Air handling solutions

In these days of rising energy prices and more stringent regulations, it is important to investigate all options for reducing the energy profile of any building. It’s also sensible to take a holistic approach to all the systems in any building particularly major energy consuming installations such as HVAC. Only by taking a total view can maximum energy efficiency be designed into each building.

Consider chilled beam systems, for example. These are increasingly on the agenda, and with good reason. Not only are they low energy systems but they also create an extremely stable and comfortable indoor environment. They therefore achieve the two major targets of any HVAC design: indoor air quality and energy efficiency.

However, with a chilled beam system you have to dehumidify the fresh air. Such systems typically have a chilled water flow temperature of about 14ºC. But if the air inside the room has a higher dew point than this, there is a risk of condensation. For best practice, the dew point of incoming air should be reduced to about 12ºC.

Another key consideration is to ensure the temperature of air supplied to the beams is not too low, or condensation could also occur on the duct. Hence the air is normally reheated to about 16ºC.

Here, then, is a low energy system that needs to cool and reheat at the same time almost a cardinal sin when designing energy efficient systems unless a twin wheel system is used.

Thermal wheels

There is general awareness that thermal wheels are the most efficient energy recovery device for air handling units.

These will recover up to 90% (typically 75-85%) of the energy in the extract air. As the extract air passes through the wheel, the warmth or coolth transfers into the thermal mass of the wheel. As the wheel turns, it moves into the supply side where the supply air picks up the energy.

The two most common types of wheel are the hygroscopic and non-hygroscopic. Non-hygroscopic wheels generally only transfer sensible heat, although, in extreme cases, they can also transfer moisture. These are the most common type in the UK and transfer the sensible energy from the extract side to the supply side.

The hygroscopic wheels are similar to the non-hygroscopic wheels but are treated so that latent energy is transferred as well as sensible energy. This has the advantage of transferring moisture. In winter, this adds humidity to the supply air; and in summer, it takes moisture away from the supply side.

However, used together, they become a twin wheel system.

Twin wheel system

For systems where dehumidification and reheat are necessary, the twin wheel system is not only designed to eliminate any heating requirement, but also to reduce the chiller load by about 50%.

The system works using two thermal wheels, one hygroscopic and one non-hygroscopic, as well as a cooling coil .

In summer outside air at 28ºC first passes over the hygroscopic wheel, as it does heat and moisture are removed from the air and transferred to the exhaust air.

The air then passes over a cooling coil to chill it to 12ºC, dehumidifying it further.

As it leaves the cooling coil the air then passes over the second thermal wheel where it is reheated to 16ºC. This wheel is non-hygroscopic and so only transfers sensible heat from the exhaust air stream to the supply air - effectively providing free reheat.

As a bonus, as the wheel reheats the supply air it transfers cooling into the exhaust air stream. This cooled exhaust air then passes over the hygroscopic wheel, pre-cooling it before it is transferred to outside. The wheel transfers this cooling back to the supply air, completing the cycle.

This re-cooling means the load on the cooling-coil is about half what would be expected if the wheels had not been present.

What's more, the installation no-longer requires a reheater battery.

In winter operation, the benefits are also substantial: heat recovery levels are high and most of the humidification load is supplied by the hygroscopic wheel, so energy demand is further reduced.

The twin-wheel system should also be considered for fan coil and displacement ventilation systems.

For more information contact Craig MacFadyen at Flakt Woods. craig.macfadyen@flaktwoods.com