Manual

BÜCHILabortechnikAG Descriptionoffunction|4
UltrasonicController 17/46
4 Descriptionoffunction
4.1 Purposeanddesign
TheUltrasonicNozzlegenerateswetdropletswithameansizeof50–200μm
afterfeedinjectionintotheMiniSprayDryerB290.Thedropletsizedepends
onthefeedpropertiesandprocessconditionssuchassolventtype,viscosity,
surfacetension,typeoffeed,atomizationpowerandfeedrate.
4.2 Function
4.2.1 UltrasonicAtomizingNozzle
Anultrasonicatomizingnozzleisadevicethatvibratesatfrequenciesbeyond
thoseofhumanhearing;thatis,inexcessof20kHz.Theatomizedspraypro
ducedresultsfromthebreakupofunstablecapillarywavesdevelopedintheliq
uidintroducedontotherapidlyvibratingatomizingsurfaceofthenozzle.Ultra
sonicatomizationissolelyasurfacephenomenon.Theamountofliquidatom
izeddependsexclusivelyontherateatwhichliquidisintroducedontothesur
face.
Flowrate
Ultrasonicnozzleshaveinfinitevariabilitywithrespecttoflowrate.Although
practicalconsiderationsrelatedtothenozzledesignlimitthisvariability(typi
callytheachievableratioofmaximumtominimumflowratesisatleast5:1),the
abilitytopreciselyadjustflowratesbyadjustingtherateatwhichliquidisdeliv
eredtothenozzleisoftenuseful.
Sprayvelocity
Thesprayvelocityistypicallylowwith18–36cmpersecondascomparedto
10–20m/secondforstandardpressureatomizingnozzles.Thisapproximately
100foldreductioninsprayvelocityisequivalenttoa10’000timesreductionin
kineticenergy.
Thereducedvelocityandkineticenergyenablesimprovedfloatingofthe
dropletsinthedryinggas.Thisreducestheriskthatdropletscollidewiththe
spraychamberespeciallyatthebottombeforecompletedryness.