Technical Deep Dive: Centrifugal Jet Fan vs. Axial Jet Fan for Underground Ventilation

Table of Contents

• 1. Fundamental Mechanical Architecture

• 2. Headroom & Clearance Requirements (The Height Battle)

• 3. Airflow Velocity, Thrust & Throw Distance

• 4. Acoustic Performance and Noise Control

• 5. Side-by-Side Comparison Matrix

• 6. Application Engineering: Which Fan Should You Specify?

Ventilation for underground parking garages or tunnels is not as simple as just selecting a fan with sufficient airflow.In actual projects, engineers often face three conflicting requirements: limited ceiling clearance, strict noise requirements, and insufficient airflow behind beams, columns, ramps, or other obstacles.

Although traditional fans can provide sufficient airflow at the fan outlet, air stagnation zones may still exist in other areas of the building. The problem becomes even more complex when the system must operate during a fire: airflow must be controlled to direct smoke to designated exhaust points without obstructing evacuation routes.

For this reason, jet fan ventilation has emerged as an important alternative.

Jet fans utilize high-speed airflow to induce and propel the movement of a larger volume of surrounding air. Unlike distributing the entire ventilation airflow through large ductwork, strategically placed jet fans can create airflow pathways in open spaces. The AMCA notes that jet fan systems reduce the need for ductwork and allow fans to be installed between structural members, thereby helping to reclaim overhead space.

But should axial-flow jet fans or centrifugal jet fans be used in a project?

This article will delve into the technical differences between centrifugal and axial-flow jet fans and explain how to select the appropriate type for underground parking garages, tunnels, and smoke control systems.

1. Fundamental Mechanical Architecture

The primary difference between centrifugal and axial jet fans lies in how the impeller directs airflow through the casing.

Axial Jet Fans (Straight-Through Airflow)

Axial jet fans operate on a simple inline design: air enters the cylindrical casing horizontally and exits along the same axis.

• Impeller: Aerodynamically pitched blades configured around a central motor hub.

• Flow Path: Parallel to the fan shaft.

• Key Characteristic: High airflow volume and direct, concentrated high-velocity jet streams. However, to accommodate the impeller diameter and built-in silencers, axial casings are typically round and deep.

Centrifugal Jet Fans (90-Degree Low-Profile Airflow)

Centrifugal jet fans draw air in from the bottom or rear and discharge it laterally at a 90-degree angle using a specialized curved casing.

• Impeller: Backward-curved or forward-curved radial blades.

• Flow Path: Radial discharge flattened into a high-momentum planar sheet of air.

• Key Characteristic: Ultra-flat, low-profile mechanical design engineered specifically to hug slab ceilings.

2. Headroom & Clearance Requirements (The Height Battle)

In modern underground car park design, every inch of ceiling height counts. Structural beams, sprinkler networks, cable trays, and drainage pipes fight for space under the concrete slab.

• Axial Jet Fans: Because axial units require cylindrical silencers on both ends to manage noise, total unit heights typically range from 400 mm to 600 mm or more. In low-ceiling basements, installing axial fans may force developers to deepen excavation or lower car clearance levels, drastically increasing civil construction overhead.

• Centrifugal Jet Fans: Centrifugal units feature a flat profile, with overall heights often under 250 mm - 350 mm. They can be installed directly flush against the ceiling slab, clearing major ceiling beams and easily complying with strict 2.2 m or 2.4 m vehicle height clearance codes.

3. Airflow Velocity, Thrust & Throw Distance

Impulse ventilation relies on thrust (measured in Newtons, N) to drive air across wide, un-ducted floor plates.

• Axial Jet Fans (Long Range Thrust): Axial fans excel at generating a high-velocity, cylindrical core of air. They provide high thrust output per unit size, making them capable of "throwing" air over long distances (30 m - 50 m). They are ideal for long, straight driving lanes or linear tunnel passages.

• Centrifugal Jet Fans (Wide Area Induction): Centrifugal jet fans discharge air as a wider, flat air jet stream. While their maximum throw distance is slightly shorter (20 m - 35 m), their broad discharge profile prevents dead spots in wider, irregular basement layouts, efficiently entraining larger volumes of stagnant ambient air.

4. Acoustic Performance and Noise Control

Noise levels are a major concern in commercial complexes, residential basements, and mixed-use developments.

• Axial Jet Fans: Operating at higher rotational speeds with direct blade tip turbulence, axial fans generate significant noise in the mid-to-high frequency bands. They require heavy acoustic baffling (sound attenuators) on both inlet and outlet ports to meet standard NR-55 / NR-60 acoustic limits.

• Centrifugal Jet Fans: The centrifugal impeller design naturally dampens high-frequency blade pass noise. Air decelerates inside the scroll housing before discharge, resulting in significantly lower overall decibel levels (dB(A)) and a smoother sound profile without requiring bulky external attenuators.

5. Side-by-Side Comparison Matrix

6. Application Engineering: Which Fan Should You Specify?

Specify an Axial Jet Fan when:

1. You are ventilating long, straight road tunnels, transit corridors, or deep warehouse bays.

2. Ceiling height is generous, and structural beams do not restrict vertical clearance.

3. Maximum thrust force (N) is required to throw air over extreme linear distances.

Specify a Centrifugal Jet Fan when:

1. You are designing low-clearance underground commercial or residential car parks.

2. The slab is crowded with HVAC ducts, sprinkler pipes, and electrical cable trays.

3. Acoustic constraints require ultra-quiet operation without bulky external silencers.

4. You need to eliminate air dead-zones across wide, non-linear floor plates.