Flex Duct: Balancing Convenience Against Efficiency and Safety

Flexible duct (or flex duct) is a common component in dust collection systems. It is typically used to make the final connection between a piece of equipment and a rigid duct from a dust collection system. This is necessary when connecting equipment that slides, vibrates, or must otherwise move, since rigid duct often will not work in these applications. Flex duct is also quick and easy to install and provides additional flexibility for routing. However, flex duct is often overused in applications where it is not necessary. While it provides a convenient and low-cost method for routing dust collection ducts, overusing it can compromise a dust collection system’s effectiveness and safety.

Dust collection systems have two main design parameters: Total Airflow (measured in Cubic Feet per Minute) & Total System Static Pressure (measured in Inches Water Gauge). The total airflow is determined during the design phase by establishing the type and quantity of equipment that will be connected to the system. Total static pressure is determined by adding up the pressure losses from various system components including hoods, dust collectors, transitions, elbows, total duct length, etc. Total airflow and system static pressure are used to select an appropriate fan that will provide the required airflow at adequate static pressure to overcome the pressure losses in the system.

So How Does Overusing Flex Duct Affect a Dust Collection System?

Flex duct, like any other component in the system, will affect the total system static pressure. The AVERAGE amount of static pressure loss per foot of STRAIGHT flex duct can be estimated by the following equation:

0.0311[V^0.604/Q^0.639]                                                [Equation 1]

Where (V) is air velocity in feet per minute and (Q) is airflow in actual cubic feet per minute.

NOTE: This equation is an average but actual pressure losses can vary by manufacturer based on materials of construction, depth of corrugations, etc. This equation also only applies to flexible duct installed in a straight position.

By comparison, the amount of static pressure per foot of rigid metal duct can be determined by the following equation:

0.0307[V^0.533/Q^0.612]                                    [Equation 2]

The same variables in Equation 1 are used in Equation 2.

Just looking at these two equations, it is difficult to get a sense of how much difference flex duct makes when used instead of rigid duct. To better illustrate the difference in static pressure drop between the two types of duct, the following graph shows static pressure losses per foot of rigid duct vs. flex duct for the following duct sizes: 4”, 5”, 6”, 8”, and 10”. The velocity was set at 4,500 fpm which is a common minimum conveying velocity for certain combustible dusts, and the airflow was calculated based off the respective duct sizes at 4,500 fpm.

As you can see above, the flex duct has 50-60% greater static pressure loss per foot. It is also worth noting that this assumes the flex duct is straight, meaning this is the minimum impact that can be expected. Flexible duct is rarely installed straight, and bends in flexible duct can produce extremely large pressure losses that cannot be easily predicted. This is why the ACGIH Industrial Ventilation Manual recommends keeping flexible duct segments as straight and as short as possible.

NFPA 660 states that :

Flexible hose and connections shall be permitted in the minimum length necessary to achieve functions such as but not limited to material pickup, material transfer, vibration isolation, weigh bin isolation, or equipment movement.

The additional static pressure for flexible duct can and should be accounted for in design and with fan selection. If additional flex duct is used beyond what was accounted for in a system’s design the effectiveness of the system can be compromised.

Let’s Look at an Example

Consider a simple dust collection system with 3 pieces of equipment needing 6” duct connections. The design indicated that each piece of equipment would have 10 ft drop of 6” diameter rigid duct and 2ft of flex duct for connection. But instead, the contractor decides to use flex duct for the complete length of the drop, resulting in 12 feet of flex duct for each piece of equipment. The two calculations below show the resulting static pressure for design vs actual. (900 cfm for 4584 fpm duct velocity)

Design: [Flex (2 ft x 0.066 in. wg/ft) + Rigid (10 ft x 0.042 in. wg/ft) ] x 3 = 1.656 in. wg

Actual: [Flex (12 ft x 0.066 in. wg/ft)] = 2.376 in. wg

Difference of static pressure between design and actual: (2.376 – 1.656) = 0.72 in wg

By installing excessive flex duct, the static pressure of this branch of the system was increased by 0.72 in wg. or 43%.

So What Does This Mean in Terms of System Performance?

This amount of additional pressure loss will negatively impact the system’s ability to effectively capture dust at the connected equipment and convey it to the dust collector. We know from the Fan Laws that changes in static pressure are proportional to the square of the change in flow rate.

SP₂ = SP₁ x (CFM₂/CFM₁)²

1.656 = 2.376 (CFM₂/900)²

CFM₂ = 751 cfm

This means that a 43% increase in static pressure will result in roughly a 17% decrease in flow to each of these machines. This will lower the capture velocity at the equipment and will also decrease the velocity in the duct to 3824 fpm which is significantly less than the minimum transport velocity recommended by NFPA or ACGIH (4500 fpm), for this hypothetical example. And remember, we also assumed the flex duct is installed straight, which it almost never is!

Improper Use

Proper Use

Summary

Flex duct can be a great option when it comes to installing dust collection systems, however it comes at a cost of increased statistic pressure losses. When necessary, flex duct should be as straight and short as possible. Flex duct manufactures should be contacted to obtain the static loss per foot, and this should be considered in the design to ensure an effective dust collection system. Contractors and owners should avoid using more flex duct than is called for on design drawings to prevent unintended decreases in flow rates, less effective dust capture, and settling of dust inside ductwork, all of which are hazardous situations, especially when dealing with combustible dust.

About the Author

Zack has been with Hallam-ICS for 5 years and recently obtained his Professional Engineering license. He is the Combustible Dust Safety Team Lead and also works on a variety of mechanical engineering projects. He enjoys disc golf, ultimate frisbee, and board games.

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About Hallam-ICS

Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina  Texas, Florida and our projects take us world-wide.