ENERGY MANAGEMENT, OPTIMISATION AND SUSTAINABILITY
Free air – the enemy of an efficient heating system
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Mike Pitt, Spirotech technical advisor UK, explains the role of deaerators and vacuum degassers in helping to ensure a heating system runs at optimal performance, thereby cutting the cost of energy bills and repair and maintenance expenditure
t the heart of an efficient heating system is the elimination of free air circulating in the water. Failure to trap and remove gases will – over time – result in the
build-up of magnetite and other undesirable elements which have a very detrimental effect on performance. Deaerators are installed on the flow side of the boiler and, as the water heats up, the gases dissolved in solution are then liberated. The resulting microbubbles are caught, coalesce on our Spirotube, then rise up and are expelled through the automatic air vent. Microbubble deaerators remove the microbubbles and circulating air left behind by automatic air vents, or after bleeding valves. They also prevent dirt formation and related negative effects on energy efficiency, failure sensitivity and wear and tear. Today’s highly efficient heating and cooling systems are designed to work at their most effective with ‘air-free’ fluid. For larger buildings, such as factory spaces and tall buildings, a vacuum degasser or series of vacuum degassers should be integrated with a pressurisation unit. As the degassers do not require thermal intervention to liberate the gases these are best situated on the return pipework with cooler temperatures and neutral system pressures. Water by its very nature absorbs gases easily until saturated, with the level of saturation depending on the pressure and temperature. A vacuum degasser subjects batched volumes of system water to very low pressure – far below normal atmospheric pressure – and this results in the release of the dissolved gasses. In essence, we are taking the vacuum chamber
pressure down to negative values and the atmospheric pressure is removed from the fluid. This means there is no longer any pressure holding the gases in the water and so they are liberated from solution within the chamber and expelled in a controlled manor. Vacuum degassing uses the principles of Henry’s law - the effect of pressure and temperature on the absorbent capacity of water in degassing situations, to keep the entire system fluid at its lowest possible dissolved gas content.
As buildings have grown in size, the
requirements for larger, more powerful degassers has grown. The height of a building determines how much pressure – from top to bottom - will be created in the system. Therefore, it is vital to correctly size the units and ensure the pump within the degasser is powerful enough for the job – it’s no good using a small pump if pushing against a system pressure of 10 bar, for example.
Our most popular degassers operate at pressures ranges from 1 bar to 6 bar, but we have units that can deal with pressures up to 10 bar and even 16 bar. A small system in terms of volume and pressure should only need one vacuum degasser. However, in the case of large volume systems, then the application would benefit from multiple units to handle such fluid volume. What’s important is how many litres of fluid a unit can deal with over a set time. For example, the Spirotech S250 operates at a pressure of between 0.5 to 2.5 bar pressure and can handle a fluid volume of 5m3 cubed (5,000 litres). It is purely a degasser and doesn’t come with a refill option. The bigger units have a refill function, which means they can act as both a degasser and pressurisation unit. We also offer an option for clients wishing to separate pressurisation and degassing functions. The level of routine maintenance will depend on how much air has got into the system and this will depend on whether the system specification was right in the first place and, obviously, if you keep taking water out and replacing it with mains cold water, it’s going to exacerbate the problem of magnetite and dissolved gases being liberated throughout the system. Our units have a build in smart switch which detects whether gases were removed or not on each cycle, this enables the unit to operate efficiently and potentially stop the degassing process till its next programmed cycle should it not remove any gasses for a period of 10 minutes consecutively. Furthermore, our refill degassers run a degassing cycle when filling a system – degassing the fluid as we put it in. This ensures as much gas as possible is liberated before the water is even put in the pipework. Getting the design specification right from the outset is key to optimising water quality and ensuring a heating system operates efficiently and cost-effectively. This means being provided
22 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2024
with all the correct data at the commissioning stage including any future expansion plans that need to be taken into consideration. Pressurisation units and fixed gas expansion vessels need to be fitted on the return leg and on the suction side of circulating pumps. However, from time-to-time, we find them incorrectly installed on the discharge side of circulating pumps. In such cases, the pressurisation units see the discharge pressure of the pumps, not the true pressure of the neutral point in the system.
Often companies don’t specify a reserve volume in their fixed gas vessel calculation. We follow the requirements laid out in BS12828 which is to say our fixed gas vessel sizing is calculated to have a reserve fluid volume. This means that even when at cold fill pressure the system will still have a pressure resilience greater than just the static height of the building. It’s not just a matter of sizing the vessel
correctly, it’s about sizing the vessel to allow a stable working pressure hysteresis. You do not want large or erratic fluctuations in pressure.
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