The design and function of a heating and cooling system in a tall building present unique challenges for design consultants and mechanical engineers. Soulla Paphitis of Danfoss explores the key issues and offers an innovative solution using floor level substations to create a functional, safe and energy efficient tall building design.
What is a tall building?
The ASHRAE Design Guide describes a tall building as anything over 100m (approximately 30 storeys). However, from an engineering point of view a ‘tall’ building is essentially any structure where the engineering is required to change the design of the mechanical systems. High Pressure is one of the key challenges in tall buildings and, from our experience, this should be a consideration in buildings of around 20 storeys.
We have seen a significant increase in the construction of tall buildings in the UK, particularly in the capital. Demand for housing in London is growing in line with the population and urbanisation. With not enough vacant land to build across, we are having to build upwards, and in order to keep pace with demand we are likely to see even more high-rise, mixed-use developments that combine domestic, social and commercial Areas. Many newly designed developments are being run from district heating and or district cooling systems which in turn could have a number of different buildings drawing from their energy, high-rise buildings create complex and challenging system designs that require special attention to the working pressure limitations of all mechanical equipment, so as buildings continue to reach for heights previously unimagined by designers, the need for innovative solutions to the increasing system pressures has become more of a necessity.
Clear strategies
The mechanical and electrical strategies in any tall building present special design/construction issues that need to be addressed. Co-ordinating space for services and mechanical plant equipment in commercial buildings always presents a challenge but this is particularly true in tall buildings. Electrical and mechanical engineers will often compete for the same space, while the architect has to balance the space required for mechanical, electrical and plumbing (MEP) systems and the optimum lease area. In addition, the equipment must fit in the spaces provided but also be freely accessible for maintenance.
Energy efficiency is another critical factor. The tall building energy strategy should aim to minimise energy usage, whilst maintaining comfort and minimising plant space. This can be achieved in a number of ways such as designing the façade/envelope to minimise solar energy gain, using daylight to minimise artificial lighting, installing low-energy terminal devices on occupied floors, and using district heating as a low carbon heat supply.
Key challenges
Pressure is the main problem that needs to be overcome when designing mechanical systems for tall buildings, generally speaking, the taller the building the higher the pressure, essentially, the same rules of minimum and maximum pressures for buildings over three storeys’ tall also apply to high-rise buildings, In order to achieve the required pressures at the top of the building equipment is designed based on the demands of the building which vary depending on the type of development for example between commercial and residential buildings, this then creates the issue where the bottom floors are exposed to very high pressures PN40 or PN25 depending on the overall building height. Selection of plant equipment can also cause problems due to under or oversizing the pumping system, this in turn creates issues with maximum and minimum pressure zones. The need for higher rated components can also create increased costs on a project, and there may be increased corridor heat due to maintaining constant flows.
Commonly used Solutions
Some designs of tall buildings call for the use of intermediate plantrooms
The positioning and specification of these will have an impact on the system’s pressure and temperature characteristics. Positioning will depend on the requirements of the project, mid-level has the advantage of having full control over pressures by creating low pressure zones to split the building but utilises valuable floor area. Also, the heat exchangers, commonly used as pressure breakers, will be connected in series, resulting in the chilled water temperatures rising throughout the building and the heating water temperature reducing due to the number of passes through each heat exchanger, commonly we see 1 degree in CHW and 3-5 degrees in LTHW systems.
Another solution is to use large basement plantrooms, these feed a core system generally running through the riser, this option has the advantage of using less valuable floor space and enabling the connection of the heat exchangers in parallel. However, parts of the system will be exposed to very high pressures, which requires components to have a high-pressure rating and high system pressurisation. The high pressures are then reduced by way of valves at the run outs to each floor allowing for the fit out to use standard PN10/PN16 rated components. Whilst both of these options provide a practical solution to the system challenges they do not always meet the needs or desires of the end client.
Innovative solutions
A solution we at Danfoss have been working on is to reduce the pressure in stages across the system
By installing a substation on each level within the riser space, the system is hydronicaly balanced and the pressure is reduced. Each zone’s services would be fed by the individual substation that is sized to deliver the duty for that floor. Not only does this provide the pressure break needed in a tall building,.it also reduces the floor space taken up by large plant equipment and provides a sensible commissioning point for engineers. It reduces the need for expensive plant equipment and high-pressure components and provides a risk free and safe option for end users.
The substation design is specific to each project and can deliver the LTHW, CHW and DHW services through the entire building, coupled with Pressure Independent Control Valves (PICV), smart controls and energy meters this solution can help the building work more energy efficiently. The stations can also include pumps and equipment for , pressurization, dosing, degassing, dirt and air removal.
One of the most important aspects to a substation is the control of the heat exchanger and the balancing of the systems.
A variety of methods exist for balance and control of heat exchangers, PICV are a widely used and highly effective method of achieving such control. They enable a very high control authority and flow balance within a system with variable differential pressure, at all load conditions. PICVs also have the ability to control at very low flows in proportion to the design flow (control ratio). As a further benefit, less space is used by combining the balancing and control valve in one device.
Conclusions
With tall buildings a growing sector in the UK, Danfoss has sought to increase its knowledge of the unique characteristics and challenges when designing the hydronics of such buildings. Controlling pressure, ensuring shell and core integrity, DHW production and key valve arrangements are just some of the critical factors in achieving tall buildings that operate efficiently for their owners and occupants. From working with HVAC designers and engineers we believe that prefabrication has a key role to play in the design and delivery of this type of project. Prefab packages incorporating key components such as heat exchangers, in-built controllers, PICVs, and even energy meters, can be delivered to site already (PED) pressure tested and certificated. These plug and play solutions are not only easier to install but also offer time and cost saving advantages for contractors and the convenience of using one supplier and one point of contact for the whole assembly. For more information or to arrange a CPD Seminar on overcoming the challenges of tall building design visit www.heating.danfoss.co.uk or e-mail ukheating@danfoss.com