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A deaerator is a device that is widely used for the removal of air and other dissolved gases from the feedwater to steam-generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.

There are three basic types of deaerators: tray-type, spray-type, and steam injecting type:

• The tray-type (also called the cascade-type) includes a vertical/horizontal deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.

• The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.

• The steam injecting type consists only of a horizontal cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank like the spray-type deaerator.

Theory Of Oxygen Removal

Oxygen is soluble in water according to Henry’s Law in proportion to the partial pressure of the gas when it contacts the water. The normal source of oxygen in water is the atmosphere, which is 21% oxygen, and contributes about 3 psi to the normal atmospheric pressure of 14.7 psi. At 60ºF, water in contact with the atmosphere will contain about 9.7 ppm of O2. The solubility of oxygen in water decreases as the water temperature increases. As the water temperature increases, the amount of water vapor present in the atmosphere above the liquid also increases. Values for saturated O₂ (ppm) for a given temperature can be found in the table below. This double effect means that less O2 can be held by water as its temperature is increased, theoretically becoming ‘zero’ when the water reaches its boiling point (saturation). As a result of these physical characteristics, oxygen can be removed from water (or better, the amount of O2 that the water can hold can be reduced) by raising the temperature and reducing the concentration of O2 in the atmosphere above the water.

Cardinal Principles of Efficient Deaeration

1. The water must be heated to full saturation temperature to create an atmosphere of 'zero' gas solubility.
2. Water must be agitated (by trays, atomization, or other means) to allow the gases to be 'scrubbed out' in a reasonable amount of time.
3. Released gases must be diluted by sufficient steam to produce minimum partial pressure above the water surface.
4. The released gases must be vented from the deaerator.

Temperature (°F)	O2 (ppm)
32	15.2
40	12.5
50	11.0
60	9.7
70	8.8
80	8.0
90	7.2
100	6.7
110	6.1
120	5.5
130	5.1
140	4.7
150	4.2
160	3.7
170	3.1
180	2.7
190	1.8
200	1.1
210	0.5

Types of Deaerators

There are many different horizontal and vertical deaerators available from a number of manufacturers, and the actual construction details will vary from one manufacturer to another. The following sections with detail the differences between the three major deaerator types and sub-types within each design.

Tray-Type Deaerator

A tray type deaerator incorporates two stages in order to completely deaerate the incoming water to 7 ppb O2 content. The first stage is a spray type pre-heater which includes the water inlet nozzle, a water box or header to house the spray nozzles, and vent baffles to collect and dispose of the concentrated non-condensable gases that have been released from the water. Since the gases are a little more corrosive when concentrated, it is normal practice to construct the entire pre-heating section of corrosion resisting material, usually 18-8 stainless steel.

Directly below the spray section is the tray section which includes a set of trays and a tray enclosure which supports the trays and provides the baffle needed to contain and control the direction of flow of water and steam through the trays. Since most of the oxygen present in the water is removed by the spray pre-heater, the water that enters the tray section has a low corrosive potential and the use for corrosion resistant materials of construction for the tray section is not necessary to ensure a long service life unless the parts are constructed of very light gauge material.

The size and arrangement of the deaerator trays varies among the manufacturer but, in general, deaerator trays are press formed from corrosion resisting steel. Thin material is used to decrease handling weight and to allow forming of the trays. Trays may be furnished as individual tray units or as assemblies of a number of individual tray units. The surface of each tray is provided with adequately sized openings to provide a path for steam and water flow. The remaining metal surface serves to collect the water and cause it to flow as films to provide the steam-water contact area. The trays are installed in multiple layers to develop a ‘tray stack’ of sufficient height, according to each manufacturer’s process, to ensure that the contact time needed for complete deaeration is available. Tray type deaerators are relatively unaffected by high ‘turndown’ (can be operated 0-100% of rated capacity), have a low steam side pressure loss, and can be built to handle any capacity (largest so far about 16,000,000 pph).

Tray type deaerators may be further classified with respect to how the steam and water flow through the tray stack. Two schemes are mainly used in tray type deaerators– counterflow and parallel flow. Both flow schemes can achieve 7 ppb final O2 performance and it is left to the deaerator manufacturer to explain the advantages of his design approach.

Parallel Downflow

File:Parallelflow.png

In parallel downflow units, water enters the pre-heating compartment through stainless steel spring loaded spray nozzles. The partially deaerated water falls to the bottom of the pre-heating compartment from which it flows through water seals located between the sprays and the top of the tray stack. The water flows from the water seals to the trays. The water seals serve to prevent the bypassing of steam from the tray compartment to the pre-heating compartment and ensure that no non-condensable gases pass from the pre-heater to the tray compartment. The water seals are designed with troughs that evenly distribute the water over the trays. It is the water seals that permit the operation of the deaerator in a parallel downflow mode. The water seal also eliminates direct impingement of sprays on trays, ensuring even and consistent distribution of water over the tray stack. It is designed with troughs that evenly distribute the water over the trays. In addition, water delivered from the spray area to the trays is at a uniform temperature and low dissolved oxygen gas level. Cold spots and stagnant areas are eliminated, and proper water distribution onto the trays is assured at all loads.

Steam enters the deaerator in the space provided below the water seals and above the trays. The steam is forced to flow downward through the trays, co-current with the water flow, due to the water seals. The water and steam mixture is agitated in the tray stack to scrub out the final traces of dissolved oxygen. The water leaving the tray stack is completely deaerated and falls to storage where it is available for immediate use. The steam leaving the tray compartment is directed around the outside of the compartment to the pre-heater where it is condensed by the incoming water with only a small amount vented to atmosphere along with the removed non-condensable gases.

Counterflow

Figure 1: A schematic diagram of a typical counterflow tray-type deaerator.

The typical horizontal tray-type deaerator in Figure 1 has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical dearation section above the perforated trays and flows downward through the perforations. Low-pressure dearation steam enters below the perforated trays and flows upward through the perforations. Some designs use various types of packing material, rather than perforated trays, to provide good contact and mixing between the steam and the boiler feed water.

The steam strips the dissolved gas from the boiler feedwater and exits via the vent at the top of the domed section. Some designs may include a vent condenser to trap and recover any water entrained in the vented gas. The vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam.

The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm. External insulation of the vessel is typically provided to minimize heat loss.

Spray-Type Deaerator

The first stage of deaeration in a spray type deaerator is the same as a tray type deaerator, where water is sprayed into a steam atmosphere via a spring loaded spray nozzle. The difference occurs during the second stage where deaeration is accomplished by dividing the water flow into small pieces (droplets, strings) and mixing it intimately with the steam flow. In contrast to the tray type, the second stage of deaeration is accomplished with a very much shortened contact time, maybe 1/10 second compared to few seconds in a tray design. This allows a spray type deaerator to be more compact than a tray type of equal capacity. Most spray type deaerators are housed in a single, horizontal tank, where the deaeration section (first and second stages) and stored water share the same volume in the tank. Spray deaerator designs though come at the expense of a small, but increased, steam pressure loss. With respect to obtaining sufficient ‘spray’ action to ensure full deaeration of the water, there are different approaches that divide the spray type deaerator second stage designs into two classifications, fixed orifice or variable orifice.

Fixed Orifice

In the industry, it is common to refer to fixed orifice designs as ‘scrubber’ types which causes the water to be atomized by steam in a parallel (co-current) mode. The sprayed water from the first stage is collected into a downcomer pipe which moves the water to the second stage where the atomization with the steam occurs. The water is introduced into the scrubbing section where it mixes with the steam at the bottom of an enclosed chamber. The energy from the steam forces the water upwards through a series of baffles where the remaining oxygen is removed from the water. The water leaves the scrubbing portion and falls into the tank where the stored water is held to be pumped to the boiler feed pumps. The non-condensable gases will then leave the deaerator through the vent condenser. A fixed orifice design has a limited range of operation because at low load the mixing and atomizing effect is reduced due to reduced steam velocity through the orifices.

Variable Orifice

The need for a spray type deaerator with improved turndown capability led to the development of a variable orifice design. Variable orifice designs are commonly referred to as 'atomizing' deaerators and required an atomizing valve to perform the second stage of deaeration. The variable orifice feature maintains the steam velocity needed for optimum atomization of the water even as the load decreases. This is accomplished by causing the steam to flow through a spring or weight loaded valve (atomizing valve) just prior to making contact with the water where the high speed jet of steam atomizes the liquid. A turndown of 20:1 is readily attainable with a variable orifice spray type deaerator.

Steam Injection Type Deaerator

Figure 2: A schematic diagram of a typical spray-type deaerator.

As shown in Figure 2, the typical steam injection deaerator is a horizontal vessel which has a preheating section (E) and a deaeration section (F). The two sections are separated by a baffle(C). Low-pressure steam enters the vessel through a sparger in the bottom of the vessel.

The boiler feedwater is sprayed into section (E) where it is preheated by the rising steam from the sparger. The purpose of the feedwater spray nozzle (A) and the preheat section is to heat the boiler feedwater to its saturation temperature to facilitate stripping out the dissolved gases in the following deaeration section.

The preheated feedwater then flows into the dearation section (F), where it is deaerated by the steam rising from the sparger system. The gases stripped out of the water exit via the vent at the top of the vessel. Again, some designs may include a vent condenser to trap and recover any water entrained in the vented gas. Also again, the vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam

The deaerated boiler feedwater is pumped from the bottom of the vessel to the steam generating boiler system.

Deaeration Steam

The deaerators in the steam generating systems of most thermal power plants use low pressure steam obtained from an extraction point in their steam turbine system. However, the steam generators in many large industrial facilities such as petroleum refineries may use whatever low-pressure steam is available.

Oxygen Scavengers

Oxygen scavenging chemicals are very often added to the deaerated boiler feedwater to remove any last traces of oxygen that were not removed by the deaerator. The most commonly used oxygen scavenger is sodium sulfite (Na₂SO₃). It is very effective and rapidly reacts with traces of oxygen to form sodium sulfate (Na₂SO₄) which is non-scaling. Another widely used oxygen scavenger is hydrazine (N₂H₄

Other scavengers include 1,3-diaminourea (also known as carbohydrazide), diethylhydroxylamine (DEHA), nitriloacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and hydroquinone.

See also

• Boiler
• Air preheater
• Economizer
• Feedwater heater
• Fossil fuel power plant
• Thermal power station
• Degasification
• Defoamer