Design & Construction
This article is the third in a series of Design & Construction columns.
by Richard V. Pavlotsky Ph.D., P.E.
Data from 20 cleanroom projects around the U.S. is analyzed in an effort to produce an initial cost evaluation for a variety of cleanroom projects
In the first two parts of this column we've emphasized the need for all parties involved in a cleanroom design/build project to comprehend the affect that each design feature will have on the overall cost.
In the last column (see CleanRooms, March, 2002, page 16) we considered 15 key factors, including air changes, unidirectional or turbulent airflow and minienvironments, air filtration, air handlers, air pressure, temperature control, humidity control, exhaust systems, vibration, magnetic and electromagnetic flux, electrostatic charge, energy consumption, form and function, particle and process piping and utilities.
Now, let's analyze the high-tech construction cost data in relation to the key factors and variables that influence cleanroom construction cost.
The data analyzed here comprises the construction cost of 20 cleanroom facilities in the United States. The projects considered consisted of the following: eight microelectronics manufacturing facilities with ISO Class 5 and 6 cleanrooms built between 1983 and 1996; four pharmaceutical and biopharmaceutical facilities with ISO Class 5 and 7 cleanrooms built between 1983 and 1996; six microelectronics manufacturing facilities with ISO Class 3, 5, 7 and 8 cleanrooms built between 1994 and 2000 and two biopharmaceutical facilities with ISO Class 4, 5, 7 and 8 cleanrooms built between 1994 and 2000.
Mechanical (HVAC, process piping) construction cost data was extrapolated and adjusted for site location, labor cost and inflation factors to obtain an overall “snapshot” of the mechanical construction cost breakdown per generic ISO Class 3, 5, 7 and 8 cleanroom.
All analyzed facilities were approximately the same size, between 40,000 and 60,000 square feet with 30 to 40 percent of the facility space occupied by the classified cleanroom, cleanroom corridors and airlocks. Similar arithmetic was applied to “mechanical equipment” operating costs to obtain the cost breakdown shown in Table 1.
This approach allows the unifying of the mechanical construction cost without differentiation between microelectronics, pharmaceutical and biopharmaceutical cleanrooms. The resulting data is shown in Tables 2, 3, 4, 5 and 6.
Based on these tables, the “key value factor” chart was constructed with an ISO Class 5 cleanroom mechanical construction cost as a baseline, and cost factor variables shown in Table 7 were obtained. These empirical variables may be applied as percent multipliers to the square-foot construction cost for different types of cleanrooms and may be helpful for the initial cost evaluation.
By maintaining the live database of design alternatives, staying close to facilities' needs and engineering valuable improvements to the base cleanroom design and criteria, we're able to reduce construction costs.
1. Air changes (cost factor variables -0.08 to 0.19)
The air distribution devices in cleanrooms (usually ceiling filters) are designed to provide a uniform “shower” of pure, filtered air. The quantity of air supplied to the room brings to mind the analogy of a fire hydrant as opposed to an ordinary showerhead.
ISO 14644-1, “Cleanrooms and associated controlled environments-Part 1: Classes of air cleanliness,” provides guidelines for cleanroom parameters, classification and testing, but does not tell you how to get there economically. This is because each process-and, therefore, its cleanroom requirements-is different and desired conditions may be reached using different quantities of recirculating air.
There are always at least five different “possible” designs to achieve the desired results. However, there is only one design that “fits” best. Finding the optimal and most economical solution for the project is a fundamental goal of the project concept for cleanroom air changes.
2. Unidirectional, turbulent, vertical or horizontal airflow (cost factor variables -0.073 to 0.18)
Generally, unidirectional flow is necessary only over small, sensitive areas of the cleanroom and may be handled with minienvironments. Sources of contamination may be localized with gloveboxes and filtration modules. The majority of cleanrooms designed for total unidirectional flow can achieve it only at rest without workers, equipment and room exhausts. The choice of vertical or horizontal airflow depends on room configuration and equipment layout. In many facilities, turbulent airflow with properly engineered exhaust and return-air locations works fine for contamination removal.
Raised floors were developed to help in the distribution of electrical wires, communications cables, utilities and piping between tools, equipment and utility sources. For economy purposes, they were used for returning air from the bays to the chases in a bay-and-chase cleanroom configuration. In a pure unidirectional cleanroom, the particles are expected to flow from the working space toward the holes in the floor.
Another option in product contamination control is to isolate an ultra-clean area with a thin air jet moving at 500 to 800 fpm; it functions the same way as the plastic curtain.
3. Air filtration (cost factor variables -0.032 to 0.10)
Incoming air is filtered with terminal HEPA filters providing efficiency of 99.99 percent at 0.3 – 0.5 micron, HEPA filters providing efficiency of 99.999 percent at 0.12 micron or ULPA filters providing efficiency of 99.99995 percent at 0.12 micron.
Other devices may also be required depending on cleanroom purpose, such as HEPA filtration in recirculating and make-up air handlers, VOC absorption with charcoal-type or similar synthetic filters and electrostatic filtration chambers.
Fan filters aren't new to the cleanroom market. Properly applied they should provide an excellent and economical solution for many high-level cleanrooms, especially in buildings with ceiling-height limitations. Applying higher-grade terminal filters should be economically justified and weighted against first cost and lower pressure drop of more expensive filter media. It is not always obvious that lower-grade filters with higher pressure drops are more economical in the long run.
4. Air handlers (cost factor variables -0.07 to 0.15)
Make-up AHUs: Typically, the make-up air-handling units (primary AHUs) provide the necessary make-up air for the recirculating air-handling units (secondary AHUs). The make-up air AHUs consist of draw-through centrifugal, vane axial or plug fans with filters, hot-water coils for preheat and reheat, chilled water coils for cooling and dehumidification and steam or adiabatic humidifiers. Also, there are many available add-ons and variations, including static-air mixers, steam-preheat coils, ultrasonic humidifiers, brine, DX or glycol sub-cooling coils for dehumidification, VOC absorption filters, sound attenuators and VFD drives.
Typically, the make-up AHUs discharge into a common header with ductwork laterals balanced to supply the required make-up air to the recirculation AHUs.
Air-measuring stations should be installed in the primary and secondary air-supply main ducts to modulate the supply fans VFDs or inlet vane dampers to maintain constant air flow. Magnahelic gauges monitor the loading of the HEPA filters, bag filters and prefilters located at each air-handling unit. Where ceiling fan filters are provided, the make-up air should be evenly distributed in the space above the fan filters. Air from the make-up AHUs enters through recirculation AHUs.
Recirculation AHUs: Each recirculation AHU typically consists of an energy-efficient centrifugal fan with filters and sensible (dry) cooling coil. Add-ons and variations are available, including reheat coils for zone temperature control, steam or ultrasonic humidifier for zone humidity control and constant volume-control boxes.
Multiple recirculation AHUs discharge into supply ducts feeding the cleanroom through ULPA or HEPA filters that typically cover 100 percent of the ceiling in the ISO Class 3 and 4 areas. This vertical unidirectional flow passes down through the room, through the perforated raised floor tile into the return air space under the floor, and then up through vertical return air shafts, which are open to the ceiling return air plenum. The air then reenters the recirculating AHUs and the cycle repeats. In the case of ceiling fan filters, the sensible cooling may be provided by water or DX-cooled fan-coil units in the ceiling space above the fan filters.
5. Air pressure differential (cost factor variables -0.041 to 0.08)
Cleanroom pressurization is necessary to protect the cleanroom against contamination from adjacent areas, to control the flow of unwanted contaminants, to prevent cross-contamination between areas and to help maintain temperature and humidity at required levels.
Typical air-pressure differential between cleanroom and reference corridor and other areas of the facility is maintained at 0.25 – 0.005 in. water gage (wg). The higher number usually applies more to pharmaceutical facilities that have cascading air pressures between areas to avoid cross-contamination. Such areas normally require a series of cascading airlocks between them with pharmaceutical doors that allow air to escape at a high velocity creating a pressure differential.
A well-designed microelectronics cleanroom normally operates at 0.02 – 0.005 in. wg, with a semi-hermetic air lock at the cleanroom entrance. The clothes change and gowning rooms often serve as airlocks. Mechanical air showers are usually at the entrance of a cleanroom because of facility protocol rather than necessity.
Many microelectronics and photolithography cleanrooms successfully operate with passive air-pressure control and maintain only minimum air velocity of 50-100 fpm over entrance with the door fully open. Some biopharmaceutical facilities require an active differential pressure control and automatically supplement air escaping through the door with make-up air.
Differential pressure monitors may be mounted outside the cleanroom with a small LED inside the cleanroom with an audible alarm and two indicators to show when pressure is normal or abnormal. The device detects negative pressure in biocontainment areas common in biopharmaceuticals and the positive pressure common to electronic facilities.
6. Temperature control (cost factor variables -0.032 to 0.15)
Temperature and humidity variations cause process equipment misalignment, impact the repeatability of the developed process and eventually reduce the useful output of the product and increase the quantity of waste. Understandably, the goal is to achieve the most stringent cleanroom temperature requirements. Cost, however, often dictates otherwise.
Often, in an attempt to lower the construction cost, engineers are asked to design a precise temperature-control area within a large space where the temperature is allowed to swing +/-4-6 degrees Fahrenheit. A warehouse area with roll-up doors is a good example.
Without hard walls and airlocks it may be an expensive alternative with virtually uncontrollable variations of humidity. Thus, the quality of this cleanroom is substandard and cost remains relatively high. Commonsense tells us that cascading levels of cleanliness, temperature, humidity and pressure are easier to achieve and maintain. The allowable tolerance should be evaluated carefully.
Mechanical equipment and control systems for cleanrooms with stringent temperature control requirements (+/-0.1 degrees Fahrenheit) may cost 20 to 50 percent more than a cleanroom with typical requirements (68 to 72 degrees Fahrenheit) and setpoint (70 degrees Fahrenheit +/-2.0 degrees Fahrenheit).
Typically, a zone thermostat controls the design temperature in each cleanroom zone. It actuates the duct-mounted zone reheat or recool coil to satisfy the room sensible-load conditions.
7. Humidity control (cost factor variables -0.034 to 0.19)
The design relative humidity for each cleanroom is controlled by a zone humidistat. If there is high relative humidity in the room, the humidistat lowers the cooling coil discharge air temperature to provide more dehumidification.
Concurrently, the reheat coil provides heat to maintain the room temperature. If the relative humidity of any cleanroom falls below the design limit, the zone humidistat actuates the duct-mounted zone humidifier. When precise humidity control is required it typically can be achieved with adiabatic humidification of make-up air in the air handler and by maintaining the cleanroom dew point.
Local variation in humidity levels may be handled with ultrasonic humidifiers located in the ductwork plenum before the filtration terminals. The ultrasonic humidifiers for cleanrooms work well with RO/DI water quality and water resistivity near 3 – 5 mgm. The design relative humidity for each cleanroom in that case is usually controlled by a zone humidistat. If the relative humidity falls below the design limit, the humidistat actuates the humidifier to increase the moisture content of the supply air. If there is a high relative humidity in the room, the humidistat lowers the make-up air handler's cooling-coil discharge air dew-point temperature to provide more dehumidification.
The next installment of this column will consider exhaust systems, vibration, magnetic flux, electrostatic charge and energy.
Richard V. Pavlotsky, Ph.D., P.E. is a director of advanced technology for San Jose, CA-based ENCOMPASS Facility Services. He can be reached at [email protected].