This article explores the advantages of condensate polishing systems—part of a robust industrial water treatment solution that helps prolong equipment life, boost efficiencies, and reduce water, energy, and chemical consumption.   

In power generation and other industrial applications that involve high-pressure, high-temperature boilers, improvements in the water treatment process can yield substantial benefits. Previous articles have discussed the advantages of deploying a robust demineralized water treatment system for boiler feed water. This article explores the function of a condensate polishing system—an inline water treatment system that helps prolong equipment life, boost heat-transfer efficiencies, and reduce energy, water, and chemical consumption.

Condensate? What’s That?

Condensate return water is the liquid water that forms when steam in industrial boilers cools and condenses at the end of the steam cycle. The condensate water is combined with demineralized municipal ‘makeup’ water and then re-introduced (“returned”) into the steam cycle.

Why Does Condensate Return Water Need ‘Polishing?’

Water—especially the demineralized water used in high-pressure, high-temperature boilers—is an efficient solvent that can dissolve the surfaces of piping and equipment, including in boilers and condensers.

The amount of condensate return water that can be recycled to the steam cycle is highly dependent on the water’s purity. The concentration of impurities—including dissolved solids, hardness, alkalinity, chlorides, and silica—increases each time the condensate water goes through the boiler’s steam cycle. These impurities result in scale, corrosion, and plugging of piping and equipment. The degradation of the system results in lower heat-transfer efficiencies, leaks, equipment failures, and potential safety issues.

In order to control the boiler chemistry and manage the ever-increasing concentration of impurities and dissolved solids, a portion of boiler feed water which includes condensate return water is “blown down” (removed) from the boiler during the steam cycle. Blowdown of impure water helps remove salts, metals, oils and greases, and other contaminants from the steam cycle, but also increases the amount of demineralized makeup water that must be added for boiler operation.

This is where a condensate polishing system becomes a vital part of an efficient industrial water treatment system. The polisher removes impurities from the condensate return water— reducing scale, boosting heat-transfer efficiency, reducing damage that impurities can cause to expensive boiler equipment, and increasing the amount of condensate return water that can be reintroduced into the steam cycle.

What Are the Steps in the Boiler Steam Cycle?

For industrial boilers, the steam cycle consists of five basic steps:

1. Raw water is demineralized, de-aerated, chemically treated, and combined with condensate return water at low pressure.
2. This low-pressure liquid is pumped to high pressure via the boiler feed pumps.
3. The high-pressure liquid enters the boiler where it is heated at a constant pressure by an external heat source—coal, natural gas, oil, biomass, etc.—to become a pressurized, dry vapor.
4. The high pressure vapor reaches the turbine which converts the thermal energy to mechanical energy. As the vapor expands, temperature and pressure reduce, and oftentimes a low-pressure turbine is used to further increase efficiency. Some condensation occurs at this step, forming wet or saturated vapor.
5. The wet vapor finally enters a condenser where it condenses back to a constant pressure liquid known as condensate return water. The condensate return water combines with fresh demineralized makeup water, and the steam cycle then resumes with step 1.

Where Does the Condensate Polishing System Fit Into the Steam Cycle?

As part of an industrial water treatment system, the condensate polishing system comes into play between steam cycle step 5 (the forming of condensate return water in the condenser) and step 1, when condensate return water is combined with demineralized makeup water to begin the steam cycle again.

The condensate polishing system consists of a mixed bed ion exchange polisher with specialized resin for high-heat applications. In some cases, an activated carbon filter can serve upstream of the condensate polishing mixed bed as a filter for suspended solids and organics (oil and grease).

Why Deploy a Condensate Polishing System?

• It provides better protection for the boilers, turbines, heat exchangers, and steam piping from scale, corrosion, and deposits while using more condensate return water.
• Treating condensate return water requires less acid and caustic to regenerate compared to treating raw water.
• It reduces the amount of demineralized makeup water utilized.
• It helps remove iron particles from the return water: Steam that comes into contact with metal equipment and piping can pick up iron particles—namely iron oxides—which, when cycled up, deposit on heat exchangers, turbines, and other equipment.
• It may support “green” initiatives because it reduces raw water consumption, reduces chemical consumption, recycles water, and reduces operating expenses.
• It is a necessity for high-pressure, high-temperature boilers that operate at pressures greater than 2,400 PSIA. Above the 2,400 PSIA threshold, contaminants in the boiler feed water (sodium, chloride, iron, calcium, silica, etc.) will carry over in the vapor of the steam produced and affect downstream processes.

Condensate Polishing = A Sound Industrial Water Treatment Strategy

Deploying a condensate polishing system is a wise water treatment strategy that can yield substantial economic payback in terms of improved boiler equipment life and operation, and controlled water, power, and chemical costs. Be sure to refer to the GAI website for more articles discussing water treatment technologies, trends, and news.

Contact Arica DiTullio, PE at 412.399.5455 and find out more about GAI’s water treatment and management services—message GAI online and start the conversation about how our multidiscipline professionals can meet your unique project needs.

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Arica DiTullio, PE specializes in civil and environmental engineering with experience in managing multidisciplinary projects in collaboration with civil, structural, electrical, mechanical and process engineers. Her expertise spans wastewater, water quality analysis, hydrologic and hydraulic computations, site grading, erosion and sediment control, and stormwater management with a comprehensive understanding of federal, state, and local permitting and regulations.