CPD Programme
CPD Programme
The CIBSE Journal CPD Programme
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Reducing operational energy consumption by deaeration and dirt removal in water systems
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 cooling systems creating 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 article will outline the methods for deaeration and dirt separation and illustrate the benefits of appropriate equipment application
Air in systems
Air 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 depend 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 (ie 0 bar gauge) that
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is initially full of water at 10°C potentially has about 22 litres air dissolved for every cubic metre 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. Similarly, 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 metres in a building) there is potentially about 11 L air released for every cubic metre of water.
180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10
10°C
7 bar
6 5 4 3 2 1 0
Gauge pressure
20°C 40°C 60°C 80°C 100°C 120°C 140°C 160°C 180°C
60°C
> Figure 1: The solubility of air in water
May 2010 CIBSE Journal
63
Solubility L/m3
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