The big question – active or passive?

A key design decision in every chilled beam project is whether active or passive beams will best suit the application. There is sometimes a preconception that an active chilled beam is just a passive beam with more bells and whistles. But, as the operating principles of the two are so different, we miss a trick as system designers if we overlook the specific benefits that each can bring depending on the requirements of the project.

The best outcome will depend on a number of factors, such as the comfort condition requirements, the architect’s intentions or other aesthetic considerations, the energy efficiency ambitions, the ceiling type, and the dimensions of the space.

Operating modes

The most obvious difference between active and passive chilled beams is the functionality. Passive chilled beams deliver cooling whilst active chilled beams are designed to provide both cooling and heating. The ventilation capabilities also vary. A passive chilled beam is not a ventilation device. An active chilled beam, however, delivers cooling by inducing room air through its coil by the help of the primary air supplied into the space through nozzles. Fresh air is delivered via an integral plenum.

The desired functionality of the beam may be enough to make up the system designer’s mind, but the decision isn’t always so clear cut. There could be considerations such as the usage patterns of the occupied zone, the required fresh air and cooling outputs, and specific demands of the site.

Changing levels of occupancy

The expected usage patterns of the occupied zone can have a significant impact.

Active chilled beams need to be sized according to fresh air volumes specified at the design stage. If the space is one in which occupancy levels are subject to extreme variations, the system designer may decide that the stepped control of cooling made possible by an active chilled beam does not give the desired levels of flexibility.

Passive chilled beam applications however, working together with a complementary ventilation system such as a floor mounted displacement system, will be able to cope with extreme variations in ventilation requirements in spaces.

In addition, whilst the passive beam is not a ventilation device, it provides increased flexibility for system designers, particularly for refurbishment projects, often with fresh air supplied into the space at low level.

In demand-based ventilation applications with a separate air supply, a cost-effective option can be to use passive chilled beams alongside a VAV air system. We have a number of extremely successful references for this product combination and can help system designers with air distribution design.

Tackling building services challenges

Another particularly effective application for passive chilled beam technology can be to install beams at the perimeter, near to glazing (see Figure 1). Passive chilled beams can be operated at higher outputs at the perimeter as they will not cause discomfort in the occupied zone. This has the advantage of maintaining comfort conditions in the internal occupied areas with downsized components and lower air change rates.

Hot air, from the vicinity of the glazing, rises and hits the coil of the passive chilled beam and moves downwards through the coil to provide cooled air. A cavity barrier prevents warm air ‘overshooting’ the beam. The result is the delivery of cooled air into the space without causing discomfort in the occupied zone.

Passive chilled beams can also suit environments where extraneous noise has to be eradicated. For example, Trox has worked on projects with passive chilled beams for sound recording studios.

High output at high efficiency

Active chilled beams, on the other hand, can excel where there are demands for higher cooling outputs.

Passive chilled beams (operating with natural convection) are not aerodynamic air distribution devices. So the combination of design and thermal output is paramount in achieving comfort conditions by means of velocities and temperatures. The limitation on how hard a natural convective device can be run, determines the maximum output of a passive chilled beam.

In contrast, active chilled beams can deliver much higher cooling outputs due to their air distribution characteristics. The Trox DID 632 active chilled beam, for example, has an output approaching 800W/m. Its high induction design gives a cooling capacity of up to 2.5kW with 70l/s primary air. A wide range of nozzles is available to suit specific applications. The chilled beam can handle a wide range of primary air volumes while still maintaining comfort conditions in the occupied zone. Adjustable control blades inside the chilled beam enable air direction to be set specifically to suit the space.

Practicalities

System designers, however, frequently find their decision dictated by practical considerations, such as the type and height of the ceiling. The operating principle of the beam will determine how and where it can be installed.

The Trox DID 632 active chilled beam is an example of how restrictions on site can be solved by good product design. Its dimensions make it suitable for installing in replacement for a standard ceiling tile. The unit height (210mm) has advantages for buildings with clear room heights of 2.6m to 4m, where there are space limitations on ceiling voids. It is perfect for applications where a large cooling capacity is required in a small space, as its adjustable horizontal air discharge can ensure the delivery of air into the occupied zone at acceptable air velocities, while also allowing flexibility for future partitioning.

Practicalities – passive beams

Passive chilled beams can be installed recessed as well as flush mounted or exposed. They are complemented by a central mechanical ventilation plant and air distribution equipment, to supply fresh air into the conditioned space. This allows flexibility in ventilation and system design, but could prove a limitation when accommodating a separate ventilation system. It is important that the air distribution design is done adequately, so that it doesn’t disrupt the operation of the chilled beams. Expertise and value engineering input is very important here to ensure the system works effectively.

The way in which a passive chilled beam is installed also plays a key role in its operation. The air gap between the ceiling and the beam, the design of the beam, and the type of ceiling, are all important factors needing careful consideration.

Practicalities – active beams

An active chilled beam directs air through nozzles into the space and there are some important design guidelines to ensure that comfort conditions are not reduced for those occupants l
ocated nearest to the beam when cooling loads are high.

On-going development of both passive and active chilled beams has gone some way towards removing these practical limitations. For example, Trox High Induction Top Discharge beams re-circulate high volumes of air within the room and achieve high cooling outputs, at the same time as maintaining desirable velocities for comfort of occupants. The nozzle design and the beam selection should be in accordance, so that coanda effect is achieved, and the discharged air moves across the ceiling, before descending into the room at the room boundaries (see Figure 2).

Key differences between passive and active technologies are shown in Figures 3 and 4.

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