QIYUE FAN · MULTIPLE-FAN PROCUREMENT

Fans in Parallel or Series: What to Confirm Before Ordering

“Add a second fan” is not a complete specification. First identify whether the units share a common system, operate one after another, or serve separate zones. Then ask for the evidence for each intended operating case.

Buyer takeaway: distinguish a theoretical combined curve from a verified installed duty. A second fan is not an automatic promise of twice the airflow, twice the usable pressure or full standby capacity.

Two different arrangements, two different questions

Parallel

Fan A branch + Fan B branch
↓
Common system

Ideal combined curves add the individual flow rates at a common pressure. The operating point still depends on the system curve; each fan must remain within its permitted stable range.

Ask: what does each fan deliver when both run, and what changes when only one runs?

Series

Air path → Fan A → Fan B

Ideal combined curves add pressure rises at a common flow. Connecting losses and disturbed inlet flow can reduce the actual benefit, so simply adding catalog pressures is not an installation guarantee.

Ask: what combined performance is supported for the proposed connecting arrangement?

Conceptual relationship diagrams only. They omit ducts, dampers, controls and protective equipment; they are not installation drawings or measured curves. Technical basis: ASHRAE Handbook, Fans, sections 11–12.

Do not turn “two installed” into “one fully redundant”

Parallel operation may support changing demand or partial operation during an outage. The remaining capacity is project-specific. Idle branches also need review to prevent recirculation; branch differences can upset load sharing. The DOE/AMCA Fan System Sourcebook, Multiple-Fan Arrangements discusses these benefits and limitations.

For procurement, replace the word “backup” with a written requirement: which process must continue, at what duty, for which unavailable unit? Ask the system designer and supplier to confirm the answer. This guide does not authorize operating a safety-critical system with equipment unavailable.

Request a case register, not just a two-fan total

Our suggested review format keeps the request and the supplier’s returned evidence together. List only cases required by the approved project design; do not invent a switching sequence.

Normal demand

Name the fans running, required system duty and each fan’s proposed operating point. Reference the selected configuration and curve revision.

Reduced demand

Identify the permitted fan combination and control mode. Record the supplier’s operating-range assessment and unresolved conditions.

One unit unavailable

Identify the unavailable tag, required remaining duty and whether that requirement is confirmed, restricted or still open.

Transition review

Name the qualified party responsible for start/stop logic, interlocks and acceptance. Request the approved control document; do not infer settings from this article.

Keep equipment and control responsibilities explicit

Request a marked layout showing common connections and individual branches.
List dampers, actuators, sensors, drives and controls as included, optional, excluded or by others.
Assign the designer of idle-branch isolation and its interfaces; confirm the selected equipment rather than assuming a generic damper is adequate.
Record who confirms the combined performance, motor loading, installation clearances and commissioning evidence.

A pair of fans serving independent cooling zones is not automatically a parallel common-system installation. Likewise, a double-inlet fan is a single fan arrangement, not evidence of two independently available units.

Copy this multi-fan RFQ record

Project / system reference:
Purpose: capacity / variable demand / standby / other:
Arrangement and layout drawing revision:
Fan tags, models and quantity:
Operating cases required by the project:
Duty and flow / pressure basis for each case:
Gas conditions and curve reference:
Required duty with a specified unit unavailable:
Branch, connecting-duct and accessory scope:
Control / isolation design owner:
Supplier's returned case-by-case evidence:
Open issues, owner and decision reference:

Mark unknowns as pending. Compare offers against the same case register before comparing price. Use the fan curve guide for performance boundaries and the quotation comparison guide for supply exclusions.

Discuss a multi-fan duty

Send the system layout, operating cases and available fan curves. Project-specific review is needed before configuration or operating changes are approved.

Send your multi-fan project data →