We begin this month’s product focus on air handling by taking a look at twin thermal wheel systems

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

Fast-kill disinfectant

Advanced Engineering has added a new-generation fast-kill disinfectant, QX-60, to its treatment for evaporator coils of air-conditioning and refrigeration systems.

In tests QX-60 killed 99.9% of all bacteria and fungi within 60 seconds.

QX-60 is effective against micro-organisms including Legionella, P aeruginosa, E Coli, E hirae, S aureous, A Niger and C albicans.

It is in new formulations of CondenCide and RTU combined cleaner and disinfectant treatments.

Fan power

The new XF100HLV fan from Manrose Manufacturing is a low-voltage unit with integral humidity sensor. It is powered by a standard transformer that can be wired up to 8m away.

Designed for bathrooms and toilets, it can be adjusted from 40% to 95% relative humidity.

The unit has an extract rate of 85 m3/h 23 litres/s. Its single phase induction motor has pre-oiled bearings for a long maintenance-free life.

Hot topic

Flakt Woods has developed a range of large-diameter aluminium impellers that in emergency situations can operate at higher speeds than normal for two hours at 400ºC.

The fans range from 315 mm diameter to 710 mm diameter two-pole fans that can work at higher speeds for tunnel jet thrust fans, and much larger 1250 mm diameter fans.

Traditionally used for tunnel and metro venting, the new fans are now proving themselves in HVAC applications for retail and office use.

Airside fan coils

Airside fan coils offer improved room temperature control and no valve maintenance but there is potential energy loss due to heat transfer. Trox damper blades have special seals with a high level of insulation and are operated by individual actuators to ensure positive closure.

This way the four damper blades two per coil close off both sides of the heating or cooling coil.

Industrial air Curtains

Dimplex’s IAB industrial air curtains provide powerful performance while maintaining visibility through the door.

Maximum mounting height is 6 m above floor level, making the range ideal for high doorways intended for vehicular access. The range comes in 1 m and 1.5 m lengths and in water-heated, electric and ambient models with heat outputs up to 41 kW.

All models come with a built-in energy management system interface.