Physical Water Treatment Devices: Understanding and Selecting ‘NCDs’

Water Treatment Devices
The goals of cooling water treatment systems are to minimize corrosion, inhibit scale formation, control biological activity, avoid fouling, and minimize water consumption.  Physical water treatment devices, often referred to as non-chemical devices or NCDs may target all or some of the same requirements as a traditional chemical water treatment program.

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The term ‘non-chemical device’ focuses on what the device is not as opposed to what it actually is –  so going forward, we will refer to these devices as PTDs.

By definition, physical treatment devices (PTDs) manage aspects of the water treatment program using physical principles.  Physical Treatment includes widely accepted practices such as side-stream filtration, reverse osmosis, and conductivity controllers.  However, there are other technologies currently offered that look to control cooling water quality that are not as well understood.  Market offered PTDs commonly utilize electro-magnetic waves to achieve chemistry control for desired results in water treatment systems.

Since it is peak cooling season now, we’ll look specifically at PTDs used in evaporative cooling water systems. PTDs can be a great fit for a certain type of applications, but like any water treatment program, they will require service and maintenance.  One common misconception is that these devices are ‘plug and play’ and do not require monitoring.  Unfortunately, this has led to instances where the end result did not meet expectations.  To ensure expectations are met it is of utmost importance to have proper selection and ongoing maintenance of any PTD.

The goal of this article is not to assess the effectiveness of any given PTD technology but to discuss the considerations for design and control of the program to ensure the best chance of success when utilizing any PTD.  The PTD may be a standalone device that targets all aspects of a water treatment program, or it may work in conjunction with other treatment products.  As with more traditional chemical water treatment, ‘one size fits all’ does not exist with PTDs, and it is important to consider all of the application requirements.

Targeted Water Chemistry

A good water treatment program has defined water quality targets that, when maintained, achieve successful results for the facility. This requires knowledge of the incoming water quality and overall system heat exchange temperatures and metallurgy.  Your water treater, and the PTD manufacturer should be able to model your water chemistry to help you determine what will be needed to achieve successful results in your system.  Just as not every system can be treated with the same chemistry, not every PTD is compatible with every makeup source or cooling application.  You should beware of PTDs that do not take this approach and claim that a device can work for any system.

Service Program and Inspections

All of these devices depend upon water quality to achieve results, and thus there should be components that control water quality through conductivity measurement and bleed control.  Like with any water treatment program, these components need to be calibrated regularly and tested to verify proper operation.  Products that claim to not require any maintenance or service program, but that simply work once installed should be avoided.  Inspection of the entire PTD should be part of your regular preventative maintenance program, as this is often the best way to evaluate the success of the treatment program.

Local Representation

If you have chosen to use a PTD, you should be aware of the local representation provided by your supplier.  Local representation is important during installation, start-up, and operation to ensure you have the support required to initiate and maintain a successful water treatment program.

Program Targets

In order to ensure that a PTD will meet your water treatment program needs, you must know exactly what function the device is designed to perform.  Some devices may only be targeting certain aspects of a water treatment program, while leaving others to be handled separately.  Integrating these alternative technologies into a Water Management Plan requires execution according to manufacturers’ recommendations, combined with proof of performance that your system is effectively controlling scale, corrosion, bacteria and biofilm while achieving your water conservation goals.  As with all program selection, evaluations should consider the local water supply, the operating environment, the cooling system type, load, risk characterization, and industry standard performance metrics.
NCD Device Engineers

Conclusions

Using physical treatment devices, you can achieve successful water treatment results.  There are many well documented and third-party validated PTD success stories.  As with any developing technology, it is common for a quality PTD to be used in the wrong application, resulting in poor functionality and an unfortunate misrepresentation in the marketplace.  All water treatment programs, whether chemical or physical, require proper service and maintenance programs, and are vital to the success of the system. Work with your water treatment professional to ensure you have the proper solution for your application needs.

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  1. I would like to know if Kelox 2000 and Klensol S1100 are safe to use near food products. If they are could you please provide information in that regard? We are looking for products that fall under a W1 classification. Thanks

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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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