All local ventilation systems can, in principle (and many are in practice), be manu­factured for use in one or more of three different modes: fixed, flexible, and mobile.

Fixed systems are those where movement of the hood or other changes to the system, except perhaps opening and closing of lids and doors, is not possi­ble. One example is the hood with a sliding door surrounding a drilling or a milling machine; another is the laboratory fume hood; and another is the can­opy hood above or the enclosure around a paper machine.

Flexible systems are those where the suction opening or supply device may be placed at different locations inside a limited area or volume. This is per­formed by connecting the exhaust (or supply) opening and duct (or tube) to

The fan by flexible connections (ducts with movable elbows or flexible tube). One example is a wall-mounted hood for welding exhaust; another is the ex­haust connected to a portable grinding machine; and another is the exhaust for soldering fumes connected to soldering equipment.

Mobile systems are those where the exhaust (supply) opening may be placed almost anywhere inside a workroom. This is performed by placing the whole system (exhaust/supply opening, duct, fan) on wheels or a portable frame. It can also be accomplished by having a separate exhaust part (opening and short tube) which could be connected to a central duct system at many places (in walls, on the floor or from the ceiling). One example of the former is a welding exhaust (with filter) on a small carriage and an example of the lat­ter is a centralized exhaust system for connection to car exhaust pipes. An­other example of a mobile system is the sliding connection between an exhaust tube and a fixed duct, which makes it possible to let the exhaust follow the car (or any contaminant generator) around the room.

Mobile exhaust or supply systems also include portable systems—for ex­ample, the systems consisting of a fan and a flexible tube used in construction work, either for exhausting or supplying air in work areas without any venti­lation system. The work area must have some type of connection, such as win­dows, to the surroundings.

It is difficult to determine the most common mode for local ventilation and naturally many systems fit into more than one mode.

All the different local ventilation systems can be manufactured in many different materials depending on demands for strength, vibration, abrasion, cleanliness, economy, etc. Most of the time, the material has very little influ­ence on a local ventilation system’s performance as long as the original design and flow field is maintained (no holes from abrasion, no clogging, etc.). How­ever, different duct materials could change the friction losses and thus the re­sulting flow rates.