The research project (RP-660) entitled "Determination of Critical Air Velocities to Prevent Smoke Backflow at Stair Door Openings" sponsored by T.C. 5.6, Fire and Smoke Control was started on June 1, 1990. The objectives of this proposal were to determine critical velocities at non-fire condition that prevent smoke backflow at open stair door on the fire floor for various fire temperatures, venting of the fire floor either with exterior wall vents or with an exhaust fan, multiple and bottom injections of supply air to the stairshaft, and for a number of open stair doors.

This research project is a follow-up of the ASHRAE research project RP-559 entitled "To Study Methods of Over- Pressure Relief for Stairwell Pressurization Systems." The objectives of the latter project were to assess stairwell pressurization systems with exit door relief, barometric damper relief, and variable supply air with feedback control. While carrying out this research project, it became apparent that a more comprehensive data on critical air velocities particularly at low fire temperatures and other fire conditions were required for designing stair pressurization systems.