Energy Recovery Wheel 8"Heat recovery wheels reduce energy costs for building owners by retaining heat during peak consumption. The latest wheel technology uses a purge section to almost entirely eliminate cross-contamination between incoming and outgoing air. 

Here’s how purge sections work to ensure fresh incoming airstreams without recirculating foul air, industrial pollutants, and airborne diseases.

Traditionally, thermal wheels within the HVAC industry have had a reputation for being luxury components. Nowadays, you can use cost-efficient heat recovery wheels for many commercial applications to meet stricter building codes, reduce energy costs, and most importantly—keep incoming fresh air unpolluted. 

As the COVID-19 pandemic increases the demand for better indoor air quality, building owners should consider introducing heat recovery wheels with purge sections. This step will not only help them keep tenants safe and healthy but also reduce their operational expenses.

How heat recovery wheels work

Think of thermal wheels as a rotating heat exchanger composed of many layers of grooved materials–often corrugated aluminium sheets. 

These layers create a matrix of tiny air passages that oppose each other in alternating patterns. You’ll often find them in an air-handling unit positioned where the supply and exhaust air streams meet, transferring heat (and humidity) between the outgoing and incoming airstreams. 

This process effectively recovers about 85% of the energy spent heating or cooling a building’s interior, which yields less operational HVAC costs over time. However, it’s a common myth that when heat recovery wheels rotate, they can mix extract and intake air streams, thereby contaminating the incoming air supply. While traditional wheels had anywhere from 4-5% leakage, this issue can now be almost completely eliminated with a newer technology called purge section.

An additional benefit of thermal wheels over plate heat exchangers is the transfer of humidity. They can transfer moisture from exhaust air to the intake supply, which makes them suitable for air-conditioned buildings, and places with humidity-controlled, hotter climates.

Why air leakage matters in a post-COVID-19 world

When we talk about leakage in the air-handling unit, HVAC specialists are usually referring to the exhaust particulates ‘leaking’ back into the incoming air supply. In buildings with a traditional plate heat exchanger–with no purge section–mixed air streams can account for anywhere between 4-6%. To keep tenants safe from airborne disease and reduce industrial pollutants, ASHRAE guidelines recommend lowering this potential risk to below 1%. There are two primary ways that air leakage can occur in a heat recovery wheel setup.

#1: Airtightness leakage  

The corrugated metal inside a thermal wheel allows both exhaust and intake air streams to transmit heat energy, but not particulate matter. However, leakage can occur when there are gaps between the opposing streams. Tears, breaks, or improperly sealed capillaries allow small amounts of the air to mix. Design strategies often solve these issues. 

For example, Swiss Rotors wheels use proprietary crimping strategies and highly automated manufacturing processes, which significantly reduce material construction flaws. Compliant with EUROVENT 17/11-2015, they contain the airtightness parameters and adhere to acceptable leakage limits.

#2: Carry-over leakage 

Rotary carry-over leakage occurs when the thermal wheel rotates, causing pockets of air to get trapped and then release back into the intake supply. This exhaust air is held within a part of the wheel, then gets dumped back into the intake stream when air pressure changes occur. 

This is where the purge section comes into play, reducing the possibility of contaminated air supply.

How a purge section works

Swiss Rotors purge sections reduce carryover leakage of heat recovery wheels to under 1%. This threshold is the maximum possible for commercial applications, outdone only by hospital or laboratory settings which require absolute levels of air supply containment.

The purge section component blocks off a small section of the wheel exactly where the exhaust air rotates back to the fresh intake supply. By blocking this section, nearly all of the outgoing air within the wheel makes its way to the exhaust-supply side, greatly reducing contamination. This technology passively uses the pressure differential between outdoor and indoor air streams. As a result, it enables purging the air section with clean outdoor before it makes a full rotation to the supply stream.

Every purge section is tailored to your HVAC unit, allowing you to optimise the necessary purge volume and angle regardless of any pressure differentials. Your HVAC engineer will calculate the exact angle and size of the section. Installation can be done as an additional retrofit to your thermal wheel, or come as a part of a new HVAC project. With Swiss Rotors heat recovery wheels, minimal efficiency losses are measured with the installation of a purge section. At the same time, the reduction of contamination volume goes anywhere from 4-6% to well under 1%. This quality means that energy efficiency loss can be kept at minimum, while health and safety metrics go to maximum levels for a commercial application.


When to use heat recovery wheels with purge sections

You should consider thermal wheel applications when you want to improve your air-handling unit efficiency, especially in climates where heating and cooling are a significant source of energy consumption. Heat recovery wheels provide high levels of energy savings and thus reduced carbon footprint emissions to meet increased regulatory limits for even the most demanding scenarios.

You want a better AHU footprint 

Heat recovery wheels are very slim, often under 250mm wide. This feature lets you reduce the size and operational capacity of your AHU. In large plate-heat exchanger applications, more space is needed and higher volumes of air must be applied to achieve the same effect.

You have tenants asking about indoor air quality 

By virtually eliminating contamination scenarios, you ensure that your energy recovery medium also serves as a way to protect the health and safety of tenants. Purge sections have been used across a broad spectrum of high-demand scenarios, such as industrial paint manufacturers, to more commercial applications like hotels, offices and shopping centres where large volumes of air need to be recycled.

You want to reduce consumption of humidity components 

Heat recovery wheels control humidity passively. This feature means that your energy usage for heating and cooling coils such as boilers and condensers can go down even further. Passive energy recovery ventilation has higher upfront costs but ultimately lowers your fuel consumption over time.

You value ease of maintenance in your HVAC unit

Heat recovery wheels operate at low rotational speeds which means there is reduced wear-and-tear over its lifetime. Spray coatings can further extend their operational life, while automated cleaning systems can keep them running at peak efficiency over the entire span of its lifetime.

Fitting heat recovery wheels for future savings and air quality

With proper energy recovery systems and ventilation technologies for improved indoor air quality, building owners can get ahead of their tenants’ demands. Heat recovery wheels with purge sections make it possible to achieve competitive market standards in indoor air quality while protecting your energy demands. Swiss Rotors offers Energy Recovery Ventilation solutions backed by the latest industry certifications. For instance, AHRI 1060 is the most important standard for thermal wheel compliance measures, and our product ranges can ensure you meet these specifications in your next building project. Our specialists help you consult the best possible products for your applications that can be further tailored to fit your region’s regulations or building code specifications. As heat recovery wheels can recuperate anywhere up to 85 percent energy loss, they’re becoming the most popular option in modern AHU retrofits and new constructions. The true benefit is that they also create better indoor air quality conditions, controlling humidity and virtually eliminating contaminants from re-entering incoming fresh air supply. 

These qualities make thermal wheels a perfect choice for the time of pandemic and beyond, so please get in touch to learn more.