How to Size a Flowbar Diffuser: Airflow per Metre, Throw and Noise
To size a Flowbar diffuser, divide the zone airflow by the available slot length, then check slot velocity, throw and noise against the manufacturer's data. A worked example and the data to ask for.
To size a Flowbar diffuser, divide the zone's supply airflow by the length of slot available to get airflow per metre, then check the slot velocity, throw and noise against the manufacturer's data for the model. Goodwind's Flowbar (model GT-H-SLT) is built to a specific shadow gap or reveal detail, so the slot length is set by the architecture before the airflow is checked.
What Is the Sizing Sequence for a Flowbar?
| Step | What you work out | Where the input comes from |
|---|---|---|
| 1 | Zone supply airflow | The load calculation and the supply temperature difference |
| 2 | Available slot length | The shadow gap or reveal length on the architectural drawings |
| 3 | Airflow per metre of slot | Zone airflow divided by slot length |
| 4 | Slot velocity | Airflow divided by the free opening area of the slot |
| 5 | Throw, pressure drop and noise | The manufacturer's data at your airflow |
| 6 | Connection points | The manufacturer's recommended plenum and connection arrangement |
How Do You Calculate Airflow per Metre of Slot?
Airflow per metre is the total supply airflow divided by the total slot length. Slot velocity is the supply airflow divided by the free opening area of the slot. The zone airflow itself comes from the room's sensible load, as set out in our air distribution design guide.
As an illustration, a zone with a 6 kW sensible load supplied 10 K below room temperature needs about 500 L/s. If 12 m of Flowbar runs along the reveal, the airflow per metre is 500 ÷ 12, or about 41.7 L/s per metre. If the effective opening is assumed to be 20 mm wide, the free area is 12 × 0.020 = 0.24 m², so the slot velocity is 0.5 ÷ 0.24, or about 2.1 m/s.
The 20 mm width is an assumed figure for this example only, not a Goodwind dimension. The real opening width and free area come from Goodwind's data for the model.
What Affects Throw and Noise?
Throw is the distance the air travels from the slot before its speed falls to a stated terminal velocity, and data sheets commonly quote it at set values such as 0.25 m/s. In general, a higher slot velocity lengthens the throw but adds noise and pressure drop, so the three have to be checked together at your design airflow. Our glossary entries on throw, air drop and the Air Diffusion Performance Index (ADPI) explain the terms.
How Long Can One Run Be, and How Many Connections Does It Need?
That depends on the plenum and connection arrangement, so the maximum run length, connection spacing and plenum size should come from the manufacturer for your reveal detail, not from an assumed standard run. In general, a long slot fed from a single point can discharge unevenly if the plenum is not designed for it. Ask Goodwind for the recommended arrangement before fixing the run lengths.
What Should You Send Goodwind to Size a Flowbar?
- The length of each shadow gap or reveal run
- The architectural drawing of the reveal detail
- The supply airflow for the zone served by each run
- The ceiling height and the use of the room
- The finish: powder coated, anodized, or a custom color
How to Size a Flowbar on a Project
For MEP Consultants at the Design Stage
Show Airflow per Metre on the Schedule
List the zone airflow, the slot length and the resulting airflow per metre for each run, so a substitute can be checked against the same figures.
Check Slot Velocity Against the Noise Target
Compare the slot velocity and the manufacturer's noise data with the room's noise target before fixing the run lengths.
For Contractors at Procurement Stage
Match Run Lengths to the Drawings
Check each ordered run against the reveal length on the drawings, since the Flowbar integrates with a specific detail and not a standard module.
Confirm Connection Points Before Ordering
Confirm where the duct connections sit behind each run so they can be coordinated before the ceiling closes. Our Flowbar installation and coordination guide sets out the sequence, and how a hidden slot diffuser works explains the air path.
FAQ
How do you calculate airflow per metre for a slot diffuser?
Divide the total supply airflow by the total slot length. For example, 500 L/s over 12 m of slot is about 41.7 L/s per metre.
What is slot velocity?
Slot velocity is the supply airflow divided by the free opening area of the slot. It affects throw, pressure drop and noise, so check it against the manufacturer's data for the model.
Does a Flowbar come in standard lengths?
The Flowbar is built to integrate with a specific ceiling detail rather than a standard ceiling grid module, so confirm the dimensions with Goodwind against your drawings.
How many Flowbar runs does a room need?
It depends on the zone airflow, the room layout and the reveal lines in the ceiling design. Divide the zone airflow across the available slot length and check the result against the data for the model.
What data should I ask for to size a Flowbar?
Ask for the free opening area, throw, pressure drop and noise level at your design airflow per metre, plus the recommended plenum and connection arrangement.
About Goodwind
Goodwind Technologies Private Limited is an HVAC air distribution products manufacturer based in Coimbatore, Tamil Nadu, India. Its range includes grilles, diffusers, louvers, dampers and ductwork accessories. Contact Goodwind with your reveal drawings and airflow, or read the Flowbar specification guide.
Last reviewed: October 2026.