New age de-gassers
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. However as the in-flow rate via flow restrictors is lower than the out-flow a vacuum is produced in the vacuum column allowing air to be released under the low-pressure conditions.
This is further assisted by pall rings in the base of the vacuum column. The released air is then forced out of the column through an automatic air vent situated at the top of the column by the next incoming charge of water.
The cycle frequency can be adjusted so that continuous deaeration cycles are programmed during the early life of the system and reduced to a maintenance level when the majority of the air has been removed. Furthermore, certain models also incorporate a make-up water facility that can be controlled on-board or by a compressor expansion control automat. The pumps are also used to maintain system pressure.
Pressure Step Degassers are compact units with a footprint of only 710 mm x 540 mm. Multi-language real time system status and diagnostics are displayed on a clear LCD screen. Programming is achieved through four push buttons on the controller. A common alarm volt-free is included for remote indication of a fault.
Installation is extremely simple requiring only two ¾” connections and a 230V 50Hz 1ph electricity supply. By installing a valve in the main line between the two PSD connections de-aeration may also continue even when the circulation is switched off.
While there is no theoretical maximum system volume that can be deaerated by a single ENA for practical purposes each unit is recommended for a system volume of 100m³. For larger systems the number of ENA’s can be increased accordingly. Costs are similar to that for a DN 300 tangential air separator. However with the added functionality of an ENA such as pressurisation and make-up supply the two pieces of equipment should not be compared directly.