FAN COIL DESIGN | TERMINAL UNIT There is little point sizing units correctly for the system specification if the material approval process is not also done with care upwards at 50 degrees to the horizontal, to help prevent dumping. The lower velocity helps to limit the pressure drop to 20Pa to 30Pa (compared with a more normal 50Pa) so allowing, in most cases, a smaller FCU. This will not only provide better thermal comfort, but also reduce noise. Other benefits of using lower-velocity systems include reducing the possibility of particulate matter getting carried through the system into the space, and a reduced opportunity for draughts, as well as lower turbulence. Shaikh was clear that there is little point in taking care to size terminal units correctly for the system specification if the material approval process is not also undertaken with care. At this stage, he undertakes a psychrometric analysis to ensure there are no fundamental issues with the selections received from the manufacturers or the contractors. He then does a capacity comparison with the specified units by employing the psychrometric plots to verify that the total and sensible loads can be met with the offered units. If they cannot, that raises questions and these concerns need to be taken to the manufacturer in terms of the offered equipment and the suitability for the application. In his full presentation (which can be viewed at bit.ly/CJNov20IS), Shaikh provides examples from real-world case studies and concludes with some dos and donts for proper terminal sizing: n Always employ calculated values in the equipment schedule n Do not apply the worst-case orientation heat load for similar, but different aspect, spaces in a development n Closely size units based on accurate design conditions and dont over specify the FCUs n When employing multiple FCUs as a group, psychrometrically assess the on-coil and off-coil conditions and the sensible and total capacities for each FCU n Although debatable, he suggested not to control on a return sensor for guest rooms his preference is to control based on the room-temperature sensor in the space. CJ TIM DWYER is technical editor at CIBSE Journal 58 November 2020 www.cibsejournal.com CIBSE Nov 2020 pp57-58 Terminal unit AC spec.indd 58 23/10/2020 16:39