Steam Humidification: Choose Your Own Adventure

Author: Louis-Philippe Beauchemin - Professional Engineer

Steam Humidification
The relative humidity level in our built environments is critical to prevent occupant discomfort and/or damage to building materials. Choose to read the quick high level summary, or test your knowledge by following this interactive blog that delves deeper and deeper into the details of the steam that we inject into our air.

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Why is humidification important?

Outdoor temperatures vary greatly between the summer and winter months, and humidity levels also fluctuate with changing weather conditions and proximity to lakes and oceans.  Modern buildings rely on mechanical ventilation systems to maintain consistent indoor temperatures and humidity levels.

Hot and humid weather during summer months will cause the air to feel heavy and damp, requiring air conditioning to increase occupant comfort and decrease the risk of mold and mites in building materials.

During winter months, heating systems are put into service that tend to decrease the relative humidity (RH).  If the RH value falls below 30%, the air will feel dry, and may lead to occupant discomfort and/or damage to building materials.  Low RH values also increase the potential of static charge build-up, which may affect negatively impact sensitive electronics and machinery.
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.

How are RH values maintained?

Large buildings rely on air handler units (AHUs) to temper the recirculating air and introduce a portion of fresh outside air.  These AHUs are comprised of multiple sections with different functions including:
  • Air Filtration
  • Heating
  • Cooling
  • Humidification
The humidification function is often accomplished using Direct Steam Injection, whereby pressurized steam is sprayed directly into the air stream.  This method is most common in facilities where a centralized steam system is already in place due to other process requirements.
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.

Is there more to steam than just water?

Yes.  There is always something other than pure H2O in steam.

Steam carries gases and may also contain specially formulated water treatment compounds used to protect piping from corrosion.

The gases are primarily carbon dioxide (CO2) and oxygen (O2).  The concentration of these gases is dependent on the composition of the water being fed to the boiler.  It is desirable to minimize the presence of these gases in a steam system to minimize the risk of corrosion.

Carbon dioxide is particularly harmful to steel piping because it forms carbonic acid as the steam condenses into condensate.

Water treatment compounds, called amines, are added to boiler water to combat the effect of carbon dioxide.  Amine compounds are designed to evaporate with the steam to neutralize the acidic properties of condensate.  This maximizes the useful life of piping infrastructure.

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.

Should I be concerned about steam purity?

The concentration of amines in your steam is primarily dependant on your incoming water quality and is typically low.
In the order of a few parts per million, or 10,000th of 1%

Air can hold a surprisingly large volume of water as humidity.

Consider a 600 square foot area with 8 foot ceilings.  If the room is at 22.5 degrees Celsius and RH is increased from 20% to 40%, a total of 500ml of steam was used.
Water Bottle
Your water treatment professional is responsible for ensuring the concentration of amines in the air meet two main concentration guidelines to ensure there are no complaints from occupants, especially from those with sensitivities:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
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.
Lobby
In situations where the incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs, or even PELs.  Implementing pre-treatment alternatives can reduce the incoming alkalinity and thereby reduce 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%.

Conclusions

Humidifiers are a critical component of a properly operating HVAC system to ensure occupant comfort and prevent damage to building materials.  The multi-levels of this blog discussed the design and operational considerations to ensure safe and effective humidification at your facility.  Consult your water treatment professional to; confirm that your amine dosage rates are below recommended concentration guidelines, and discuss the implementation of pre-treatment equipment to reduce occupants’ exposure to amines and lower your water treatment costs.

Louis-Philippe Beauchemin is a Professional Engineer and has a Mechanical Engineering degree from the University of Ottawa.

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