by John Haystead
Energy-efficient HVAC design can add up to millions in savings.
Like any business, performance and productivity are the primary concerns in cleanroom manufacturing environments. And though this necessarily makes any potential cost savings from energy efficiency measures a second-tier priority, given their extremely high energy consumption per square foot, even modest attention to the area can result in a significant contribution to the bottom line. Up-front investment in energy-efficient HVAC system design can deliver millions of dollars in total cumulative savings over the life-cycle of a facility.
Given the number of potentially devastating yield-limiting factors, such as leaking filters, inadequate airflows, fluctuating room pressures and ultimately high-particle counts, it's not surprising that the first concern of users is ensuring the efficient operation of their cleanroom environments.
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Nevertheless, many are finding they can have the best of both worlds. According to Bill Edwards, president, Mechanical Environmental Systems Analysis & Adjustment Agency (MESA3), a cleanroom monitoring and energy efficiency company in San Jose, CA, “Companies seek us out because they're having problems with performance as opposed to looking for ways to reduce energy consumption. During the course of a performance evaluation, we can also point out to customers where energy is being wasted or where a performance-based enhancement may also have a bi-product benefit in energy efficiency. It's one more selling point to help justify the enhancement, and regardless of the type of facility, the savings numbers can be significant between paying attention to HVAC efficiency and not.”
Rob Darnell, project engineer, Advanced Technology Division, Black and Veatch (Kansas City, MO), an international engineering and project management company, agrees, noting that while energy-efficiency considerations generally come up after the fact, “energy efficiency is becoming a more prevalent concern for most companies and has always been a high priority for facility managers whose job it is to keep operating costs down.”
Because each user has different goals and priorities and ultimately dictates how the facility will operate, it's difficult to put a general figure on cost-reduction goals. Darnell says that a 10 percent cost reduction is a frequent target.
Likewise, the difficulty of achieving this goal will vary by user. As noted by Darnell, sometimes this is as straightforward as examining how the facility is being utilized and finding places and times where systems can be turned back or shut off to save energy. “Particularly in locations where operations are not running 24 hours per day. Things like reducing cleanroom airflow velocities at night, or during reduced-usage periods, can offer substantial savings,” says Darnell.
Although there is an associated capital-investment premium (25 to 50 percent) associated with the use of variable-speed fans and drives, they can deliver significant operating cost savings over constant-speed units. As pointed out by Jim Hoffman, principle, Hoffman Environmental, a cleanroom enclosure specialist, “When constant-speed fans are used, a cleanroom's energy usage at rest will remain close to that during full operation, while variable-speed systems will allow this to be cut back to less than 50 percent during off hours. And, when integrated with automated environmental monitoring and management systems, they can generate further savings.”
Another important factor is the trend toward reducing the percentage of coverage associated with different cleanroom classifications. As observed by Hoffman, “Whereas a few years ago, the practice was to provide close to 100 percent coverage in an ISO Class 3 (Class 1) room, experience has shown that in almost all applications, this is just not necessary, with coverage frequently cut back safely to 60 percent coverage.” Similarly, Hoffman notes that air velocities often don't need to be 85fpm in an ISO Class 5 (Class 100) room, where 60 fpm will work just as well and result in a 30 percent reduction in air-handling capacity requirements.
Bill Hurley, president at design-engineering firm Alfatech (San Jose, CA), which specializes in HVAC systems, says operating cost savings will vary depending on the approach taken, and users should look carefully at both the up-front cost of the HVAC equipment and the projected operating costs. “Sometimes the payback period for an up-front investment in energy efficiency is short term, whereas other times it's not.” Hurley also points out that system or configuration familiarity is another factor ultimately impacting HVAC system selection. “Oftentimes, the customer will just want to stay with what's been working for them, and not want to take a chance on something new or unfamiliar,” says Hurley.
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Finally, according to Brian Halsey, Hoffman Environmental production manager, sometimes operating costs just aren't a factor in the decision process, particularly for startup companies where available cash is tight. “Despite the significantly higher operating costs, we've had some customers opt for fan-powered HEPAs versus a much more efficient centralized system, strictly because of the relative up-front capital equipment costs.”
Products and processes
As with everything related to cleanroom design and operation, any potential improvements to HVAC energy efficiency must begin with the process and the product.
 Airflow model of a simulated fire in a chase showing path of smoke to determine the best location of a smoke detector. |
Here, Dennis Grant, mechanical department manager at IDC (Portland, OR), a one-stop, international engineering and architectural firm, observes there have been historical differences in the pace and extent to which different industries have addressed energy efficiency. For example, whereas the pharmaceutical industry has generally followed a linear, if not aggressive path to improving efficiency, the semiconductor industry has tended to be more cyclical, reacting to fluctuating market conditions. For example, Grant points out that “over the last 1 1/2 to 2 years, it's certainly been getting more attention in the semiconductor industry as reductions in new facility investment have lead to a greater focus on the operating costs of existing plants.”
Clearly, HVAC requirements and considerations are dramatically different between the ISO Class 3 (Class 1) environments of microelectronics fabs and the ISO Class 7 (Class 10,000) rooms (with blocks of ISO Class 5 (Class 100) space) of pharmaceutical and biotech plants. Notes Hurley, “The amount of air you have to move and the approaches to getting there require that the systems be looked at entirely differently.” Along with the 500 to 600 air changes/hour and 90 foot/min laminar airflows of ISO Class 3 (Class 1) microelectronics plants versus the 20 air changes/hour of a biotech facility, other important differences play a role in HVAC design, such as the need for compartmentalized pressure controls.
Hurley notes that while, on a per-square-foot basis, HVAC is generally more expensive to implement in a microelectronics fab, installations can also get expensive in biotech facilities because of the complexity involved in servicing a number of small rooms with individual pressure controls, the cascading of air from cleaner to less-clean areas and the requirement for negative pressure zones.
Capacity and equipment
Any attempt at improving cleanroom HVAC efficiency must incorporate the air-delivery system, which can range from ducted HEPAs to fan-powered HEPAs, to large, pressurized plenum-type recirculation units.
Regardless of the system type, however, Michael O'Halloran, director of technology at IDC, says energy use and efficiency can be analyzed by looking at three specific performance factors: pressure drop, the efficiency of the fans and the total volume of air being moved.
IDC has conducted studies that show that over the last 15 years improvements in all of these areas have combined to reduce total energy usage by as much as 1/16 to 1/8. Specifically, immensely improved efficiency of HEPA filters, together with more open floor designs, have reduced the pressure drop factor by as much as 50 percent, while at the same time losses due to fan inefficiencies have been reduced by about 25 percent.
Particularly in the semiconductor industry, O'Halloran observes there have also been significant reductions in total airflow volumes where users are generally dropping from 100fpm velocities to 80 or 70fpm. “Using airflow modeling, we can demonstrate the impact of different airflow velocities in the cleanroom” says O'Halloran. “And although our models can't connect directly to yield, users have been finding that these kinds of changes don't impact yield.” Another factor leading to reduced airflow volumes is the trend toward minienvironments where airflows can drop to 20 to 25fpm ranges, which alone deliver a 75 percent reduction in airflow volume.
Overall, O'Halloran says that airflow modeling is proving to be an extremely economical method for doing up-front HVAC requirements analysis and design “It's much more efficient to do a complete environment requirements analysis as opposed to working with one system at a time.”
Not all trends are in the positive direction, however. Another important factor now beginning to play a role in the efficiency of semiconductor industry HVAC systems is the growing interest in controlling molecular-level contamination. Says Dennis Grant, “Here, the solutions may drive us back the other way, because most solutions to controlling molecular contamination involve fine-mesh filters which in turn lead to increased pressure drop and higher energy-consumption to drive the air.”
The domino effect
Air delivery is only one part of the HVAC equation, however. As pointed out by Darnell of Black and Veatch, in addition to supplying the air, there are follow-on cost factors all down the line relative to conditioning that air such as chillers, boilers, humidification etc.
Once a cleanroom's airflow requirements have been determined and met, the chiller plant is often the most critical factor in maintaining energy efficiency. Chillers, however, are also a major up-front cost factor, and as observed by Hurley of Alfatech, “Some clients just don't want to buy a big central chiller plant, even though that would lower their overall operating costs.”
Hurley says the size of the cleanroom will be a big factor in making this decision. “For example, where you're looking at a large, foundry-size cleanroom, the cost of chiller capacity can be favorably stacked against operating costs,” he says.
Grant of IDC notes that improvements are also continuously being made in the efficiency of central outside-air conditioning plants. One system developed by IDC and marketed by the York Company can significantly reduce operating costs. Known as Optimah, the system precisely controls leaving air temperature (± 0.5 degrees), humidity (3 percent), and filtration, down to a 45-degree dew point, while reducing annual energy costs of chiller plants by as much as 25 percent and eliminating reheat energy cost.
York claims Optimah can provide $42,000 annual chiller energy savings per 1000 tons of chilled water, and up to $31,600 annual reheat energy savings per 10,000 cfm. O'Halloran says Optimah is particularly beneficial in very humid climates or in applications where a depressed humidity spec is required in some portion of the cleanroom where it can provide a significant efficiency gain to the entire central plant.
Balancing the ins and outs
There is one other all-important factor to consider when addressing HVAC energy efficiency. Regardless of the cleanroom, the cost and efficiency of the total replacement clean-air delivery system, including filters, chillers, boilers and filtration, all correspond directly to the size and efficiency of its exhaust system(s). As observed by Grant of IDC, “Exhaust requires make-up air, and make-up air is expensive.” As such, finding ways to minimize exhaust should be a key objective in any cleanroom design.
In fact, according to Tim Potvin, division manager, Quality Air Control (Colchester, VT), one of the first things that needs to be looked at when designing a cleanroom is the equipment matrix, including the exhaust requirements of each piece of planned production equipment.
Unfortunately, Potvin also acknowledges that accurate data is often difficult to obtain, particularly for new equipment. “Since the manufacturers are sometimes in doubt as to how much exhaust will be required by their machines, they frequently raise the specification.” This, together with the fact that the list of equipment and requirements initially planned for a cleanroom typically differ significantly from the final configuration, often leads to a last-minute juggling of the exhaust and make-up air requirements. Still, Potvin emphasizes that it's important to determine what the actual minimum exhaust requirements are so that additional expense is not incurred needlessly.
In theory, equipment selection based partially on minimizing exhaust requirements could lead to substantial cost savings. In practice, this is usually impractical because, regardless of industry, process specialists typically select tools based purely on their process performance. When feasible, comparative exhaust requirements should certainly be considered when choosing process equipment.
Another way to minimize the impact of exhaust systems is to place support equipment outside the clean space whenever possible. This allows the exhaust to be placed outside as well, and as noted by Potvin, “Whenever you can move exhaust out of the controlled environment, it can then be serviced with less-conditioned, less-expensive, air.”
Exhaust systems themselves are also consumers of energy, and poor duct design with sharp fittings, or unnecessarily short reducers, can impact their operating efficiency, creating static areas that will require more horsepower and more energy to pull air through the system. As described by Potvin, “A bad design may have started out great, but what frequently happens is that in response to changing tools or tool-system requirements, the user then tries to get more capacity out of a fully utilized system. Sometimes people don't realize that they just can't continue to tap off existing ductwork and expect to continue to get good airflow out the other end.”
Other than good duct design, the only other place to improve on exhaust system efficiency is in the blower motors used. Here, Potvin points out that most states offer some form of high-efficiency payback program where they will help offset the cost of installing more efficient motors. “Other than that, the idea is to keep the horsepower down by maintaining static pressure at the minimum needed by the tools.”
Initial construction vs. retrofit
Good up-front planning, requirements analysis, and careful design will pay off down the road in HVAC system efficiency and lower operating costs. “You can get your operating costs way out of whack if you don't start your approach right. For example, you can really inflate your operating costs if you mis-design your cooling plant and end up re-heating air,” says Hurley.
According to Darnell of Black and Veatch, “You may spend a little more up front on things like higher-efficiency motors, additional variable frequency drives (VFDs) etc., but the paybacks will always be a lot better if you plan for and implement energy-efficiency features up front than when you go back and retrofit,” he says.
Hurley of Alfatech agrees, adding that there's also usually considerably less latitude in the types of improvements that can be made through retrofit. “In the case of an existing building, a prime consideration has to always be just making something fit.”
Still, the replacement or retrofit of outdated, inefficient HVAC systems can also sometimes offer economic advantage. Says Grant of IDC, “It varies by sophistication of user and the age of the plant, but we've seen numbers in the 25-30 percent range in operating cost savings through retrofitting state-of-the-art equipment into 10- to 15-year-old plants.”
Even so, the advantages of such follow-on improvements can't compare in terms of life-cycle costs to initial installations. Although the payback for a typical greenfield facility is in the two to three year range, the retrofit of an existing facility inevitably pushes the payback period further out and depends on each situation. For example, as pointed out by O'Halloran, “If you're already replacing an existing fan system for other reasons, you can very cost-effectively increase its efficiency at the same time. But if the original system can be salvaged, it's more difficult to justify the investment just to save energy.”
As observed by Potvin of Quality Air Control, “Although most cleanroom users generally have a pretty good idea of where they want their operation to be in the foreseeable future, they may not be nearly as intuitive with regard to the relative value of up-front investment in efficient HVAC systems versus long-term operating costs.”
Says Hurley of Alfatech, “Everyone wants you to consider energy efficiency, but it ultimately depends on the customer whether up-front or operating costs will take precedence.”
John Haystead, a former editor of CleanRooms, is a freelance writer in Verona, ME.