Q: What are some key points in HVAC engineering for cleanrooms?
A: "Good things, when short, are twice as good." ~Baltasar Gracián, The Art of Worldly Wisdom, translated from Spanish
This month's topic poses a dilemma. The HVAC system in any controlled environment is the kingpin of clean. Given its criticality, and the myriad considerations in engineering an HVAC system for a cleanroom, this column could end up rivaling War and Peace. To save your sanity (and mine), I decided old Baltasar must be right, so following is the CliffsNotes version, touching on some of my favorite topics. Just remember to consult your favorite engineer when undertaking your next project for "the rest of the story."
When it comes to cleanroom HVAC, it's all about contaminants and environment control. The nasty "C-word" lurks everywhere. Contaminants —invisible to the eye—can pack a punch and wreak havoc on product yield and integrity if not controlled. This sets up a potential double whammy: hitting the company's yield and bottom line, or a research team’s multi-year effort.
The optimal HVAC design solution is determined by desired temperature and humidity control, air flow and pressure, and filtration requirements and air change rates, among other considerations. These design factors are dictated by the requirements unique to your process, facility, and regulatory requirements.
Whether creating a controlled environment for an electronics manufacturer or a life sciences environment free of pathogens, the HVAC system controls your success and will significantly impact your operating costs.
Contamination lurks everywhere
Whether your goal is to create an environment at ISO 9 classification—not tremendously different from outside air—or drive the cleanliness to ISO 1, the most stringent of controlled environments, a few fundamentals apply:
• Contaminants are not your friend—start by not allowing them in from the outside.
• Those that infiltrate your environment must be eliminated quickly—don't let them accumulate or hang out. Your cleanroom is not the neighborhood bar.
• Besides worrying about particulate interlopers from the outside environment, make sure you have your own house in order. This means minimizing contaminants that your manufacturing or research processes—including the equipment integral to your operations—throw off, whether through biological, chemical, or operating processes. And make sure your employees consistently follow protocols developed to minimize contamination.
A tool you can use: Computational Fluid Dynamics (CFD) analysis
CFD is a software modeling tool that can provide an accurate view of both existing airflow conditions while also modeling projected airflows of a variety of HVAC solutions or system adjustments. CFD is a precise and valuable tool both to design new controlled environments and also to diagnose and solve problems in existing cleanrooms.
CFD can trace its roots to the 1930s, when two-dimensional models were developed to solve linearized potential equations. The development and continued advancement of computing not only enabled the analysis and modeling of any gas or liquid fluid flow in 3D, but also enabled analysis of complex equations. Los Alamos National Labs lays claim to being the first to use computers to model fluid flow using the Navier-Stokes equations.
A valuable tool for today's controlled environments engineer evolved from that rich history. Not only can CFD modeling assist in selecting the most efficient and effective air handling system when designing a new controlled environment, it is extremely useful in dealing with underperforming or problematic controlled environments. CFD can analyze airflow problems, humidity issues related to airflow, temperature gradient issues, the impact of tool sets and other equipment on airflow, and identify air pressure differentials throughout a space or track contaminant flow. This analysis is priceless when working to identify root causes of underperforming cleanrooms.
Figure 1: An example of CFD modeling of a Class 1,000 cleanroom that was experiencing inconsistent airflow and stagnation issues.
Figure 1 shows CFD modeling of a Class 1,000 cleanroom that was experiencing inconsistent airflow and stagnation issues, while the "after" shot in Figure 2 validates the effectiveness of the solution.
Figure 2: After the problem was identified, CFD modeling validated the effectiveness of the solution.
Energy is king in the HVAC kingdom
Cleanrooms rank among the top energy consuming facilities in the world, driven in large part by their HVAC requirements to meet stringent airflow and pressurization requirements within strict temperature and humidity controls. And as existing technologies increase in sophistication and new technologies create additional demand, controlled environments have proliferated around the globe in more than 30 different industry sectors and are a mainstream feature of academic, medical, industrial, and defense research facilities. It’s been estimated that cleanrooms demand between 10 and 100 times more energy than standard office spaces—mainly driven by air cleanliness standards—and the HVAC system can account for more than half of the facility's energy costs. This impacts operating costs, on top of an already costly capital facility.
Following are a variety of strategies to help reduce energy costs related to your HVAC system:
1. To begin, minimize demand. Take a look at your building. Can you increase the efficiency of the shell? When building new, carefully orient and develop the building form. Is there an opportunity to reduce the volume of your cleanroom? Less volume equates to less air re-circulation with resulting HVAC savings.
2. Make sure you accurately scope the level of cleanliness and the square footage required. Going overboard in either category will drive up your costs. Considering reducing positive pressurization where prudent.
3. Flexibility is key. Design your HVAC system with an eye towards flexibility, not only for sustainability, but for future product line and expansion capabilities as well. Don’t forget to plan your HVAC equipment to accommodate part load scenarios.
4. Subdivide your facility's space classifications. Carefully examine the proposed process and product requirements when determining your required cleanroom classification. Don’t shoot an ant with an Uzi. Do you really need the entire space to be stringently controlled?
5. Mini- and micro-environments are your friends; stick or prefab? Consider the use of micro- or mini-environments (see the May 2013 issue of Controlled Environments) and a mix of stick built and prefabricated areas—determined by process specifications and flexibility needs. Utilize these tools to meet your process requirements instead of upgrading your entire cleanroom.
6. Invest in high efficiency equipment. Your upfront costs are an investment with surprisingly short payback periods. And don't forget to use high efficiency filters.
7. Consider energy recovery and waste recovery strategies. Energy recovery strategies such as an exhaust energy recovery system, co-generation, and equipment or other heat recovery systems can cut demand and costs.
8. Use alternate energy appropriately. You can reduce the load on your HVAC system by carefully analyzing and appropriately using alternative energy sources throughout your facility. Consider solar heating and power, daylight, wind energy, and thermal where technically sound and fiscally responsible. "Green for a reason" is the mandate at SMRT, ensuring that alternate energy sources are operationally sound, financially responsible, and appropriate to the application. Don’t let anyone sell you on being green for green's sake.
9. Analyze the viability of reducing air change rates (ACR). The sizes of your motors and fans are driven in large part by the air change rate in your cleanrooms. Larger motors and fans drive increased HVAC investment and operating costs. You can reduce power usage by approximately two-thirds if you reduce your ACR by approximately one-third.
10. Adjust your airflow to match your production load. Scheduling software and timers can be used to decrease air recirculation and the HVAC load during periods of reduced production. Ditto the wonders of occupancy sensors that can make automatic adjustments depending on the occupancy levels of your biggest contaminant source—people.
11. Locate equipment outside the cleanroom where appropriate. This is a triple bonus strategy. When you locate process tools in an adjacent chaseway and provide critical clean access on the cleanroom side, you will reduce heat gain as well as the square footage required in your cleanroom, resulting in less demand on the HVAC system. You will also make future equipment maintenance easier and less costly.
12. Use variable frequency drives (VFDs). Variable frequency drives, which adjust HVAC equipment speed to match conditions, can cut your energy up to a third compared to constant speed drives.
13. Use particle counters to manage airflow in real time. Carefully located optical sensors provide 24/7 particle counts to the building management system, allowing the HVAC system to operate with efficiency matched to need.
14. Analyze your air distribution system to reduce pressure drop. Your HVAC fans have to work harder in a restrictive air distribution system, raising energy consumption. Keep the freeway open with straight ductwork where possible, eliminating obstructions and carefully sizing duct diameters. Consider the pressure drop properties of supporting equipment like coils, fans, and filters.
15. Don't be overly conservative or cautious. Don't overdesign your HVAC system, or build in too many safety nets. Those behaviors compromise operating efficiencies.
A final word
The world of HVAC design for controlled environments is an always-evolving field with new equipment and constantly emerging operational innovations. While this article provides an overview of some key considerations, the unique properties of your process, product, or research requirements—coupled with those of your physical plant—will determine best practices.