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 di!erent 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 di!erent diameters, sudden or gradual contraction or expansion pipes. Local wall shear stress values, obtained by an electrochemical method, were shown to be in9uenced by the loop arrangement. Geometry as the gradual expansion pipe had an e!ect on the upstream straight pipe where the wall shear stress values were two times lower than in other con:gurations as with a sudden expansion. In sudden geometry, the bacterial removal during cleaning was improved by the increase of the 9ow 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 9ow rate. The e!ect of the mean wall shear stress on the removal has been con:rmed. Moreover, this work demonstrated a clear positive e!ect of an increase of the 9uctuation 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 9uctuation rate (around 21%). These results underline the importance of the design and the 9ow con:guration. ? 2002 Elsevier Science Ltd. All rights reserved.