THERMAL WHEELS
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. Our 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. In short, the same concept of good, basic engineering practice applies as in any health and well-being 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 which
are 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.
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How does the Swegon Gold heat exchanger work?
T
his 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. ■ Air Quality Control (AQC) function is used to guarantee that the heat ex- changer’s direction of air leakage and the purging sector work correctly.
■ Carry Over Control, that 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.
March 2021 17
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