When it comes to indoor comfort and humidity control, HVAC systems are becoming increasingly intelligent, with displacement ventilation and radiant chilled beams making an impact.
Today’s computerised control systems track occupant levels, season, time-of-day, outdoor temperature and humidity levels.
For example, setback thermostats adjust thermostat levels to the building’s occupancy schedule, saving up to 20 per cent of HVAC running costs without sacrificing comfort.
Two highly efficient air distribution techniques are gaining traction and making a real difference.
The first is displacement ventilation (DV), which is the supply of a ground level flow of conditioned air into an interior space to push out stale air at ceiling level.
In cooling applications, incoming chilled air forces up warm air which is expelled at the ceiling.
In heating applications, DV operates like conventional mixing ventilation – warm air is supplied near the ground level, then mixes with the present air as it rises and the mixed air is expelled at the ceiling.
In either application, the supply point should be located and oriented so as to avoid air flow directly to the extraction point or into drafts.
DV is best suited to spaces with a ceiling height greater than three metres and a high airflow requirement, which is usually due to high occupancy or a high contamination rate.
Often the supply air is provided via a cavity under a raised floor (under-floor air distribution).
Compared with conventional mixing ventilation, it saves energy in three ways.
Firstly, it controls the climate of only the occupied zone, rather than the whole space from floor to ceiling. Less space to control means that a smaller volume of conditioned air is required.
Secondly, the incoming air is delivered close to the occupants so the supplied air temperature can be closer to ambient temperature. Overall, ventilation efficiency is typically 0.5–0.8 for DV, as compared to 0.3–0.45 for mixing ventilation.
Greater ventilation efficiency also means better indoor air quality. Thirdly, the lower airflow rate can, with appropriate fan control, reduce fan energy.
Radiant chilled beam systems cool interior building spaces through ceiling pipes.
The pipes carry chilled water that directly cools the space via natural convection and heat transfer.
Radiant chilled ceilings are best suited to indoor spaces in large commercial buildings with low constant latent cooling loads (reducing latent cooling loads requires removing moisture from the air).
The primary challenge with these systems is that the pipes and cooling coils may accumulate condensation, potentially damaging ceiling materials and fostering biological growth.
Typically, this challenge is met by managing the moisture of the indoor air by dehumidifying it before it enters the space.
Compared to a conventional variable air volume system, radiant chilled beams use less energy, are simpler to control and operate, and take up less of the building’s volume for energy distribution.
Radiant chilled beams can reduce cooling energy by as much as 20 per cent through:
• a targeted delivery of cooling to only those areas that require it
• cooling with water rather than air; water requires 1/4000th the volume of air to deliver the same cooling effect (the volumetric heat capacity of water is about 4000 times greater than that of air)
• using radiant cooling, so partially decoupling thermal comfort from air temperature and thereby losing less cooling when exhausting stale air from the building.
Source: www.eex.gov.au
