COOLING | HYBRID AIR CONDITIONING The Artus unit has been popular for healthcare applications, because everything that needs to be accessed can be reached from below The air conditioning units low noise levels make it suitable for residential applications interaction of the fan blades with the shroud means that the air does not flow straight down; instead, the fan generates a swirl pattern that induces and mixes the air in a similar fashion to that of a swirl diffuser, says Olsen. The benefit of this arrangement is that it eliminates the additional pressure drop created by the secondary ductwork and diffusers, as Olsen explains: A conventional FCU will have a 25Pa pressure drop across the coil, plus an additional 30Pa on external ductwork totalling 55Pa whereas we have just 7.5Pa pressure drop across the entire unit. This is fundamental to how the very fan powers are achieved. A consequence of the low pressure drop is that typical specific fan power (SFP) for the unit is 0.06 to 0.08. Ed Sayce, projects director at Artus Air, says a typical fan coil unit would have an SFP of 0.2 at best, while the backstop for Part L for nondomestic services is an SFP of 0.25. Were only using 25% of what is allowable, which is an 80% improvement in the SFP. Weve modelled for a model office scenario (see panel, Modelled energy use in an office), where it saves 21% of the regulated energy against Part L which is a phenomenal saving for what is a simple concept, adds Sayce, who points out that the SFP in the Part L calculation is 100% of load, but for most of the time, the fan will be running much lower than that figure. Furthermore, the fan speed is variable, to save energy and reduce fan noise. Maximum fan speed is set based on the noise level required in the space. The unit can achieve noise levels as low as NR22, which makes it suitable for use in residential applications. In an open-plan office, however, an acceptable noise level is NR35, with a 5dB relaxation allowed in the British Council for Offices Guide to Specification for a variable speed system. We start with NR40; because weve done loads of acoustic modelling, we know that if the units are spaced serving 25m2 the fans can run up to 140L.s-1 per unit and we work backwards from there to calculate the cooling capacity, says Olsen. The unit comes packaged with the pressure-independent control valves, the controller, and condensate pump pre-fitted, to enable predetermined software in the controller to always run the components in the lowest energy-use mode. When you buy these components separately, as is common with a conventional fan coil system, you are reliant on onsite coordination of these components, whereas weve got certainty of component compatibility and performance because weve tested it all together, says Sayce. In this version, the unit is fitted with a single, large heat exchanger coil to provide both heating and cooling via a changeover system, which is different from the UK convention of having FCUs with separate heating and cooling coils. The coil is sized to provide 2.4kW of sensible cooling based on chilled water at 6C flow/12C return, which Olsen says is a bit smaller than the duty of some conventional fan coils. The consequence is that more units will be needed for a Cat A office fit-out. We are developing a new unit of double the cooling capacity to match typical FCU coverage, says Olsen. Despite the higher number of units needed, Sayce claims the installation is cheaper than one based on conventional FCUs. Weve done the exercise several times with external quantity surveyors: the capital cost is cheaper because you need to add in the cost of the secondary ductwork, grilles, controller, condensate pump and control valves to the cost of a conventional fan coil, he says. Thats before you add in the huge contractor savings on quicker install time, reduced procurement, less controls work and less commissioning because there is no secondary air balancing, and the operational savings from low energy use. The unit can also run at an elevated chilled water temperature of 14C/19C enabling excellent central plant efficiencies, however, 52 November 2021 www.cibsejournal.com CIBSE Nov21 pp51-52, 54 Artus.indd 52 22/10/2021 18:14