Deaerators

Deaerators

Deaerators are mechanical devices that remove dissolved gases from the boiler feed. They minimize corrosion by reducing the concentration of dissolved oxygen and carbon dioxide and protect the boiler from the effects of corrosive gases.

To prevent corrosion in most high-pressure boilers, a dissolved oxygen level of (5 ppb) or less is required. Although oxygen concentrations of up to 43 ppb are tolerated in low-pressure boilers, limiting the oxygen concentration to 5 ppb will extend the life of the equipment.

Insulation:

The deaerator and storage tank and all piping carrying hot water or steam must be adequately insulated to prevent condensation of steam and heat loss.

Henry’s Law:

According to Henry’s Law, the solubility of gases is directly proportional to the partial pressure.

Therefore, by reducing the partial pressure by adding steam to the deaerator, the solubility of gases decreases. Gases dissolved in the feed water can be removed by spraying water into the steam environment where the partial pressure of the gas is reduced.

Types of deaerators:

• Tray-shaped deaerators
• Spray deaerators

Tray-shaped deaerator:

It consists of a dome-shaped vertical section, which is installed above the boiler water storage tank.

The boiler feed water flows from the dome-shaped section onto perforated trays and moves downwards through them.

The deaeration operation is carried out by low-pressure steam entering from the bottom of the perforated trays.

This operation separates the gases dissolved in the boiler water; these gases are sent out through the outlet valve at the top of the dome-shaped section.

Spray Deaerators:

It has a preheating section (E) and a discharge section (F). The two sections are separated by a plate (C).

Low pressure steam enters the deaerator through the openings at the bottom of the drum.

Boiler water is sprayed into section E (preheated by the rising steam).

The purpose of using the boiler water spray nozzle and the preheating section is to more easily separate the gases dissolved in the water and send it to the deaeration section.

The preheated water then enters the deaeration section (F). There it is completely deaerated with the steam exiting the sprayers.

The gases separated from the water are sent to the atmosphere through the outlet at the top of the deaerator tank.

In some cases, a condenser may be used to recycle the outlet water along with the gas.

Deaerator design parameters:

• Oxygen level reduction to a minimum (5ppm)
• Working pressure equivalent to 1.2 atmospheres
• Tank volume equivalent to 10 to 20 minutes of boiler water supply
• Inlet water with a pressure of at least 2 bar and a temperature of approximately 85 C
• Outlet water at saturation temperature (approximately 105 C)