The science behind deaeration and what it means for heating systems

In the final instalment of Spirotech’s three-part blog series, Spirotech’s Kevan Peaker takes a more scientific look at deaeration and the importance of fitting the right kit in the right size building.
In our first blog post, we took a closer look at dirt separation and dispelled a few myths about functionality, performance and installation. We also faced head-on the importance of deaeration, outlined how air effects a heating system and looked at the need to adopt a combined approach to dirt and air removal in order to minimise, and where possible, eradicate air and the associated problems of dirt from heating systems.
In part two we addressed the effect that air has on a heating system, principally the noise and cold spots it causes. These are problems that are becoming more widely understood in the domestic heating industry, and it’s promising to see that there’s a definite upward trend in the number of installers who now install dirt and air removal as a matter of course on new and retrofit boiler installations.
Acknowledging that deaeration should be done is one thing, but understanding the practice is quite another. This deeper understanding is essential, especially for installers who install or plan to fit deaeration products on larger properties, notably ones that are more than three floors in height.
As we move beyond the domestic family home into larger domestic and semi-commercial properties, issues of pressurisation, vessel sizing and system degassing may need to be factored into designs, or at least considered. The average family home, for example, heated by an energy-efficient boiler can be fitted with a standard SpiroTrap MB3 and SpiroVent RV2 combination, which will successfully remove air (including microbubbles) and dirt from a system.
In case of an excessive static head (pressure) above a deaerator, dissolved air cannot be released from the fluid.  In these circumstances, it is very hard to predict where in the system air bubbles will emerge from the fluid.  Apart from that, the point where microbubbles emerge can change depending on fluid temperature and hydrostatic pressure (Henry’s Law).  Rule of thumb for maximal static height is: heating< 15 m, cooling < 5m.  Above the critical height, a vacuum degasser is generally a more effective solution.  Degassing and managing pressurisation is key to good system design, which is commonplace in the commercial arena and is increasingly being adopted by installers with engineering backgrounds in the domestic market too.
What then is the science behind deaeration, how does air react in a system and how does it impact performance?
Deaeration is based on Henry’s Law (or Henry’s coefficient), which states that the partial pressure of the gas within a liquid will come to equilibrium to the partial pressure of that same gas external of the liquid; which explains why a fizzy drink will eventually go flat.
If we take 1litre of water as an example: at 20oC under atmospheric pressure, 1litre of water will contain 35ml of dissolved gases. Through high temperature and low pressure these gases come out of solution, so if you increase the pressure on water, not only do you increase its boiling point but you also increase the amount of gases that it retains or absorbs. When gases are dissolved in water, they hold no volume, but when they come out of solution they do.
Water will try to find its equilibrium, so it will reabsorb the gases that have been removed, if given the opportunity. The gases (oxygen and nitrogen) re-enter the central heating circuit via various means: through micro leaks, connection points (diffusion) and via expansion vessels.
Oxygen and nitrogen can both cause problems within a system, but it is oxygen that causes the corrosion. Nitrogen is not detrimental to the circuit from a corrosion perspective, but from a noise perspective, it is the chief culprit and is the root of irritating radiator and system noise. Because of the combined problems caused by these gases, it is important therefore to remove them all.
By installing a deaerator, positive gases can be expelled when they come out of solution, having an impact on corrosion.
Where a deaerator is fitted onto an existing system, homeowners will still enjoy the same benefits of air removal, which includes a quieter system, less friction and better transference of heat in the main heat exchanger; the latter being key to avoiding long-term system damage. When a boiler is fired up and heat is introduced, microbubbles will immediately start to form on the wall of the heat exchanger and can therefore hinder the transfer of heat between the heat exchanger and the system fluid.
Air, and its effective removal, is at the heart of good system functionality, regardless of whether a heating system is being designed for a two bed flat, a family home or an office block. The effects of air on a system will start to have a detrimental impact long before dirt will, so deaeration should be part and parcel of all new installations. System design must also be factored in for larger properties, and it should start with a good understanding of how air works within a system and the tools needed to effectively remove it. Armed with this knowledge, the UK market will be well placed to tackle an issue that has for too long hampered system performance.
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