Passive vs. Active Chilled Beams

Passive vs. Active Chilled Beams

By Greg Cunniff, Applications Engineering Manager, Taco, Inc.

Chilled beams are available in three variations: passive, active and integrated/multiservice beams. The difference between passive and active beams focuses on the way airflow and fresh air are brought into the space. Both of these systems are now enjoying significant attention here in the U.S.and Canada.

Passive Chilled Beams - natural convection.

Passive Chilled Beams - natural convection.

Passive chilled beams require ventilation air to be delivered by a separate air-handling system. With active chilled beam systems – sometimes referred to as “induction diffusers” – a building’s ventilation air is continuously supplied to chilled beam terminal units by a central air-handling system. Ventilation is cooled or heated to partially handle temperature –driven sensible loads, while in summer it is sufficiently cooled and dehumidified to handle all of the internal moisture-driven latent loads. With active chilled beams, chilled beam air is introduced into the space through a slot diffuser creating a Coanda effect – that is, the tendency of a fluid jet to be attracted to a nearby surface like the ceiling.

Active Chilled Beams - forced convection.

Active Chilled Beams - forced convection.

Inducing warm air to blow through the chilled beam coil substantially increases its capacity. Active chilled beam capacities (in the range of 350 to 600 Btuh/sq. ft.) get a boost in capacity from the primary air from the DOAS. Depending on the temperature and the quantity of this primary supply air, the capacity can amount 300 Btuh/sq. ft. An active chilled beam can deliver from 500 to 900 Btuh sq. ft. between the chilled coil and the primary air.

Primary/ventilation is introduced into the active chilled beam through a series of nozzles. This induces room air into the chilled beam and, in turn, through a water coil. Induced room air is cooled and/or heated by the water coil and then mixed with ventilation air and released, which controls room temperature.

Next up…More on active chilled beams.

4 Responses

  1. what about condenseted water accumalated drain pan and it will affect to grown funges and blocks drain water after al that drain water will come out above the false cealing or the motor will strike for pump the drain water

  2. Thank you for the question!

    Chilled beams typically are sensible cooling devices and do not have drain pans.

    – Greg

    • As sensible cooling “machines”, the ability of chilled beams to condition the space is will be limited by the surface area provided and the overall heat transfer coefficient. The surface area is constrained by the available capital for investment in the chilled beam apparatus. The overall heat transfer coefficient will be limited by the flow rate of the induced room air. The efficacy of the inducer is usually quite poor unless the jet velocity is high. A Second Law analysis should be performed to determine the relative merit of chilled beam cooling vs variable air volume cooling.

      • Thank you for joining the discussion.

        The central air system airflow supplied to a chilled beam is approximately 1/10 of that needed for a VAV box or fan coil. However the air flowing over the coil in a chilled beam is only half because the induction ratio of a chilled beam is 4 or 5 to 1. Manufacturer’s publish the capacity of their chilled beams and the design engineer can select the required number of units to satisfy the sensible load.


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