DEAERATOR ( PART 1 )

DEAERATOR ( PART 1 )

FUNCTION

-The function of the deaerating heater is to remove dissolved non-condensable gases and to heat boiler feed water. A deaerating heater consists of a pressure vessel in which water and steam are mixed in a controlled manner. When this occurs, water temperature rises, and all non-condensable dissolved gases are liberated and removed and the effluent water may be considered corrosion free from an oxygen or carbon dioxide standpoint. Free air or other non-condensable gases should be vented prior permitting the fluid to enter the deaerator.

-A deaerating heater is the watch dog of boiler plant as it protects the feed pumps, piping, boiler and any other piece of equipment that is in the boiler feed and return cycle from the effects of corrosive gases, i.e. oxygen and carbon dioxide, to a level where they are no longer a corrosion factor.

Principles of Deaeration

-There is physical law which states that the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. Another law states, the solubility of a gas in a liquid decreases with an increase in temperature of the liquid. Experience has shown that more rapid and more complete removal of non-condensable gases from a liquid is obtained when the liquid is vigorously boiled or scrubbed by condensable or carrier gas bubbles.

-Therefore, essentially the deaerating heater must first heat the feed water to as high a temperature as possible, i.e. to the temperature corresponding to the steam pressure, It must vigorously boil and scrub the heater water with fresh steam, which can carry to the liquid surface any traces of oxygen and carbon dioxide.

-The partial pressure of the oxygen and carbon dioxide in the steam atmosphere must be maintained as low as possible, particularly at the point where the deaerated water separates from the steam. Non-condensable gases must be continually withdrawn from the heater at the rate at which they are being liberated.

Deaerator Heater

-A deaerating heater utilizes steam by spraying the incoming water into an atmosphere of steam in the preheated section (first stage). It then mixes this water with fresh incoming steam in the deaerator section (second stage).

-In the first stage the water is heated to within 1.1C of steam saturation temperature and virtually all of the oxygen and free carbon dioxide are removed. This is accomplished by spraying the water through self-adjusting spray valves which are designed to produce a uniform spray film under all conditions of load and consequently a constant temperature and uniform gas removal is obtained at this point.

- From the first stage the preheated water, containing minute traces of dissolved gases, flows into the second stage. This section consists of either a distributor or several assemblies of trays,

Here the water is in intimate contact with an excess of fresh gas-free steam. The steam passes into this stage and it is mixed with the preheated water. Deaeration is accomplished at all rates of flow if conditions are maintained in accordance whit design criteria. Very little steam is condensed here as the incoming water has a high temperature caused by the preheating. The steam then rises to the first stage and carries the small traces of residual gases. In the first stage most of the steam is condensed and remaining gases pass to the vent where the non condensable gases flow to the atmosphere, A very small amount of steam is also discharged to the atmosphere which assures that the deaerating heater is adequately vented at all times.

- The water which leaves the second stage falls to the storage tanks where it is stored for use. At this time the water is completely deaerated and is heated to the steam saturation temperature corresponding to the pressure within the vessel.

VENT

-Efficient removal of the non-condensable gases from the deaerating heater requires that the vent valve be opened sufficiently to allow complete discharge of the gases passed to the vent condenser outlet pipe. The maximum concentration of the non-condensable gases such as oxygen, carbon dioxide passing out the vent depends on the degree of condensation produced by the steam and gas mixture passing through and around the spray created by the special spray valve.