Semiconductors, Pharmaceuticals Drive Advances in Air-Handling Equipment
Desiccants and electronic controls supply cleaner, drier air to cleanrooms.
By Susan English
Like other segments of the cleanrooms industry, cleanroom air-handling equipment has had to keep pace with escalating demands for purer and more efficiently circulated air. The most critical technological developments are dictated by the semiconductor industry, with its concern for the effect of contaminants on smaller wafer geometries, and the pharmaceutical market, always looking for more sophisticated containment methods to safeguard its processes from the ravages of bacteria, yeast, mold, and mildew. Even with the trend toward mini and microenvironments, the HVAC segment of the cleanroom industry will continue to experience a healthy rate of growth, according to surveys conducted by The McIlvaine Co. (Northbrook, IL). HVAC orders for the worldwide semiconductor industry in 1995 for Class 1-10 systems totaled $207,775. By the year 2000, that figure is expected to reach $727,661. Orders for Class 100 HVAC pharmaceutical installations for 1995 totaled $56,609, with a projection of $92,223 by the year 2000.
The air-handling system is the largest mechanical system in the cleanroom, and each increment of cleanliness will require a greater air change rate. Since it is not uncommon for a cleanroom to recirculate more than one million cfm of air per day, the energy efficiency of cleanroom air-handling systems must be carefully considered, with specific design and construction techniques able to significantly reduce costs. Each individual component of a system must be looked at. For example, the twin factors of temperature and humidity must be constantly monitored so that the temperature inside the cleanroom ranges between 65° to 75°F, controlled to a tolerance of at least ۬°F and, in some critical processes, ۫°F. Relative humidity is usually at 30 to 60 percent, and humidity controls generally have a tolerance of ۯ percent RH–۪.5 RH for critical applications. The need to supply a constant flow of clean, dry air has led to the increasing popularity of desiccant dehumidifiers, which provide control at low moisture levels without the load sensitivity or maintenance associated with air-conditioning. (Desiccants pull moisture out of an air stream before it gets to the cooling coils that cool return air to the cleanroom.) They are most effective at the 45° to 50°F dewpoint–where refrigeration loses its effect. They also require a somewhat larger investment in reactivation energy in return for humidity control below 10 percent relative humidity, or when hot air must be dehumidified. “Desiccant cooling has really gotten hot within the last two or three years,” says Mario Ranieri, a product expert at Munters Corp.`s Cargocaire Division (Amesbury, MA). “Everyone`s scared of CFCs, so what`s happening now is that with desiccants, you`re using a natural gas or steam to regenerate it vs. an electric flash, which is an ozone depleter.” The company manufactures a complete turnkey system that hooks up to in-house utilities to control temperatur
Another trend driven by steeper cleanroom performance specs is seen in a shift away from pneumatic-actuated to electronically controlled valves and controls for maintaining temperatures in air handling equipment. “Demands on the equipment have been increased greatly,” says Steven Linn, OEM sales manager for San Diego-based Staefa Controls. “Four or five years ago, obtaining variations in room temperature of maybe a degree and a half Fahrenheit was acceptable, whereas now, people are looking at tenths of a degree. We`ve even seen some tests where they`re trying to hold temperature to ۭ/10ths of a degree Centigrade, which is about half a degree Fahrenheit!” While he concedes that pneumatics (the use of compressed air to operate valves or dampers) is still an important part of the industry, “quite a few” of the major chipmakers and circuit board manufacturers, he says, are now going to electronic controls, including electronically controlled valves. Pneumatics has two advantages. One is that it is an “open protocol.” “It doesn`t matter whose valves or whose controls you`re trying to tune loops or controls with,” says Linn. The other advantage is speed, and while very tight control can be achieved, the disadvantages are the need for additional equipment–compressed air. The other disadvantage is that the valves and other mechanical controls making up the system–bellows, packing, etc.– require a high level of maintenance. To bridge the transition between the two, Staefa has revised one of its valves to operate in synch with the pneumatic protocols PLC programmers are more accustomed to.
Air-handling units
Makeup air-handling units are often used to precondition and supply outside air to recirculation air-handling units, which push or pull conditioned air to the final cleanroom environment. Factors that must be considered are location of the unit, humidity, vibration and sound levels, filtration needs and any architectural constraints. In addition, the needs of a specific application will dictate construction materials, maximum weight and capacity. Air Enterprises (Akron, OH) specializes in providing custom air moving and conditioning units to cleanroom environments in the chemical, pharmaceutical and semiconductor industries. Its units address the need for lower temperatures, low humidification/ desiccants, DI/RO atomizing clean humidification, build clean/ship clean protocols, low sound and vibration control, and high static pressure.
Designed for use in minienvironments, Clestra Cleanroom, Inc.`s, (N. Syracuse, NY) AdvancAir III, offers a self-contained, pre-engineered and pre-fabricated modular air-handling unit for OEM minienvironment applications. The AdvancAir utilizes environmentally friendly refrigerants in capacities of up to 18 tons and 5,000 cfm in a single, direct-drive plug fan module, with temperature controls to ± 0.1°F and humidity control to ± 0.5°F RH. For use in conventional, modular and softwall cleanrooms, Enviroflex`s (Anaheim, CA) freestanding package airhandlers provide an easy solution. Using available chilled water, the unit sits on the floor next to the cleanroom space, recirculating air and providing optional temperature and humidity control. It can also accommodate ducted makeup air.
Stratus Air Handlers from Daw Technologies Inc. (Salt Lake City, UT) offer start-up service for each project. Each unit is equipped with a custom-designed sound attenuation section to provide maximum attenuation at absolute minimum air pressure drop. Clean Air Technology, Inc.`s (Canton, MI) line of Whisper Quiet recirculating air handler equipment is specifically designed for HVAC applications that require quiet operation. It is available in a variety of air volume capacities from 1,600 cfm to 25,400 cfm. PACE (Clackamas, OR) manufactures all the major components for its Clean-Pak RAH recirculating air-handling units, including fan wheel assemblies, heating/ cooling coils, dampers and all cabinetry. The units offer the P-Cone air measurement device, only available on PACE Plug Fans, which allows accurate air flow measurement (۫.5%) at the face of the fan with no increase in noise or decrease in efficiency.
The new Modular Line precision air-conditioning units from Stultz of North America, Inc. (Frederick, MD) are designed in over 190 microprocessor-driven configurations for simultaneous control of cooling, heating, humidification and dehumidification. Air Energy Systems (Willow Grove, PA) provides temperature and humidity control within a small footprint. Its ComfortClean cleanroom air-conditioning units are designed specifically for an existing space that is not meeting spec for temperature and humidity requirements where the current air-conditioning system can`t accommodate the heat load for new equipment or processes.
Breakthrough technology in humidification systems
Two new technological tools from Herrmidifier Co., Inc. (Lancaster, PA) have recently been introduced to the market. The first, G.E.D. (Guaranteed Evaporative Distance), is a comparison of evaporative distance under changing conditions of air velocity and steam intensity, now available in published form. Using standards of 55°F and 70 percent relative humidity (RH) downstream of the humidifier, the chart will allow an engineer or end-user to configure a humidification system by evaluating all the potential variables: temperature, velocity, downstream relative humidity and steam intensity. At the company`s 32,000 cfm test facility, Humidified Air Research Environment (H.A.R.E.), Herrmidifier engineers can build a complete performance matrix to duplicate real-world situations.
The second tool, developed from thermodynamic and psychrometric relationships, determines the correction factor needed to compensate for psychrometric conditions (downstream temperature and RH) which are different from the standard conditions. Once a customer`s humidification needs are pinpointed on these matrices, the resulting steam intensity requirement is combined with system specifications such as duct or air handler width and height, air handler capacity, and tube and nozzle configurations to determine the Herrmidifier system best suited to meet the requirements. Besides being able to guarantee evaporative distance at the initial installation of a system, H.A.R.E. research provides useful tools for additional performance analysis. Because industry standards do not exist for measuring evaporative distance, system suppliers have, until now, had to make rough estimates of system performance. None of the previous methods, however, allow for an accurate determination of true evaporative distance (the actual point at which condensation will no longer occur on objects within the duct at duct air temperature) or steam distribution of the humidification system.
Desiccant dehumidifiers
More than 30 years ago, the Swedish scientist Carl Munters (the inventor of Styrofoam and foam rubber) invented HoneyCombe rotor technology, or “desiccant wheel” technology. A rotating wheel containing thousands of honeycomb-shaped cells removes water directly from humid air as it flows through and contacts the desiccant material impregnated into the cell walls. Then the wheel rotates to a sector where the water that was adsorbed (or absorbed) is discharged by heated air, regenerating the desiccant material before rotating back into the humid air stream. This method provides a very constant outlet dewpoint for humidity control.
The Cargocaire Division of Munters Corp., which manufactures its own desiccant wheels on site, has recently introduced a titanium-enhanced silica gel desiccant wheel for use in installations operating in highly saturated air streams. The company also offers wheels of lithium chloride, zeolite (molecular sieve) and HPX. Lithium chloride wheels are especially useful in applications requiring high moisture removal with minimal energy requirements over a broad range of humidity levels. A natural inhibitor of microbiological growth, it is often used in hospitals and pharmaceutical manufacturing and can attract and hold over 10 times its weight in water. The zeolitic molecular sieve desiccant is made of a crystalline material of aluminum silicate, which is capable of separating molecules of different sizes. Used in very dry environments, molecular sieve materials are suitable for applications that require a very low level of humidity and can reach extremely low dewpoints (-40°F to -80°F). The HPX, designed for specialty applications, is an adsorbent that provides an extremely low dewpoint (-60°F and lower).
Dehumidification systems from Kathabar Inc. (New Brunswick, NJ) operate on the principle of chemical absorption of water vapor from the air using a liquid desiccant. The liquid desiccant is non-toxic, does not vaporize, and is not degraded by common airborne contaminants. It removes and kills up to 94 percent of all airborne bacteria, molds and viruses. Liquid desiccant dehumidifiers are also available from Engelhard/ICC (Philadelphia, PA) in sizes from 1,000 to 84,000 cfm. The company has just introduced its next-generation Desert Cool system.
Modular, packaged desiccant dehumidification systems from Airflow Co. (Frederick, MD) provide low dew point temperature control for cleanrooms using a non-toxic, non-granular desiccant rotor-cell. The company`s Dryomatic line of modular desiccant wheel dehumidification systems include units with 600 cfm to 20,300 cfm capacities. The company adds pre- and post-cooling, post-heat, and post-humidification, along with programmable microprocessor control to offer a packaged, integrated system that can be customized.
The Balston Membrane Air Dryer offered by Whatman Inc. (Haverhill, MA) will dry compressed air to dewpoints as low as -100°F at flow rates of up to 40 scfm. The air dryer has no moving parts and can be easily mounted in an existing pipeline. Dry air is produced by returning a small portion of the dry product air to sweep out moisture, which selectively passes through the membranes. By varying the compressed air throughput, the degree of drying can be controlled. Moisture-laden sweep gas is vented into the atmosphere, eliminating potential liquid-handling and freezing problems. The dryer requires no electrical connections, making it suitable for remote and point-of-use installations or for flammable and explosive applications.
Exhaust equipment
Harrington Industrial Plastics (Chino, CA) features a full line of fiberglass exhaust equipment for ventilating corrosive fumes from cleanrooms and removing contaminants from the airstream more efficiently before discharge. Harrington`s HPCA centrifugal fan motor is coupled directly to the wheel, eliminating power loss, vibration, noise and maintenance created by the drives and pulleys of a V-belt system. The company offers several types of corrosion-proof ducts: polyester, vinylester, and ICBO-approved fiberglass, as well as PVC and Polypropylene.
Ausimont USA, Inc. (Thorofare, NJ) offers Halar ECTFE fluoropolymer-coated, stainless steel ductwork for corrosion protection from cleanroom chemical fume exhaust, eliminating the need for sprinklers. The ductwork drastically reduces salt buildup during operation, maintaining a uniform, constant airflow. It is resistant to mineral acids, oxidizing acids, bases, halogens, and solvents over a wide range of temperatures.
Continental Fan (Buffalo, NY) introduces newly created fan curves for its `TEK` backward-curved blower wheels. The reverse-sloped, backward-curved airfoil impeller is molded from high strength glass-reinforced polyamid (GRP) resin, which is stronger than steel and half the weight. The impellers are available in CW, CCW, and DWDI configurations. McLean Engineering`s (Princeton Junction, NJ) forward- and backward-curved motorized impellers are designed with the air inlet over the motor for efficient heat dissipation for use in tough HVAC applications. Backward-curved impellers are designed to produce high static pressures up to 2 in. with airflow up to 1,200 cfm discharged in 360° and will operate without a housing. Forward-curved motorized impellers function within a housing, allowing airflow to be directed. The impeller, motor and shaft are manufactured as a single unit, minimizing rotational vibration.
Ross Cook, Inc. (Los Angeles, CA) utilizes CAD/CAM technologies to optimize blower internal air/gas flow and pressure in higher static pressure applications. These include all types of process systems, fluid bed aeration, high velocity industrial applications, free-flowing pneumatic conveying and central vacuum cleaning systems. Overall vibration levels are under 1.5 mils. Blowers feature pressure ranges from 0.5 psig to 10 psig, with flows to 7,000 icfm or vacuums from 3 in. Hg to 14 in. Hg.
CRP`s (Ronkonkoma, NY) IBF is a modular, self-contained HEPA filter and blower unit designed for use in new cleanrooms or facility upgrades. It may also be incorporated into CRP`s laminar flow softwall and straddle units. At 53 lbs, and only 13 in. high, it can be eyebolt-suspended or installed on a reinforced ceiling “T” grid. Air Crafters (Ronkonkoma, NY) manufactures a complete line of motorized blower filter modules in three standard sizes: 2 ¥ 2 ft; 2 ¥ 4 ft, and 2 ¥ 6 ft. The company`s BFM series offers as standard features 115 V, 60 Hz 1 ph electrical requirements HEPA filtration at 0.3 µm, variable motor speed control, knife-edge-to-gasket trough seal, and 30 percent efficient pre-filtration. Its Arrayed Air Delivery System performs at air velocities of 70 to 140 fpm via speed control and Db levels are 56 dba, making it one of the quietest in the industry, according to the company. The UltraGuard HEPA/ULPA fan module from Airo Clean Inc. (Exton, PA) is a self-powered blower and filter packaged together in a compact housing for use where full ceiling coverage is required to achieve Class 10 and Class 1 performance. A 1/4-HP motorized impeller fan unit operates at a mere 180 watts to achieve efficiencies up to 99.99999 percent.n
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Class 100 HVAC orders for all contamination-control related industries will almost double by the year 2000.
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The inside of a Whisper Quiet recirculating air handler from Clean Air Technology, Inc., shows the forward-curve, double-inlet centrifugal blower and motor with pre-lubricated, sealed ball
bearings.
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Herrmidifier engineers test humidification systems in the company`s Humidified Air Research Environment (H.A.R.E.) to obtain Guaranteed Evaporative Distance (G.E.D.).
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Airflow Co.`s Dryomatic line of modular desiccant wheel dehumidification systems includes units with 600 cfm to 20,300 cfm capacities.
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The Kathapac 1,600 FV dehumidification system from Kathabar Inc. uses a non-vaporizing desiccant. To achieve air cooling and dehumidification, Kathapac equipment uses a cooling-tower type contact surface and external heat exchangers in lieu of cooling coils.
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Enviroflex`s freestanding air handlers servicing a modular cleanroom, also by Enviroflex.
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The UltraGuard HEPA/ULPA fan module from Airo Clean Inc., a totally self-powered blower and filter packaged together in one compact housing, features a 1/4-HP motorized impeller fan unit that operates at only 180 watts.
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Halar-coated ductwork from Ausimont, USA Inc. provides corrosion protection from cleanroom chemical fume exhaust, eliminating the need for sprinklers.