Choosing the optimum approach

One of the impacts of the new Part L has been the opening up of new builds to a wider range of HVAC approaches. Over the last decade, use of glazing in building facades increased steadily, creating solar gain challenges for HVAC system designers as cooling demand crept higher. This led to certain HVAC options (such as chilled ceilings and displacement ventilation) being outpaced in favour of equipment able to handle these high cooling loads with efficiency and comfort.

Today, however, we can expect to see solar gain being brought under closer control, through better facades and reduced use of glazing. This brings certain technology approaches into play once more, giving the HVAC system designer more choice.

As a child in a sweet shop will tell you however, more choice doesn’t necessarily mean an easier decision. Customers are more eco-aware than ever before, and time and margins are under increasing pressure. So decisions have to be technically robust and made in a fraction of the time.

In this article I am reviewing the basic principles that speed up this decision process, particularly with regard to choosing between passive chilled beams, chilled ceilings and active chilled beams.

Let’s begin with the ‘no brainers’ – the factors which immediately determine one approach over all others.

Functionality

This is the obvious first process of elimination. All three options can provide multi-service capability, but the need for heating or flexibility in ventilation could immediately rule out certain approaches.

Other parties in the supply chain might also pre-determine the HVAC approach, for interior design or environmental reasons. Architects frequently have aesthetic preferences. We also see chilled beams and chilled ceilings being specified for their potential when it comes to energy efficiency. They can be utilised to reduce carbon emissions and increase the energy performance of buildings. This helps achieve more stringent targets as part of the sustainability agenda, and to promote to speedy letting of commercial properties.

In many situations, however, the choice may not be clear cut between the type of chilled beams or chilled ceilings.

Cooling demand

In many instances, the required cooling or heating load is the most useful method for ruling out those approaches that are not technically advisable for the application. Specific ‘beyond the envelope’ products within manufacturer’s ranges may also be considered at this point. For example the Trox high induction beam, which delivers high cooling outputs with lower fresh air rates, enables you to reduce ductwork riser and central AHU plant size for applications with high cooling loads.

If the choice of HVAC approach is still wide open, then the final decision might involve comparing advantages and limitations of each technology. Some of these are outlined in figures 1&2 attached below.

Each technology has applications in which it can deliver the goods.

Passive beams for perimeter cooling

When installed at the perimeter, near to glazing, passive chilled beams can be operated at higher outputs, as they will be situated outside the defined boundaries of the occupied zone. This allows achieving higher outputs without risking high and uncomfortable air velocities around the occupants. 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 space which is outside the area affected by the perimeter beam.

Active chilled beams for energy efficiency

Latest generation high induction beams achieve an induction ratio of around 8:1. The Trox DID 632 active chilled beam, for example, has an output approaching 650 W/m. Trox HITD active chilled beam can deliver 500 W/m of cooling at typical design conditions with fresh air rates as low as 1.3l/s/sq m. These high induction active chilled beams are capable of reducing the loads placed on energy consuming components elsewhere in the system.

This is achieved as the aerodynamic capabilities of these units help achieve the highest cooling outputs and the optimum comfortable ventilation rates with minimum air and water pressure drops and volumes. Therefore the fan power required in the air handling unit plant can be reduced as a result. In addition to this, the system is designed for chilled water at higher temperatures, which helps reduce the energy consumption in chillers, avail potential free cooling or provide a great fit for heat pumps using renewable heat sinks or sources.

Possibilities to save energy, provide aesthetics and comfort at component level through Trox innovation when coupled with the overall system potential of high temperature cooling, are an important driver in the evolution of chilled beam technology.

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