CPD Programme
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Air removal
Air will be typically present in piped water systems as a result of the inadequacies of bleeding the system after filling (for example, radiators have a 15mm air pocket that cannot be manually bled out). 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 so increase pumping costs or reduce the pump capacity. Components will be damaged such as the ‘pitting corrosion’ of pump impellors from microbubbles (bubbles just a few microns in diameter) and drawing of valve seats. Following a correct design process,
and using a combination of manual and automatic air vents (AAVs), 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 AAVs, as the momentum of the water/air passing under the AAV 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 AAVs are normally located at the top of risers. 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. As air (particularly microbubbles) cannot
be efficiently removed in a turbulent zone, specific low velocity regimes are needed so that the bubbles can be eliminated by buoyancy forces (outside the main water flow). Hence the development of the deaerator as in Figure 2. Deaerators need either to be
tall to create this ‘still’ water environment, or of an increased bore, to create a laminar flow area outside the turbulent waterflow, and thus allow the air to rise naturally and be vented from the system. The simplest (and most common) deaerators, known as a temperature differential deaerator, should be installed close to the point where the bubbles are formed – the system’s hottest point (in a heating system this would be the flow header). As the water cools it will absorb air (for example, air trapped in the top of a radiator) 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 deaeration 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 so minimised that it almost does not exist.
As air (particularly
microbubbles) cannot be efficiently removed in a turbulent zone, specific low velocity regimes are needed so that the bubbles can be eliminated by buoyancy forces (outside of the main water flow)
The deaerator should always be installed
at the hottest point in the system (on a boiler flow or a chiller return; chilled beams or ceilings require further consideration to locate the hottest point.) 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. These devices operate successfully on
Figure 2: Section through deaerator
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CIBSE Journal May 2010
heating systems (flow temperature 80°C) with up to a 15-metres static head above the hottest point. With the lower temperatures used in condensing boiler systems the maximum static head for successful temperature differential deaeration is reduced somewhat below 15 metres. For chilled water installations this static head is about
Figure 3: Pressure differential deaerator
five metres. Where the static head is greater than the capabilities of the temperature differential deaerator (as discussed above)
a pressure differential deaerator (Figure 3)
may be applied. A small volume of water is removed from the system water, exposed to a vacuum of 0.05 bar absolute (by using a separate pumping device), deaerated, and returned to the system. This process is repeated until the entire system is fully deaerated equivalent to the 0.05 bar absolute. The unit would normally automatically start each day to produce a deep level of deaeration throughout the system life. Pressure differential deaeration is automatic, but does require annual maintenance because of the active components. While temperature differential deaeration probably accounts for 90 per cent of commercial installation deaeration, the remainder uses pressure differential deaeration. In conventional domestic installations temperature differential deaeration is normally suitable.
Dirt removal
Dirt (such as sand, swarf from pipe cutting, plastic, welding slag etc) will enter a piped water system while it is being fabricated. Systems should thus be properly flushed prior to use. However, inefficient flushing will leave some of this debris in the pipes and, once in operation, scale and particles from corrosion will also accumulate. This can block heat exchangers – particularly more modern low water content heat exchangers; heat emitters and underfloor heating pipes become partially blocked and the heat output is reduced. A common way of reducing particles in
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