ABSTRACTMathematical models are presented which make it possible to calculate pressure drop and mechanical cleaning effect for plant design of vacuum dairy milking pipes. Experimentally, air‐water mixtures were sent through a test plant, consisting of glass pipes 0.034m in diameter and60m in length (93m in some experiments), where a soiled piece of pipe could be inserted without disturbing the flow. The soiling was milk‐based and dried on application. Light absorption of the soiled glass tube was measured before and after the cleaning operation, which was performed for45 s with plain water at room temperature. To calculate the pressure drop, the so‐called Dukler constant‐slip‐method was used. A holdup correlation according to Hughmark was improved to fit the pipe system used.Through linear regression analysis an equation was obtained, describing the cleaning effect as influenced by the Reynold number and the liquid fraction of the air‐wa