The technical dimensioning of an air curtain often rests on an insufficient basis when it comes to the influence of the building envelope and the ventilation system. The air curtain has a vital connection to both these elements and this has to be taken into account properly in the dimensioning procedure. The goal of the work was to bring new perspectives to such questions as well as to provide certain practical tools for the dimensioning process. This first part of the paper is focusing on simple fluid mechanical approaches. Several dimensioning methods based on the momentum balance are presented. The performance of the methods is compared. The breakthrough phenomenon of the jet is analysed and classified into three types. The factors determining each type are quantified. Equations for the breakthrough momentum flux are derived. The breakthrough ratio is introduced and its connection to the jet discharge angle demonstrated. The tightness, the vertical leakage distribution and the height of the building all have an influence on the dimensioning of jet parameters. The ventilation and process air flows should be in balance at all times to achieve working conditions for the air curtain. The most advanced dimensioning method is based on the moment-of-momentum principle enabling consideration of the jet impact point. Small discharge angles leading to a high jet momentum flux as well as a tight building envelop provide more tolerance against the wind-induced breakthrough. (C) 2002 Elsevier Science B.V. All rights reserved.