This article we will learn about the deaerator working principle and types of deaeration.
What is Deaerators?
Low pH levels in feed water cause severe acid attack throughout the boiler system. The dissolved gasses and low pH levels in the feed water can be controlled by the addition of chemicals. This is more economical and thermally efficient to remove these gasses mechanically. This mechanical process is known as deaeration. Deaeration is taking place in a vessel is called deaerator.
By removing the dissolved gasses in the boiler feedwater, increases the life of a steam system dramatically.
Deaerator Working Principle
Deaeration in deaerators is based on two principles.
The first principle can be described by Henry’s Law.
As per Henry’s Law, the gas solubility in a solution decreases as the gas partial pressure above the solution decreases.
The second principle that governs deaeration is the relationship between ‘gas solubility’ Vs ‘temperature’.
The gas solubility in the solution decreases as the temperature of the solution rises and approaches the saturation temperature.
Deaerator utilizes the above two principles to remove dissolved oxygen, carbon dioxide, and other non-condensable gasses from boiler feedwater. The feedwater is sprayed as a thin film into the steam atmosphere thereby the feedwater quickly heated to saturated condition.
The feedwater is supplied as a thin-film, thereby the surface area of the feedwater in contact with steam increase. Due to this the dissolved oxygen, CO2 and lower gas concentrations are removed quickly. This process reduces the solubility of all dissolved gasses and removes it from the feedwater. The liberated gasses are vented from the deaerator.
Types of deaerators
Deaerators are classified as three types, they are
- Spray type deaerator
- Tray type deaerator
- Vacuum type deaerator
Spray Type Deaerator
The above figure refers to Spray type deaerator. In which the Un-deaerated boiler feedwater is sprayed in the steam space by a recycle pump. Under all kind operating conditions, the sprayer assures heating up of the condensate to saturation temperature and a very large area for mass transfer. As the solubility of oxygen in water at saturation conditions is practically zero, oxygen transfers from the water droplets to the surrounding steam. As the steam condensates on the water, the concentration of oxygen in the direct vicinity of the sprayer is increased making it possible to vent a small amount of steam with a relatively high oxygen concentration.
Final deaeration in the water tank is achieved by injection of steam in the water tank. Depending on the conditions of the heat source(s) used (steam, pressurized hot water, steam/water mixture) one (or more) types of steam rake(s) are chosen. A properly designed steam rake, taking into account the hydrodynamics in the water tank, will result in good contact between water and steam, allowing the oxygen to move from water to steam as per Henry’s law.
Tray Type Deaerator
The above figure represents the horizontal tray-type deaerator. The vertical domed deaeration section mounted above the horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical dearation section above the perforated trays and flows downward through the perforations. The low-pressure dearation steam enters below the perforated trays and flows upward through the perforations.
The steam strips the dissolved gas from the boiler feedwater and exits via the vent at the top of the domed section. The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam-generating boiler system.
Vacuum Type Deaerator
The vacuum type deaerator consist of a deaerator storage vessel, vacuum pump and a boiler feed water pump. The boiler feedwater enters at the top of the tray section, the deaeration steam enters at the bottom of the tray section and flow in the upward direction. Thereby the dissolved gases in the boiler feedwater are carried out by deaerated steam.
A vacuum pump (liquid ring type vacuum pump) is connected with the deaerator for sucking the liberated dissolved gasses. Thereby maintained a negative pressure in the deaerator system to enhance the liberation of dissolved gasses from the boiler feedwater.
The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam-generating boiler system.