Pressure step de-gassers

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Figure 1 depicts the inflow of water from the system whilst the pump is returning the previous charge back to the system after de-gassing. A vacuum is created in the vacuum column.

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In Figure 2 the pump is switched off and the vacuum column is refilled with the next water charge. The incoming charge forces the released air from the column through the automatic float vent.

Jim Martin looks at the role of pressure step de-gassers in the removal of air from a closed chilled or heated water system.

Air that is present in a closed, chilled or heated water systems can cause numerous problems such as noise, cavitation at the pump impellor, corrosion when ferrous components are present as well as circulation and thermal inefficiency.

It is for this reason that for a closed system to work efficiently, air should be removed not only during the initial filling stage, but on a continual basis throughout the operating life of such a system. The reason for continual deaeration is that air can enter a system either through leaks or by the addition of make up water, which is typically saturated with air.

Air, which is largely made up of oxygen and nitrogen, is present in three forms, free air, micro-bubbles and dissolved air. The concentration of air in water can be determined using Henry's Law. To summarise this law the amount of air that can dissolve in water decreases with temperature and increases with pressure.

Conversely air will be released from solution in conditions of reduced pressure or increased temperature. Therefore for an air separator to be most efficient is should be situated accordingly. In a chilled water system a typical tangential air separator is ideally situated on the return line to the chiller/heat exchanger and on the suction side of the pump.

Although the use of tangential air separators is common within the Middle East region there are more efficient solutions available including micro-bubble pall ring de-aerators such as the in-line Flamco Flamcovent which uses pall rings to attract and remove micro-bubbles as small as 18µm.

This method also applies equally to dirt removal with up to 99% of particles >63µm being removed. However Pressure Step Degassers are now gaining region wide acceptance even though they have been in use in European markets for many years. Cost, space and efficiency are just some of the reasons that ENA's are becoming more attractive.

By applying Henry's Law and reducing the water pressure air is allowed to come out of solution and be removed effectively from the system. As an ENA is installed on a branch and not in-line as with a tangential air separator the issue of pressure drop is reduced dramatically. The method of operation is relatively simple.

A programmable micro-processor controlled is set to remove a quantity of water from the system during each cycle. This water is then fed through to a vacuum column and then pumped back into the system.

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