COOLING | EVAPORATIVE SYSTEMS DUAL PURPOSE Switching between modes in dual evaporative coolers can result in large operational savings, according to a recent B&SERT paper. Tim Dwyer looks at the latest research on hybrid direct and regenerative evaporative cooling A lthough particularly effective for drier climes, the simple direct evaporative cooler (DEC) is successfully employed in many climates to reduce incoming outdoor air drybulb temperature towards its wet-bulb temperature, as discussed in CPD 135, in the November 2018 CIBSE Journal CPD special. This has the benefit of not requiring mechanical cooling devices. However, as the dry-bulb temperature is reduced, the associated increase in absolute humidity of the cooled air may not be desirable in some situations. Various forms of indirect evaporative coolers (IEC) overcome this, where cooling is provided from the evaporation of water into a secondary airflow, from a continuously wetted surface running along the channel length of a heat exchanger. This reduces the temperature of the water film and the dry-bulb temperature of the secondary air that is separated by a thin, impervious heat-transferring layer/membrane from the counterflowing primary system air, which is then cooled by passing over the low-temperature heat-transfer membrane surface. Both primary and secondary airflows often originate from the same outdoor air, and the secondary air will be typically rejected to outdoors after it has achieved its work of cooling the membrane by absorbing the evaporated water vapour. The primary airflow is cooled towards the wet-bulb temperature of the entering air, but gains no humidity, so it is a sensible cooling The cooling energy in the DEC mode is higher than REC, so the direct mode may be more suitable for places with high sensible heat loads where the increase in moisture content is not an issue process, often referred to as dry cooling, but is less effective than a DEC. (James Falconer discusses such systems in Making the case for evaporative cooling in June 2017 CIBSE Journal.) The so-called regenerative evaporative cooler (REC), based on the principles as variously published by Maisotsenko1 (and sometimes referred to as an M-cycle cooler) is a development of the IEC. The secondary airstream in an REC is supplied by splitting off a proportion of the conditioned air from the outlet of the primary airstream and enables the cooler to reduce the outlet primary air towards its dewpoint temperature (ie, cooler than its wet-bulb temperature). This is discussed in Clothes to zero in the March 2018 CIBSE Journal, with examples showing how it can be applied to satisfy cooling loads in a wide range of climates. This technology continues to be developed, both by academics and commercially, and a recent research exercise, reported in BSER&T, provides a useful reminder of the growing interest in applications www.cibsejournal.com March 2022 39 CIBSE March 22 pp39-40 Evaporative cooling.indd 39 25/02/2022 15:20