Cleanrooms are used extensively in the manufacturing of integrated circuits and in the biological and pharmaceutical industries. For particle concentrations to remain low, for example, less than 100 particles/ft3 at >0.5 micrometers (Class 100), the air in the cleanroom must be filtered. Typically, the air is circulated through high-efficiency particulate air (HEPA) filters at a very high rate, such as 400 to 600 room air volumes per hour, to maintain low particle concentrations. The combined effect of high recirculation and a high pressure drop through HEPA filters is higher power costs per unit floor area to operate the cleanroom than to ventilate a commercial building. Cleanrooms are usually ventilated constantly and turned off only for maintenance, contributing to high energy costs.
The Center's Indoor Environment Program, along with LBNL's In-House Energy Management Program and Engineering Division, conducted a research project to study ways of reducing cleanroom energy use. We used a technique called demand-controlled filtration (DCF) which controls the particle concentration in a room by changing the recirculation flow rate based on the real-time measurement of particle concentrations. As the measured particle concentration rises above a threshold, the speed of the recirculation fan increases. As the concentration falls below the limit, the fan speed falls with it. Lower average fan speed reduces fan motor energy consumption.
To evaluate DCF, we used a Class 100 cleanroom at LBNL with a floor area of 300 ft2. The facility is used by researchers two to four times a week for one to four hours per day, and its primary activity is fabricating silicon detectors. A $2,500 particle counter measured the concentration of two size ranges (>0.3 and >0.5 micrometers in diameter) of particles in the cleanroom and was located on a benchtop near a machine that operators used frequently.
|Control Strategy||Average energy savings per day||Average energy savings during occupancy|
|Baseline 1*||Baseline 2**||Baseline 1*||Baseline 2**|
|Preexisting||10% ± 5%||60% ± 2%||0% ± 5%||56% ± 2%|
|Ten percent||64% ± 1%||84% ± 1%||63% ± 2%||84% ± 1%|
|Proportional||64% ± 3%||84% ± 1%||60% ± 6%||83% ± 2%|
*Average power for the first baseline is 2.6 kW (3.5 HP). Based on fans operating at 75% of full speed.
**Average power for the second baseline is 5.9 kW (7.9 HP). Based on fans operating at full speed.
The cleanroom's preexisting control strategy (see "Preexisting" in the table) was to maintain a set pressure drop across one bank of HEPA filters. The daytime setpoint was established to maintain the cleanroom's Class 100 certification. At night, the setpoint changed so the fans could run at a lower speed, cutting energy costs by about 10%.
In the first DCF control scheme ("Ten Percent" in the table), the counter read particle concentrations once per second. Each second, if particle concentrations in either size range exceeded the upper limit, fan speed increased by 10%. Likewise, if the particle concentration in either size range was below the lower concentration limit, fan speed decreased by 0.13%. With a second DCF scheme ("Proportional" in the table), if particle concentration in either size range was above the upper limit, the fan speed changed in proportion to the magnitude of the difference between the concentration and the upper limit. If the counts were below the lower limit, the fan speed decreased by 3%.
Depending on the choice of baseline energy use, the two DCF control routines reduced the energy consumption of the recirculating fans by 60 to 84%, with no significant difference between the two methods (see table). The percent energy savings while the cleanroom was occupied was nearly the same as when it was unoccupied (and thus equal to the daily energy savings). When the cleanroom was occupied, the fan speed sometimes increased for only short periods, hardly affecting the daily average power consumption.
Changes in recirculation fan speed by 10% or more did not cause a noticeable particle release from filters or resuspension from indoor surfaces, as many cleanroom users had predicted. Cleanroom users' prediction that it would take hours to reestablish proper conditions after recirculation fans speeds were lowered was also found not to be true. With either of the two DCF control routines in operation, there were occasional (usually fewer than 10 per day) nonconsecutive one-minute episodes in which the particle concentration exceeded Class 100 status. Thus the DCF maintained Class 100 specifications at least 98% of the time while saving significant amounts of energy.
Depending on the energy consumption baseline used in the calculations and whether or not variable fan drives are in the facility, the simple payback time for retrofitting a 1,000-ft2 Class 100 cleanroom to use DCF methods is from one to four years. This assumes a cost of electricity of $0.08/kWh.
DCF holds the most immediate promise for small cleanrooms that are not continuously in use and in which users are performing one activity at a time such as in universities and research institutions. Implementing DCF in large manufacturing cleanrooms would require a more sophisticated control system than the one outlined here, but the potential savings in energy may justify its development.
This research was sponsored by DOE's Office of Energy Research and the California Institute for Energy Efficiency.
Indoor Environment Program
(510) 486-7326; (510) 486-6658 fax
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