Handbook of Hygiene Control in the Food Industry

function of temperature, strength of chemicals and contact time between
detergent and soil.The contact timeis the timea specifiedtemperatureand
strengthof detergentare present at the interface between soil and detergent. The
mechanical effectis a shearforcedraggingin the soil at the surface. The shear
forceremovesthe soil fromthe surface. For a mildsoiling the mechanicaleffect
mightbe sufficientto removethe soil using onlya mild(or evenno) detergent
solution. In caseof a hard soiling, bondsbetween surface and soil needto be
weakened by a strongerdetergent to allowthe mechanicalforce to removethe
soil fromthe surface.Themechanicalforceis appliedfromliquidmoving
relatively to the surface(moreon this later). Holah(2003)givesdetailson the
combination of the relative energysources for openequipment and different
cleaning techniques.
Whencleaningclosedequipment,all fourof the componentsof Sinner's
circleare influenced,eitherdirectlyor indirectly,by the flowof detergent inside
the equipment. The directinfluenceis throughthe mechanicalforce actingon
the soil. The forceis generatedfrommotionof the detergent acrossthe surface.
The force is alsoknownas the wallshearstress(w). Wallshearstressis a
consequenceof a velocity gradientoccurringbecause of non-slipconditionsat
the wall.For fully developedpipe flow the wall shearstressis givenby (Shames,
1992):

w;lamàˇ

du

dr

ràR

à

4 U

R

[Pa] Re< 2300 Ö 11 : 1 Ü

w;turbà 0 : 03325 U^2



RU

  0 : 25

[Pa] 2300 <Re< 3  106 Ö 11 : 2 Ü

whereis the dynamicviscosity(N s/m^2 ),uis the axialvelocity (m/s) at a
distancer(m) fromthe centreof the pipe,Ris the radiusof the pipe (m),Uis the
averagevelocityof the fullydevelopedflow (m/s),is the density (kg/m^3 ) and
is the kinematic viscosity(m^2 /s).
The indirect influenceof the flowon cleaning arisesfromthe fact that the
detergent (temperature and chemical)needsto be `delivered'to the soil by heat

Fig. 11.1 The influenceof the transportphenomenonon cleaningtimeillustratedby
Sinner'scirclefor (a) a fullydevelopedturbulentpipeflow,(b) a recirculationzonein,
e.g.,a dead-endwithpoorexchangeof detergentand (c) a recirculationzonein, e.g.,a
valvewitha goodexchangeof detergent.

Improvingthe hygienicdesignof closedequipment 193