What is Deaerator – Working Principle and Applications

Boilers are used in many industries to heat the water. The applications of boilers mainly include water heating, central heating, cooking, sanitation, and boiler-based power generation systems. The essential part of this boiler operation is the feedwater. This water is recycled throughout the system and is never exposed to the external atmosphere. This water has to be treated to prevent corrosion, scaling of the boiler inner surface. To overcome this, aeration has been proved to be an effective process to remove the oxygen and other dissolved gases from the water. Deaerator is the device used to treat the feedwater before moving it into the boiler.

What is Deaerator?

Water is a universal solvent that contains many dissolved gases which are highly corrosive when exposed to the boiler and boiler systems components. In addition to these dissolved gases, water also contains many dissolved minerals. So, when water is used as feed water for boilers it will damage the boiler.

When the water containing dissolved oxygen and it is added in the boiler, then corrosion and rusting forms at an accelerated rate. Iron starts to dissolve when it comes to contact with water forming Ferrous Hydroxide. Carbon dioxide present in the steam flows through all the steam piping. When this steam gives up its latent energy resulting in condensed water it combines with the free carbon dioxide and forms carbonic acid.

Aeration Process

Carbonic acid in boilers leads to the corrosion of pipes and heat transfer units. Carbon dioxide when working along with oxygen leads to 40% more corrosion and scale formation thereby damaging the boiler. Aeration process has proved to be the key to attaining highly efficient and long-lasting boiler systems. This is the device in which the aeration process takes place. It is used to remove the oxygen, carbon dioxide and other dissolved gases from the water before moving it into the boiler system. These are essential in thermal power plants, steam power generation system, petrol refineries, etc. The feedwater is first treated in the deaerator and then moved into the boiler system.

Functions of Deaerator

One of the properties of water is its surface tension, as it contains a high degree of surface tension which holds all the things together. The application of a surfactant can reduce the surface tension of water. Aeration is the process that breaks the surface tension of the water.

This function starts with the reduction of the surface tension of water through spraying or filming. Then heat is applied to the condensed water. After applying heat, the agitation process takes place. The corrosive gases separated from the water are liberated back into the atmosphere through vents.

Design and Components

Deaerator requires a high temperature and low-pressure settings to function properly. They must have the capacity to hold hot condensate returning from the system in addition to the cold makeup water. A deaerator must be mechanically designed to remove oxygen from water to 7ppb and remaining oxygen is removed chemically using the oxygen scavengers such as sodium sulfite and Hydrazine.

The design contains a makeup water inlet to let the raw water into the deaerator. A pressure relief valve and Vaccum breaker are also present to adjust the pressure in the system. A condensate inlet allows the condensed steam into the system. An operating vent is provided with an orifice plate to liberate the gases into the atmosphere. The steam is passed into the deaerator through the steam inlet.

A deaerator working with 0.5bar or 7psi pressure requires a temperature of 217 degrees Fahrenheit. The temperature and pressure values may vary depending upon the design.

Working Principle

The main aim here is to remove the dissolved gases. Applying heat is the proper way to remove the dissolved gases from the water. Oxygen comes in contact with water either from the external atmosphere or the leaks in piping. Carbonic acid is formed inside the boiler when water is heated. For corrosion-free carbon dioxide levels in the water, its pH value should be maintained greater than 8.5 pH.

Removal of Oxygen and Carbon Dioxide

The solubility of dissolved gases present in water decreases with the increase in temperature of the water. That means more oxygen and carbon dioxide will be liberated from the water with an increase in temperature. So, we need to increase the temperature of water to the value close to the saturation temperature of the water. By heating the water below the boiling point the liquid state of the water is maintained.

The makeup water is sprayed into the spray shroud through a spray nozzle. At the same time steam is also released into it. Spraying of water increases the contact surface area of water with the steam. This leads to a faster heat transfer rate. Thus, water gets heated quickly and many non-condensable gases are liberated fast. These non-condensable gases travel through the vent.

Removal of Non-condensable Gases

The water heated up by the steam gets collected in the preheating section of the deaerator. Once the water level reaches the operating level of the tank, then the steam is passed through a steam pipe into this section. This steam bubble ups through the water thereby heating the water and releasing the non-condensable gases. These gases are then liberated into the atmosphere through the vents.

Types of Deaerator

Deaerator design differs from one manufacturer to another. There are three popular types of deaerators like thermal type, Vacuum rotating disc type & the ultrasound type. The vacuum rotating disc type is used for low to high viscous products whereas the ultrasound type is used with very viscous products.

Based on their design, the thermal deaerators are classified as two types like the spray type deaerator & the cascade type deaerator. The spray type deaerator consists of a vertical or horizontal cylinder that serves as both a deaerator section and a storage section. In cascade type deaerator the deaerator section is separated from the storage section. Here, a vertical or horizontal doomed deaerator section is placed on top of a horizontal storage cylinder vessel. This deaerator is also known as the Spray & tray type deaerator.

 Spray Type Deaerator

This deaerator contains a preheating section denoted by E, deaerator section denoted by F separated by a baffle denoted by  C. The low-pressure steam is passed into the system through the sparger present at the bottom of the vessel. To facilitate the stripping of out of the dissolved gases in the deaeration section, the water is preheated in the E section by the stream. The water is then deaerated in section F. The released gases are liberated into the atmosphere through the vent. This water is then pumped into the steam-generating boilers using a pump at the bottom of the vessel.

Cascade Type Deaerator

In this deaerator, a vertical doom deaeration section is mounted above a  horizontal feedwater storage section. The deaeration section contains perforated trays. Water enters this section trough the spray valves present above these trays and move downwards. The water passes from the trays into the storage vessel. Preheated steam is applied to the water from the perforated pipeline present at the bottom section. This steam heats up the water and the separated gases flow upwards. These are liberated trough the valve present on the deaerator section.

Advantages and Disadvantages

There are many advantages and disadvantages associated with the different types of deaerators.

When compared to the other types with the same capacity, spray deaerator is inexpensive & less weight. This deaerator also requires less Headroom. Its capacity ranges from 7000 to 280000 pounds per hour.

The disadvantages of spray deaerator are its large amount of moving mechanical components which may require more mechanical maintenance. This increases the routine operating cost & reliability of the deaerator. In this deaerator, aeration is done in two stages. Here in the spray head area, about 90 percent of the aeration is done whereas the remaining 10 percent is done in the scrubbing or spring-loaded nozzle area. Critical miss alignments to the steam nozzle will affect the reliability of this type of deaerator. This also has limited High-pressure returns compared to other types.

The advantages of cascade type of deaerator are its high reliability, higher HP returns, high DA consistency, and High capacity. The disadvantages of this deaerator are its low Headroom, high weight and high price compared to the spray type deaerator.

Applications

Some of the applications of the deaerators are as follows-

• These are used for the boiler plants that are operating at 75 pounds or higher capacity.
• Plants with no standby capacity.
• Boiler Plants with critical loads.
• Plants operating with 25 percent makeup or more.
• Thermal power plants.
• These can also remove various dissolved gases from products such as food, personal care products, cosmetics, chemicals, etc…
• Deaerator are used in pharmaceuticals to increase the dosing accuracy in the filling process.
• These are also used with products to increase their shelf stability, to prevent decolorization of the products, etc..

Deaerator is usually used with boilers in the chemical process industry or power generation industry. The use of deaerator before feeding water into the boiler highly increase the efficiency and reliability of the boilers. The corrosion caused to the boiler can be highly reduced. The temperature of the preheated steam used in the deaerator should also be kept in check. For every 10 degrees rise in the temperature of the feedwater 1 percent rise in the gain can be observed. The amount of carbonic acid formed in the deaerator also depends on the number of bicarbonates present in the water. What are the working temperature and pressure values for a deaerator?

FAQs

1). Why Deaerator is placed at heights?

Deaerator is placed at a certain height to maintain the optimum pressure before suction.

2). Why Deaerators are used in boilers?

Water contains many corrosive dissolved gases. When this water is directly supplied to the boilers, it causes high corrosion and rusting of the boiler metal components. This damages the boilers thereby decreasing their reliability. To prevent this deaerator is used in boilers, to remove these non-conductive gases present in water.

3). Is Deaerator a pressure vessel?

Yes, it is a pressure vessel. These are available in the market in different pressure ratings.

4). What is Deaerator pegging?

During several strat up events, the pressure of the deaerator drops. To stabilize the pressure fluctuations during start-up/ramp up/ down conditions pegging system is maintained as a backup. This maintains the pressure of deaerator above 3PSIG.

5). How is it used to remove oxygen?

Oxygen gets dissolved into the water either during the contact with the external environment or through the leakages in the piping system. The solubility of oxygen decrease with the increase in temperature. So, to remove the oxygen from water, the temperature of the water is increased in the deaerator section. This separated oxygen is then vented out through the vents present on the top.