Heating and cooling make up a big part of building operation costs. Well-insulated and airtight building designs help, but efficient and effective ventilation is still needed. That’s where counterflow heat exchangers come in. Counterflow heat exchangers get fresh air into a building while using little energy with a technique called heat recovery ventilation. In this article, we’ll discuss how counterflow heat exchangers work and help you choose between aluminium and polymer core materials for a heat recovery ventilation system.
Heat recovery ventilation
The word recovery is key. Heat recovery ventilation uses energy that is already in the system, requiring less energy to heat or cool outside air. This quality makes heat recovery ventilation a good match for energy-efficient HVAC projects. Energy-efficient buildings are good at keeping the internal temperature stable. Doing that requires thick insulation and minimal energy losses to the outside environment. Rather than directly letting in cold or hot air from outside, heat recovery ventilation allows heat exchange between indoor and outdoor air as they pass each other. The fresh air still gets in, but the indoor temperature is less affected by the outdoor air. If it’s hot outside and cool inside, the hot outside air is cooled by the inside air before coming in. If it’s cold outside and warm inside, the cold outside air is warmed by the interior air. The only energy used is by fans to push and pull air through. Most of the energy used to heat or cool the building stays inside, where it should be. Heating and air conditioning systems do less work, so they need less energy. Another plus of heat recovery ventilation is that it’s easily reversible. The same installation will heat or cool fresh air from outside, depending on the temperature gradient. Heat recovery ventilation saves energy in any type of climate.
Putting heat recovery ventilation in context
When surveyed at this year’s AHR Expo, attendees and exhibitors were unanimous that reliability is the most important factor when choosing products. No surprise there. What’s more surprising is that energy efficiency was also near the top of the list. This survey is not the only indicator pointing towards a growing trend. The 2019 Energy Efficiency Indicator survey revealed that 75% of organisations plan to invest more in energy efficiency, renewable energy, and smart building technology. This result was a 16% increase over 2018. The trend is in part due to more demanding standards all over the world. Dubai is aiming to reduce its energy use by 30% by 2030. Gulf News reports that as much as 80% of electricity use in the United Arab Emirates goes to buildings for essentials such as air conditioning. Some of Dubai’s energy reductions are sure to come from more energy-efficient HVAC systems. Looking further north, the story is similar. The European Commission found that over 70% of energy use goes towards heating. Also, energy use for cooling is on the rise. Heating and cooling will have to use less energy to meet EU climate and energy goals. The economic situation demands careful budgeting, however. It’s more true now, but even in February, AHR Expo attendees put first costs above energy efficiency. Everyone knows about the potential savings from energy-efficient HVAC systems, but many assume that the cost of entry will be high. Maybe too high. Luckily, heat recovery ventilation systems have become more affordable thanks to modern, automated manufacturing and new, advanced materials. We’ll come back to this point after discussing how counterflow heat exchangers work.
How counterflow heat exchangers work
Counterflow heat exchangers are an efficient way to do heat recovery ventilation in air handling units. They offer high thermal efficiency per unit area and up to 90% energy recovery. The name comes from the fact that inside and outside airstreams flow in opposite directions, counter to each other. Heat transfer occurs as the two airstreams pass each other in the air-to-air heat exchanger. Let’s look at Swiss Rotor’s line of hexagonal heat exchangers to examine the details of a specific design. In the Swiss Rotor design, the return air does not mix with the outdoor air. Indoor pollution of any kind stays in a separate airflow channel from the fresh supply air (video here). Eliminating cross-contamination of airstreams means the heat exchangers are suitable for chemically harsh environments such as health care facilities, laboratories and food-processing plants. This also makes it possible to introduce dry air and remove moisture from damp environments such as swimming pools and sewage treatment plants. Inside the heat exchangers, fin spacing is either 2 mm or 3 mm. Closer fin spacing means higher efficiency. Looser fin spacing means minimal pressure loss. Which of these two factors is more important depends on your system design. External dimensions range from 496 mm × 271 mm to 1182 mm × 959 mm to fit a wide range of industry standards. Regardless of the size, the whole line of Swiss Rotors’ counterflow heat exchangers functions according to the same principles. All are also Eurovent-certified. Finally, there is the question of core material: aluminium or polymer. The core material changes little in how the heat exchanger works, but each choice has pluses and minuses.
Aluminium vs polymer counterflow heat exchangers
Price: Polymers are easier to handle in most manufacturing processes. The raw polymer material is also usually less expensive than aluminium. Together, this means that the price for polymer core heat exchangers is lower.
Temperature limits: Aluminium cores have a wider range of operating temperatures. Swiss Rotor hexagonal heat exchangers with aluminium cores can operate from -40ºC to +70ºC. Those with polymer cores have tighter limits, from -20ºC to +50ºC.
Corrosion: Both materials are suited to a wide variety of environments with little risk for corrosion. That said, polymer cores are generally more resistant to the process of corroding. If corrosion could be an issue, you should check what the corrosive agents are and choose the material to minimise any possible risk.
Weight: Polymer cores are lighter and easier to handle. Often, this will only be a benefit during installation. But there may be building projects in which lighter heat exchangers have other advantages.
Maintenance: Both materials need regular cleaning. Polymer cores require a pressure washer, while the aluminium ones need an air compressor.
Ensuring high system performance and efficiency
Whatever your choice of equipment, tight manufacturing tolerances and verified performance specifications are crucial. You don’t want to install the elements of your system only to find that performance or efficiency is worse than advertised. This requirement is especially important for energy-efficient HVAC designs. Such systems need to operate efficiently for a long time to get the maximum benefit from the initial investment. Manual manufacturing processes can achieve such tight tolerances but at a higher cost. High lead times are also more common with suppliers relying on more manual labour. Modern, automated manufacturing processes are the best way to guarantee tight tolerances, system performance and shorter lead times at a lower price. This is the Swiss Rotors approach. Organisations all over the world are looking for ways to cut energy costs and be more environmentally friendly. Counterflow heat exchangers are a good option for doing just that as part of a retrofit or new-build HVAC project. Thanks to recent advancements in the field, the first costs to invest in the technology and realise energy savings are lower than ever.
If you’d like to learn more about counterflow heat exchangers and heat recovery ventilation systems, get in touch with Swiss Rotors. We’d be happy to answer any further questions you have.