Boiler deaerator: Working principle, types and some key design calculationsTemitayo Oketola | March 30, 2022
Deaerators are essential devices in chemical process industries or facilities where boilers are used for steam production from boiler feedwater. They protect the boiler system from the effects of low pH and corrosive gases by reducing the concentration of dissolved carbon dioxide and oxygen to a level where water pH is increased and corrosion is minimized.
As simple as the deaerator’s function may seem, a lot goes into the design of this system, especially when it is to be used in large chemical facilities or power plants. This article presents the working principles on which deaerators are based, common types of deaerators and some critical design calculations for deaerator systems.
Operating principles of deaerators
Deaerators work based on:
- Henry’s Law
- The relationship between gas solubility and temperature
Henry’s law simply asserts that the solubility of any gas in a liquid (or solution) is directly proportional to the partial pressure of the gas at the liquid surface (or above the solution). So, if the gas partial pressure above the solution decreases, there would be a corresponding decrease in gas solubility.
Henry’s law is expressed as:
Ctotal = Total concentration of the gas in solution (in ppm)
P = Partial pressure of the gas above the solution or at the liquid surface (atm)
k = Henry’s law constant
Let’s consider a scenario where 9 ppm of oxygen can be dissolved in water when the partial pressure of oxygen is 0.3 atm. This would mean that only 6 ppm of oxygen can be dissolved in water if the partial pressure of oxygen was reduced to, say, 0.2 atm.
As Henry’s law suggests, engineers can remove dissolved gases in a solution by reducing the partial pressure of the gas in the atmosphere contacting the solution. This can be achieved by applying a vacuum to the system to vent the unwanted gas.
Relationship between gas solubility and temperature
The second principle that governs the operation of deaerators simply asserts that the solubility of a gas in a solution decreases as the temperature of the solution increases up to its saturation temperature. To achieve this, engineers typically use steam to prepare boiler feedwater when designing deaerators.
Steam helps to heat the water, reducing the solubility of oxygen and venting unwanted gases. In addition, steam is affordable, readily available and does not contaminate the system.
Types of boiler deaerators
For the deaerating process to occur, here are a few systems that must be in place:
- A vessel in which the deaeration process will occur and that protects the deaerated water from contamination.
- Large surface area to allow the heated steam to contact the water
- A path that allows the escape of oxygen and non-condensable gases
With that being said, boiler deaerators typically come in two types:
- Tray-type (or spray-tray) deaerators
- Spray-type deaerator
The tray-type deaerators feature a deaeration chamber mounted horizontally above the water storage section, as shown in Figure 2. Water is dispersed into the deaeration chamber through a spring-loaded nozzle (usually manufactured from stainless steel) and onto a specially designed tray.
The water cascades down through the openings in the tray. At the same time, steam is made to flow upward through specially designed openings, making contact with the water from the spray nozzle and heating the water close to its saturation temperature. This, in turn, strips dissolved gases from the boiler feedwater up to a level of 7 ppb.
The deaerated water flows to a storage tank from where it is pumped to the boiler, while the unwanted gases (including steam) are made to exit the vessel through a vent valve (see Figure 2).
Spray-type deaerators feature a vessel, spring-loaded inlet spray valves, a direct contact vent condenser, and a baffled scrubber. Water is sprayed into steam in the atmosphere before being made to fall into a baffled scrubber section, where it is scrubbed and heated to remove unwanted gases.
The feed pump’s base head (at duty point flow rate) can also be estimated using:
Deaerators are essential devices that offer numerous benefits in a wide range of industrial applications. However, these advantages will only be felt if engineers correctly size the deaerators and feed pump to suit their applications.
While this article presents valuable information about deaerator sizing, engineers are advised to reach out to industrial deaerator and boiler manufacturers to discuss their needs and projects.