([i]This article originally appeared in the October, 2000 issue of[/i] Tunnel Business Magazine.) Tunnel ventilation control dampers (TVCDs) are classified in several different ways. Terms such as fan and track isolation, by-pass, and motorized fire are just a few terms used to describe their function. Today's automotive and mass transit tunnels differ in design, ventilation rates and requirements. A TVCD appropriate in one job might not work in another. Choosing the right damper is so critical, that the decision needs to be made during a project's design stage and not as an afterthought. Choosing the proper application, construction and damper features are key factors in the successful installation, commission and cost-effective performance of ventilation and emergency systems. When identifying types, functions and qualifications of a track or natural ventilation damper, several things need to be considered. TVCDs are constructed from various types of materials, such as galvanized and stainless steel, and designed based on system conditions. These dampers are generally used to supply or exhaust intake air from the tunnel. They have the additional capabilities to extract smoke in case of a fire. These machines could possibly be classified as true fire dampers, depending on their application and location. TVCDs need to qualify under National Fire Protection Agency 130 standards, requiring 250 degrees C heat exposure for one hour for fixed guideway transit systems. The damper's system location also has a bearing on the length of time they need to be tested to meet requirements. For example, if dampers are required for smoke control with exits located a considerable distance away, a test for the duration and time the damper is exposed to temperature may be required. This extended period gives people caught in the tunnel more time to reach emergency exits. Most direct mounted electric or pneumatic actuators require heat protection devices to meet elevated temperatures and extended exposure times. Sealing devices along the blade edges and between the ends and damper frame may be needed to keep excessive air or smoke from infiltrating through the TVCD.