Water conservation through the use of recirculating chillers
10/01/1997
Jeffrey P. Mills, Lytron, Woburn, Massachusetts
Lise C.H. Laurin, Clear Tech, Newton, New Hampshire
The SIA National Technology Roadmap for Semiconductors cites water conservation as a key goal for future circuit manufacturing. Although the SIA included this goal (and others in the environment section of the Roadmap) to keep the industry ahead of governmental and public pressures for conservation, water conservation offers a significant return on investment. The use of recirculating chillers has the potential to reduce water consumption significantly at the tool level, providing rapid payback and long-term savings. Where the process is sensitive to fluid temperature control, other benefits accompany the installation.
Wafer fabs consume huge quantities of water each day. A typical wafer fab consumes as much water as a town of 60,000 [1]. A striking example, Sematech, is one of Austin's top ten water consumers [2], even though it is an R&D facility. Including purification and effluent removal costs, the industry spends as much on water as it does on wet chemicals [1]. With this high usage and the associated cost, any reduction can be significant. In an existing facility, it is possible to approach use reduction on a tool-by-tool basis, providing rapid payback with minimal disruption to manufacturing. In the case of a vertical diffusion furnace, installation of a recirculating chiller typically provides a payback in less than four months, with savings/year exceeding $10,000.
Water usage in a vertical diffusion furnace
A typical diffusion furnace uses a heat exchanger to exhaust excess heat from the heater element cabinet. This application requires approximately 6 gpm of water [3]. For many low-pressure applications, water also cools the elastomer seals at the rate of about 0.5 gpm [4]. Pyrogenic oxidation systems may include a condenser, using water at about 0.5 gpm [5]. The figure shows how the water flows through a diffusion furnace.
 A typical vertical diffusion furnace uses at least 6 gpm of water, adding up to over 8500 gallons/day. |
By installing a recirculation system on a furnace using only 6 gpm, water and sewer savings will easily pay for the recirculation system. Furnaces using more than 6 gpm will pay back more quickly.
Water costs
Commercial water costs around the country range from $0.55/1000 gallons in North Carolina [6] to $2.22/1000 gallons in Austin [7]. Wastewater rates are higher: $1.80/1000 gallons in North
Carolina to $3.13/1000 gallons in Austin [2]. Without recirculation, the diffusion furnace draws 6 gpm from the city water supply and discharges 6 gpm into the sewer. For a 24-hr-a-day, 7-day-a-week operation, the yearly cost/furnace is $7411 in North Carolina and $16,872 in Austin.
Payback
A typical recirculating unit costs less than $5000. Return on investment is less than one year in areas of low population density, and much faster in areas where water costs are higher. Operating costs are minimal, allowing the user significant savings on water costs over the life of the furnace. The furnace user can achieve incremental savings by retrofitting furnaces one at a time, with minimal disruption to manufacturing. (If the retrofit takes place during scheduled downtime, manufacturing may be unaffected.)
Sewage treatment costs are escalating in populated areas, making recirculation not only cost-effective, but, in many cases, required. To reduce the load on the sewage treatment plant, many municipalities have already barred discharging clean water into the sewer system [8]. Since recirculation is cost-effective by itself, the wise furnace user will install a system before legislation is enacted.
For CVD systems, the use of a recirculating chiller can improve system reliability (see "How a recirculating chiller works"). Without the chiller, the incoming water supply temperature may change from one season to the next. If the unregulated water temperature becomes too high, the elastomer seals will degrade faster. If the water temperature drops too low, moisture from the air can condense on carrier lines and the door assembly, posing additional reliability and contamination problems. A recirculating system also allows the use of more efficient cooling fluids such as a glycol-water mixture, Fluorinert, or Galden.
Conclusion
The SIA goal of a 50% reduction in water consumption for 0.25-µm facilities will reduce the cost to manufacture these devices. Fabs running larger geometries stand to gain from reduced water consumption as well. Recirculating chillers offer these facilities cost savings with minimal production downtime. The incalculable benefits to the environment provide further justification that water conservation is the right thing to do.
Acknowledgment
Fluorinert is a trademark of 3M; Galden is a trademark of Ausimont.
References
- Michael C. Lancaster, "Ultrapure Water: The Real Cost," Solid State Technology, p. 70, September 1996.
- Jan Gerston, "Sematech Reclaims Plant Wastewater: Shows 7-Month Payback," Texas Water Savers Newsletter, Vol. 2, No. 4, Summer 1996.
- SVG Thermco.
- Bruce Technologies International.
- Progressive Technologies Inc.
- North Carolina Agricultural Extension Service.
- City of Austin.
- Massachusetts Water Resource Authority.
Jeffrey P. Mills received his BS degree in marine engineering from the Massachusetts Maritime Academy. He has more than 13 years of managerial and design engineering experience in a wide range of industries. Mills is currently engineering manager at Lytron, where he oversees all aspects of engineering, including R&D and new product development, and is responsible for all product and agency approvals, such as CE marking. Lytron, 55 Dragon Court, Woburn, MA 01801; ph 617/933-7300, fax 617/935-4529.
Lise C.H. Laurin received her BS degree in physics from Yale University. She began her career as a process engineer at Intel, subsequently entering the field of semiconductor equipment marketing. After 15 years of technical and marketing experience, she founded Clear Tech in 1996, a marketing services business for the semiconductor and other technology industries. Clear Tech, 14 South Main St., Newton, NH 0385; ph and fax 603/382-7682, e-mail [email protected].
How a recirculating chiller works
Each recirculating chiller consists of two liquid flowloops (Fig. 1). The process cooling loop — the loop used to cool the furnace or other application — is the simpler of the two. Typically, this loop consists of a reservoir, a pump, a heat exchanger, contamination removal in the form of filters and resin beds, and a temperature probe. Most chillers can operate using water, deionized water, glycol mixtures, or high dielectric materials such as Fluorinert and Galden.
 Figure 1. Flow through a recirculating chiller; the hot gas bypass reduces wear on the compressor. |
The heated process fluid returns from the furnace into the reservoir. The pump circulates the heated water from the reservoir into the heat exchanger, where it is cooled to process temperature. The chilled fluid then flows through the contamination-removal media and back to the furnace.
The heat exchanger transfers heat from the process-cooling loop to the refrigerant loop. From the heat exchanger, the compressor turns the low-pressure, superheated return vapor into a high-pressure, high-temperature vapor. The air-cooled condenser condenses and cools the refrigerant. (Some models allow for liquid cooling of the condenser, using a house water recirculation system to reduce the heat load on the cleanroom.) The subcooled liquid refrigerant then flows through a reservoir ( "receiver"), filter/dryer, sightglass, liquid line solenoid valve, and to the thermal expansion valve at the heat exchanger inlet. The thermostatic expansion valve meters the amount of refrigerant to the heat exchanger coil in response to the temperature at the heat exhanger outlet. If the temperature is higher than that of the preset value at the valve, the valve will open, allowing more refrigerant flow.
A typical control system cycles the compressor on and off to control the refrigerant flow as needed to control the process fluid temperature. Alternatively, Lytron chillers use a hot gas bypass. Instead of turning the compressor off, this option flows refrigerant gas and the refrigerant liquid alternately through the heat exchanger to control the process-cooling water temperature actively. By eliminating the on-off cycling of the compressor, this technique lengthens compressor lifetime. All chiller manufacturers offer chlorofluorocarbon-free refrigerants. This type of chiller will maintain the cooling liquid temperature within ±0.5°C, sufficient for most diffusion furnace applications. Higher-grade chillers control within ±0.1°C for applications requiring tighter temperature control (Fig. 2).
 Figure 2. Lytron offers chillers with temperature control ranges from ??0.1??C to ??0.5??C. |
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