The amount of pressure differential that a duct balloon can withstand before it moves or leaks is based on the method used to install it in the duct work. Using a test program formulated by the Department of Aerospace Engineering at Penn State University, a thorough analysis was made by testing fabrics for coefficients of friction, as well as reviewing current product designs. Being that their wind tunnel was a loop design and could not be used for our application, a custom built pressure chamber was constructed that could be rated up to 20” w.g. Duct balloons to match the chamber size were supplied in various depths that could be connected to lab instrumentation to measure external and internal forces exerted on the duct walls. After the tests were conducted, they were computer modeled to typical industry duct work sizes.

Two methods shown below were used to secure the duct balloon in the test chamber. One  method used steel chain secured to the duct wall and then to the anchor rings. Another method used steel chain between the anchor rings and anchored steel backing bars:

Note: When using a tie down or support restraint system, they should always be designed by a professional engineer who is experienced with structural design.

The chart below shows that different holding values can be achieved by using different perimeter thickness as well as different attachment methods inside the duct:

It must be understood that a number of variables can affect the accuracy of these predictive holding values such as duct balloon size, perimeter depth, fabric selection, duct surface conditions, method of restraint, atmospheric pressure, temperature, quantity and size of blowers used, as well as the over all integrity of the installation.   

If you are considering installing a duct balloon in an area where it will see pressure differential, you should also be aware of the “double block and bleed” method commonly advocated by safety professionals which can be seen in an artist rendering by clicking here.  Always consult your company's safety department or the person in responsible charge before making a decision to use ANY type of temporary barrier!

The test program showed that predictive holding values can vary between 0.5” w.g. by friction alone but can be as high as 10” w.g. if properly secured in the duct work. Generally speaking, the larger the size of the duct balloon the less pressure it will hold back before it starts to leak.  As an example, the predictive holding capability for a 10’ high x 10’ wide x 3’ depth size using one 120V or one 220V 2 HP high pressure blower is as follows:

Friction Only Method of Holding inside duct
Duct Pressure: .078 in H20
Force pushing on Duct Balloon: 407 pounds at this pressure

Chain and Ring Method of Holding by securing inside duct to anchor rings
Duct Pressure : 4.80 in H20
Force Pushing on Duct Balloon: 2497 pounds at this pressure

Horizontal Method of Holding by providing steel backing bars behind balloon
Duct Pressure 5.13 In H20
Force pushing on Duct Balloon: 2668  pounds at this pressure