Case study: Energy-efficient clean rooms
- Learn about the energy-efficient design strategies that should be considered for a clean room HVAC system.
- Understand the energy use of each clean room design strategy and how each one compares to a traditional design approach.
- Evaluate the most common HVAC system design approaches to consider for maintaining a clean room environment.
Heating, ventilation and air conditioning systems account for a large percentage of the total energy use in a typical commercial building. Functionally intensive buildings, such as pharmaceutical and biotechnology manufacturing facilities, consume much more energy per square foot and often take exception to energy-efficient building codes that typically apply to the design of commercial buildings.
The average commercial office building built after 2000 has an average energy use intensity of 81.4 Btu/square foot (257 kilowatt hours/square meter) The average pharmaceutical plant has an EUI of 1,210 Btu/square foot (3,819 kilowatt hours/square meter). This is due to the fact that these types of facilities consume energy to maintain clean room environments, to power production equipment and to power large utility generation equipment.
An increasing number of projects have attention focused on energy reduction initiatives, such as green building certification and client-mandated energy benchmarks. These are structured around corporate goals and strategies for sustainability and reduced carbon footprint. It’s no surprise that energy reduction requirements similar to the design of commercial buildings are unavoidable and must be accomplished using atypical strategies that take engineers, designers and clients out of their realm of comfort.
Overdesigning and oversizing mechanical systems will directly result in higher capital and operating costs as well as facilities with higher carbon footprints. Increased emphasis on total building performance and energy code compliance, coupled with corporate sustainability goals, will be the mandate for the industry moving forward.
Energy codes are adopted at a local or state level. These codes establish minimum energy-efficiency requirements and other baseline requirements related to building construction. The International Energy Conservation Code is the most widely adopted energy code in the United States. The IECC establishes a baseline for energy efficiency by setting performance standards and requirements for the building envelope as well as the mechanical, electrical and plumbing systems (in both residential and commercial settings). ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings has been the benchmark for commercial building codes for the past few decades and is often adopted as code through the IECC.
The IECC contains three basic options or approaches for commercial buildings:
- Meet the requirements of ASHRAE 90.1.
- Meet IECC prescriptive provisions.
- Meet IECC total building performance provisions.
In the past, there has been debate over the applicability of energy codes developed for commercial buildings as they relate to more industrial facilities. As states have adopted green building and energy codes, industrial facilities are generally directly included and fall under the jurisdiction of the code.
There has been a rapid progression and development of energy codes and standards since 2010. Selective compliance and questions over code applicability will become a distant memory as energy codes are further refined.
According to Pacific Northwest National Laboratory, the three most recent editions of the IECC and ASHRAE 90.1 have the potential to generate almost a 30% reduction in energy use compared to codes a decade ago. Their analysis, published in 2016, forecasts that energy codes will save U.S. homes and businesses $126 billion between 2012 and 2040. These savings correspond to 841 million tons of avoided carbon dioxide emissions, which equate to the annual emissions of 245 coal power plants.
The new challenge for the design engineer is advocating for the application of cost-effective design practices and technologies that minimize energy consumption and carbon footprint without compromising the critical environmental parameters of industrial facilities.
The design of HVAC systems to serve pharmaceutical and biotechnology facilities is completely custom and is uniquely dependent upon the specific manufacturing process, raw materials and drug product as well as the project sponsor/client specific standards and requirements. Some clean rooms require very strict temperature and humidity control based on raw material or drug product requirements while other clean rooms require high levels of air containment or air filtration to protect the product from personnel or vice versa. Some clean rooms require all of the above as well as active pressurization controls.
Therefore, not all clean room facilities are the same. Each project’s HVAC system requirements are unique and must be custom engineered in detail to implement energy-saving strategies that align with the scheme of the facility.
The facility presented in this case study consists of approximately 3,600 square feet of clean room space consisting of ISO-7, ISO-8 and common circulation corridor. The study compares four different approaches, which are all common among engineers and designers for an HVAC system design of clean room spaces. The study assumes identically configured central plants serving the HVAC system for each model and provides the following central utilities for the purpose of this comparative analysis:
- An air-cooled chiller with distribution pump to serve a chilled water-cooling coil.
- Natural gas-fired steam boiler to serve steam heating and direct steam injection humidification requirements.
- Natural-gas fired hot-water boiler to serve heating hot water requirements of the air systems.
The four cases evaluated in this study each assume that the space will be maintained at 68°F plus or minus 2°F and relative humidity is controlled between 30% and 60%. Each air system of this study has its own purpose for supporting a clean room environment and is described in further detail.
Case 1: 100% outside air unit
A once-through air system that provides 100% outside air to the clean room environment typically is required when the designer wants a system to serve suites where a biosafety level must be contained or where segregation is necessary to prevent cross-contamination between products or processes. A once-through system design is usually required as a result of suite pressurization relationships, dust extraction or fume containment needs or process equipment requirements.
The design used in this case study consists of a central station air handling unit with cooling, heating, dehumidification and humidification capabilities that provides filtered, preconditioned outside air directly to the clean room environment. An exhaust system also has been provided in the analysis to remove air directly from the suite or from process equipment within the suite.
The air handler is typically a custom unit with multirow heating and cooling coils using steam and chilled water, respectively, for temperature and humidity control. The fans require higher motor horsepower ratings to accommodate the necessary supply airflow rates of the clean room suite(s). This is due to higher air-change rates inside the clean rooms. The larger fans need to overcome higher static pressures from high-efficiency particulate air and ultralow particulate air filtration.
These types of systems are extremely energy-intensive and are usually designed to include provisions for energy recovery to comply with international codes and ASHRAE standards.
Case 2: Primary 100% outside air unit with secondary recirculation units
A primary/secondary air system is a traditional approach for clean room HVAC system design and consists of a 100% outside air primary unit for preconditioning makeup air to multiple recirculation-type secondary units. This design allows for a smaller 100% outside air unit, which provides only enough air to maintain the ventilation and pressurization requirements of the secondary units.
Most facilities that have multiple clean room suites designed for multiproduct or campaign production use this approach for their HVAC system design. The secondary units contain smaller coils and fans that are designed to meet the specific temperature/humidity requirements and air-change rates of the individual clean room suites.
Case 3: Recirculation units with enthalpy-based economizers
Dedicated, recirculation-type units with modulating (0% to 100%) enthalpy-based outdoor air economizers are being used for the clean room environment due to the advancement of direct digital control technology. HVAC systems for clean room environments operate best under constant conditions. Once the designer introduces modulating technologies for energy optimization, such as the dampers required of an economizer, the clean room suite risks controllability of pressurization and temperature/humidity excursions.
However, due to the advancement of DDC technology for HVAC controls, systems are now programmed with complex algorithms capable of maintaining temperature, humidity and pressurization while compensating for modulating sequences related to fan speed, temperature reset strategy and outside air control. If the designer must meet more stringent energy conservation goals, such as U.S. Green Building Council LEED or more recent versions of ASHRAE 90.1, then economizers may be considered mandatory in the clean room’s HVAC system.
Case 4: Fan-filter modules with a makeup air system
Fan-powered HEPA filter modules are very popular in clean room environments and use energy-efficient electronically commutated motors to maintain the air-change requirements of the suites. The once-through air system for this configuration, which is more commonly referred to as a makeup air unit, would provide preconditioned outside air to the fan-powered HEPA modules. This design allows for a MAU that provides enough outdoor air to maintain the ventilation, pressurization and cooling air requirements of the clean room suite.
Fan-powered HEPA modules would either be directly ducted or pull air from a common distribution plenum to serve the suites. The MAU would provide enough air to overcome any fan motor heat gain and other sensible heat gains within the space. If spaces have a larger latent heat gain (typically from a wet process), then this HVAC system design is not ideal considering dehumidification is provided by the MAU and is not directly controlled for each suite.
If the designer must meet more stringent energy conservation goals, such as LEED or more recent versions of ASHRAE 90.1, then fan-powered HEPA modules with a MAU should be considered for the design of a clean room’s HVAC system.
It also should be noted that these types of systems are notorious for having balancing issues and it may be difficult to meet pressurization requirements if plenums are not properly sealed or balanced.
Study results, achieving energy efficiency
This study produced comparative energy modeling calculations of four different air-system design approaches and how they relate to a model clean room environment. Modeling was conducted using a third-party computer-based program to perform an 8,760-hour energy simulation to determine the energy consumption of each case.
The study has concluded that airflow rates remain relatively constant among different design strategies due to the mandated air change requirements of the facility; however, fan motor efficiency and air delivery method for conditioning the clean room environment is very different in each model and proves to be where the majority of energy usage occurs within each design strategy.
Fan energy and space conditioning typically account for about 47% of energy use in pharmaceutical manufacturing facilities. Therefore, even small reductions in the required air-change rates can garner large energy-saving opportunities over the annual energy usage profile of the facility.
As shown in Table 1, each system’s total supply flow rate is relatively equivalent; however, varying the air delivery and conditioning methods across each case presents opportunities for energy-conscious design. The model shows that case No. 4 provides the greatest amount of energy savings due to the extremely efficient ECM fan motors that maintain the required air-change rates within the spaces.
The MAU only requires two 5-horsepower fans to deliver the optimal amount of outside air to the clean room environment for space conditioning; the designer must be careful to properly size the MAU for conditioning the heat gains and heat losses in the spaces to ensure space temperature and relative humidity can be maintained. The designer must also account for pressurization air gains and losses into the clean room suites, as the MAU and associated exhaust fan will need to maintain the required pressurization of the facility, which hasn’t been factored into this analysis.
It should be noted that this type of system design doesn’t work well for large clean room areas with lots of separate rooms at different pressurization levels. In addition, this type of design could require more maintenance due to a large quantity of fan filter units.
The second alternative in pursuit of a more energy-conscious HVAC design is a traditional primary and secondary air system strategy, which is identified as case No. 2. The designer using this system must ensure the design meets the latest version of energy code, which may now require alternative design methods, such as HVAC systems with 0% to 100% economizer controls, to be considered.
If economizers are considered for a clean room facility, the designer would need to carefully identify a design method for managing the economizer system functionality and would need to provide a means to exhaust the necessary airflow from the clean room during that mode of operation. Therefore, the designer must be aware that this design strategy would likely result in more controls and equipment to achieve the economizer function.
Case No. 3 shows the annual energy use of three air handling systems with integrated economizers. The cost of heating and cooling energy use reduces significantly when considering the use of economizers of case No. 3 compared with the primary/secondary system of case No. 2; however, the designer must be aware that the facility’s pressurization strategy could be jeopardized with the introduction of air-side economizers. Water-side economizers may be a better alternative to comply with energy code and are typically preferred over air-side economizers when designing clean room HVAC systems.
Twelve months of energy data based on the simulated energy model were compiled and the estimated energy use of each HVAC system design are summarized in Tables 2 and 3. Table 2 shows component costs based on an average electric rate of $0.973 kilowatt hours and an average natural gas rate of $10.13/mille cubic feet.
Table 3 summarizes the component costs in an estimated dollar per square foot metric that can be extrapolated and used by designers looking to baseline the annual energy use of a clean room facility based on HVAC system type.
Best approach and recommendations
As with all building design, energy efficiency needs to be evaluated at the beginning of the design process. The approach should be a collaborative one that involves the design and construction teams. Thoughtful consideration should be applied to developing key building attributes. This needs to be discussed early in the design process due to the major impact they can have on the overall building energy consumption and more specifically, the HVAC systems.
A methodical approach to the overall building design can have a significant impact on reducing the HVAC loads and the overall energy consumption. Slight changes in key attributes, such as glazing types and amounts, can be easily be modeled in various design platforms showing real-time impact of the design decisions.
Another key aspect for the design process is defining meaningful and measurable energy performance benchmarks and setting project-specific goals. With clearly defined goals, the design team can develop energy-saving strategies for the various building systems.
When approaching HVAC design for a clean room application, the first step is documenting key performance parameters of the critical environment, such as cleanliness levels, temperature, humidity, pressurization and air changes per hour. The definition of these requirements is a vital initial step in the HVAC design process as it has the largest impact on system sizing and configuration and the overall complexity of the facility.
Often, the owners of the facility have established ACH that are associated with their unit operations and the required cleanliness level. Advocating for a risk-based approach and challenging established practices will be the required paradigm shift for the design team moving forward. Without designers advocating for lower air-change rates, many owners will remain averse to changing their guidelines despite energy and cost savings due to perceived risk.
In addition, the design teams must challenge common misconceptions associated in the industry with respect to the zoning of HVAC systems and the effectiveness of proper filtration. Unless there is potential for free-floating viruses, pushing for increased unit segregation with HVAC systems should only be considered in extreme situations. Maintaining the status quo, using high ACH rates and increasing the complexity of HVAC systems to mitigate perceived risk directly correlates to increases in initial facility cost, ongoing operational costs and a carbon-intensive facility.
Another key aspect for consideration when designing HVAC systems for clean rooms, although not the intent of this article, is understanding the impact of hazardous raw materials, such as flammable, combustible liquids and potent compounds. As a designer, it is vital to understand the quantities and types of materials used in the process, as they have design and code implications related to personnel safety, building safety and environmental impact.
Building codes, fire codes and adopted standards will dictate the design of facilities using such materials in their processes. In general, more stringent ventilation requirements will be the result of using these materials and will take precedence over some of the energy code requirements due to the impact on life safety.
A risk-based approach combined with analytical and economical models can assist the design team in determining the appropriate HVAC system design that not only meets the critical design requirements of a facility, but also achieves an energy-efficient and code-compliant design.