How to Choose a Side Channel Blower
Most wrong blower orders trace back to one number that was never pinned down: the duty point. These are the five checks we run on every enquiry before a model is quoted.
Step 1 — Define the duty point, not just the flow
A blower is specified by two numbers that both have to be true at the same time. A quote built on one of them is how undersized machines get sold.
Airflow alone says nothing. A blower that delivers 300 m³/h at 100 mbar will deliver a fraction of that at 400 mbar — the machine has one curve, and your process sits at one point on it. That point is the duty point.
| What is usually sent | What is missing | What we ask for instead |
|---|---|---|
| "I need 500 m³/h" | At what pressure or vacuum? | Flow at the operating pressure, or the two numbers separately |
| "Give me a 5.5 kW unit" | Power is an output, not an input | The duty point — we size the motor from it |
| "It is for a vacuum table" | How deep a vacuum, and at what leakage rate? | Target vacuum in mbar plus the flow at that vacuum |
| "Same as the one we have" | Is the old unit actually correct? | Nameplate data, measured flow, and what the process needs now |
| "For aeration, 3 m deep" | Water depth gives pressure; load gives flow | Depth, tank volume, and diffuser type |
The duty point is the only thing that lets us put you on the right curve instead of on the nearest available size.
Step 2 — Convert the units before you compare anything
Half of the model mismatches we see are unit errors, not engineering errors. These are the conversions that matter for blowers.
| Quantity | Conversion | Note |
|---|---|---|
| Volume flow | 1 m³/h = 0.2778 l/s = 0.5886 cfm = 16.67 l/min | cfm is the most common mistake source — it is 1.7× larger than m³/h |
| Pressure / vacuum | 1 mbar = 0.1 kPa = 0.4015 inH₂O = 0.0145 psi | kPa and mbar are the same scale, one decimal place apart |
| Vacuum depth | 1 mmHg = 1.333 mbar | Medical and lab specifications often arrive in mmHg |
| Percentage vacuum | −50 % vacuum ≈ −506 mbar at sea level | Percent is relative to local atmospheric pressure, so it changes with altitude |
| Temperature | 0.1 bar ≈ 1 m of water column | Handy on site: 1 m of water is about 100 mbar |
Convert both numbers to the same system before you compare two quotes. A "500" from one supplier and a "300" from another may be the same machine.
Step 3 — Let the pressure band narrow the field
Side channel, centrifugal and Roots blowers live in different pressure ranges. Knowing your band removes most of the catalogue before you read a single curve.
| Pressure band | Typical duties | Technology that fits |
|---|---|---|
| Up to ~130 mbar | Aeration, air knives, light conveying, vacuum lifting | Single-stage side channel (2RB) |
| ~130–300 mbar | Pneumatic conveying, plating agitation, drying, hold-down | Side channel (2RB / 4RB) |
| ~300–650 mbar | Pressure drying, dense-phase conveying, high-vacuum hold-down | Multi-stage side channel (4RB) |
| ~650 mbar – 1 bar | High-pressure blowing, gas boosting, combustion air | High-speed centrifugal (CX / PF / TB / HTB) |
| 0.5 bar and above | Wastewater aeration, gas transfer, bulk conveying | Roots blower (NSR series) |
These bands overlap on purpose — the choice inside a band is made on efficiency, noise and duty cycle, not on pressure alone.
Step 4 — Check the supply, the ambient and the derating
A model that looks correct on the curve can still be the wrong machine if the power supply or the site conditions were not declared.
- Phase and voltage: single-phase (115 / 230 V) is practical to about 2.2 kW — above that, three-phase is smaller, cheaper and more efficient
- Wide-voltage three-phase units (200–480 V, 345–720 V) suit markets where the supply varies between sites or over time
- Ambient temperature: ratings are quoted at 40 °C. Above that the available power falls, so the frame must be selected on the derated figure, not the nameplate
- Site altitude: above roughly 1 000 m the air is thinner, so a volumetric machine moves less mass for the same shaft speed — declare the altitude
- Suction-side filter: a filter adds pressure loss that belongs in the duty point. Specify the clean pressure drop and the change-out drop
- Duty cycle: continuous duty and frequent start-stop need different motors and different thermal margins
Step 5 — Verify on the curve, then add margin — but not too much
The last check costs nothing and prevents most returns: put the duty point on the published curve at your actual voltage and frequency.
A correctly sized blower has its duty point sitting on or just below the curve, never at the extreme end where the curve turns down. At the far left the machine is operating near its pressure limit: flow becomes unstable, temperature rises and the motor draws more current for less air.
A 10 % flow margin is normal practice and covers filter loading and small leakage. Beyond about 20 % you are paying for air you never use — and, because these machines draw their highest current at high pressure, an oversized blower on a throttled duty can consume more energy than the correctly sized one.
- Free-air delivery is not what arrives at your process — subtract filter, piping and silencer losses
- If flow must be adjustable, say so at RFQ stage: a frequency inverter changes the derating and the mechanical limits
- For vacuum duties, tell us the leakage rate; systems that hold a vacuum at zero flow need a different selection than continuous extraction
- Ask for the curve at your voltage and frequency, not the nominal one
Frequently Asked Questions
What is a duty point on a blower?
The duty point is the combination of airflow and pressure (or vacuum) at which the blower has to operate at the same time. Every blower has a performance curve; the duty point is where your process sits on that curve. Two numbers, not one — a flow quoted without a pressure is not a specification.
Can I choose a blower by its kW rating alone?
No. Power is a result of the duty point, not an input to it. The same 2.2 kW motor can be fitted to machines with very different flow and pressure characteristics, and two suppliers quoting "2.2 kW" may be offering machines that cannot both do your job. Always compare the curve at your duty point.
How much safety margin should I add?
Around 10 % on flow is standard and sensible. More than 20 % usually means you are buying capacity you will not use, and because a side channel blower draws its highest current near its maximum pressure, an oversized unit running throttled can use more energy than a correctly sized one.
Do I need a filter on the suction side?
Yes, in almost every application. Dust erodes the impeller clearances that produce the pressure, and the clearance is what the performance depends on. Include the filter's pressure drop in your duty point, and replace the element when the drop reaches the figure in the manual — usually 25 mbar.
Single-phase or three-phase?
Single-phase at 115 or 230 V works up to about 2.2 kW and is convenient for small workshops and lab equipment. Above that, three-phase is smaller, more efficient and cheaper for the same output. If your site has both, choose three-phase.
What do I need to send for an accurate quote?
Flow and pressure (or vacuum) at the same operating point, the supply voltage and phase, the ambient temperature where it will run, the application, and the duty cycle. A photo of the old nameplate helps if you are replacing a unit. That is enough for us to come back with a model and a price, usually within 24 hours.
Send Us Your Duty Point
Airflow, pressure or vacuum, voltage and application are enough for us to come back with a model and a price — within 24 hours.
