Stagnancy in process equipment does not only increase the concentration of microorganisms in perishable products (Lelieveld, 1976a) but also considerably increases the time required for cleaning in-place. Frequently, cleaning is stopped when most of the line is clean although dead spaces still contain soil residues. Such residues may protect microorganisms against disinfection treatments, whether with steam, hot water or cold biocidal solution. Survivors may infect a sterile product passing the dead space (Lelieveld, 1976b) A badly designed line then forces manufacturers of foods and pharmaceuticals to apply excessive heat-treatments, of up to two hours at 140 -'C. This means a waste of energy and a loss of production time.
Dead spaces can be avoided by selecting the right type of components (e.g. welded IDF-couplings instead of RJT-or DIN-couplings; Timperley and Lawson, 1979).
If dead spaces are unavoidable, for instance if appendages (pressure gauges, temperature probes, safety discs etc.) have to be installed, the dead spaces should be kept small and the constructions be such that cleaning is easy.
Flow visualization experiments enabled us to measure to what extent the distances travelled by particles in a cleaning fluid in the dead leg of a T-piece depend on the direction of flow. Some of the results are shown below. The time lapsed between two dots is 0.02 s. The flow rate was 1.5 m/s. Details of the experiments are described by Timperley and Lawson (1979).
Cleaning was most efficient if the flow was directed towards the dead leg.
l'xNN\\\~r iN LELIEVELD, H. L. M. 1976a. Influence of stagnant areas in process equipment on the increase of micro-organism concentrations in the product : A tentative mathematical approach. --Biotech-