No fan dampers are gas tight however they are all available in close fit designs and can be manufactured in many materials allowing them to be used in arduous industries like offshore or high temperature applications.
Inlet Vane Control
These work by creating a vortex on the fan inlet in the same direction as the direction of rotation of the fan. Thus, the relative rotational speed of the air at the inlet is the fan speed less the swirl speed, and the fan performs as though it has been slowed down. However, as the device closes, it starts to choke the mass flow. There are, therefore, two components to control by this type of device. At near full open, the control is all by swirl with little reduction in fan efficiency. At near closed, the control is all flow restriction. At any point between, flow is by a combination of these effects.
Single Blade Circular Damper
These could be re-named flow restrictors because their control is due entirely to the production of an increase in system resistance to reduce pressure or volume. The simplest variant is a single blade device. As can be seen from the illustration, these are inexpensive dampers used for crude system control.
Opposed Bladed Louvre Damper
These offer efficient control in straight/laminar flow ducting, on the outlets of fans or fitted before inlet-boxes; these dampers can offer good isolation and can be manual or electronically controlled.
Often used as a shut off device. Butterfly dampers are restrictive gates that offer good isolation when closed. Can be manual or electronically controlled.