Air Separators

Air, or more specifically the oxygen content in the air, that has entered a piped water system during installation or operation, corrodes the steel surfaces in Heating and Chilled water systems, which creates the black sludge known as magnetite. The magnetite collects in comparatively still areas, wears out pump seals, blocks up heat exchangers and fouls valve seats. Entrained air affects the pump’s ability to efficiently circulate the water, so increasing the power required to drive the pump. This CPD Presentation is designed to outline the methods for deaeration and dirt separation and illustrate the benefits of installing Stainless Steel air and dirt separators to your new or existing systems.

Air in systems

Air in Systems - Chart 1Air will be present in piped water systems both as a result of incomplete purging after the system is filled but also due to the release of dissolved air. The amount of air dissolved in the water depends on the temperature and pressure that may be determined and explained using Henry’s Law. Henry’s Law is that at a particular temperature the amount of gas that will dissolve in a liquid is proportional to the partial pressure of that gas over the liquid – the potential solubility of air in water is shown in Figure 1

For example, a heating system open to atmospheric pressure (i.e. 0 bar gauge) that is initially full of water at 10°C potentially has about 22 litres air dissolved for every cubic meter of water (22 L/m3). When the low temperature system is heated to 60°C the volume of dissolved air falls to about 10 L/m3 – this released air (12 L/m3) circulates around the system to create the air pockets at high points such as tops of radiators. considering the effect of pressure, for example at a system temperature of 60°C, for every reduction of 1 bar pressure (equivalent to a pipe rise of 10 meters in a building) there is potentially about 11 L air released for every cubic metre of water.

Air and Microbubble removal

Air is typically present in piped water systems as a result of the incorrect bleeding of the system after filling, In addition no matter at what pressure the system is operating, air will leak into the system through ‘microleaks’, seals, glands and by diffusion through the pipe walls. Air will increase system noise and pressure drops, and also increase pumping costs or reduce the pump capacity due to cavitation. Components will be damaged such as the ‘pitting corrosion’ of pump impellors from microbubbles and drawing of valve seats.

Following a correct design process, and using a combination of manual and automatic air vents the bulk of air can be removed from a piped water system. However, when water is being pumped around the system microbubbles cannot be readily removed by our automatic air vent, as the momentum of the water/air passing under the automatic air vent connecting tee does not allow the air to rise into the air vent. When the circuit is not operating air can gravitate in the still water to the top of the system – this is why automatic air vents are normally located at the top of risers.

The Coalescence of Microbubbles In heating & chilled water systems

The explanation for this is simple; the unique concentrator inside the Deaerator allows no direct passage of water straight through the unit. Due to the multitude of fine wires, it is impossible for the flow water to pass directly through. Therefore, all microbubbles eventually come into contact with the filter and the coalescence process takes place.

In the event of increasing head of water the microbubbles do decrease in size. Nevertheless just because the microbubbles are getting smaller this does not mean the microbubbles cannot be removed in this range of water. The explanation is because the microbubbles collide together to make one larger bubble then rise to be vented. However, above 60 meters the efficiency of the unit is reduced.

The principle above is the same for chilled water systems except the 40 meter head of water applies.

Our Separators have been tested & they can remove microbubbles down to 20 microns (0.02mm) in size at six bars with a water temperature of 10 degrees centigrade.

Air separators are normally cheap and rely on relatively low centrifugal forces. They can separate out the larger air bubbles circulating around the system, but will not be able to remove microbubbles because the environment in the vessel remains turbulent, not still.

CVA - The DeaeratorThe Deaerator

The simplest and most common deaerators, should be installed close to the point where the bubbles are formed. This is normally the system’s hottest point (in a heating system this would be the flow header). As the water cools it will absorb air that then returns back to the boiler, in solution. The absorbed air is then released as the boiler reheats the water and then the following deaerator removes a proportion of the released air, until eventually all the air pockets have been automatically removed by the Coalescence process. The water is deaerated to an extremely deep level to the extent that at no point in the circulating system can air be released. The absence of air means oxygen corrosion is also minimized to the point it does not exist.

The deaerator should always be installed at the hottest point in the system (on a boiler flow or a chiller return, for the deaerator to operate properly it must also be located where the static pressure is lower – preferably on the suction side of pumps).

Temperature differential deaeration requires no input from operatives (except for the initial manual venting procedure), and is fully automatic.

CVA - Chart 2Case study: Reduction of pump power through deaeration – London UK

An existing heating system was designed to supply 103m3/hour (28.6 L/s). However when in operation large fluctuations in flow rate were observed, although, the pump was maintained at a constant speed. The system was monitored both before and after the fitting of a deaeration system. The effect of the deaeration may be clearly seen on the outline pump/system curves shown in figure 7. Deaerating the system effectively reduced the system resistance so moving the operating point from one to two, employing a smaller impeller and reduced pump head. This reduced the power input to the pump by 31% compared to the original system without a deaerator.

 

Air and Dirt Separators Clients 1
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