Demand-controlled Filtration

One of the major energy uses in cleanrooms is in the management of particles in the space. The acceptable level of particles depends on the use of the space, as indicated by the cleanroom specification. Recirculation of air is a means to the end, but the question of whether higher air change rates necessarily yield higher levels of cleanliness has not been adequately addressed. LBNL began its research on this topic in the mid-1990s, which focused on analyzing the potential for dynamically managing air change rates in response to real-time particle-count measurements. By doing so, desired environmental conditions can be maintained without excessive energy use. The results of a lab study were very positive, indicating an economic payback time of 1 to 4 years, depending on whether or not the facility's recirculation system is already equipped with variable speed drives. If a facility already has variable speed drives on the recirculation fans for the cleanroom, then the payback will be closer to 1 year. Because the energy used by a fan is approximately proportional to the cube of the fan speed, small changes in fan speed can translate into large energy savings. The concept of varying ventilation speed has been applied at a major industrial site, which setback cleanroom fan speeds at night and on the weekend.

Industry receptiveness to this thinking is evident in a site benchmarked by the LBNL cleanrooms project, where substantial nighttime air change setbacks in a Class-100 cleanroom were found not to compromise the process. In another study, energy savings were evident while maintaining required cleanliness levels.

LBNL has developed a methodology for controlling cleanroom airflow based upon contamination levels in the room. Using commercially available particle counters, fan speeds are directly controlled by sensing particle counts in real time (rather than full-time, full-speed ventilation based on little more than rules of thumb.) Since fan energy varies with the cube of fan speed, small changes in fan speed will lead to large changes in fan energy. In a pilot study, LBNL implemented the strategy in a 300 ft2 ISO class 5 cleanroom, measuring particle concentrations using multiple particle counting instruments while changing recirculation system fan speeds. The results validated our expectation that DCF can save energy, i.e. higher fan speeds (step curve to left) do not necessarily mean lower particle counts (jagged curve to left). There may be an optimum recirculation fan speed that is unique to each facility and/or processes occurring in each facility. Following the pilot study, the strategy was demonstrated in an industrial cleanroom where again, large energy savings were realized through lower air flow when contamination levels were low. Other control strategies using timers and occupancy sensors were also demonstrated and similarly showed the potential for energy savings.

Case Study

LBNL completed a case study of the potential for demand-controlled filtration, conducted in a 300-square-foot Class-100 cleanroom located at LBNL.

  • Lasair Model 1003 by Particle Measuring Systems with particle sizing from 0.1 to 2.0 microns separated into 8 size bins and a sample flow rate of 0.001 cfm.
  • Integrating Nephelometer, Model M903 by Radiance Research
  • Climet Ultimate 1000, 0.10 to 1.0 micron with 6 size bins (0.1-0.15, 0.15-0.2, 0.2-0.3, 0.3-0.5, 0.5-1.0, & 1.0) at a sample flow rate of 1 cfm

Study Objectives

  • Identify a type of particle counter that is well suited for DCF and enables fast control response. In our previous study of DCF in the LBNL cleanroom, the particle counter used could detect particles no smaller than 0.3 microns, and the number of counts in that bin was not adequate for good control of recirculation fan speed.
  • Examine how changes in recirculation fan speed control affected particle concentrations.

Major Findings

  • Particle counts of particles <0.3 microns were about 10 times as great as for those of larger-sized particles.
  • The counts for 0.1- to 0.15-micron particles and the 0.15- to 0.2-micron particles were nearly the same. Thus the lowest size particle detected needs be no smaller than about 0.15 to 0.2 microns.
  • The particle counter should have a sample flow rate of 1 cfm or higher.
  • Higher speeds of the recirculation fans did not always produce the lowest particle counts. Thus, there may be an optimum recirculation fan speed for minimizing particle counts.