD denoted by d 32 Damaschke 1999 was calculated The spatial variability of these spray para meters was considered with regard to the determination of several spectra within the spray area Local measure ments of droplet size and velocity spectra at a designated initial cross section plane of the sprays were achieved by radial moving of the detection volume mean area a p 0C133mm 2 of about 35 or40mm in each case Taking into consideration the tolerance range of the pump and the measurement inaccuracy the variation range between droplet size measurements under the same spray conditions ranged from 7 5 up to 10 Lipthal 2002 3 Distribution of the maximum spray impact pressure With the help of a matrix based tactile sensor Type 5051 Tekscan Inc Boston USA Herold et al 2001 the impact pressure distributions were measured The sprays were thereby impacted onto the surface of the sensorforan exposure time of 120s Mulugeta et al 2002 The entire impact area of the spray was scanned in the respective standoff distance by shifting the sensor in steps of 25mm The measured values of the impact force F max Tekscan less than 0C1002N have been ignored as the operating mode of the sensor makes it impossible to verify the conditions of origin for these smaller load values The average of two measurements taken for each set of values of standoff distance nozzle diameterand spray pressure was used as the representative spray parameter To protect the tactile sensor it was covered by thin lm polyethylene 4 Distribution of the erosion depth on a standardised sand binder mixture plate A procedure has been developed for obtaining standardised sand binder mixture plates following the spray analysis in the high pressure area Scott et al The following spray characteristics caused by the variation of nozzle parameter and operating conditions have been determined 1 the droplet size distribution 2 the distribution of volume intensity per unit area and 3 the mean impulse distribution in the spray which is responsible for further spray dispersion and thus the spray geometry and which affects the erosion processes on the impact surface 3 1 Effects of spray pressure nozzle diameter and standoff distance on spray parameters per unit area and time 3 1 1 Spray structure variables at a standoff distance of 10 cm Compared to the A 90 the ow rate of nozzle I 90 was approximately ve fold higher The A 90 generated sprays with a larger width whereas a smaller radial spray dispersion characterised the I 90 Table 1 For example under a pressure of 3bar a spray produced by the A 90 was dispersed over an area with a width of s Ve nozzle in terms of volume removed in mm 3 on the plates at a de ned exposure time per unit of energy in Nm expended by that nozzle tional E descriptive term the area washing effectiveness in mm 3 Nm C01 has been proposed which rates a different The program permits a coupled analysis of the spatially related data Furthermore one addi Agricu 2002 s data obtained from the experiments were entered an evaluation program developed at the Institute of ltural Engineering Bornim Mulugeta et al scan measurements of depth distribution of unloaded plate 1981 Krautter Obara et al 1995 Meng et al 1998 The sand bindermixture plates 300mm by 250mm by 20mm were used as a target to observe the washing mechanisms and process due to the erosion pattern on the plates see Fig 6 They were placed perpendicularly to the central axis of the spray nozzles The exposure time for each sample lay with 300s The resulting eroded areas and depths on the plate surfaces were recorded by a laser scanner Five measurements for each set of parameters were taken and the average of them was used as the representative value Records made with less than 0C13mm of scan depth values were around 8cm larger than the spray of the I 90 Consequently the spray area A s of the A 90 C2471C19cm 2 was larger than the area of the I 90 C2430C14cm 2 For the set of nozzle diameters and spray pressures used in this experiment a mean droplet diameter d d within the range of 20 900mm and a mean velocity within the range of 5 35ms C01 were recorded The volume density distributions q 3 x in mm C01 as a function of the mean droplet diameters d d measured at a spray pressure p of 3barin a standoff distance of 10cm are shown in Fig 2 The droplet spectrum of the A 90 provided de nitely more ne droplets o300mm with the volumetric content of 49 than the one of the I 90 with 15 of the total spray volume There were remarkable differences in Sauter mean diameterSMD between the nozzles Table 2 This was due to the fact that the sprays generated by the I 90 contained more large droplets 4700mm resulting in a clearincrease of the SMD On the otherhand an increase of the operating pressure only slightly affected the formed droplet size spectra Thus the droplet size spectra generally showed a tendency to move towards the range of ne droplets and the SMD values slightly decreased ARTICLE IN PRESS Table nozzle 5 a b 0 0 002 0 004 0 0 002 0 004 0 300 600 900 Mean droplet diameter d d m 0 300 600 900 Mean droplet diameter d d m Volume density distribution q 3 x m 1 Volume density distribution q 3 x m 1 Fig 2 Volume density distribution q 3 x as a function of mean droplet diameter d d for two nozzle sizes A 90 and I 90 at the two standoff distances h a 10 cm b 20 cm spray pressure p 3 bar WASHING PROCESSES OF VEGETABLES AND POTATOES 445 Sauter mean diameters and mean velocities of droplets for two distance Standoff distance h cm Sauter diameter Nozzle Spray pressure 3 10 A 90 277 256 I 90 418 407 20 A 90 266 239 I 90 375 361 3 1 2 Energetic view of the sprays at a standoff distance of 10 cm Results of the spray analysis from measurements of volume distribution droplet diameter and velocity spectra are shown in Table 3 Based on the measure ments of spray volume distribution a mass weighed mean speci c volume intensity of spray per unit area C m spec of 1C18mm 3 s C01 mm C02 has been determined for the entire spray area of the A 90 operating at a pressure of 3 bar Under the same spray situation the spray area of the I 90 had approximately 18 fold C2432C14mm 3 s C01 mm C02 the C m spec of the A 90 As 2 sizes showing the effect of spray pressure p and standoff h d 32 mm Droplet velocity n ms C02 p bar Spray pressure p bar 8358 251 13C1619C1624C13 391 18C1123C1127C17 230 11C1216C1216C14 319 17C1722C1425C18 The mean velocity distribution of droplets in sprays corresponded to earlier measurements Ludewig 1998 and to normal distribution function Comparing the values between the two nozzles at the same operating pressure the mean droplet velocities in the sprays of the I 90 increased by about 14 33 The curves of I d calculated from Eqn 1 versus mean droplet diameter d d with nozzle diameter d 0 as a parameter are presented for the spray pressure p of 3 barin Fig 3 Comparing these curves there is a clear difference in mean impulse between the same droplet size groups shown in Fig 2 the ratio of droplets larger than 0C13mm to total spray volume signi cantly increased as the nozzle diameterincreased The speci c impulse rate of effective droplets per unit area I d spec revealed marked differences between the sprays formed by the different nozzle diameters As shown in Fig 4 at a spray pressure of 3bar the I d spec found in a spray released from the I 90 is 0C148gms C02 mm C02 Underthe same operating condi tions the I d spec for the spray generated by the A 90 is determined to be 0C102gms C02 mm C02 about 96 lower than the above given value 3 1 3 Effects of a standoff distance elevated to 20 cm on spray parameters A numberof attempts by the standoff distance elevated to 20cm are made to nd the optimal critical standoff distance for each set of spray pressure and nozzle diameter by considering the formed spray structure From the data presented in Table 1 it can be concluded that the spray areas for a standoff distance of 20cm are about three to four times higher than those at a distance of 10cm The obtained droplet size spectra generally showed a tendency to move towards the ne and middle size ARTICLE IN PRESS mm 0 3 6 0 300 600 900 0 3 6 Mean impulse I d mg m s 1 Mean impulse I d mg m s 1 0 300 600 900 a b Mean droplet diameter d d m Mean droplet diameter d d m Fig 3 The mean impulse I d as a function of mean droplet diameter d d for two nozzle sizes A 90 and I 90 depending on the standoff distances h a 10 cm b 20 cm spray pressure p 3 bar 0 300 600 900 0 0 4 0 8 0 0 4 0 8 0 300 600 900 b Mean droplet diameter d d m Mean droplet diameter d d m Specific impulse rate I d spec g m s 2 mm 2 a Specific impulse rate I d spec g m s 2 mm 2 Fig 4 The specific impulse rate of effective droplets I d spec as a function of mean droplet diameter