INDUSTRIAL & COMMERCIAL HEATING


Key considerations in tall buildings and the role of pressurisation


Keith Mileham, technical sales engineer from Spirotech, takes a look at the key elements that need to be considered when installing a heating or cooling system that can meet the demands that will be placed upon it in buildings that are 15 metres high or more. At the heart of this is the pivitol role that system pressurisation will play


W


ith an ever-increasing need for additional accommodation within the UK, it is hardly surprising that tall buildings are providing vertical solutions


to accommodate the growing population within our cities. In today’s modern world, tall apartment blocks have become a prominent feature of urban landscapes, capable of accommodating a significant number of residents. As buildings reach new heights, it’s not surprising that specific challenges need to be met including that of temperature or pressure management - especially now that we are being encouraged to reduce our Carbon footprint. Meeting the demands of tall buildings requires advanced engineering solutions to provide comfortable living conditions for the occupants. Once you get above a certain static height, then the pressure becomes too great to easily release dissolved gas from the heating system liquid. A build-up of air will negatively impact on a system and therefore it is a fundamental requirement to prevent this from happening in the first place. Having infiltrated the system, air will have an adverse impact resulting in diminishing efficiency, higher energy bills, corrosion, additional maintenance and, finally, breakdown!


When designing a standard LTHW system with boilers installed within a ground floor plant room, the BS7074 and BSEN12828 standards ask for an overfill of +0.3/ +0.2.


However, if the ASHP is on the roof, you should be questioning whether this overfill is enough? Therefore, when designing a system of this type, should the system be overfilled by a further 1bar to stop cavitation in the ASHP? The key to creating the optimum HVAC system is at the design and planning stage. Far too often the wrong ‘kit’ is specified due to not understanding the system parameters and then, moving forward, the problems in the system mount and, increasingly, repair and maintenance will be required.


So, what does a pressurisation unit do?


A pressurisation unit is comprised of a make-up unit and an expansion vessel. The make-up unit constantly monitors system pressure and, when needed, adds refill water. It must also always ensure positive pressure at all points and be fitted with system-warning indicators.


As the fluid in the system heats up, it expands and the expansion vessel will absorb the expanded volume of water. When the water cools, the vessel will allow the expanded water volume back into the system. Should the pressure move outside the designed operating range, an alert can be sent to advise of a problem with the system.


When it comes to methods of control for maintaining and improving the condition of the system fluid, pressurisation is the most important element, followed by air, then dirt, and finally chemicals. Each of these has a role to play in a heating system’s hydronic stability. If the pressurisation unit is set-up incorrectly, an air problem will be created. This leads to dirt and, typically, a high chemical usage. You cannot fix any of the ‘upper’ levels without first correcting the foundation - the pressurisation.


Degassing a closed heating or chilling system


A fundamental element at the design stage is to understand Henry’s Law which states that: ‘Gas will dissolve in a liquid until there is a balance between the partial pressure of the gas and the pressure of the liquid.’ This means that as the temperature rises, or the pressure drops, the mass of gases that dissolves in a liquid will be reduced. Therefore, at certain points within a system, the amount of gas absorbed, or dissolved gas emitted, will depend on the pressure and temperature. It is recognised that Inline Deaerators can operate at heights up to approximately 15 metres, at an operating flow temperature of around 82 degrees. If the flow temperature is designed at a lower temperature, the Static height will be reduced.


Releasing dissolved air


Today’s highly efficient heating and cooling systems are designed to work at their most effective with ‘air-free’ water. For larger buildings this requires a vacuum degasser to be integrated with the pressurisation unit. Vacuum degassers work by reducing the


pressure in the vacuum vessel that sits within the unit. This liberates dissolved gases from the system liquid in the vessel and the accumulated gases can then be expelled. These powerful vacuum degassers are needed because ‘ordinary’ in-line deaerators will not work anywhere near as effectively in high-rise offices and other tall buildings. By continuously degassing quantities of the system liquid, the vacuum degasser keeps the negative effects of dissolved and liberated gas to a minimum. As buildings have grown in size, so the


26 BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2023 Common system issues


There are four issues that regularly ‘crop-up’ around pressurisation in under-performing systems:


Poor design – sometimes it’s a lack of truly understanding what a pressurisation unit does. It is, therefore, important to recognise that the pressurisation unit and expansion should not be treated as a commodity. Quite simply, it’s the heart beat of the system, dictating what the final working pressure of other equipment on the system will be.


If the expansion vessel and/or the specialisation unit is located in the wrong place; sometimes the connecting pipework is the wrong size.


Commissioning – to ensure the unit is correctly commissioned by the manufacturer and failure to understand the pre-charge and cold fill pressures.


Poor maintenance – a failure to check and adjust the pre-charge pressure of a vessel at least every 12 months.


All the above can be avoided if those involved in the process – from specification through to installation – are aware of the latest developments, ensure that calculations are fully worked through and checked; and follow best practice – which includes regular maintenance checks.


requirements for larger, more powerful vacuum degassers have grown. The latest can degas up to 1,000 litres of liquid an hour. The drive for efficiency and energy-saving has also seen manufacturers increase the level of connectivity, with more sophisticated vacuum degassers including ‘intelligent’ controls enabling remote access and operation via ‘the cloud’.


Read the latest at: www.bsee.co.uk


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