It's not the heat; it's the humidity

Most industrial processes can handle high heat conditions, but many require some control over the amount of moisture in the air. Whether manufacturing musical instruments, semiconductors, processed foods, or simply heating a warehouse, the amount of moisture in the air impacts product quality and worker comfort.

08/08/2002


Key concepts
Indoor air quality must be considered when selecting a humidification system.
Systems that make their own steam for humidification avoid problems with volatile amines.
Corrosion inhibitors in humidifying steam must be chosen carefully.
Sections:
Humidifier design and operation
Corrosion inhibitors
Systems
Sidebars:
A health problem solved


Most industrial processes can handle high heat conditions, but many require some control over the amount of moisture in the air. Whether manufacturing musical instruments, semiconductors, processed foods, or simply heating a warehouse, the amount of moisture in the air impacts product quality and worker comfort.

A number of mechanisms exist for maintaining proper humidity. Picking the right method depends on specific needs. There are also some environmental factors that need to be taken into consideration. Corrosion inhibitors, commonly used in steam boiler distribution systems, need to be considered when thinking about humidification.

Humidifier design and operation

Direct steam injection humidifiers , the most common humidification system, control the introduction of steam into heating ducts in response to a signal from a humidistat. Steam enters the body of these humidifiers, passes a number of baffles, which force any moisture or particulate out of the vapor, and is introduced into the duct through a manifold (Fig. 1). Direct steam humidifiers require very little upkeep.


Fig. 1. Direct steam humidification provides immediate response.

This system provides excellent response to demand and pinpoint control of output. Because it uses steam directly, no bacteria are produced. However, any volatile amines used as boiler corrosion inhibitors are introduced into the steam, which could be a problem.

Steam-to-steam humidifiers use a steam coil immersed in a water bath to heat the water to boiling (Fig. 2). The boiling water produces humidification steam at atmospheric pressure. This system avoids the problems presented by volatile amines in boiler steam since there is no contact between boiler steam and humidification steam, barring failure of the heat exchanger.


Fig. 2. Steam-to-steam humidifiers avoid problems with volatile amines in boiler water.

Steam system operators make a tradeoff, eliminating the risk of odors and exposure but reducing the energy efficiency of the humidification system.

Because of common concerns about exposure to neutralizing amines or odor complaints caused by them, many consulting engineers specify these systems for new construction.

Steam-to-steam humidifiers are susceptible to problems caused by water impurities such as calcium, magnesium, and iron. These impurities can deposit as scale, necessitating cleaning. Scale also insulates the heating coil, reducing the thermal efficiency of the humidifier. Because time is required to boil the water, response to control input is generally slower than with direct steam injection systems.

Electronic (or electrode) steam humidifiers are used to generate humidification steam when a source of steam is not available. Electrode-type units pass an electric current through water to provide steam (Fig. 3). The water used in these units must conduct electricity. Use of pure demineralized, deionized, or distilled water alone will generally not provide sufficient conductivity for electrode units.


Fig. 3. Electronic steam humidifiers are used when steam is not available.

As with steam-to-steam humidifiers, water impurities can adversely impact the operation of the humidifier. Water with a high mineral concentration necessitates routine cleanings; water with a low mineral concentration is corrosive to the electrodes. These systems do provide a viable humidification option when a source of steam is not available and the advantages of a steam-to-steam humidification system are desired.

Electronic (ionic bed) steam humidifiers use immersed resistance heating elements to boil water and generate humidification steam much like electrode and steam-to-steam humidifiers. Since current does not pass through the water, high-purity water can be used. Ionic bed humidifiers allow the use of almost any water source.

Ionic bed cartridges, containing a fibrous media to attract solids from water as the temperature rises, minimize the buildup of solids inside the humidifier. Maintenance of these units generally involves little more than periodic replacement of the cartridges.

As with other indirect humidification methods, control is sometimes adversely impacted by the time needed to boil the water.

Water spray humidifiers inject water directly into the air duct. This method generally results in poor humidification and temperature control because water spray contains virtually none of the heat of vaporization required to increase the relative humidity of the air. Water spray humidification is a virtually constant enthalpy process, making it different from all other humidification systems that increase the relative humidity with almost imperceptible increases in dry bulb temperature.

Water spray humidifiers respond to control input slowly because evaporation must take place before humidified air can be recirculated. Unvaporized water can cause deposits to build up on surfaces, creating sanitation hazards. These systems also tend to suffer from corrosion and bacterial contamination.

Corrosion inhibitors

The choices for chemical corrosion inhibition in steam systems have changed little in the past 50 yr. They are:

  • Neutralizing amines

  • Filming amines

  • Oxygen corrosion inhibitors

  • Blends of emulsifiers.

    • Of the four choices available, the first three have been in common industrial use since the 1940's.

      Neutralizing amines are the most common of the chemical inhibitors. They are volatile, nitrogen-bearing compounds that condense with the steam, neutralize any acids present, and raise the condensate pH sufficiently to reduce corrosion.

      Neutralizing amines have a number of drawbacks. For one, they cannot inhibit oxygen corrosion, the most common corrosion mechanism found in heating systems. Often, these systems operate intermittently and oxygen is introduced during any offline periods. The combination of oxygen, moisture, and elevated temperature has serious corrosion implications.

      These compounds are volatile, and sometimes contribute odors, which many users find troublesome. Because they are volatile, they pass readily through direct steam humidifiers. Noncondensed amines pass the baffles to the outlet manifold. Condensed amines exit the humidifier through the steam trap at the bottom.

      Although neutralizing amines have a long history of good technical performance, they can offer some environmental, health, and safety concerns. (See sidebar on "A health problem solved").

      Filming amines are long-chain amines that adsorb onto piping surfaces, effectively inhibiting oxygen and carbonic acid corrosion, but they create feeding and control challenges that make them unattractive to many potential users.

      Because of their strong cationic charges, they act as detergents, "scrubbing" old deposits and loose material out of the condensate system. These corrosion products are commonly deposited in low points in the condensate system and routinely clog strainers and other steam equipment.

      Oxygen corrosion inhibitors , most commonly erythorbic acid and diethyhydroxylamine (DEHA), offer oxygen scavenging and passivation capabilities. Unfortunately, in instances where oxygen concentrations are high or carbonic acid is present, their use can become uneconomical.

      Blends of emulsifiers commonly used in the food industry have been shown, over the past several years, to effectively inhibit oxygen and carbonic acid corrosion. They form a non-wettable, substantive barrier on metal surfaces. The blends perform much like filming amines, but because they are essentially non-ionic, they do not flush out corrosion products or form sticky deposits in the condensate system.

      These inhibitors are also non-volatile and contribute no odors, an important property in HVAC installations. Experiments have shown that nonvolatile corrosion inhibition chemicals will not pass through direct steam humidification systems, eliminating the problems associated with neutralizing amines while maintaining the high efficiencies of the direct steam humidifiers. This approach shows promise as a method for returning high efficiencies without exposure to neutralizing amines.

      Systems

      The type of humidification system installed depends on the availability of plant steam, the need to avoid amine exposures, requirements for pinpoint control of humidity, cost, and other factors specific to any particular facility.

      New treatment options, coupled with highly efficient humidification systems, can meet many needs for safety and efficiency. Direct steam humidification offers greater efficiency than other options, but operational concerns often make other humidification choices more attractive.

      — Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com PLANT ENGINEERING magazine and the author extend their appreciation to Armstrong International, Inc. for supplying the illustrations and table.

      Comparison of humidification methods

      Direct steam Steam-to-steam Electronic steam Ionic bed electronic steam Water spray
      Effect on temperatureVirtually no changeSubstantial temperature drop
      Unit capacity per unit sizeSmall-very largeSmallSmall to mediumSmall to mediumSmall
      Vapor qualityExcellentGoodGoodGoodPoor
      Response to controlImmediateSlowFairFairSlow
      Control of outputGood to excellentBelow averageAverageAverageAverage
      Sanitation/corrosionSterile medium corrosion-freeBacteria can be presentProgrammed to not promote bacteriaProgrammed to not promote bacteriaSubject to severe corrosion and bacteria problems
      Maintenance frequencyAnnualMonthlyMonthly to quarterlyQuarterly to semiannuallyWeekly to bimonthly
      Maintenance difficultyLowHighMediumLowHigh
      Cost per unit of capacityLowHighMediumMediumMedium to high



      A health problem solved

      NIOSH completed a Health Hazard Evaluation at a manufacturer of electrical circuit boards and electrical equipment. In this case, the company had been using neutralizing amines for boiler lay-up. When the boilers were brought online, high concentrations of amines were introduced into the work areas.

      Symptoms consistent with acute toxic effects of diethylaminoethanol and cyclohexylamine were noted in many of the employees. These included nausea, dizziness, vomiting, and eye, nose, and throat irritation. A significantly higher risk of having several symptoms was noted among the employees in the areas humidified with boiler steam than among employees in nonboiler steam humidified areas.

       



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