New Practices Improve Cleanroom Bottom Line
Laura Peters, Lead Technical Editor -- Semiconductor International, 10/1/2007
The days of overbuilding and overspecifying cleanrooms may be over, at least, if Tengfang (Tim) Xu, researcher and manager in energy programs at Lawrence Berkeley Labs (Berkeley, Calif.) has anything to do with it. "For every 10% increase in airflow rate, there is a 30% increase in energy usage by fans, in theory. If facilities scale down to the necessary level needed for contamination control, a great deal of energy cost can be saved." Xu recently presented an online course, "Sustainability Considerations in Cleanroom Design and Operation," as part of the Institute of Environmental Sciences and Technology (IEST) Access the Experts educational series. The goal was to present recommendations and guidelines for the sustainability of cleanrooms and mini-environments (MEs) — from planning, design, construction and operation through decommissioning.
Process tools typically consume one-third of a cleanroom's energy, while chillers and pumps consume another 20% and the airflow system consumes an additional 20–30%. The rest is consumed by deionized and process water pumping, exhaust fans, nitrogen generation and other support. Actual air velocities and air change rates in cleanrooms vary greatly from one facility to another, and some have unidirectional airflows while others have non-unidirectional airflows.
Xu presented real-world examples of half a dozen ISO Class 4 cleanrooms and a set of Class 3 MEs, all of which would have been overspecified in terms of recommended airflow rates and pressure differential requirements inside the ME per current industrial specific documents. He explained that existing recommended practices are ruled by overspecification because of risk aversion and conservatism, as well as the attitude that if a certain practice has worked in the past, why change it? He proposed a hierarchical approach of information gathering for cleanroom planning to provide guidance on decisions during the planning stage and reinforce that energy is an important consideration. Xu also provided recommended practices for some specific systems (Table) that have been newly adopted in IEST-RP-CC012.2. "Anything we can do to better use energy will improve our environmental quality. For cleanrooms, and as an industry, energy efficiency, facility reliability, productivity and future guidelines have a great impact on current practice," he said.
Xu also encouraged integrated decision making by owners throughout the lifecycle, for example, to size systems (process, mechanical and electrical interface), ensure low pressure drop airflow (mechanical and architectural interface) and settle on sensible airflow requirements (mechanical and controls).
In one example, Xu looked at a dozen models of a 4 × 2 ft fan filter unit and compared energy efficiency of the units across a common airflow range. Not only was there great variation in performance, but some units did not even perform well within the desired range. For HVAC air systems, some of the measures to consider include low pressure drop, appropriate recirculation system type, air change rates, demand-controlled filtration, fan efficiency, fan filter units, exhaust optimization and exhaust systems. Xu pointed out that by optimizing airflow within the ME and cleanroom, energy savings in the 10–85% is possible while maintaining effective particulate filtration control.
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