Why reinvent the wheel?

Rising fuel costs, growing environmental concerns and ever increasing Building Regulations. It’s a potent mix that is turning up the heat regarding the efficiencies of air handling units (AHU) and has the industry searching for a breath of fresh air. Yet, according to Andrew Patch of Reznor, we’re overlooking a forgotten hero, the thermal wheel (or rotor) – a solution that is proven to recover up to 85% of lost energy and dramatically reduce a building’s carbon footprint.

With over three decades under its belt, the thermal wheel principle isn’t new, but it has never been more appropriate than now. For years, it was regarded as a highly specialised and costly heating component, suitable only for larger AHUs. But, with the spotlight now shining brightly on delivering sustainable and energy efficient solutions to modern buildings, the thermal wheel is currently more viable than ever.

In fact, it is a product so efficient, it means a reduced capacity boiler and/or cooling condenser can be used – effectively paying for itself during installation, so the cost benefits can be enjoyed immediately. Plus, with the newest fresh air ventilation requirements under Part F of the Building Regulations, it is the ideal answer for architects, HVAC contractors and developers looking for a solution that will dovetail seamlessly with the many air handling systems available on the market today. However you look at it, the thermal wheel is set to start a new revolution.

Its growing popularity is part of a significant trend that is looking at heat recovery as a genuine vehicle to deliver real cost savings. After all, it makes perfect sense to harvest the air we’ve already paid good money to heat, doesn’t it?

The thermal wheel does this better than virtually any other technology on the market – it is particularly effective with larger air volumes – turning standard AHUs into ultra efficient systems. Similarly, the thermal wheel is just as effective at cooling incoming fresh air during the hotter summer months. Perfect for cutting cooling costs and reducing reliance on energy hungry mechanical cooling methods, such as air conditioning.

Here’s the spin
Thermal wheels that are found in standard Heating, Ventilation and Air Conditioning (HVAC) systems, such as those supplied by Reznor, are normally constructed from an aluminium matrix. Aluminium is favoured due to its outstanding conductive characteristics, corrosion resistance and lightweight composition.

The matrix is formed by winding very thin aluminium sheet in alternating flat and corrugated layers. Despite the thin aluminium – where it offers optimal heat transfer properties – the matrix comprises an exceptionally strong and rigid construction, with a large number of small air passages.

Situated in counterflow fresh air and exhaust air streams – often within the AHU – the wheel rotates slowly, passing from the exhaust air stream to the fresh air section. These sections are sealed off from each other. As the matrix rotates through the exhaust air, the warm gases pass through the narrow channels and heat the aluminium.

The wheel then continues its rotation until it enters the fresh air flow, where the reverse effect occurs. Here, the cooler incoming air absorbs the latent heat recovered within the matrix and is carried into the building. This process is known as regenerative heat transfer.

The matrix continues to rotate until it re-enters the exhaust air, thus completing one full cycle. The process is then repeated, commonly at a speed of 10rpm.

Green solutions take a new turn
With the world waking up to the need for CO2 emissions to be drastically cut, and Building Regulations demanding more energy to stem from renewable energy sources, the thermal wheel’s environmental credentials are shooting it to the top of many specifiers’ wish lists.

A key reason for this is the thermal wheel’s small footprint. At typically only 200mm deep in the direction of air flow, it can easily be slotted into most AHUs. In many cases, it also means a smaller AHU can be used. In doing so, capital expenditure is substantially reduced, with a quicker return on investment. Furthermore, it has been our experience at Reznor that thermal wheels also reduce the need for large capacity boilers and/or cooling condensers to be fitted – again offering a greater return on investment.

In comparison, a large plate heat exchanger would demand additional section being added to the AHU, which would have clear cost implications and, from our experience, could make that system impractical for some applications.

The wheel’s slow rotation speed creates very little wear throughout its working life, requiring minimal servicing. Plus, when the wheel reaches the end of its serviceable life, it is completely recyclable.

Vive la revolution…
While we’ve already mentioned that the technology isn’t new, new developments and a greater awareness mean the scope is constantly widening.

Since their introduction all those years ago, thermal wheels used in AHUs have demonstrated their exceptional durability in even the most demanding environments. These include marine applications and spray-paint plants. Significantly for the latter, the addition of automated air or water cleaning systems mean the wheel never becomes clogged. It is, therefore, able to perform at peak efficiency throughout its operational life.

More recent developments include a sorption coating, which is designed to enhance the thermal heat transfer. As such, it has made the thermal wheel more efficient than ever.

It is hard to imagine AHUs in the very near future that don’t incorporate some form of heat recovery method. Thankfully, the industry already has the perfect solution, which has turned full circle to improve economic efficiency and minimise the carbon footprint of future buildings.

www.reznor.co.uk