Pressurization

Some systems normally do not have any specific supply airflow, though they could have some air supply. These are used when a volume (normally a room) is intended to have a lower pressure than the surrounding rooms. This so-called pressurization of zones is used when a high level of safety against spreading of contaminants is needed.1 A high pressure difference between two rooms causes the air to flow into the room with the lowest pressure if some cracks exist or some connection between the two rooms is opened.

The pressure difference is created by using a large difference between the sup­ply and exhaust flow rates (in either direction). Higher pressure than the sur­roundings is used to prevent airborne transport into the room and lower pressure to prevent transport from the room. The airflow is then directed from the room with the (initial) higher pressure to the room with the lower pressure, and the transport of contaminants in the other direction is largely prevented. The pressure difference will disappear, for example, when a door is opened between the two zones. When the pressure difference has disappeared, which could happen in frac­tions of a second, the preventive effect diminishes since, as has been pointed out earlier, it is impossible to have total isolation only by using air.

Closed volumes (confined spaces) exist where supply air is necessary, though not in a specific way. The air could be treated (like in a general ventilation system) to increase its usability or the workers’ comfort. The supply air could enter through an inlet device of any form. This could be similar to any normal general ventilation supply device or just an open end of a duct or a tube. The air then exits from the closed volume where possible. This system could be used for temporary work, without high contaminant generation, inside storage tanks, and the supply opening used is usually only the open end of the duct or tube.

There are other small closed volumes where it is necessary to supply a limited flow rate, but where the air sometimes has to be treated carefully through filtering, gas cleaning, heating, cooling, etc. Some examples are the air supply inside a car and airplane and train cabins. These systems are seen mostly as general ventilation systems. Ventilation of control cabins in industry such as crane driver’s cabins, with specific demands on heating, cooling, and cleaning, is usually named local ventila­tion and consists of a supply system only. The air could be treated and distributed through a system in the cabin or from a central system, connected in a way similar to how some mobile local exhaust hoods are connected to a fan (see Section

10.1.2) . These cabins have similar functions as cabins used for control purposes, such as traffic toll booths, pay booths, money exchange booths, ticket booths, etc. These have some differences with general ventilation systems regarding choice ot opening size of air supply, air velocities, airflow rates, air quality, and temperature. Mostly the demands are similar to demands on general ventilation systems, the dif­ference being that these small volumes have no exhaust system and let the air out of the volume through small openings in the walls. In this way they also to some extent use pressurization of the volume to limit the entrance of contaminants.

When using supply inlets it is more important than for exhaust hoods how the person, working with a process, is placed, relative to the contaminant source and to the inlet. It is nearly always better to keep the person between inlet and source than source between inlet and person. For supply systems it is even more efficient than for exhausts to have the flow passing in front of the person instead of from back to front. The airflow from behind the person could generate a wake, which includes the source or the generated contaminant and thereby in­creases the person’s exposure. This phenomenon is more common with large flow rates and large supply openings than with small flow rates and small inlets. Placing the worker beside the path from the inlet to the source and on to the ex­haust is a general rule. It is possible to counteract the wake around a person by using supply air, directed downward around the worker. In this case, the air is normally sucked into an exhaust hood (see Section 10.4).

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