Air handling units
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Sam Birch, Big Foot Systems
Air Handling Units (AHUs) can vary in size and complexity depending on project demands. Once specified, the next challenge for HVAC contractors is where to site them. Sam Birch, technical manager at Big Foot Systems explains how to overcome the challenges of siting AHUs on flat roofs
I
t’s complicated. Modern buildings have complex needs; they need to be fit for purpose
while providing a healthy and comfortable indoor environment for employees. When selecting an AHU, the specifier has to weigh up and prioritise a wide range of factors including performance, efficiency, maintenance, space constraints and, not to forget, costs.
With a rise in focus on health and wellbeing, HVAC contractors also need to ensure air handling units address all the different areas that help ensure an ideal working space which includes considering ventilation rates, temperature, humidity, air purity and noise. However, once specified, the complexity does not end there since the next challenge is where to site large units in buildings that are often short on space.
With the UK’s highly populated
cities continuing to expand, in over- congested areas such as London, the logical (and sometimes only) place to install mechanical plant is on the roof. Specifiers are therefore challenged with designing complex, busy rooftops with a mixture of plant and pipes that all need to be safely accessed at height. Air handling units can vary in size and function to meet the individual needs of projects from the smallest fan coil units, to packaged, modular and custom AHUs. The latter can be so large that they can include walkways and service areas within them and even have space for skid-mounted equipment like pumps or heat exchangers. This variety means that HVAC contractors have a challenge when it comes to siting AHUs on flat roofs since it is not
September 2018 COMPLEX ROOFTOP PROJECT
ig Foot Systems supplied a complete solution for rooftop plant support on a new commercial office building near Kings Cross train station in central London. The split-level project used a range of non-penetrative frames including Custom Safe Access systems, Bespoke Frames, Custom HD Frames, HD Cubes and Custom HD Supports to provide safe access and support air handling units, chillers and high level pipes. At the start of the project, the technical team held several meetings with the
B
contractors Kier Construction to design a solution for this large and multifaceted project. After an initial consultation it was decided to increase the complexity of the systems being used in order to combine many different supports and access ways, so that the project fully met the needs of the customer. After detailed surveys were completed, drawings and 3D assemblies were provided to the customer along with technical calculations to back these up. The size and complexity of this project proved to be a real challenge since Big
Foot had to design a system with over 500 different parts to fit in an unusual space. That meant the project team had to be innovative and think outside normal design practices to provide a solution that worked well. On the project, the roof was split into two levels, level 9 and level 11. The supports for level 9 included a custom HD frame supporting an air handling unit (AHU) weighing over 7,800kg, custom access to service the AHU, and integrated access and pipe support for seven large volume water pipes at 4m above the roof level. Meanwhile, supports for level 11 included HD Cubes supporting four chillers weighing 5,700kg each, two bespoke frames supporting air handling units weighing 6,500kg each, a custom HD support with integrated access for a 15,750kg generator and over 100m² of custom safe access. The deliveries of the frames were staggered over six months to allow all the different services to be coordinated with Big Foot Systems, with full assistance offered to aid the install by the site team.
only the physical size of the AHU that needs to be accommodated, but also the weight and there is always the question about whether a roof is strong enough to support the plant.
So what is the answer? Faced with the problems of mounting AHUs and plant on flat roofs, manufacturers have set out to provide an economical alternative. A solution to these installation headaches is to use non- penetrative, repeatable support structures mounted on anti- vibration mats. These structures are configurable in that they can be adjusted according to the demands of each individual project, therefore offering great flexibility and time savings.
Ideal for both new build installations and retrofitting, non- penetrative systems are quick and easy to install, ensuring project cost and time clarity, negating a need for time consuming and costly penetrations through the roof in the form of poured or pre- cast concrete plinths, steels or fabrications. Non-penetrative support systems thus have the advantage of maintaining the integrity of the roof membrane, therefore avoiding thermal bridging and the potential for moisture ingress, removing worrying risks. For example, Big Foot Systems has developed its Heavy Duty modular framework systems for retrofitting heavier plant on flat roofs, such as AHUs. This presents an opportunity to offer an array of solutions to support a variety of plant, such as AHUs and chillers as an effective alternative to on-site concrete constructions.
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