What Is a Deaerator?

What Is a Deaerator?

Oxygen is the main cause of corrosion that occurs in boilers, boiler equipment, pumps and other piping components. If carbon dioxide is also present in the boiler feedwater along with oxygen, it lowers the water's pH, making the water acidic and increasing the corrosion rate. Oxygen corrosion appears as small pitting that can be difficult to see with the naked eye.

Devices used to remove oxygen (O2) and carbon dioxide (CO2) from boiler feedwater are called deaerators.

Removing dissolved oxygen can be achieved through chemical and physical methods. If the feedwater's pH is fixed between 9 and 12, there will be no CO2 in the feedwater. To remove oxygen, the boiler feedwater must be held at a certain temperature under constant pressure.

Using condensate returning from the piping system allows less fresh feedwater to be used. Water returning as condensate is both hot and chemically pure. Water that contacts air absorbs oxygen; the oxygen concentration in water varies with temperature: the higher the temperature, the lower the oxygen content.

How Does a Deaerator Work?

When treating boiler feedwater, the first step is to heat the water in order to remove oxygen from it.

The feedwater tank or condensate tank is generally operated between 85°C and 90°C. This leaves approximately 2 mg/liter (ppm) of oxygen content. Operating at higher temperatures at atmospheric pressure is difficult due to the possibility of cavitation in the feed pump.

DEAERATOR OPERATING PRINCIPLE

A deaerator heats water to saturation temperature, reducing the solubility of the oxygen it contains to zero. If a liquid is at saturation temperature, the solubility of gas within it is zero; however, the liquid must be boiled to fully remove its air content. A deaerator is a device that performs this process.

The dome at the top of the deaerator breaks the water into small droplets and particles and heats these droplets with steam. This provides a large surface area and rapid heat transfer from the steam to the water; the water quickly reaches saturation temperature. Dissolved oxygen carried by the steam is vented to atmosphere. The deaerated water then flows down into the tank section of the deaerator.

Parts of a Deaerator

Water must be broken into small droplets to increase its surface area. This is necessary to raise the water temperature and release gases in the deaerator dome.

To break the water into small droplets, the dome contains trays called cascades. As water flows over these trays, it breaks into droplets and releases the oxygen it contains to atmosphere.

A level control probe is located in the deaerator tank to maintain the water level.

Steam injectors are located at the base of the tank and spray counter to the flow of water. Steam is sprayed to achieve good distribution within the deaerator dome.

DEAERATOR DESIGN

To design a deaerator correctly, it is important to properly size the steam supply line and calculate how much steam is needed to heat the deaerator.

If feedwater is heated to a saturation temperature of 100°C in a condensate tank open to atmosphere, the oxygen remaining in the water drops to approximately zero; however, this method is not considered economical due to the high steam and heat losses involved. For this reason, pressurized deaerators are preferred, especially at large capacities.

Pressurized deaerators are generally designed to operate at 0.2 bar; deaerators designed this way have very low losses.

To ensure proper system design, it is important to size the steam supply line and know how much steam is needed to heat the deaerator; this steam is used to heat the feedwater.

The required steam flow rate is calculated using the conservation of mass and heat equation. The mass/heat balance is based on the principle that the sum of the initial heat in the feedwater and the heat added by the injected steam mass must equal the sum of the final heat in the feedwater and the heat of the steam mass that condenses during the process.

Oxygen is not the only thing that needs to be removed; as a result, along with a certain amount of steam, the deaerator also vents other air components, predominantly nitrogen. The rate of air removal from water is approximately 3.5 grams of oxygen per 1,000 kg of water; since this amount mixes with the steam in the space above the tank's water surface, it is expelled together with the steam released from the deaerator.

In deaerator design capacity calculations, a steam-to-air ratio of 2 kg/h is assumed per 1 t/h of deaerator capacity. The heat balance in deaerator design is generally calculated based on a 20°C rise in feedwater temperature.

Before calculating a deaerator's dimensions and proceeding to design, it is important to know the feedwater requirement. This is determined by calculating the boilers' evaporation rate, which depends on the initial feedwater temperature; determining the boiler evaporation rate gives the maximum evaporation rate.

BENEFITS OF USING A DEAERATOR FOR THE BUSINESS

• Saves on operating costs by reducing the use of feedwater conditioning chemicals. In high-capacity, large water-tube boilers, maintaining low TDS levels (<1,000 ppm) in the feedwater is becoming increasingly important.
• Saves on both chemical use and heat energy by reducing boiler bottom and surface blowdown.
• In production processes where steam is in direct contact with the product (such as food products or sterilization applications), prevents unwanted substances or microorganisms from contaminating the product by various routes.