One Large Axial Fan or Several Small Ones? Redundancy, Efficiency and Maintenance Compared

By the NextAir Systems engineering team. Last reviewed October 2026.

Factory and tunnel ventilation can usually be done with one very large fan or a bank of smaller ones, and the quotation for the large fan is usually the cheaper of the two. The cheaper quotation is not always the better installation. The choice trades efficiency, redundancy, maintenance access and handling, and the arithmetic of redundancy is surprising.

We split one duty across one to eight geometrically similar fans, state the efficiency assumptions, and show how the result changes when you require the plant to keep running with one fan out of service.

The short version

• For 150,000 m³/h at 250 Pa, one 1,600 mm fan at 960 rpm does the job. Four fans of the same family need 800 mm wheels at 1,920 rpm, with the same tip speed.
• A single fan that must be backed up needs 200% installed capacity. Six duty fans plus one standby needs about 117%, and eight plus one about 112%.
• Smaller fans are usually a little less efficient, since tip clearance and surface effects weigh more. We use an assumed penalty of 1.5 points per halving of diameter, which you should replace with supplier data.
• More fans mean more motors, bearings and starters, which raises the number of things that can fail but lowers the consequence of any one of them.

Same duty, different splits

Take a duty of 150,000 m³/h at 250 Pa. A single 1,600 mm fan at 960 rpm meets it, in this example family. If we split the flow across n geometrically similar fans, each carries 1/n of the flow at the same pressure. Similarity says the pressure fixes the tip speed, so each fan has the same tip speed and a diameter smaller by √n, running faster by √n. The table follows that logic.

Number of fans	Diameter each (mm)	Speed (rpm)	Assumed efficiency (%)	Total shaft power (kW)	Capacity with one fan out	Installed capacity for N+1 (percent of duty)
1	1,600	960	72.0	14.5	0%	200%
2	1,131	1,358	71.2	14.6	50%	150%
4	800	1,920	70.5	14.8	75%	125%
6	653	2,352	70.1	14.9	83%	117%
8	566	2,715	69.8	14.9	88%	112%

Efficiency is an assumption, with a loss of 1.5 percentage points for each halving of diameter from 1,600 mm. Real figures depend on the fan family and the supplier. The point is the shape: smaller fans cost a little in power, and the loss is much smaller than the saving that redundancy can bring.

The redundancy arithmetic

If the process must keep running at full duty when a fan is out of service, you install N+1 fans, each sized for 1/N of the duty. Each fan is sized for 1/N of the duty, so the installed capacity is (N+1) ÷ N of the duty.

Worked example. With one big fan, full redundancy means a second identical fan: 200% installed. With four duty fans plus one standby it is 125%, with six plus one about 117%, and with eight plus one about 112%. If the duty can fall to 75% on a failure, which many ventilation duties tolerate, four fans with no standby already give 75% on one failure. The number of fans is a way of buying exactly as much redundancy as the process needs.

The same logic applies to maintenance: with several fans you can take one out for service while the others carry the load, and you do not have to wait for a plant shutdown. With a single fan, every bearing change is a shutdown.

A cost check before you decide

The efficiency penalty in the table is small. For the 150,000 m³/h duty, going from one fan to eight raises shaft power by only about 0.4 kW in the model, a few percent. Against that, the extra motors, starters, cables and fan housings add to the capital cost, and the extra bearings add to the maintenance. The honest comparison is a short spreadsheet: capital cost of each arrangement, the annual energy at your hours and tariff, and the value of the redundancy to the process. If a stoppage costs more per day than the whole difference in capital, redundancy wins. If the process can pause, a single fan may be enough.

What favours one large fan

• Higher efficiency, as the table assumes, and less to maintain: one motor, one set of bearings, one starter.
• Lower first cost per unit airflow, in most cases.
• Less space for the fan housing and ducting in some layouts.
• A narrow duty range, where the fan runs at one point and redundancy is not needed.
• Lower sound power per unit airflow, in some designs. Check the data, because the count of fans adds noise, as shown in sound power to sound at the fence.

What favours several fans

• Redundancy at low extra cost, as above.
• Turndown by staging. Switching fans on and off follows a varying load closely, with each running near its best point. This competes with speed control, discussed in adjustable pitch versus VFD.
• Handling and access. A 1,600 mm fan with a heavy motor needs lifting gear that smaller fans do not.
• Layout. Banks of wall or roof fans fit buildings where one large opening does not.
• Phased installation. You can add fans as the load grows.

What to watch with several fans

Fans in parallel must be arranged so they do not interfere with one another, and a stopped fan in a bank can let air flow backwards through itself unless it has a backdraught damper. How the fans interact on a shared system is the subject of parallel operation of axial fans. Also keep them clear of walls and of each other, as the manufacturer advises, because crowding creates the inlet losses described in the system effect article.

Where this fits with NextAir

NextAir builds axial fans from small wall units to 1,600 mm and 185,000 m³/h, so we can quote either arrangement and show the redundancy trade-off. Send the duty, the tolerance for a failure and the layout to our axial fan manufacturer team. Our axial fan guide explains selection, and tunnels, metros and car parks is a duty where redundancy is often a requirement.

Sources

1. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.
2. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

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