Clearing the Air on Confined Space Ventilation

Clearing the Air on Confined Space Ventilation

To help clear the air, here is some advice for selecting a proper ventilation blower for a confined space, as well as some tips on usage.

For all of the attention that confined spaces receive, many safety managers are still uncertain about ventilation requirements. They may not understand how much airflow is needed to properly ventilate a given space. They may overlook ventilation principles allowing them to eliminate pockets of bad air or dead spots where workers are positioned, or they may fail to monitor air quality before and during employee entry into a confined space. To help clear the air on this subject, here is some advice for selecting a proper ventilation blower for a confined space, as well as some tips on proper usage.

Know Your Space

First, it is necessary to understand what defines a confined space. A confined space is not designed for continuous worker occupancy but is large enough for a worker to be able to enter and perform work. It also has limited or restricted means for entry or exit. Common examples of confined spaces include tanks, vessels, silos, storage bins, hoppers, vaults, pits, manholes, tunnels, equipment housings, ductwork and pipelines.

A confined space may have natural ventilation, but it may not be sufficient enough to provide the proper working atmosphere. A confined space may have no ventilation at all. In both instances, forced air ventilation may be required. A confined space also isn’t necessarily small; on the contrary, it can be quite large. Several factors may contribute to defining a space as confined, so having a thorough understanding of OSHA regulations is the best way to determine if a particular space qualifies.

Hazardous or Non-Hazardous?

A confined space will be one of two types: hazardous or non-hazardous. Any area where the possibility of fire or explosion may exist due to the presence of flammable material is what the National Electrical Code (NEC) defines as a hazardous location. It can also be deemed hazardous if the confined space atmosphere contains toxic or hazardous substances that can create an oxygen deficiency or harm a worker as defined by OSHA standards. Sites such as petrochemical plants, fuel-servicing facilities, industrial 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 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.


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 are made from either molded plastic or cast aluminum, and the two types serve different purposes.

Plastic units are lightweight and cost-effective, but they’re only designed for light to moderate use. Although plastic does well at resisting scratches and dents, the housing may crack easier if abused or operated in cold weather.

Blowers that incorporate cast aluminum housings come with higher price tags as well, however, these units also have a demonstrated reputation for withstanding much more abuse and should be expected to last longer than plastic blowers.


Airflow depends heavily on the duct. The standard diameter for portable ventilation blowers is eight inches, although larger sizes are available, and the most popular duct lengths are 15 and 25 feet. Duct lengths that exceed 25 feet significantly decrease the airflow that can be delivered to a confined space. If more than 25 feet of duct is needed, consider other options to place the blower closer to the confined space opening. Otherwise, one can invest in a larger, more powerful blower, add another blower inline (mid-duct run), or set up separate duct runs with multiple blowers to compensate for the reduced airflow.

Bends in the duct — especially those with an angle of more than 90 degrees — also reduce the airflow rates produced by any blower. Manufacturers understand that providing airflow in a confined space environment often requires making a minimum of two 90-degree bends in the duct, so they often account for the airflow loss when rating the CFM of their products. Be sure to check the specifications provided by the manufacturer for an explanation of how each specific blower was rated.


While everyone must deal with meeting budget requirements, worker safety always comes first. If a blower doesn’t meet the requirements of a confined space, it will not be able perform adequately and will put workers at risk with the possibility of causing serious injury or even death. Conversely, don’t think that purchasing the most expensive blower automatically means it will be the best one for a particular space. The biggest thing to keep in mind is that a price tag can’t be placed on employee safety and wellness.

Despite the several factors that go into the process, properly selecting and using a ventilation blower for a confined space isn’t terribly complicated. While it does require some thought and preparation, meeting regulatory compliance and ensuring worker safety is well worth the effort.

This article originally appeared in the May 1, 2021 issue of Occupational Health & Safety.

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