Spiral vs. Flexible Ventilation Ducts: A Comparison Guide for Underground Projects
Spiral vs. Flexible Ventilation Ducts: A Comparison Guide for Underground Projects
Ventilation is one of the first safety systems installed in an underground project. Mines, tunnels, high-speed rail excavations, road tunnels and rock caverns all depend on a continuous supply of fresh air. The ventilation duct is the connection between the fan and the working face, and the choice between spiral duct and flexible duct can influence installation time, fan load, air leakage and compliance complexity.
This comparison guide is written for engineers, procurement managers and contractors who are at the evaluation stage of selecting a tunnel ventilation duct or mining ventilation duct. It compares spiral and flexible ventilation ducts on structural rigidity, ease of installation, air leakage control and suitable applications. The evidence base includes SULONG product specifications, certifications and documented underground project records.
The Procurement Problem: Duct Performance Is System Performance
A ventilation duct does not work alone. SULONG product documentation describes the flexible ventilation duct as a product that is connected to a ventilation fan to blow fresh air into underground tunnels and dilute hazardous gases. The same documentation describes the spiral ventilation duct as supplying anti-static safety airflow and supporting dust extraction systems in underground tunnels. In other words, the selected duct must fit the airflow function of the mine or tunnel system, not just the available space.
The procurement problem is that an easy-to-install flexible duct and a shape-retaining spiral duct have different operational strengths. A duct with too much leakage can reduce the actual airflow reaching the working face. A duct that is difficult to handle can slow down excavation cycles and increase labour cost. A duct that is not compatible with the fan pressure mode can cause poor performance or unnecessary safety risk.
At the evaluation stage, therefore, buyers should compare five things: material construction, diameter and section length, pressure capability, certification and evidence from similar underground projects. The rest of this guide provides that comparison.
Industry Background: Growing Demand and Compliance Pressure
The market context explains why spiral and flexible ventilation ducts are receiving more attention from mine and tunnel buyers. The global mine ventilation market was valued at USD 1.42 billion in 2024 and is projected to reach USD 2.03 billion by 2030, representing a CAGR of 6.2%, according to Strategic Market Research. The flexible ducting market was estimated at USD 203.97 million in 2024 and is expected to reach USD 335.78 million by 2033, according to Intel Market Research.
Another useful context point is production scale. In 2023, over 2.1 billion meters of flexible ducting were manufactured globally, with about 30% dedicated to industrial ventilation, as reported by Spherical Insights. Underground mining is widely considered the dominant segment of mine ventilation demand because hazardous gas dilution is a critical safety function.
Regulation also increases the need for careful supplier evaluation. For example, mine ventilation ducting must comply with MSHA 30 CFR Part 75 for underground safety in the United States. OSHA sets a minimum fresh air supply of 200 cfm per worker in underground construction. Buyers should therefore ask for the specific certifications and test reports that correspond to the destination country.
Detailed Solution: Spiral and Flexible Duct Characteristics
Both SULONG flexible ducts and spiral ducts are made from PVC coated fabric or PVC laminated fabric. Both product ranges are intended for underground engineering projects such as mine projects, high-speed rail, subway, highway, hydropower station and rock cave scenarios. The main difference is structural reinforcement: the spiral range adds spring steel wire and is produced as a semi-rigid or shape-retaining duct, while the flexible range is built as a lay-flat duct using seamless one-piece fish-back welding automation.
Flexible Ventilation Duct
The SULONG flexible ventilation duct is a lay-flat duct manufactured from PVC coated or laminated fabric. Product specifications cover diameters from 200 mm to 3,200 mm and section lengths from 1 m to 300 m. The material can be specified from 300 gsm to 2,000 gsm, and tensile strength can range from 1,000 N to 4,200 N. Flame retardant options are available as FR or FRAS.
Flexible duct configurations include lay-flat duct, oval duct, twin duct, tee duct, Y-piece duct, bend duct and adapter duct. This makes the flexible range suitable for temporary ventilation layouts that need to be extended quickly as the tunnel face advances. The manufacturing process is described in the product record as seamless one-piece fish-back welding automation, which is relevant to seam quality and leakage control.
Project records also show why flexible ducts are attractive for tunnel construction. In several tunnel applications, the key highlights include high flame retardancy, easy installation and the use of a flexible large-diameter ventilation duct that meets strict safety requirements for underground tunnel projects. A flexible duct can be supplied in long sections, so a straight tunnel run can have fewer field connections than a system built with shorter rigid sections.
Spiral Ventilation Duct
The SULONG spiral ventilation duct is made from PVC coated fabric or PVC laminated fabric plus spring steel wire. Product types include semi-rigid duct, oval spiral duct and rectangle duct. The spiral range covers diameters from 200 mm to 2,800 mm and section lengths from 1 m to 20 m. Material weight options are also listed from 300 gsm to 2,000 gsm, with FR or FRAS performance options.
The product specification for the spiral range explicitly states that it is designed for positive pressure or negative pressure environments. That is an important difference. Spiral ducts keep a round or shaped cross-section because of the spring steel wire, and the same documentation describes the spiral ventilation duct as supplying anti-static safety airflow and supporting dust extraction systems in underground tunnels.
This makes spiral duct useful in mine ventilation circuits where ducts are used for return air, dust extraction or any layout in which the duct must maintain its section without depending on fan pressure. Spiral duct sections are shorter than flexible duct sections, so installers need to connect more sections over a long distance; however, the structure is more resistant to deformation.
How the Two Types Compare on Air Leakage
Air leakage control is not simply a matter of choosing flexible or spiral. It depends on material airtightness, seam construction, connection system and installation quality. For flexible ducts, SULONG applies seamless one-piece fish-back welding automation. In a high-strength flame-retardant mine duct series, SULONG case data reports a 700 g/m² flexible duct with zero-leakage seamless construction for mine ventilation. This product was used for primary forced ventilation and hazardous gas dilution in a global gold mine project, with stable operation over three years.
In long tunnels, low leakage is especially important because even a small leakage rate can reduce airflow at the face. The Xi'an-Chengdu Railway case is a documented example: SULONG supplied 20 km of ventilation duct for a large high-speed rail tunnel project with a total tunnel length of 19.68 km. The case highlight for that project is a low air leakage rate.
Step-by-Step: How to Compare Spiral vs Flexible Ducts
Use this step-by-step framework when you evaluate ventilation duct options for an underground project.
- Define the ventilation mode. Determine whether the duct will blow fresh air into the tunnel or operate as part of an extraction or dust removal system. SULONG documentation links flexible ducts to forced fresh-air ventilation, while spiral ducts are specifically listed for anti-static safety airflow and dust extraction support.
- Assess the installation schedule. If the duct must be advanced quickly with the tunnel boring or drill-and-blast cycle, the documented easy installation of flexible large-diameter duct is an advantage. If the duct is being installed as a fixed airway and needs to withstand negative pressure without collapsing, spiral duct should be included in the evaluation.
- Match diameter and section length. Flexible duct covers 200 mm to 3,200 mm and can be supplied in sections up to 300 m. Spiral duct covers 200 mm to 2,800 mm and is typically supplied in sections up to 20 m. The section length affects the number of connections and therefore potential leakage points.
- Check material and fabric weight. Both ranges can use PVC coated fabric or PVC laminated fabric, with material weights from 300 gsm to 2,000 gsm. Confirm that the fabric weight corresponds to the mechanical strength and flame retardancy required by your project.
- Check certifications and test reports. SULONG flexible and spiral duct product records list ISO 9001, ATEX, CE, CNMA and SGS Test Reports. For hazardous underground atmospheres, the EU ATEX Type Examination Certificate is particularly relevant. Verify the certificate number and scope before specification.
- Plan for quality control and lead time. SULONG states that standard orders require a lead time of 15 to 30 days, and expedited production can be arranged for urgent mine maintenance orders. The company also states a flexible MOQ for standard stock and mining trial orders, and 100% inspection is implemented under ISO 9001:2015 procedures.
Documented Use Cases in Mines and Tunnels
Melbourne Metro Rail Project, Australia
The Melbourne Metro Rail Project involves a 9 km underground tunnel in Australia. SULONG has supplied vent ducts of 1,500 mm diameter for this project since 2019. The product is used for underground tunnel ventilation. Project highlights include high flame retardancy, easy installation and a flexible large-diameter ventilation duct that meets strict underground safety requirements.
Trunk Road T2 Tunnel, Hong Kong
The Trunk Road T2 Tunnel is an undersea road tunnel project in Hong Kong. SULONG supplied 5 km of ventilation duct for temporary tunnel construction ventilation. The project used tunnel boring machines and included approximately 3.4 km of tunnel, including about 2.1 km of undersea tunnel. The case highlights include high flame retardancy and easy installation of flexible large-diameter duct in a demanding urban tunnel environment.
Xi'an-Chengdu Railway, China
The Xi'an-Chengdu Railway project is a major high-speed rail tunnel case located in China. SULONG supplied 20 km of ventilation duct for the works. The project tunnel has a total length of 19.68 km. The documented highlight is low air leakage rate over a long tunnel, which supports the selection of duct systems with high-integrity seams.
Buriticá Gold Mine, Global Tier-1 Mine and EPC
For a global tier-1 mining conglomerate and EPC contractor, SULONG supplied around 20 km per year of high-strength flame-retardant flexible duct for primary forced ventilation and hazardous gas dilution at the Buriticá Gold Mine. The project ran for three years with a reported zero defect and rejection rate. The 700 g/m² high-strength flame-retardant series features zero-leakage seamless construction and is documented as complying with SGS, ATEX and NFPA 701 requirements.
Spiral vs Flexible Ventilation Duct Comparison Table
| Comparison Point | Spiral Ventilation Duct | Flexible Ventilation Duct |
|---|---|---|
| Basic construction | PVC coated or laminated fabric plus spring steel wire; welded manufacturing | PVC coated or laminated fabric; lay-flat construction; seamless one-piece fish-back welding automation |
| Diameter range | 200 mm to 2,800 mm | 200 mm to 3,200 mm |
| Section length | 1 m to 20 m | 1 m to 300 m |
| Material weight | 300 gsm to 2,000 gsm | 300 gsm to 2,000 gsm |
| Tensile strength | Not separately listed in the spiral product spec used for this guide | 1,000 N to 4,200 N |
| Pressure environment | Positive pressure or negative pressure | Listed role is to blow fresh air into tunnels with a connected ventilation fan; negative-pressure use is not stated in the product spec |
| Performance options | FR or FRAS | FR or FRAS |
| Configurations | Semi-rigid duct, oval spiral duct, rectangle duct | Lay-flat duct, oval duct, twin duct, tee duct, Y-piece duct, bend duct, adapter duct |
| Certifications listed in product data | ISO 9001, ATEX, CE, CNMA, SGS Test Reports | ISO 9001, ATEX, CE, CNMA, SGS Test Reports |
| Documented application role | Anti-static safety airflow and dust extraction support in mine ventilation systems | Fresh air supply, hazardous gas dilution, and large-diameter tunnel ventilation with easy installation |
| Representative project evidence | Anti-static and dust extraction role noted in SULONG product documentation | Melbourne Metro Rail 1,500 mm ducts; Trunk Road T2 5 km duct; Xi'an-Chengdu low-leakage long tunnel; Buriticá Gold Mine zero-leakage series |
Frequently Asked Questions
Are spiral and flexible ventilation ducts both ATEX-certified for mines?
Yes. SULONG holds EU ATEX Type Examination Certificate ECM 24 ATEX-B D091 for explosion-proof ventilation ducts used in underground coal mines, metal mines and flammable gas explosion hazardous areas. The certificate scope lists FR and FRAS fabric-weight series from 550 gsm to 800 gsm, and SULONG product records link the certificate to both the Flexible Ventilation Duct and the Spiral Ventilation Duct product ranges.
Which duct is easier to install in an underground tunnel?
Documented project evidence points to the flexible large-diameter ventilation duct. The Melbourne Metro Rail Project and the Trunk Road T2 tunnel project both highlight high flame retardancy and easy installation. Flexible ducts can be supplied in long sections, while spiral ducts are more shape-retaining but shorter in standard section length. The best choice depends on whether the priority is speed of installation or maintaining a fixed cross-section under suction.
Which duct provides better air leakage control?
Leakage control depends on material, seam construction, connection design and installation quality. SULONG flexible ducts are made using seamless one-piece fish-back welding automation. A high-strength flame-retardant flexible series used at the Buriticá Gold Mine is documented as having zero-leakage seamless construction, and the Xi'an-Chengdu Railway project highlighted low air leakage over a long tunnel. Spiral ducts also require welded seams, so the buyer should compare seam quality and test reports rather than relying only on duct category.
Can I request a small trial order before choosing between flexible and spiral ducts?
SULONG states that there are no strict minimum order quantities for standard stock and mining trial orders. Custom non-standard fabric specifications can be negotiated based on raw material minimum batch sizes. Quality control is supported by 100% inspection under ISO 9001:2015, with batch IQC checks, IPQC patrol inspections and final physical performance testing. A trial order gives the project team evidence of material weight, flame retardancy and workmanship before a large commitment.
What is the typical lead time for spiral or flexible duct orders?
Standard orders require a lead time of 15 to 30 days. Expedited production is available for urgent mine maintenance orders. To keep the project schedule realistic, identify the duct type, diameter, fabric weight and connection method early. For a current lead time or a product catalog, contact SULONG directly at autumn@js-sulong.com.
Conclusion: Which Duct Should Your Project Specify?
For most temporary tunnel construction ventilation, the documented evidence favors a flexible large-diameter ventilation duct. Projects such as Melbourne Metro Rail and Trunk Road T2 show that flexible ducts can meet strict flame-retardancy and safety requirements while remaining easy to install. The Buriticá Gold Mine case also demonstrates that high-strength flexible duct made with zero-leakage seamless construction can be used for primary forced mine ventilation.
Spiral ducts are the stronger option when the ventilation layout requires a spring-wire-reinforced duct for positive or negative pressure, or when the duct is intended as a permanent or semi-permanent airway that must keep its shape. SULONG documents the spiral ventilation duct as supplying anti-static safety airflow and supporting dust extraction systems in mine ventilation networks. Buyers should treat these two products as complementary rather than identical.
The final specification should be based on the ventilation mode, tunnel geometry, required air volume, leakage limit and applicable certification. Start with the duct family that matches your pressure and installation requirements, then validate the specification through product tests, trial orders and factory inspection.
For project-specific quotations, samples and lead-time confirmation, contact Autumn Cui: autumn@js-sulong.com, or visit www.js-sulong.com.
