As many countries across Europe are still operating under heavy restrictions, facilities managers may wish to take the opportunity to review how their ventilation and air conditioning systems are operating in preparation for the re-opening of buildings.
It is now widely understood that maximising outdoor air, increasing air change rates and therefore effectively diluting the air inside a premises is the most effective way to minimise the risk of building occupation. However, one of the most asked questions we receive about building re-occupation is on the topic of rotary heat exchangers...
Rotary heat exchangers
The operating benefits of properly installed and maintained rotary heat exchangers (thermal wheels) should far outweigh any risks, particularly when air change rates can be so much higher when compared with many other ventilation strategies (such as opening windows, ‘split’ systems etc).
Until recently, some guidance suggested that rotary heat exchangers were switched off or bypassed as a precaution during the COVID-19 pandemic because of fears about the potential of cross-contamination of supply and return airflows with virus laden droplets. In practice, this is was not a realistic recommendation for physical engineering reasons and would have no tangible impact on the amount of ‘return air’ being brought back into a room.
The subsequent guidance from REHVA – the Federation of European Heating, Ventilation and Air Conditioning Associations, Chartered Institute of Building Services Engineers (CIBSE) and the UK’s Building Engineering Services Association (BESA) have clarified that rotary heat exchangers pose an extremely low risk of transferring viruses (including COVID-19) if they are “properly constructed, installed and maintained”.
REHVA said rotors should not be switched off because the leakage rate is not influenced by whether they are rotating or not. In fact, normal operation of rotary systems was a good way to allow outside air to be brought into the building to maintain air change rates, which will reduce the concentration and potential transmission of virus laden water droplets, according to the guidance.
Bypassing makes it difficult to achieve levels of comfort cooling for occupants during summer months – or to meet heating requirements in the winter months. There are also effects on the planet (thanks to the increased energy consumption) and the building owner, who will experience increased energy bills having missed out on the considerable energy efficiency benefits of this heating and cooling recovery, unless the systems are returned to normal operation.
REHVA said properly operating and maintained thermal wheels would have similar leakage rates to plate heat exchangers i.e. below 2%.
“Under certain conditions virus particles in extract air can re-enter the building…[but] there is no evidence that virus-bearing particles starting from 0.1 micron would be an object of carry over leakage,” states REHVA’s updated guidance.
Properly designed, constructed, installed and maintained rotary heat exchangers have almost zero transfer of droplet bound pollutants including air-borne bacteria, viruses and fungi. It is known that the carry-over leakage is highest at low airflow, thus higher ventilation rates are recommended. Not only does this benefit the carry-over leakage, most guidance suggests that using the maximum ventilation rates and outdoor air also has the desirable effect of ‘diluting’ any virus droplets in the air, therefore lowering the risk of airborne transmission of a virus in an indoor space.
To prevent the rotor from transporting air from the extract to the supply a purging sector should be fitted. The function of the purging sector requires that the pressure difference is correct which means, firstly, the fans need to be correctly positioned so that they are both downstream of the rotor in their respective airstreams and, secondly, that the extract air is correctly throttled to create the correct pressure difference between extract and supply. Swegon’s installation documentation explains how this should be done.
However, it is always good practice to inspect all heat recovery equipment (including plate heat exchangers) to ensure any possible leakages from the exhaust air side to the supply side are addressed. Adjusting the pressures can also minimise the potential for cross-contamination, including the use of bypass valves and/or dampers to avoid higher pressure building up on the extract side.
How does the Swegon GOLD heat exchanger work?
This may seem complicated, but it is good to know that Swegon air handling units with rotary heat exchangers from the GOLD and SILVER C series, have been designed with the fans in the correct position and are fitted with purging sectors as standard. In addition, the rotor is well sealed both on the perimeter and across the diameter where the supply and extract airstreams are separated. The return (extracted) air is filtered before reaching the thermal wheel and pressure measurement nipples are provided to make it easy to check the pressure balance and a pressure reducing perforated plate is included in the unit. This means that the leakage rate is less than 1% for both units with rotary and plate heat exchangers.
The GOLD units also feature several special control functions to help with indoor air quality.
• The air quality control (AQC) function is used to guarantee that the heat exchanger’s direction of air leakage and the purging sector work correctly.
• Carry over control controls the rotational speed of the rotor when the airflow rate is reduced in variable air volume systems. This ensures the function of the purging throughout the operating range and reduced leakage even further.
In short, the same concept of good, basic engineering practice applies as in any health and wellbeing context, if ventilation equipment is of sufficient quality and has been properly designed, installed and maintained, then the risk of virus transmission from a thermal wheel is extremely low.
What this period has taught us all, however, is to be vigilant and to further focus on the importance of quality equipment from knowledgeable manufacturers, and of comprehensive monitoring and maintaining of our equipment.
Thanks to low occupancy levels (only likely to increase gradually), this is a good time to carry out thorough HVAC system checks, look at any tweaks or upgrades that we might want to make and ensure all maintenance is up to date. In other words, doing what we must always do to keep building occupants safe and healthy.
Swegon is part of the Swedish Latour group and is a market leader in energy efficient ventilation and indoor climate products and systems. It employs 2,400 staff and has a turnover of approximately EURO500 million worldwide – over £54m of which is in the UK.