Requirements to pipes and fittings in food and dairy technology.

Pipes and fittings used in food and dairy technology must satisfy the requirements of DIN-standard sheets when planning a piping system it is necessary to take into consideration that there must not be any difficult to clean areas, in order to be able to rinse pipes and fittings. Flow apparatus, for example holding-sections, should have a residence time distribution as narrow as possible. Otherwise the quality of food would be adversely affected. In order to set exact requirements regarding residence time distribution, the physical fundamentals are described. It is possible to obtain narrow residence time distributions when Reynolds number is larger than 12,000, if the pressure drop caused by fittings is small and if the ratio L/d has values of about 200 or larger. Considering these facts it is possible to reduce the difference between largest and smallest residence time down to about 30% of the mean residence time. The behaviour of a pipe system when cleaning and rinsing can be judged by measuring the amounts of residue remaining in the system. The time for rinsing must be long enough to reach the final amounts of residue. Fittings have to be rinsed about four times longer in order to reduce the amounts of residue down to values comparable to those found in pipes. (A)