Cleaning in place:: effect of local wall shear stress variation on bacterial removal from stainless steel equipment

Local wall shears stress analysis as well as cleanability experiments on different pieces of equipment in various circuit arrangements were performed. Pieces of equipment used in this work, stainless steel made, were representative of production lines: straight pipes of different diameters, sudden or gradual contraction or expansion pipes. Local wall shear stress values, obtained by an electrochemical method, were shown to be influenced by the loop arrangement. Geometry as the gradual expansion pipe had an effect on the upstream straight pipe where the wall shear stress values were two times lower than in other configurations as with a sudden expansion. In sudden geometry, the bacterial removal during cleaning was improved by the increase of the flow rate, corresponding to the increase of the mean local wall shear stress. However, in gradual expansion or contraction pipe, cleanability was not improved by the increase of flow rate. The effect of the mean wall shear stress on the removal has been confirmed. Moreover, this work demonstrated a clear positive effect of an increase of the fluctuation rate on the spore detachment. Low mean wall shear stress zones as in sudden expansion (around 0.15 Pa) could be cleanable because of the high fluctuation rate (around 21%). These results underline the importance of the design and the flow configuration. (C) 2002 Elsevier Science Ltd. All rights reserved.