Direct Steam Injection Humidification: Is It Safe For Building Occupants?
Often when steam is available in a building, engineers rely on the direct steam injection method to provide humidification to occupants. This method involves the steam from a boiler (under pressure) being used for direct injection into a duct, air handler, or directly into the space being conditioned. Engineers have been relying on this method since the 1930s.
However, there have been concerns in recent years that the chemicals used to inhibit scale formation and corrosion in boiler systems may cause IAQ problems if the steam is injected into an occupied space for humidification purposes.
Chemical manufacturers and humidifier manufacturers alike have been working double time in order to reassure engineers and building owners that there's nothing to fear.
Why There Are ConcernsConcern over direct steam injection humidification started to grow in 1982, when the National Institute for Occupational Safety and Health (NIOSH) looked into complaints at the Herbert F. Johnson Museum of Art at Cornell University in Ithaca, NY. Employees were complaining of eye irritation and dermatitis, among other things, and NIOSH determined that these symptoms could be reactions to diethylaminoethanol (DEAE), an amine added to the feedwater of the boiler that supplied steam to the museum's humidifiers.
NIOSH looked into a subsequent complaint in 1988 regarding an outbreak of illness among employees of Cincinnati Electronics Corp. (Cincinnati). The corrosion inhibitors DEAE and cyclohexylamine had been added to the boiler water, which was used to humidify the work area. These amines were suspected as a cause of the employees' symptoms. Indeed, NIOSH concluded that exposure to these amines should be eliminated by using alternative humidification methods.
There are genuine concerns over just what kind of chemicals should be used and how much should be used when employing the direct steam injection method, notes Dan Cicero, product manager, Nalco Watergy Group (Naperville, IL). "These concerns typically fall into four categories: Regulatory, safety, technical, and economic," says Cicero.
From the regulatory perspective, there are laws that prohibit the use of some steam system corrosion inhibitors. The best example is the dairy industry, where until recently, only ammonia was acceptable for use in these systems. Facilities regulated by the Food and Drug Administration (FDA) are required to maintain the concentration of corrosion inhibitors in the steam within limits set by the FDA. Exceeding those limits could constitute an adulteration of the food being processed.
There are also guidelines specified by the Occupational Safety and Health Administration (OSHA) covering the maximum allowable concentration of corrosion-inhibiting chemicals allowed in room air humidified with steam. Exceeding those levels could have adverse health effects in some individuals.
From a safety standpoint, most corrosion-inhibiting chemicals can be difficult to handle, notes Cicero. "Although most would not be considered 'hazardous' by most definitions, most are corrosive. Several have 'reportable quantities' amounts which, if spilled, must be reported to the appropriate environmental agencies."
Additionally, some users or industries shy away from use of these materials because of potential problems with the products they produce. For instance, baby food manufacturers do not routinely use chemical corrosion inhibitors because the risk of adulteration of their product is too great; it is more prudent to simply repair equipment as it corrodes.
Concerning the technical aspect, some corrosion inhibitors simply work better than others, which can cause further confusion. In some systems, especially those that operate intermittently, oxygen corrosion can be a real problem. For instance, in a heating system, the system can be shut down for half the year, and during that time, oxygen corrosion will occur because the corrosion inhibitors are not present.
Finally, economics sometimes play an important role. "Some waters used to generate steam contain high concentrations of alkalinity - carbonate and bicarbonate ions - which break down in the boiler to form carbon dioxide gas. When the carbon dioxide condenses with the steam, forming carbonic acid, corrosion inhibitors are needed to both neutralize the carbonic acid and raise the condensate pH to a noncorrosive value," says Cicero.
Chemical OptionsMost corrosion inhibitors are applied stoichiometrically; that is, for each mole of carbonic acid present, a mole of corrosion inhibitor is needed at the point of condensation. In some cases, that can be very expensive, notes Cicero. There are three chemical options for corrosion inhibition. By far, the most common is the use of neutralizing amines. Because they're nitrogen-bearing compounds, neutralizing amines are volatile organic materials that, because of certain physical properties, condense with the steam, neutralizing carbonic acid and raising condensate pH.
A variation of this chemistry is the filming amine. Again, these are nitrogen-bearing materials, but they are less volatile and, instead of neutralizing carbonic acid, form a protective barrier between the corrosive condensate and the condensate piping. Finally, there are blends of emulsifiers, which have historically been used in the food industry.
"These materials form a protective barrier, like a filming amine, but do not have some of the drawbacks of filming amine chemistry. They also meet many of the other regulatory, safety, technical, and economic needs of steam system operators," says Cicero.
There Are BenefitsEven with all these concerns and surrounding confusion, there are still good reasons to consider direct steam injection humidification, says Jon Bingaman, sales manager for hvac products, Armstrong International Inc. (Three Rivers, MI, see sidebar #1).
"There are established OSHA and ACGIH guidelines for permissible exposure limits to four specific neutralizing amines for concentration levels in steam humidified room air. There are also published test results from various sources documenting actual concentration levels in steam humidified room air that suggest, with proper treatment, the actual amount of amine in the air is well under the guidelines."
Another good reason to use the direct steam injection method is better control of room relative humidities, notes Bingaman. Direct steam injection humidifiers control the introduction of steam into heating ducts in response to a signal from a humidistat or building management system. Steam is introduced in inverse proportion to the relative humidity measured, maintaining the desired level. Humidification control by other methods such as steam-to-steam humidification may not give the tighter control of the direct steam injection method. "The control of relative humidity with steam-to-steam humidification is not quite as good - it's not as responsive, because it is a heat exchanger, and it's always going to be one cycle behind. If your controls say don't give any more steam, even if the control valve to the steam-to-steam humidifier closes, there's still this big pot of boiling water," notes Bingaman.
He adds that in addition to excellent control, comparisons of humidification methods typically suggest that direct steam injection humidification provides a low cost per pound of output, requires very little maintenance, and offers consistent and reliable performance.
Proper Maintenance Still An IssueBecause there are so many concerns surrounding the materials used in direct steam injection humidification, operators must be extremely aware of the amounts of chemicals that are to be used in the system. In fact, even though this type of humidification equipment is relatively easy to maintain, it is because of the operational concerns that engineers and building owners alike may turn to other kinds of humidification equipment.
Chemical overfeed is always a concern, notes Cicero, but proper testing, proper product selection, and proper feed and control methods can eliminate a lot of problems. He stresses that it's important to know that the levels at which these chemicals become a concern are very high.
"In the case of odor complaints, the level at which most people start to smell an amine is an order of magnitude lower than the point at which they become a health concern. And the health concerns are not caused by short-term exposure. The OSHA Permissible Exposure Limits (PELs) for amines are defined as the concentration of amine in air to which a health worker can be exposed for a 8-hour day and a 40-hour week without adverse health effects." In a paper entitled Regulatory Issues in Condensate Treatment, written by Cicero and presented at the International Water Conference in November 1997, the author notes that the method whereby amines are injected into the steam system has a substantial impact on the amount of amine at any specific time and location in the steam system. Therefore, it's important to have a standard procedure in place to ensure consistent feeding. Another option is the continuous feed method, as it will maintain a constant treatment level in the system.
Cicero notes that it is also vitally important to monitor the concentration of amines in several locations around the steam system. This aids in ensuring proper application of the products for corrosion inhibition and in documenting regulatory compliance. The paper states that the use of a simple, inexpensive amine titration test is useful on a day-to-day basis but should be supplemented with more accurate laboratory analysis on a periodic basis.
But if the use of amines makes you panicky, there are alternatives (see sidebar #2). As Cicero notes, carbon dioxide and oxygen are the primary causal agents in steam system corrosion. Reducing their concentrations will substantially reduce the need for corrosion-inhibiting chemicals. Most commonly, dealkalizers - ion exchange processes whereby alkalinity is removed from boiler makeup water - will reduce the amount of carbon dioxide gas liberated with the steam and, subsequently, the amount of carbonic acid dissolved in the condensate. This results in fewer chemical corrosion inhibitors needing to be used, as well as the improvement in quality of boiler feedwater.
In addition, proper deaeration of the boiler feedwater can reduce feedwater oxygen concentrations. "Deaeration is primarily a mechanical process, but there are also chemical oxygen scavengers that can more completely reduce oxygen concentrations," notes Cicero.
And remember that there's always help available. Bingaman notes that many humidifier manufacturers are usually willing to work with engineers to determine the proper type of equipment to specify for an application. In addition, a competent water treatment company can also help to ensure that the right amount of the correct inhibitors are in place and that a testing, control, and monitoring program is used to ensure the safe and effective application of the chemistry.
As Cicero notes, "It is much easier to solve a problem, should one arise, when it's identified early, potential problems have been anticipated, and contingency plans are put in place." ES
Benefits Of Direct Steam Injection HumidificationWhile it may seem that there are myriad drawbacks to direct steam injection humidification, there are just as many (or more) benefits, according to Jon Bingaman of Armstrong International:
Unit capacity per unit size. A great deal of steam can be introduced from a relatively small package, particularly true of units with integral control valves.
Vapor quality is excellent. The steps a properly designed direct steam humidifier goes through to perform separation include straining, separating, drying, and silencing the steam. Conditioned steam is discharged, which means the humidifier has removed impurities as well as separated out the condensate to provide good-quality steam.
Responsiveness to control. Conditioned steam is at the control valve, merely waiting for an opportunity to be called upon. Thus, there is not a time lag associated with most humidifiers, as they require filling with water and changing state (boiling) and periodically draining (blowing down).
Responsiveness is particularly important in situations where a high-limit humidistat or air proving switch senses a fault condition and asks the humidifier to stop discharging steam. Another scenario could include an application where a high air-change rate requires a very responsive humidifier to keep up with rapidly changing conditions (Perhaps someone opened a door to a small lab or test room).
Control of output. A direct steam humidifier can be supplied with a wide variety of control valves to be precisely sized for a given application. High turndown of the valve is often available with direct steam-injection humidifiers. This is referred to as rangeability, which is the ratio of maximum controllable flow to minimum controllable flow of steam through the valve.
Humidifiers are typically sized for outdoor "design" conditions, which are typically relatively severe. In the Three Rivers, MI area, outdoor design conditions are 78% rh at -10°F. Thus, the humidifier would be sized for some portion of the air to be at -10° before it's heated. Once it's heated, this air dries out and moisture must be added from the humidifier. But how often does it reach -10° in southwestern lower Michigan? Not very often. So a humidifier spends most of its working life being oversized relative to the actual amount of moisture it's being asked to provide. This "rangeability" or "turndown" becomes critical in making sure we avoid overhumidifying (wet spots in duct or working off the high limit humidistat) or underhumidifying.
Maintenance frequency requirements. The direct-injection steam humidifier is the most reliable and trouble-free of all the commercial/industrial humidifiers. The moving parts include only the operator, the control valve itself, and the steam trap(s) draining condensate from the humidifier. When designed and installed properly, the humidifiers typically go several years without required parts replacement. Armstrong recommends the strainer supplied with the humidifier be cleaned a few days after initial start-up and again once per year. The steam trap could be checked for proper operation at the same time.
Maintenance difficulty is typically very low with a direct-injection humidifier. With a little bit of care, many of these units installed 35 years ago are still in operation.
Lower first cost. The first (purchase) cost of a separator-type direct-injection humidifier is typically going to be lower than any other commercial/industrial humidifier. The total installed cost will typically be quite low, assuming steam is available. For some projects, the performance and reliability of these units justifies their use, even when combined with a packaged boiler (in other words, the enduser did not have steam available where needed). Direct steam injection typically is a cost-effective means of humidification in terms of cost per pound of humidity output.ES
New Technology Means There's An Alternative To AminesIn 1998, Nalco patented a new corrosion inhibitor, called Nalco Advanced Condensate Treatment (Nalco ACT(r)), which is formulated from a blend of emulsifiers commonly used in the food industry. It can be used in lieu of some of the more hazardous chemicals often used in a direct steam-injection system. The new treatment differs from conventional amine treatments in four ways, according to Dan Cicero, Nalco. First, the product meets many regulatory needs in the facilities served by Nalco:
- It is approved by the FDA for use in steam which contacts food.
- It is approved by the National Sanitation Foundation (NSF) for use in federally inspected meat and poultry plants.
- It is approved by the Chicago Rabbinical Council for use in food processing facilities requiring kosher certification.
- It is approved by a variety of Canadian and European regulatory bodies for food processing applications.
Second, ACT also contributes no tastes or odors, either to food products processed with steam treated with it or to air into which it is injected in a direct-steam humidification application:
- It has no "reportable quantity;" in other words, if it is accidentally spilled, no environmental agency - the EPA or DNR - needs to be contacted.
- It can be disposed of easily.
- It has no permissible exposure limit (PEL), which is the concentration of a substance in air to which a health worker can be exposed for an eight hour day and a 40-hour week without adverse health effects.
- It is compatible with most materials. Amines have a high pH and tend to be corrosive in some applications. ACT has a neutral pH and is not harmful to anything it might come in contact with.
Third, the product meets some technical needs. Amines cannot function in the presence of oxygen, while ACT can, forming a barrier on the condensate piping to protect it. This is important in many systems that shut down periodically.
During the down time, ACT continues to protect the system, which is beneficial in some applications in which equipment only runs during the heating season. ACT provides a small amount of protection in erosive environments, and it is easily and directly measurable. That can't be said for amines, which require some sophisticated analytical practices to measure accurately in the field. Unlike amines, Nalco ACT contributes no volatile organic chemicals (VOCs).
Fourth, ACT meets many customers' economic needs. In some applications, the demand for amines is very high and customers spend a lot of money for very little benefit. In those cases, ACT can be a relatively inexpensive treatment option.ES