FAN COIL DESIGN | TERMINAL UNIT Selecting equipment based on total cooling load can result in higherthan-desired sensible cooling capacities and higher flowrates SENSIBLE APPROACH Selecting the wrong fan coil (terminal) units can have disastrous consequences for occupant comfort, according to SSHs Imran Shaikh at the CIBSE UAE Healthy Buildings Conference. Tim Dwyer took notes P oor fan coil (terminal) unit design is an oft-neglected topic that can have a major impact on occupant comfort and health. The subject was tackled at the CIBSE UAE Healthy Buildings Conference, however, by Imran Shaikh, CIBSE UAE vice-chair and senior lead mechanical engineer at SSH. He emphasised the importance of understanding the difference between correctly sized and oversized or poorly sized units. His presentation focused on residential and hotel guest-room applications, where the sensible load is dominant, and clearly differentiated this from other applications, where latent load may be more significant. Shaikh highlighted the need to select equipment to meet the calculated sensible cooling load at appropriate design conditions. Selecting equipment based on the calculated total cooling load (that is, sensible plus latent loads) can, in some instances, result in higherthan-desired sensible cooling capacities and higher flowrates. In such instances, the off-coil temperature becomes higher, so preventing the unit from providing sufficient dehumidification. Referring to a number of common myths, Shaikh first rejected that more cooling capacity means better thermal comfort, as it can actually lead to the supply of excess air, resulting in poor air distribution, draughts and occupant discomfort. The setpoint temperature is also likely to be achieved swiftly, so as the off-coil temperature increases dehumidification reduces and the room humidity increases, adversely impacting occupant comfort and, potentially, health. Another myth is that large load safety factors will lead to a robust design. The risk is that the summation of all the excessive load safety factors in the lighting, equipment, infiltration, occupant density and activity increases the installed sensible cooling capacity significantly (together with a smaller impact of the calculated latent load). This will lead to oversizing, with the inherent problems previously discussed. The final myth that Shaikh destroyed was that a higher installed capacity results in lower humidity as discussed above, this is not likely to be the case. It could actually result in surface condensation as the room humidity rises and, potentially, mould, which will impact human comfort and health. He reiterated the importance of assessing all loads properly, including fabric, lighting, equipment, infiltration, occupant density and activity schedule; this will inform a correctly selected unit. Considering a typical air conditioning layout for a hotel guest room in the Middle East region, Shaikh said that, when selecting units, he generally considers the performance of the terminal units at medium and high speed. He then identifies five or six suitable models from two manufacturers that meet cooling performance as well as other criteria, including noise performance and physical dimensions. This will allow the unit to operate at medium speed for all but extreme load conditions, when it can be operated at high speed for a short period of time. Shaikh would normally size the final duct with a low velocity of around 2m.s-1 and the entry to grille at 1.5m.s-1, with the (vertical) supply grille single-direction blades facing www.cibsejournal.com November 2020 57 CIBSE Nov 2020 pp57-58 Terminal unit AC spec.indd 57 23/10/2020 16:39