A chilled beam cools a space by circulating chilled water (typically 14–18°C) through a coil in a ceiling-mounted beam; active beams add a small primary-air fan (10–50 Pa, 20–60 L/s) while passive beams rely on convection only. Both cut fan energy 30–50% versus VAV because they move water, not large air volumes, for cooling, which is fundamentally more efficient per watt of delivered capacity.
Active vs Passive Beams
Passive beams have no fan: warm room air rises, hits the cold coil, cools, and falls by convection, handling 100–200 W/m² of sensible load with zero fan power. Active beams induce room air with a primary-air jet at 10–50 Pa, boosting capacity to 200–400 W/m² and also delivering ventilation. Active beams use 20–60 L/s of primary air each, far less than a VAV box’s full volume, which is where the fan-energy saving comes from.
Passive beams are nearly silent and ideal for libraries, bedrooms, and labs where any air noise is unacceptable. Active beams trade a little sound for much higher capacity and the ability to also meet the ventilation code through the primary air, making them the default for open offices and classrooms where 200–400 W/m² is the typical sensible load.
Water and Air Temperatures
Chilled water enters near 14–18°C with a 2–4°C rise across the coil, so the chiller runs at a high evaporating temperature and COP improves 10–20% over 7°C chilled-water VAV systems. Supply-air temperature for active beams is 17–20°C. Because beams handle only sensible load, keep space humidity below 60% RH or condensation forms on the coil—a key limit that dictates the outdoor-air treatment strategy.
Comparing Beam Types
| Type | Primary air (L/s) | Sensible capacity (W/m²) | Fan energy | Best use |
|---|---|---|---|---|
| Passive beam | 0 | 100–200 | None | Low-load, dry climates |
| Active beam | 20–60 | 200–400 | Low (10–50 Pa) | Offices, schools |
| Multi-service beam | 20–60 | 200–400 | Low | Labs, hospitals |
| VAV (reference) | Full volume | varies | High | Variable loads |
Key Design Numbers
Plan beam spacing on a 3–6 m grid covering 9–25 m² each; a 500 m² office at 80 W/m² needs about 40 kW, met by 13–20 active beams at 250 W/m². Primary-air duct pressure is just 10–50 Pa versus 250–500 Pa for VAV, so the AHU fan is far smaller. Keep chilled-water flow 0.05–0.15 L/s per beam and control coil valve ±1°C for stable rooms without short-cycling the chiller plant.
Worked Example: A Classroom Block
A 600 m² school floor at 90 W/m² needs 54 kW. We specify active beams at 300 W/m², so 180 m² of beam length serves it—about 24 beams on a 4 m grid. Each draws 30 L/s primary air (total 720 L/s, under 1 m³/s) at 30 Pa. The AHU fan is a fraction of a VAV equivalent. Chilled water at 16°C with a 3°C rise needs 54,000 / (4.18 × 3000) ≈ 4.3 L/s flow per circuit, easily pumped with a tiny circulator.
Decision Checklist: Choose Beams When
- Choose passive beams in dry, low-load spaces (RH under 50%) where you want zero fan energy and silent operation for concentration or rest.
- Choose active beams in offices and schools needing both cooling and ventilation at 30–50% lower fan energy than VAV boxes moving full volume.
- Choose multi-service beams in labs/hospitals where lighting, sprinklers, and air merge into one ceiling module for maintenance access.
- Choose VAV instead when latent load is high or humidity regularly exceeds 60% RH and you cannot afford a dedicated outdoor-air unit.
Condensation and Controls
The beam’s hard limit is dew point: if room dew point rises above the coil surface temperature (often 13–16°C), water drips. Size the primary air to hold RH under 60% and add a dew-point cutoff that closes the valve when risk appears. Poor humidity control is the main reason beams fail, so pair them with a capable outdoor-air treatment train rather than relying on the beam itself to manage moisture.
Standards and Commissioning
Follow ASHRAE 62.1 for primary-air ventilation through active beams, and size the dedicated outdoor-air unit to hold space RH under 60% per ASHRAE 90.1 efficiency targets. Commission by measuring supply and return dew point, confirming the cutoff logic, and verifying beam capacity within ±10%. Log coil surface temperature against room dew point for the first month through the humid season.
Equipment Sourcing
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. The low-pressure primary-air AHU for active beams pairs well with stable 4-79 supply fans rated to the 10–50 Pa duty, and the factory G2.5 balance keeps the small primary fan quiet enough for the classrooms and offices where beams are most often specified.
Control Sequences and Integration With BMS
A chilled-beam system is only as good as its control sequence, because the beam itself has no active modulation—it responds to the water valve and the primary-air volume. The typical sequence modulates the chilled-water valve to hold space temperature at 22–24°C, while a separate loop holds the primary-air dew point low enough to keep RH under 60%. The two loops must be coordinated in the BMS so the valve does not open into a space that is already humid, which would condense on the coil and drip despite correct water temperature.
For active beams, the primary-air volume is usually constant (the small fan runs at fixed speed), so control authority comes entirely from the water side; size the valve for the full 200–400 W/m² range with ±1°C control. Integrate a dew-point cutoff that closes the valve when room dew point approaches the 13–16°C coil surface, and alarm on it rather than silently letting the space drift warm. A beam system that loses its humidity interlock is a leak waiting to happen, so the BMS points—valve position, supply and return dew point, and alarm status—should be trended through at least one full humid season before sign-off.
FAQ
Active or passive chilled beam?
Passive beams use no fan and handle 100–200 W/m² by convection—ideal for dry, low-load rooms needing silence. Active beams induce room air with 20–60 L/s of primary air at 10–50 Pa, reaching 200–400 W/m² and also delivering ventilation. Choose active for offices and schools needing both cooling and fresh air through the same ceiling device.
Why are beams more efficient?
Beams move chilled water (14–18°C) instead of big air volumes; the AHU fan only handles primary air at 10–50 Pa versus 250–500 Pa for VAV, cutting fan energy 30–50%. The warmer chilled water also lifts chiller COP by 10–20% over 7°C VAV systems. Water carries far more energy per watt than air, which is the root of the efficiency gain.
What is the condensation risk?
If room dew point exceeds the coil surface temperature (about 13–16°C), condensate drips. Keep RH under 60% with adequate primary-air dehumidification and fit a dew-point cutoff that shuts the valve on risk. High-humidity sites above 60% RH should use VAV or add dedicated outdoor-air treatment, because beams manage sensible load only and cannot remove moisture themselves.
How many beams per room?
Space beams on a 3–6 m grid, each covering 9–25 m² at 250 W/m² for active types. A 500 m² office at 80 W/m² (40 kW) needs roughly 13–20 active beams. Primary-air duct pressure is only 10–50 Pa, so the AHU fan is much smaller than a VAV equivalent, and each beam needs just 0.05–0.15 L/s of chilled-water flow to perform.