A Guide to Ventilation of Confined Spaces

We receive a number of questions concerning the ventilation of confined spaces. Typically, these questions are of the following variety. Why do we have to ventilate a confined space? How does one effectively ventilate a confined space? What ventilation blower is needed to effectively ventilate a confined space? In this definitive guide we answer all of these questions.

VENTILATION - THE WHY

In 1910.146 Permit Required Confined Spaces, OSHA outlines why ventilation of a confined space may be necessary. OSHA states, "An employee may not enter the space until the forced air ventilation has eliminated any hazardous atmosphere”.

This statement can only be properly understood in light of 1910.146 as a whole. Before each confined space entry and during each entry, a confined space gas monitor is used to monitor the space’s atmosphere.

If an atmospheric hazard is detected during entry, the entrant must exit immediately. If a hazard is detected before entry, no entrant can enter the space.

The reasons for this should be obvious. An elevated level of oxygen can be highly flammable. Decreased levels of oxygen are deadly. Elevated levels of carbon monoxide or hydrogen sulfide are also deadly. And the presence of combustible gases can present the threat of an explosion.

When any of these conditions are present, ventilation of the space is required. In fact, OSHA states, "The forced air ventilation shall be so directed as to ventilate the immediate areas where an employee is or will be present within the space and shall continue until all employees have left the space".

VENTILATION - THE HOW

Once it has been determined that a space has to be ventilated. The question arises: what is the best way to do this?

There are a few things to consider as we answer this question. What is the best practice for blower and duct placement? How much air is needed to ventilate a specific space? What kind of blower will provide the best result.

Blower Placement

The below picture will help understand blower placement. The blower itself is to be placed a minimum of 5 feet from the opening. This ensures that the blower is pulling fresh air from outside of the space.

The duct is then lowered into the space. The duct should be placed to terminate within the bottom quarter of the space. When done in this way, the best ventilation results are obtained. Ideally, the duct should be placed at the furthest corner from the entrance.

Duration of Ventilation

The duration of ventilation will depend upon the volume of the space (cu. ft.) and the CFM exchange rate of the blower. Just be aware that the CFM rates of any blower decrease when duct is bent at 90 degree angles. The blower’s manual will tell you what that decrease is.

There is disagreement on the ideal air exchange rate one should use to ventilate a space. Allegro recommends “…using a blower that is capable of 20 exchanges of air per hour.” RamFan, based on their purge chart, seems to advocate for an exchange rate of 7 1/2 times (and whatever the time it takes to reach that).

The below RamFan ventilation purge chart shows the time in minutes it takes to “turn over” or exchange the air in a confined space 7 1/2 times. The time, the black numbers, is based on the CFM (blue numbers) of the ventilation blower and the volume (red numbers) of the space in cubic feet. 

The chart can be downloaded here: ventilation rate purge chart.

This chart is NOT to be used in place of a gas monitor to assess the quality of the air in a confined space. Using a purge chart will help reduce the risk of harmful gases and a lack of fresh air. It will not confirm that your confined space has safe air.

Before initial entry or re-entry can be made, the space must be tested again with a confined space monitor. And per the 1910.146 standard, it must be continuously tested during entry. See our Definitive Guide to Confined Space Entry for details.

Once the space is shown to be free of any hazardous atmosphere, the space can be entered or re-entered. Note that once a ventilation blower is used to clear the space, it must be used throughout the duration of the entry or re-entry. You are not permitted to turn off the ventilation blower at this point.

WHAT VENTILATION BLOWER DO I USE?

Generally, blowers for confined space use can be understood in terms of four categories—blower diameter (8” or 12”), blower style (axial or centrifugal), how the blower is powered (AC, DC, air, or gasoline), and the CFM output of the blower. All of these have advantages and disadvantages.

The bottom line is simply this: time versus money. Faster exchange rates mean higher cfm blowers and more money. Slower exchange rates mean lower cfm blowers and less money. It is entirely your call on which way to go.

In our fifty-plus years of experience, the most common blower used for confined space ventilation is an 8”, axial, AC-powered blower. This is the less money route.

The prime example of this blower is the 9533 blower. It is AC powered. A similar version of the blower running off of 12v DC power is also available. If you would rather opt for power, we suggest the UB30. This is a 12” axial blower with loads of CFM power.

HAZARDOUS LOCATION BLOWERS

There is one huge exception to everything just stated about blower selection. If you are ventilating a confined space that is an explosion risk, you must use a blower designed specifically for this hazard.

The National Electric Code (NEC) dictates that a hazardous confined space is one that has the possibility of fire or explosion due to the presence of flammable materials.

An explosion proof blower motor is specially constructed to limit the risk of explosion. It also uses ducting that is coated with Neoprene which resists static charge build up. This ducting is also integrated with a grounding wire to maintain the ground throughout the system. 

Moreover, a special plug is needed and is always purchased separately. As Allegro states, “The receptacle (sold separately) must be hard-wired to your power source (e.g. conduit, flexible conduit) to meet explosion-proof standards.” Our best-selling Haz blower is found here—UB20xx.

EXTRAS

One final topic to cover is ease of access into a confined space when using ventilation blowers. Clearly, when one drops an 8” diameter duct into a manhole, it makes entry and exit more difficult. There is a solution to this problem. It is called the Manhole Ventilation Passthru. We encourage you to check it out.

CONCLUSION

We urge you to take ventilation of confined spaces seriously. If you need any clarification, don’t hesitate to reach out to us.

We’d like to add one final point to this discussion. It is always more pleasant to work in a ventilated confined space that an unventilated one. Even if no hazards are present, fresh air is better for the workers.