Confined Spaces - Workplace Safety and Health Council

7.12.2.1.3 Push-pull SystemA push-pull system uses a combination of both forced ventilation and exhaust ventilation. Itusually provides more effective ventilation of the space than using any of the ventilation systemalone, and is recommended for use whenever practicable. The push-pull system introduces freshair into the space while removing contaminants by exhausting them.Exhaust ventilationVenturi EductorVenturi eductors are powered by compressed air or steam.The air or steam is released into the eductor through anozzle at high velocity and this, in turn, induces air intothe inlet and forces it along the tube for discharge at highvelocity. Eductors are usually lighter, more compact andless expensive but they are not able to move large volumesof air and require a significant supply of compressed airor steam to operate. In addition, the high velocity airmovement may also generate static electricity that couldserve as an ignition source.Figure 19: Example of venturieductor.Force ventilation7.12.2.3 DuctingDucting is used widely and extensively to channel air to and from confined spaces. There are twomain types of ducting commonly used:• Flexible collapsible ducting; and• Flexible non-collapsible ducting.Figure 16: Example of a push-pull system.7.12.2.2.1 Air Moving DevicesThere are typically three types of air moving devices that are used in the mechanical ventilationof confined spaces. These are:Axial-flow FansAxial-flow fans are designed to move air parallel to the axisof rotation of the blades. The operating principle is similarto that of standing fans used at home. Axial-flow fans canbe used for both forced or exhaust ventilation and are mosteffective for moving high volumes of air under relativelylow airflow resistance, such as when minimal or no tubingis attached. However, care is necessary when flammablegases/vapours are present as the fan motor is in the directpath of the airflow and can act as an ignition source. It isimportant to consider using an explosion-proofed fan for Figure 17: Example of axial-flow fan.such an application.Centrifugal-flow FansCentrifugal-flow fans, or radial-flow fans, move airperpendicular to the axis of rotation of the blades.Centrifugal-flow fans tend to be heavier, bulkier andgenerally produced lower airflow but higher static pressureas compared to axial-flow fans. This ability to generate highstatic pressure is important in application such as in LEVespecially where long runs of ducting may be used.Figure 18: Example of centrifugalflowfan.It is important that the ducting length be as short as possible and that the number of elbowsor bends in the duct are kept to a minimum to reduce friction loss. Friction loss will reduce theefficiency of the ventilation and could potentially result in lower than the designed/calculatedcapacity.7.12.2.3.1 Flexible Collapsible DuctingFlexible collapsible ducting, such as plastic material tubing is commonly used. Such ducting lacksstructural support and can only be used for forced ventilation. While they usually cost less andare more flexible, the lack of structural support can often result in less effective ventilation due toincreased static pressure drop across the ducting.Particular care is required when using plastic material tubings as they are more susceptibleto wear and tear during use. It is also important to note that such plastic material tubings arecombustible. When they catch fire, smoke/toxic gas will be emitted and travel back to theconfined space.7.12.2.3.2 Flexible Non-collapsible DuctingAnother type of ducting used is the flexible non-collapsibleducting. It usually has a wire helix that provides the shapeand prevents collapse. Such ducting offers strength, flexibilityand can be compacted for storage. Flexible non-collapsibleducting can be used for both forced and exhaust ventilation.7.12.2.4 Effective VentilationAnother key component of ventilation in the confinedspace is to ensure that the ventilation systems in place areeffective. As a general principle, ventilation systems shouldbe set up with the following considerations:• Long confined space;• Deep confined space;Figure 20: Example of flexible noncollapsibleducting.46 47