by Neil Savage
Your choice for humidification control depends on your application
It's a comment you'll often hear on a sweltering summer day: “It's not the heat, it's the humidity.” And in cleanrooms, to a certain extent, that's true.
The control of humidity in cleanrooms is becoming increasingly important in areas such as semiconductor manufacturing, which are demanding more precise control than ever before. And as cleanrooms expand to more industries, the need for humidity control grows with them.
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“Everybody's got a cleanroom that they're manufacturing something in, in a variety of industries,” says Mike Dovich, regional sales manager for Dri-Steem (Eden Prairie, MN), which, according to market research firm Frost & Sullivan (San Jose, CA), is the market leader in steam-based humidification and number two in atomizer-based humidification.
And some of these industries have stringent demands for the level of humidity control. Semiconductor fabs, for instance, are using deep ultraviolet photolithography, with wavelengths of 193 or 156 nm to create tiny features on their chips. The photoresists for these processes—the masks that enable makers to scribe the features—are extremely sensitive to moisture while they're being exposed and require relative humidity (RH) as low as 35 percent. Semiconductor processes in general require RH in the range of 40 percent or lower and can require accuracy of as low as 0.5 percent RH. Temperatures often must be as accurate as 0.1 degree Fahrenheit. Too much moisture can cause swelling and introduce errors that are magnified by the tiny size of the features.
Pharmaceutical manufacturers need to control humidity as well. Too much moisture can cause fine powders to clump and can promote the growth of contaminating microbes. Some chemical processes can be affected by a slight change in moisture, causing whole batches of a drug to be rejected. In electronics, on the other hand, too little humidity can promote static electricity, destroying sensitive equipment. Even automobile makers have found that they need to keep RH at 50 percent to prevent static electricity from attracting dust to the cars being painted.
Dehumidification takes two basic forms—mechanical coil and dessicant. Mechanical coil dehumidifies the air by cooling it, taking advantage of the effect anybody who's ever had a cold glass of ice tea on a humid summer day has noticed. Cooler air holds less humidity, so lowering the temperature causes water to condense out of it.
Blake Hodess, president of Hodess Building Co., North Attleborough, MA, says air conditioning is the most efficient way to dehumidify a cleanroom. But to get RH down below 30 percent, and to get the fine control semiconductor fabs demand, dessicant dehumidification is needed.
A dessicant is a material that soaks up large amounts of moisture. One system, from Kathabar (New Brunswick, NJ), uses a liquid dessicant spray. But Mitchell Swann, director of facility technology at The Day & Zimmermann Group's Biopharm Technologies Division (Philadelphia, PA), says he has not seen many of those systems being installed recently. The liquid is corrosive, which challenges the mechanical design system and threatens such sensitive products as semiconductors.
More common is a dry dessicant, such as that used by Munters USA (Amesbury, MA) in its rotor system. Munters is the leader in dessicant dehumidification, according to Frost & Sullivan. A honeycombed wheel filled with a dessicant rotates through the air stream, removing moisture. The dessicant is then heated to re-dry it and used again.
David Simpkins of Munters says that semiconductor fabs are using this system both on the inlet of the fabs, to remove moisture from outside air, and on the outlet, to remove volatile organic compounds that play a role in air pollution and draw the attention of the Environmental Protection Agency. He says dessicant systems can be less expensive to operate than mechanical coil systems because of the cost of the electricity used in air conditioning. Of course, the demand depends on factors such as physical location. The dehumidifying load can be higher in the summer in the Northeastern part of the United States than in the Southwest, which, on the other hand, has a higher cooling load throughout the year.
Nelly Anderson, a research analyst at Frost & Sullivan, says dessicant systems are more expensive to buy in the first place, though she had difficulty comparing the two. Mechanical refrigeration accounts for probably 85 percent of the dehumidification market, she says.
Half the equation
But reducing moisture is only half of the equation. The flip side of dehumidification is humidification, and there the methods break down into isothermal and adiabatic. Isothermal humidification involves boiling water to produce steam. Adiabatic humidification mixes water droplets into the air.
Steam humidification grew out of the fact that many companies, particularly those in cold climates, had boiler systems already in place to heat their plants.
 The Munters desiccant HoneyCombe wheel rotates through the airstream to remove moisture. |
But it usually can't be used directly, Dri-Steem's Dovich points out. “Boiler steam in its raw form is full of a lot of nasty chemicals,” he says. Only an ISO Class 7 or 8 (Class 10,000 or 100,000) cleanroom might use boiler steam.
Typically, the steam from a boiler is run through heat exchangers to boil the secondary source of water, often 18 megohm de-ionized water, which then provides the humidification.
“Clean steam has been and continues to be part of cleanroom humidification definitely, particularly in the pharmaceutical end of things,” Dovich says.
Electronics companies that have long had plants in the Northeast, such as IBM and Xerox, often use clean steam.
But boiler systems aren't the only way to boil water, and both electric and gas-fired humidifiers exist. In fact, Dovich says, gas-fired technology is growing in popularity. For one thing, natural gas tends to be cheaper than heating oil, making it an attractive alternative to boilers. And for those who didn't have a boiler, their only option had been electric heating. But gas is more cost effective than electricity, even though the initial cost of the system is higher, Dovich says. “That difference in price is almost always paid back in the first year.”
Alternatives exist for companies that don't want to be boiling all this water. One is an evaporative humidifier, in which water runs over some medium, and air is blown across it, absorbing some of the moisture. Hodess says this is the most energy-efficient of the adiabatic systems. Another possibility is the air-blast or atomizing system, in which compressed air and water are blown out of tiny ports, mixing the air and the water vapor.
A third form is ultrasonic humidification, in which high-frequency sound waves, beyond the range of human hearing, are directed at a shallow tray of water. Tiny droplets break off the water's surface into the air, and the particle droplets produced by this method can be as small as a micron in diameter. These systems all have the added benefit of producing a cooling effect, reducing the load on air conditioning systems in hot areas.
Scott Herr, president of DGH Systems (Lancaster, PA), says his company has been building ISO Class 3 through ISO Class 7 (Class 1 through Class 10,000) cleanrooms for companies including Motorola and Intel. “They've gotten away from steam and gone more and more to atomizers because they're cleaner,” he says.
And the increments in which atomizers can add moisture to the air is getting finer, Herr adds. Systems have typically had modulating turndowns of 20 or 30:1. Now systems are available with turndowns of 500:1, and can process from 500 pounds of water per hour down to 1 pound per hour. This is important for making fine adjustments to temperature and humidity.
“In effect when you get very close to the set point, a lot of systems will essentially give you on/off, which doesn't give very good accuracy,” Herr says.
Fine-tuning also lets these newer systems control droplet size over a range from 10 to 0.3 microns.
According to Hodess, the biggest mistake people make when installing a humidification system is forgetting the way dehumidification works and neglecting to add heat to cool air before adding moisture. “If I just take the very cold air and add humidity to it, the moisture just falls out,” he says.
Another common problem is not building adequate lengths of straight ducts for the particular system. Ultrasonic systems need 10 feet of room to mix the moisture properly with the air. Compressed air systems take 12 feet, while evaporative systems take four.
If the duct is too short, the plume of moisture is going to hit the far wall and condense before it is adequately mixed. That not only throws off the humidification, it also provides a place for molds and bacteria to grow.
Swann, who is also chair of the clean spaces committee of American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE; Atlanta), says he's seen increasing use of compressed air and ultrasonic systems, especially in the pharmaceutical industry. That means, of course, that cleanrooms have to be vigilant about the quality of both the water and the compressed air the systems use. He's also seen a move toward smaller tubes to produce finer atomization and better mixing, which in turn leads to less potentially contaminating moisture build-up.
On the other hand, he says, some systems provide more humidification control than is really necessary. The International Society for Pharmaceutical Engineering, Tampa, FL, and the federal Food and Drug Administration are developing joint guidelines on water and steam systems.
“There's been kind of a tendency to reduce their risks they've overdesigned,” Swann says. “This guideline will help bring some of these systems back to reality and help control some costs.”
Swann sees the use of humidity control systems increasing as more processes move into cleanrooms. Increased concern about food safety, for instance, is leading food packagers into cleanrooms, where they need an RH of about 40 percent to retard spoilage. He also predicts that energy use will become a bigger concern as prices go up.
“There's probably going to be some push to try to find more energy-efficient methodologies for dehumidification, so that's probably going to spur some technological growth,” he says.
Neil Savage is a freelance writer in Lowell, MA.
Dehumidification: The basics of wheel technology for air treatment
by Jim Doolin
The rapid growth of wheel technology for dehumidification in air treatment applications for cleanrooms has brought with it confusion about the differences in wheels.
Often, the attempt at resolution for contractors and/or engineers is to analyze a psychrometric chart used to describe the relationship between temperature, moisture and heat commonly used in the design of heating and cooling systems.
But to end users, the chart usually sounds like a foreign language and rarely does it demonstrate how the wheels work to improve conditions and/or save energy.
The decision about which wheel system is best suited depends on the intended application. To install the right system and achieve the best performance, a number of factors have to be taken into consideration, including internal moisture content, outside air required, amount of exhaust and desired results.
Wheel function
The three types of wheels commonly used in cleanrooms perform three distinctively different functions involving sensible heat, latent heat and total heat. Sensible heat can be defined solely as the temperature of the air. The warmer the air, the greater the sensible heat.
Latent heat is defined by the humidity or amount of moisture in the air. The moister the air, the greater the latent heat.
Total heat is simply the sum of sensible and latent heat.
Desiccant wheels remove latent heat. Sensible wheels remove sensible heat. Enthalpy wheels remove total heat (sensible and latent).
What are desiccant wheels?
The vast majority of desiccant wheels are made up of silica gel or molecular sieve bonded to a substrate of fiberglass, paper or sometimes aluminum. When viewed from the air path, the wheel takes on the appearance of finely meshed honeycomb material with many small flutes.
The flutes, like the fins on a coil, force the air path evenly along the concentrated desiccant. The desiccant provides an attraction for water molecules through microporosity based on the size of its pores.
In this manner, silica gel and molecular sieve desiccants are adsorbatents, or mechanical attracters, of water like a sponge. In a typical application, 75 percent of a desiccant wheel is in the target air path with the remaining pie shaped 25 percent in a regeneration chamber.
Regeneration is accomplished by passing extremely hot air, about 250 degrees F, through the wheel and into an exhaust. The hot air provides a greater attraction for water molecules than the desiccant and the wheel is thus regenerated.
Desiccant wheels by nature are removers of moisture (latent heat) whether from outside air or internal load. They do not by themselves reduce the energy load. They simply replace latent (moisture) load with increased sensible load. As in the opposite of cooling something by pouring water on it, they increase sensible heat (temperature) by removing moisture from the air.
Common desiccant wheel applications
The three most common uses of desiccant wheels are reducing an internal moisture load, holding or lowering a specific relative humidity (RH) or specific dew point (limitation of the air's ability to hold moisture), and processing high latent outside air.
In semiconductor manufacturing, desiccant wheels are used to closely control relative humidity because too much moisture in the air corrodes semiconductors and not enough moisture in the air creates static electricity, which shorts unprotected semiconductor circuits.
In pharmaceutical manufacturing, many drugs are hygroscopic and will absorb moisture and clump up unless the humidity is controlled. Also, coating and gel caps require careful drying in the manufacturing process; thus, humidity control is imperative.
The wheels also provide humidity control in production processes for food, beverage and paint. Many of these applications require control of RH within 2 percent.
Sensible wheel technology
Sensible wheels are essentially sensible heat (temperature) collectors acting as sensible heat exchangers in two air streams. They are most commonly made from aluminum, although there are also Mylar and polypropylene versions.
When viewed from the air stream they share the honeycomb appearance of the desiccant wheels with numerous small flutes to channel air across the collecting material.
Unlike desiccant wheels, sensible wheels require both an exhaust and a supply air path.
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In the stationary position, a portion of the wheel is in both air streams. When operating, the wheel simply rotates between air streams. In summer, the wheel can pick up sensible heat from the supply and exchange it in the exhaust air path, thus lowering the temperature of the supply.
While a lot of complex arguments are made about the effectiveness and efficiency of sensible wheels in different conditions, there is a simple principle that can be applied.
The greater the sensible temperature difference between the two air streams, the more work the wheel will do. The equation is, generally, the sensible temperature difference times wheel efficiency equals temperature change in each air stream.
Sensible wheels, unlike desiccant wheels, are generally used to reduce energy loads and will do so effectively as long as the level of desirable exhaust is present.
Applications for sensible wheels
An argument can be made for sensible wheels in almost every application where there is some form of desirable exhaust and the potential energy capture great enough to justify some increased cost.
Typically sold as makeup air units or integral parts of larger systems, sensible wheels are highly effective energy savers. Because sensible wheels are most efficient when the temperature difference between exhaust and supply is at its greatest, they, by nature, are at peak efficiency when ratcheted or metered electrical cost is at its greatest.
Enthalpy wheel applications
Enthalpy wheels are a combination of sensible and latent strategies on the same wheel. Their physical appearance is very similar to sensible wheels in that a desiccant is bonded to the wheel's aluminum collection system. Air is channeled through multiple flutes across both the desiccant and the sensible collector.
Like sensible wheels, enthalpy wheels straddle both air streams and a desirable exhaust air stream is required. Also like sensible wheels, the greater the latent and sensible difference in the two air streams, the more work the wheel will do. The efficiency equation is generally identical to sensible wheels. The sensible temperature difference and the latent grain difference times wheel efficiency equals sensible temperature and latent grain change in each air stream. Many industry insiders believe that enthalpy wheels could be applied almost anywhere a sensible wheel is applied. The cost difference between the two is not significant.
Making the wheel choice
Sensible and enthalpy wheels are not common in clean manufacturing because most of the internal air is filtered and recirculated and because what exhaust air there is may contain contaminants. Specialty desiccant wheels are generally installed on the exhaust of cleanrooms for VOC removal.
It is important to remember that wheel applications are not limited to an either/or choice. Many manufacturers offer both single- and dual-wheel systems utilizing all the possible wheel combinations.
Jim Doolin is commercial/national accounts manager for Munters Corp. D DH Division.