THERMAL WHEELS | COVID-SECURE DESIGN Type 3. Both fans on the outdoor side Type 4. Both fans upstream of the rotor Type EATR without purge EATR with purge Note 1 EATR<3% EATR<1% Dependent on external pressures. See Eurovent 6/15 2 10%< EATR <20% 3 EATR <1% EATR=0% 4 EATR <3% EATR<1% Purge does not function in this configuration D AVID BLACK is global product marketing manager Table 1: Exhaust air transfer ratio of the four AHU configurations with and without purge Type Eurovent 1 This configuration is most recommended The most common configuration on to minimise internal leakages the market 2 Price-driven solution. Very high internal leakage rates and should be avoided 3 Very high supply air leakage to exhaust Supply air leakage lowers unit efficiency 4 Higher internal static pressure Very uncommon solution Additional comment Table 2: Eurovent guidance on the four AHU configurations Type REHVA guidance Comment 1 Adjust the throttle in the extract air so p11 will become at least p22 20 Pa The damper could be in the ductwork or at the AHU 2 There is no possibility to use throttling in this case Purge does not function in this configuration 3 There is no need to use throttling in this case 4 Adjust the throttle in the extract air so p11 will become at least p22 20 Pa Table 3: REHVA guidance for the four configuration types regimes. Table 1 gives the EATR based on normal Part L-compliant external static pressures. Eurovent also gives guidance for each individual configuration (Table 2) and I have added an additional comment if applicable. As can be seen in Table 2, the Eurovent-recommended configuration Type 1 the most common has an EATR equivalent to, or lower than, an EN308-compliant plate heat exchanger (EATR < 3%). There is an additional measure than can lower the EATR even further. REHVAs document4 Limiting internal air leakages across the rotary heat exchanger states that the next step to eliminate a leakage is to set the correct relation between pressures p22 and p11 [see Figure 1]. This pressure can be achieved by regulating the exhaust damper (A) correctly positioned on the system side of the exhaust airstream (Figure 1). If the correct pressure is achieved for the recommended configuration, the EATR can be considered virtually zero. In conclusion, AHUs with rotors can have an EATR equivalent to, or lower than, plate heat exchangers if the fans are in the recommended configuration, and the unit is fitted with a purge sector. Many existing installations could be put back into operation safely, but a competent engineer should ensure the recommended configuration and presence of a purge sector. For additional security, add pressure control. Specifying AHUs with all of these in place will ensure new installations reduce the already very low potential risk to a minimum, while delivering the highest efficiency and lowest life-cycle costs. CJ modular AHU at FlktGroup References: 1 H eating, ventilation and air conditioning systems in the context of Covid-19: first update, ECDC, November 2020 2 CIBSE Covid-19 Ventilation Guidance, v4, October 2020 3 PR EN308 Heat exchangers test procedures for establishing performance of air-to-air and flue gases heat recovery devices, May 2020 4 REHVA Covid-19 specific guidance document. Limiting internal air leakages across the rotary heat exchanger. July 2020 5 EN 16798-3 Ventilation for buildings. Part 3: For nonresidential buildings. Performance requirements for ventilation systems, December 2018 6 Eurovent 6/15 2020 Air leakages in air handling units. Guidelines for improving indoor air quality and correcting performance. 1st edition, 1 April 2020 p11 A p22 The damper could be in the ductwork or at the AHU Figure 1: Stopping leakage by regulating damper (A) to correct pressures 46 March 2021 www.cibsejournal.com CIBSE March 21 pp45-46 AHU Thermal Wheel Covid.indd 46 19/02/2021 15:31