Fans in series: pressure at a common flow
In a series arrangement, the air passes through one fan and then another. At a chosen flow, the fans’ total-pressure contributions can be combined to construct a combined characteristic, allowing for the actual arrangement and losses.
For an illustrative pair each providing 300 Pa total pressure at 500 L/s, simple curve addition gives 600 Pa at 500 L/s. This is a corresponding point on a combined curve, not a promise that an installed system will operate there.
Connections between the fans, air conditions and each fan’s permitted operating range need assessment. A poor inlet into the second fan can create system effect rather than the expected performance.
Fans in parallel: flow at a common pressure
In parallel, the fans provide separate paths that join into the system. At a chosen pressure, their flow contributions can be combined. Two illustrative fans each delivering 500 L/s at 300 Pa total pressure produce a combined-curve point of 1,000 L/s at that pressure.
The actual operating point is where the combined fan curve meets the system curve. If the same duct system resists the higher flow more strongly, the resulting increase in airflow will be less than simply doubling the original installed flow.
Use the full curves, not just one rated point. Unequal fans or unequal branch losses can distribute flow differently from the intended arrangement.
Compare the arrangements at the required duty
| Question | Series | Parallel |
|---|---|---|
| What is combined? | Pressure at a common flow. | Flow at a common pressure. |
| Key connection issue | The flow delivered to the next fan. | Flow sharing and branch interaction. |
| One fan off | Resistance and behaviour through the stopped unit. | Backflow or bypass through the inactive path. |
Neither arrangement is automatically a standby system. Define the required performance with one fan unavailable and assess whether that duty can actually be met.
Check stability and control transitions
Assess the range of normal operation, not only the maximum duty. Starting or stopping one fan changes the combined characteristic. The control sequence needs to avoid unsuitable operating conditions and manage any isolation devices.
Review stall and surge where the fan curves or changing system resistance suggest instability. Noise, pressure fluctuations or reverse flow are not acceptable substitutes for a designed operating sequence.
The designer should consider what happens during a fan fault, loss of power and restart. The laboratory needs a clear indication of available extraction and an appropriate response if the required duty cannot be maintained.
Keep the chemical-extraction design complete
Each fan and connection must suit the extracted substances, temperature and environment. Include access for inspection, cleaning and replacement, together with the isolation and decontamination arrangements needed for service.
Document the curves, pressure basis, airflow schedule, controls and accepted settings. Commission the planned operating modes and retain the results. A multiple-fan arrangement can be more complicated to maintain and troubleshoot than a single correctly selected unit.
For a new proposal, first establish whether a single Fumeflow fan selection can meet the required duty. Where the project needs another arrangement, give the designer the full operating and resilience requirements before deciding how many fans to use.
Questions about fans in series and parallel
Will two parallel fans double my existing airflow?
Not necessarily. The operating point changes where the combined fan curve meets the system resistance curve. Branch losses and fan interaction also matter.
Does having two fans provide automatic backup?
Only if the system is designed and verified for the required one-fan-off duty, including isolation and controls. Two installed fans alone do not establish redundancy.