Page 49 - OHS, May 2021

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They may not understand how much airflow is needed to properly ventilate a given space.
requirements. If the space has an odd shape, greater CFM in combination with ducting may be needed to obtain proper air circulation. If the CFM requirement is excessively high, multiple blower and duct configurations may be required to effectively meet the needed airflow delivery.
Confined spaces may also have only one option for power. As a result, the worksite must be evaluated to determine what blowers may be feasible before a job can begin. Where a blower can be placed from the entry point, engine exhaust fumes, stability of the local power grid and whether other large construction projects are putting a heavy draw on the available power supply all play a part. As a result, a blower with higher CFM may be required to compensate for these variables. Knowing the power requirements help decide if a gas, pneumatic or electric blower is needed.
Blowers
When browsing a selection of portable ventilation blowers for use in confined spaces, one will first notice that there are multiple power options. AC electric-powered units are the most popular, and they are generally the best choice for efficiency and performance as long as there is a readily available power source near the confined space. For outdoor jobs where there are no electrical sources nearby, gasoline-engine powered blowers are also offered. Other, less- common power sources on the market include DC electric motors and diesel engines, which are both intended mainly for niche applications, such as utility and offshore work.
After determining the best power source required for a blower, a person can then select from two types of fan configurations: radial (centrifugal) and axial. Radial-type blowers direct air in a circular motion before blowing it out at a 90-degree angle to the air intake inlet. Axial fans move air in a straight path through the blower body.
Compact size is a major selling point of radial blowers. Furthermore, if an engine-powered blower is needed, radial units are the only choice since the geometry of axial configurations does not effectively accommodate engine installations. In axial configurations, the fan thrusts high volumes of air in a straight path through the blower similar to a modern jet engine, allowing the units to produce higher CFM rates than radial fans. The main drawback is that axial blowers can be larger, requiring more room to operate.
Durability is another big consideration when selecting a ventilation blower. Although several factors, like the quality of components and electric motors, can help foreshadow the service life of a portable ventilation blower, the construction of the fan housing is the main mark of durability. Generally, fan housings
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processing centers, grain elevators and sewers are all examples of hazardous locations. Any confined space that contains serious safety or health hazards requires a permit, which documents the atmospheric conditions of the space. Permits can also be used to help determine the right type of blower needed for the job.
If the NEC determines the space is hazardous, specific equipment must be used. Hazardous location air ventilation blowers and accessories incorporate spark-proof construction into their design. The equipment works to control the potential for a random spark being the ignition source of an explosion. These types of blowers have explosion-proof motors (i.e., A motor having structural integrity to stay together if an explosion occurs within the motor), as well as special ventilation ducts, which contain carbon-based materials that conduct static charges to a proper ground location.
Furthermore, the electrical wiring within hazardous- location blowers must also be certified by a listing agency, such as the Underwriter’s Laboratory® (UL) or Canadian Standards Association® (CSA). This means that the wiring system will not create a spark when plugging it into an electrical outlet. If the confined space is not deemed hazardous, a non-hazardous blower may be used that does not need to meet the above certifications and design requirements for hazardous locations.
Size, Shape and Power of a Confined Space
The size and shape of a confined space determines the ventilation requirements. Calculating the minimum required rate is quite simple, but a few factors must be known before it can be done accurately.
First, what is the size of the space in cubic feet? Second, what are the air changes per hour (ACH) required or, in other words, how many times per hour must the air be replaced? There are different ways to arrive at the ACH number. Safety managers may use their own caution and common sense, basing it on the volume and severity of contaminants in the air, or, depending on the state in which the job is taking place, a number may already be defined. Because OSHA does not regulate ACH, each individual state may determine its own requirements. Not all states do, but if the job is taking place in a state that has an ACH number defined, be sure to abide by it.
When these two numbers are known, they can be multiplied together to get the total cubic feet per minute (CFM) delivery requirement. For example, say a state requires air to be changed six times per hour, or 6 ACH. If the confined space is 10,000 cubic feet in size, a 60,000 CFM per hour delivery is required. Divide the product of these two numbers (60,000), by 60 minutes. This will give the minimum CFM delivery rate, which in this case is 1,000 CFM. Therefore, a blower providing 1,000 CFM will be sufficient to exchange the air at the minimum rate. Always remember to choose a blower that at least meets, if not exceeds, CFM delivery requirements.
While the size of the space will impact how large the blower must be, it isn’t the only factor. The space configuration will help determine the blower size, and it will also dictate ducting
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MAY 2021 | Occupational Health & Safety 45