Hi-tech HVAC

Numerous air-management concepts have been devised over the years using different types of fans to recirculate air in semiconductor fabrication facility cleanrooms. In these facilities, three types of air-moving apparatus predominate: (1) fan-filter units, (2) distributed recirculating air-handling units (RAHs) and (3) fan tower units. To give us perspective for an exploration of some more recent HVAC approaches that have emerged, let us review these three approaches that for many years were considered state of the art air management concepts.

Fan-filter units Fan-filter units typically consist of a centrifugal plug fan driven by a fractional horsepower motor, mounted in a sheet metal box in whose lower surface is mounted a HEPA or ULPA filter. Sound attenuating material is sometimes installed inside the box, as the fans tend to be noisy unless acoustic treatment is applied. The motor often has a multi-step or continuously variable speed controller. Fan-filter units do not develop much static pressure. After air passes through the internal acoustic passages and the final filter, there is comparatively little motive force left to offset external static pressure losses. Care has to be taken to properly design each component in the external airstream to minimise these losses, which are caused by air friction. Often this means enlarging the air passages and over-sizing the sensible cooling coils to keep the air velocity, and thus the resultant air friction, to an acceptable level.

Distributed return air handlers (RAHs) Distributed air handling systems have typically been built around packaged air handling units consisting basically of a sensible cooling coil and a fan. Plug-type centrifugal fans have been employed more often than not due to their relative compactness compared to scroll fans. Sound attenuation is almost always required at the fan discharge. Centrifugal fans can develop as much static pressure as is needed to move the air through the various components of the recirculation loop. However, as the total airflow increases, so does the fan size, the fan cost, and the amount of noise that the fan generates. Therefore, multiple, smaller air-handling units, installed in parallel, typically are used. Fig. 1 contains plan and section views of the standard cleanroom module with a conventional distributed RAH air-management system.

Fan towers Fan towers are nearly always built with vaneaxial fans. Vaneaxial fans offer the advantage that they can efficiently move large volumes of air against comparatively low static pressure. This is typically the set of conditions under which any cleanroom recirculating fan must operate. Vaneaxial fans also are relatively compact and efficient and, hence, are fairly inexpensive. A disadvantage of these fans is that a large part of the total pressure generated is velocity pressure. This becomes a significant factor in the selection of the fan. Vaneaxial fans are typically the most noisy of the three fan types in this study. Extensive sound attenuation measures must be taken, which add cost and additional static pressure loss. Fig. 2 again depicts the standard cleanroom module, but this time with a conventional vaneaxial fan tower system.

Advances in technology In the last few years, fan systems have been introduced that make much more effective use of fan energy and thereby significantly reduce the annual operating cost. Today fan-filter units are available with the fan, the motor and the motor controller engineered together as a package for optimal efficiency. One such package features a brushless, electronically commutated dc motor with an external rotor. The fan impeller is fitted directly onto the rotor. This mounting configuration allows heat transfer from the motor to be optimised. A Hall-effect sensor is used to detect the position of the rotor magnet each time it rotates. Control circuitry then precisely adjusts the motor voltage to match the torque requirement of the fan, thereby minimising inefficiencies due to slip. Overall, the resultant motor efficiency is 75 – 80%, compared to less than 40% for phased split capacitor or shaded pole motor designs. With this improved efficiency comes the byproduct of quieter operation. Because the fan uses a dc motor, its speed is infinitely variable. The controller can be set up so the rotational speed of the fan is remotely monitored and controlled. The on-off status of each fan-filter unit also can be remotely controlled and monitored. Vaneaxial fan packages are now available that combine advanced fan engineering with aerodynamically and acoustically engineered sound attenuators. This type of package may be applied in either a fan tower or a distributed RAH to yield a quiet, efficient system. If needed, vaneaxial fans can be selected with additional static pressure capacity to accommodate such items as prefilters or chemical filters. Fan-filter units generally do not have this capability. For uniformity, these items are not considered for any of the fan options analysed in this paper. Distributed RAHs require the installation of a structural fan deck to support the equipment. The interstitial space is part of the return air stream and must therefore be designed and operated to clean protocols.

Using airflow modelling to select the best air management scheme

Another recent advance in assisting the strategic selection of an optimal air handling approach for a given facility is airflow modelling. This diagnostic tool is highly accurate for both the planning of new cleanroom design, as well as the optimisation of existing, under-performing cleanrooms. Airflow modelling uses the mathematical accuracy of computational fluid dynamics to calculate the degree of deviation from vertical airflow, determine area pressure differentials throughout a space, reveal air current disturbances caused by cleanroom tools and other fixtures, determine temperature gradients, and track movement of particles, smoke and airborne molecular contaminants. One of airflow modelling's greatest benefits is its ability to precisely guide the intricate art of cleanroom floor balancing, which in the past has relied upon smoke wands and guesswork performed after cleanrooms were constructed - which is too late to de-bug a cleanroom. Airflow modelling allows you to achieve the same goal on the computer screen before cleanroom construction even begins.

Energy effectiveness Historically, cleanroom air-moving systems have accounted for a large percentage of the energy budget. Referring to Fig. 3, note that the energy effectiveness of traditional fan systems is approximately 0.4 watts per cfm. For a typical large fab with a recirculation rate of 2,000,000 cfm, operating 24 hours per day, and an electricity cost of US$0.08 per kWh, this amounts to an annual cost of over US$500,000. For each air-management option considered in this article, the static pressure loss, measured in inches water column, was calculated across the recirculation loop. The static pressure losses vary depending on the nature of the air passageways, and each option has unique characteristics. In all options the static pressure was intentionally kept to a practical minimum. Fans were then selected which would meet the airflow and static pressure requirements with the lowest power input. The static pressure requirement for each option is plotted against the corresponding energy effectiveness expressed in watts per cfm. The operating point for each option is indicated. Figs. 3 and 4 show the relative energy effectiveness of the three air management options for a cleanroom with 100 percent filter coverage. As might be expected, the vaneaxial options tend to out-perform the fan-filter unit option because the vaneaxial fans and motors are inherently more efficient. Fig. 5 is a graph of the energy effectiveness for a 25% filter coverage case. Again, the performance of the vaneaxial fan option is superior to that of the fan-filter units. In both the 100% case and the 25% case, the energy effectiveness is about double that of the older technologies, which means that the annual operating cost is halved.

Capital costs Turning to capital costs, we must look not only at the cost of the air-moving equipment, but at the total cost that each air-management concept imposes on the facility in which it is installed. The equipment cost of quality fan-filter units is about half-again higher than either distributed RAHs or fan towers. The basic reason is quantities. Even though the unit cost of a fan-filter unit is low compared to the other air-moving options, in our cleanroom module there are 200 such units and the total cost quickly exceeds that of the others. The fan-filter units require larger air passageways in order to reduce static pressure losses. This increases both the overall building width and height. Distributed RAHs require a greater building height, similar to what is required for fan-filter units, but less building width. Fan tower units require a greater building width, but do not add to the height. A structural fan deck is required to support the distributed RAHs, but is not needed for the other two options. Greater cooling coil surface is needed for the fan-filter units than for the other two options in order to keep the static pressure loss down. Each distributed RAH is supplied with a sensible cooling coil, so cooling water must be piped to each unit. Fan-filter units can be set in a gasketed ceiling. We have assumed pressurised supply plenums for the other two options, which require a gel-track ceiling. The electrical distribution cost is highest for fan-filter units. Even though the motors are much smaller, there are many more of them. Finally, automation costs are higher for fan-filter units, again due to the large quantity of fans. Fan-filter automation cost can be and often are lowered by eliminating the monitoring functions, but this is done to the disadvantage of those who need to maintain the system. Even so, the comparative cost ranking of the three options remains intact.

How do you get the best value for HVAC installation? Once you have sifted through the various air handling schemes to select the approach most appropriate for your facility and production goals, the next critical decision is how best to install your preferred option. A growing trend in installation is design/build cleanroom delivery. In ideal circumstances this approach provides thoroughly integrated design, construction and systems installation services. The superior practicality of this approach has particularly gained favour in Asia and other parts of the world where high-technology contracting resources are not yet fully developed. Integrated design/build cleanroom delivery eliminates many concerns related to quality of base build materials, maintaining clean build protocols on project sites, skilled assembly of complex systems, and startup issues.

The design/build approach treats the cleanroom as an integrated, cohesive piece of the overall facility Design/build is also appealing to owners because it involves the relative simplicity of one contract between the owner and the contractor, and it shifts more of the owner's traditional risk to the design/build contractor. When properly executed, the design/build method offers owners the further advantages of single-point project responsibility, the improved cost control of single-point accounting and change management, expedited procurement, and faster delivery of the overall project. The testing grounds for design/build cleanroom delivery have been locations that are among the world's least prepared to accommodate systems as complex and temperamental as cleanrooms, yet design/build has repeatedly proven its mettle under such challenging circumstances.

Conclusion The cleanroom industry has evolved impressively over the past 20 years to offer new airflow options, and better ways to achieve those options, and better productivity and cost-effectiveness over course of the facility's life cycle. However, the benefits of today's improved cleanroom technology are not automatic. As always, owners need to remain committed to performing the due diligence necessary to analyse multiple approaches for a cleanroom's design and construction. The good news is that when owners take the time to make such an analysis thoroughly, the new cleanroom technologies can reward those owners with an outstanding return on investment.

Editor's note: Variations of this article were presented at SEMICON Singapore and CleanRooms Asia and Cleanrooms Ireland 2002 technical symposiums.

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