4 Things to Know for Increasing Steam Boiler Deaerator Effectiveness

Author: Joshua Ince | District Sales Manager

Deaerator Boiler Room

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Why should you have a deaerator?

Raising steam for your facility requires water, and depending on the steam plant’s size and design, the volume of feedwater needed can be small or quite large.  Some applications allow for steam condensate to be recycled back to the boiler which is reused as part of the feedwater, with the rest coming in as fresh water, called makeup.   The makeup water comes from your local utility or well water source.  It is often cold and contains many dissolved gases, with oxygen being the most prevalent.  Regardless of the volume of condensate returns you have, the makeup water needs to be conditioned before it can be used as boiler feedwater.  The conditioning requirement for the boiler’s feedwater quality must achieve 4 goals, as outlined in both the 2015 ASHRAE Handbook chapter on Water Treatment , and the ASME Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers (CRTD-81).  These goals are:

  1. Dissolved oxygen removal through mechanical means, in order to reduce the need for water treatment chemical oxygen scavengers (Sulfite, hydrazine, DEHA, etc)…. reduce your water treatment chemicals

  2. Dissolved gas removal through mechanical means, in order to protect the steam distribution network and condensate network from oxygen pitting and carbonic acid corrosion…. prevent corrosion and failure

  3. Dissolved non-condensable gas removal, in order to increase steam system efficiency…. increased steam potential

  4. Heat the cold water to avoid thermo-shock to the boiler system (economizer, boiler)…. longer steam boiler life
A deaerator is a specifically designed piece of equipment to achieve all 3 of these goals.  Often the deaerator is a combination of two vessels; one vessel for removal of oxygen and non-condensable gases (scrubbing section), and a second vessel for storing hot feedwater (storage section).  It is this 2 vessel design that provides superior results and efficiency as compared to simply having one steam heated “feedwater tank”.  The deaerator’s scrubbing section mechanically breaks up the water into droplets so that oxygen and dissolved gases are removed from the water and vented out of the vessel before entering the storage section.  The resulting high quality feed water has far lower concentrations of dissolved oxygen and non-condensable gases, as compared to a conventional feedwater tank, leading to minimized operating costs and improved steam quality.

How do I know my deaerator is working well?

As described above, the deaerator is far more effective than a steam heated feedwater tank because of its mechanical design and ability to use steam pressure and temperature to condition the makeup water into feedwater.  The three main parameters indicative of a properly operating deaerator are temperaturepressure and dissolved oxygen removal.  Ensuring these three parameters are within design limits is crucial for maximum performance and return on investment of the deaerator.  The temperature and pressure should be checked at least daily, and logged in an electronic database for trending analysis.  If fluctuations in temperature and/or pressure are observed, maintenance should be scheduled for the controls on the deaerator.  Dissolved oxygen removal can be assessed by regularly scheduling a Dissolved Oxygen Study (DO2 study), which is a controlled test on the system that measures the dissolved oxygen level at the outlet of the deaerator.   DO2 studies are important because they can identify when something has failed mechanically inside the deaerator, limiting its ability to remove oxygen or other non-condensable gases.  A physical inspection of the deaerator will not often reveal if something has failed internally.  DO2 studies should be scheduled at least on an annual basis, with the results logged and stored electronically for trend analysis.  The DO2 study itself is relatively simple, but does require some planning and setup:
  1. A representative sample must be obtained from the storage section of the deaerator, and cooled to 70-80°F before testing can be performed. The recommended setup for proper sampling is a stainless steel sample cooler, installed in such a way that a sample can be continuously collected during the DO2 study.

  2. The boiler chemical (Sulphite) program must be disconnected from the deaerator for the duration of the test, and ideally temporarily connected to the feedwater line(s) feeding the steam boiler(s), downstream of any re-circulating lines, such as those found on feedwater pumps and feedwater economizers.

  3. Sampling and testing should occur on a flowing sample, while testing the deaerator during all of its modes of operation. Ideally the unit should be tested when it is taking on makeup, when it is satisfied and over a period of normal operation so as to measure the effectiveness of the temperature and pressure controls on the deaerator.

  4. Testing equipment should be used for the range of dissolved oxygen to be tested. This can range from 10-15ppm in the cold makeup water, down to 7-50ppb for the storage section of a properly operating deaerator.
Boiler Water Treatment

What is my water treatment provider’s role?

Your water treatment provider should be monitoring the temperature, pressure, and demand for chemical oxygen scavenger at least monthly, and logging this data into an electronic database for trend analysis.  The easiest way to spot a failing deaerator is by observing changes in the “normal” levels of chemical consumed, or the relationship between pressure and temperature.  Your water treatment provider should also be reviewing your daily logs of temperature, pressure and chemical consumption to spot potential issues.  If you are doing your own DO2 studies, the water treatment provider should be using your data as well to help determine the effectiveness of the deaerator.  At the very least, your water treatment provider should be performing a DO2 study themselves, to ensure the deaerator is functioning properly.

What is my boiler mechanical contractor’s role?

Your boiler mechanical contractor’s role is important as well, as they need to coordinate inspections and possibly repairs, in order to act on any deviations observed during the regular monitoring of deaerator performance indicators.  A DO2 study will reveal incorrect deaerator operation, and often the fix is to repair/replace the spray head or a tray inside the “scrubbing” section of the deaerator.  Bringing your mechanical contractor and water treatment provider together as a team is a great first step in ensuring your deaerator continues to operate at maximum efficiency.

WANT TO LEARN MORE?

Download our Engineering Note on Dissolved Oxygen Studies and Deaerator Efficiency.

Joshua Ince has a Bachelor’s Degree of Applied Science – Chemical Engineering, from the University of Toronto. He is a District Sales Manager, with over 15 years of experience designing and implementing solutions for water systems. He is a recognized public speaker on topics of water and energy conservation and water safety. Joshua is a big fan of science fiction and all things motorized!

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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