INFLUENCE OF THE FILM FLOW CHARACTERISTIC ON THE CLEANING BEHAVIOUR

The cleaning of machinery and facilities is a key process to meet the safety needs in food and pharmaceutical production. The high significance of cleaning is a reaction to the rising importance of environmental protection. Due to the mostly unknown interaction between process parameters and cleaning behaviour of deposits, too many resources are used in industrial cleaning steps. This results in growing costs for the cleaning processes. This publication presents results of laboratory scale cleaning investigations of falling films with respect to the influence of the film thickness and hence predictable parameters. A fluorescence method was applied for determination of film thickness on stainless steel samples. With this publication it can be shown that a connection exists between the falling film cleaning behaviour and the wall shear stress or the mean velocity. Additionally it is shown that a low wetting rate is the most effective setup for saving cleaning fluid. INTRODUCTION Contamination free and safe consumer products are given highest priority in the food and pharmaceutical industry. Therefore meeting the requirement of consistently high product quality cleaning is a key step. Validated and mainly automatic cleaning-in-place (CIP) processes are used, however currently resource efficiency is becoming increasingly important, particularly targeting the reduction of cleaning costs and down time including consideration of environmental aspects. Environmental conservation is also demanded by an EU directive 2008/1/EG. The contradictory intensions of saving resources and producing safe consumer products, necessitates the fulfilment of a balancing act for the food and pharmaceutical industry. A better understanding of influences on cleaning, especially of the flow parameter, can provide a contribution to reach resource-efficient processes. For CIP-processes static or dynamic cleaning devices are used. Using a spray ball or a rotating head, the main mechanical cleaning effect is provided by the gravity-driven falling film. Patel and Jordan (1970) investigated cleaning for the first time under the application of falling film. They used microorganisms to assess the influence of tilt angle between 30 ° and 150 ° (relating to the horizontal) on cleaning results. They identified that a vertical substrate (90 °) leads to the best cleaning results. Additionally, they determined a higher cleaning efficiency with increasing flow rate, caused by intensified turbulence and higher mechanical scrubbing. In addition, Patel and Jordan (1970) supposed an intensified turbulence caused by eddies and roll waves on the film surface, as an explanation for better cleaning. Lerch et al. (2013) studied washing-in-place processes (WIP) by falling film. In cleaning studies they compared a particulate riboflavin test soil in dry, pre-wetted and wet conditions. These experiments concluded that a tilt angle of 67 ° leads to faster cleaning than a vertical position describing it with a constant diffusion coefficient. The coefficient is independent of the tilt angle but, with diminishing inclination of the plane, the film thickness and the concentration gradient of the rinsing film increases. The best cleaning results were obtained with a combination of pre-wetting of dried soil, inclination of the sample and a high flow rate. Finally, a general relationship for all WIP processes could not be given, thus additional studies of flow parameters influencing cleaning behaviour and cleanability of material, as well as surface finishes, should be conducted. In addition, the different experimental results of Lerch et al. (2013) and Patel and Jordan (1970) indicate that the acting cleaning mechanisms may also have an influence on the design of components. Therefore it is necessary to do further cleaning studies with different soils in order to obtain general statements. Wall shear stress is commonly used as a description of cleaning (e.g. Detry et al. 2009), which can be calculated through knowledge of film thickness (Brauer 1971). Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2013 (Peer-reviewed) June 09 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson