It is probably not unrealistic for an owner about to invest in premium priced manufacturing or research space to expect its design/construction team to provide a cleanroom facility that meets quality process needs and worker comfort requirements as well.
Over many years of consulting with cleanroom owner/operators a number of recurring problem areas that negatively affected the performance of clean facilities, HVAC as well architecture related, were identified. Such problems, if appropriately addressed during the design phase can be avoided; if identified after construction, possibly as a result of an unfavorable certification test report, or perhaps product quality concerns, can frequently be corrected, at a cost.
What follows, in no particular order, are a number of issues, frequently not obvious at the early stages of conceptual design, that can mess with an owner’s expectations.
Usually an afterthought, maintainability ought to be addressed initially, and throughout the design process. Frankly maintainability is an issue rarely integrated into the mainstream cleanroom design process because a new, freshly certified cleanroom rarely has maintenance issues to address. However over time the performance of the facility will degrade unless facility equipment maintenance requirements can be met. Pre-filters located in difficult/impossible to reach places will not be checked/changed; air handling units (AHUs) crammed into undersized equipment spaces will not be serviced and repair may be negatively affected; inaccessible evaporator drain pans will not be cleaned and will be subject to overflowing when clogged; equipment access panels that cannot be readily opened may not be opened at all; pump strainers located where a serviceman cannot swing a wrench may never be cleaned; unlighted equipment spaces will compromise careful system troubleshooting; and so it goes.
A word about the janitorial needs of the cleanroom is in order. There is an industry dedicated to providing clean garments as well as an array of cleaning materials and equipment. The brand new cleanroom will only stay clean for one shift after initial start-up. After that it must be washed (maybe) and wiped and vacuumed to restore the facility to its pristine condition. Provision should be made to store cleanroom cleaning materials within the cleanroom complex in dedicated, clean/cleanable storage areas in order to provide access to materials when needed, and to make them disappear when they are not. Thinking about installing storage cabinetry after the cleanroom is built may not yield the desired result in making materials conveniently available for use and overall janitorial effectiveness may deteriorate. Cleaning the cleanroom is probably the least fun part of ownership.
HIGH PRESSURE DIFFERENTIALS
Generally the object of creating a pressure difference between adjacent spaces in a cleanroom installation is to provide a higher pressure within the more critical spaces, relative to the less critical, to keep particles out.
How high is high? Generally, in the U.S., room pressure is expressed in inches of water column (for those embracing the metric system, the Pascal is the unit of measure) and the difference in pressure between the clean space and an adjacent space is prescribed. A sufficiently high cleanroom pressure will insure that airflow is outward and particles outside the critical space will not move through cracks and crevices, or open doors, into the more stringent clean space.
Too high a pressure differential will result in high air velocity through those cracks and crevices resulting in annoyingly high sound levels. Additionally, a high pressure tends to either make doors difficult to open or difficult to close Note that a differential pressure of 1.0 inch of water column (in.w.c.) can create a force on the order of 108 pounds on a 3’x7′ door as well as air velocity in the neighborhood of 4000 feet per minute through openings. Such pressures would make traveling through doorways a struggle and would create noise levels within the facility that would make verbal communication difficult, at best.
A cleanroom design with differential pressures between clean space and a surrounding “dirty” ambient space of approximately .04 in.w.c. has proven quite acceptable in isolating the clean space from external particle penetration. A pressure differential of .02 in.w.c. is frequently used between adjacent clean spaces with the space having the more stringent cleanliness requirement being at the higher pressure.
The keys to maintaining desired pressure differentials is tight construction throughout, with particular attention to sealing windows, doors, and ceiling as well as using personnel and material airlocks and/or pass thru boxes, to control pressure loss as people and materials move in and out of the facility.
Note that there are applications where hazardous vapors or biologicals dictate a negative pressure cleanroom. A common approach is to provide a “roomwithin- a-room” design wherein the low pressure cleanspace is enclosed in an envelope positively pressurized with clean air. Any leakage into the cleanroom will be of essentially particle free air, thereby maintaining the cleanliness classification. The clean envelope will typically maintain a positive pressure of .04 in.w.c. relative to the dirty ambient surrounding it, and a positive pressure of .02 -.04 in.w.c. relative to the cleanroom.
CONTAMINATION DUE TO PROCESS EQUIPMENT
A tightly constructed cleanroom with good control of pressure differentials may still not be particle free. A good target cleanliness class for initial, “at rest” certification of the completed facility is a rating ten times cleaner than the end user requested. If in fact that is achieved yet, upon start-up of process equipment and movements of personnel, particle counts soar beyond the design rating, it should be fairly obvious that the people and/or functioning equipment within the cleanroom are generating the particles.
In an ideal world all process equipment used in the cleanroom will have been designed and built for cleanroom use and will be made of appropriate non-shedding materials, will utilize non-shedding lubricants, will not have cooling fans stirring up air eddies adjacent to the machine, will utilize sealed control panels, and may be equipped with minienvironments at the loading and unloading ends of the equipment that will keep product and cleanroom particle free. In the real world, particularly the real world containing end users as well as equipment suppliers that upgrade existing equipment designs for use in a cleanroom environment, there may be any number of sources of particulate contamination present.
Prior to moving equipment into the cleanroom and as part of the installation process a variety of strategies can be applied. These may include thorough cleaning; painting with a non-shedding/non-outgasing finish; shrouding moving components to capture particles being generated; replacing standard drive belts with those of non-shedding alternative materials; changing lubricants to cleanroom compatible types; providing local HEPA filtered air supply over critical sections of the equipment, and then ducting that air directly out of the cleanroom; and/or thru-wall bulkheading of process equipment, are approaches that have been successfully applied.
In many, if not most, cleanroom applications people may be the biggest threat to a successful, profitable, operation. Training and discipline are vital elements in the success of working clean. People working in a clean environment should know the effect they have on the productivity of the facility and how not wearing the cleanroom garments provided for them properly can have a negative impact on product quality. Therefore it is mandatory that the appropriate garments, including footcovers, head covers, face masks, etc be donned properly, be worn at all times in accordance with standard operating procedures (there should be a formal standard operating procedure), be removed and stored properly, and be maintained in proper condition.
Detailed initial worker training followed by periodic updates and renewal of training should be part of the cleanroom SOP. By far the biggest challenge to making training work on a consistent day in and day out basis is maintaining discipline in a consistent manner to insure that the training message is fulfilled. Shrugging off violations of the SOP; management failure to respect the SOP that has been developed; failing to acknowledge positively those workers who follow the letter of the regulations; inconsistency; and demonstrating by thought and word that the rules are for “show” and are not really important, are some ways in which the formal training program can be undermined and a poorly performing workforce will result.
Summer humidity control, particularly in locales with warm, humid summers, can be a challenge. Increasingly dedicated outdoor air systems (DOAS) are being applied to condition all of the make-up air for the clean facility and then distribute this air to local air handling units, as required, where it mixes with recirculated air from the cleanroom and final temperature control is provided.
Cleanroom pressurization plays a role in humidity control. At a positive pressure of .04 in.w.c. the air velocity through any leak passages is approximately 800 fpm (9 mph) or enough to minimize upstream migration of water vapor into the cleanroom, thereby minimizing the influence of the humidity surrounding the cleanroom. This will enable the cleanroom humidity level to be controlled based on the internal load (people, process baths etc) and make-up air load only, a much more predictable, and therefore controllable, dehumidification load. The beneficial pressure differential effect becomes less pronounced at lower pressure differentials, again suggesting the .04 in.w.c. between clean space and ambient space might be optimum.
UPGRADE THAT SPACE….CAREFULLY
Upgrading a general manufacturing space to a cleanroom environment can be a popular option. A number of cleanroom compatible material alternatives exist for wall, floor and ceiling construction. It may also be tempting to use the existing air-conditioning system to maintain temperature and humidity control within the upgraded space. For less stringent cleanrooms there may even be the notion to install HEPA filtration in the system AHU thereby gaining an even greater first cost advantage.
A few words of caution. The pressure drop of a HEPA filter is significantly higher than that of the Merv 4 or 6 filters commonly found on commercial AHUs. To move the same air volume through a HEPA filter may require increasing the fan speed with accompanying increased noise; may require a larger horsepower motor; may require that larger horsepower motor to have a larger frame size requiring the AHU to be torn apart and rebuilt to accommodate the larger motor; in short, may be totally impractical. This definitely should be reviewed at an early stage of the conceptual design process.
Standard commercial air-conditioning generally does not have dehumidification control built in. The cooling system will remove moisture from the air, but usually as an adjunct to the comfort cooling process, not as a separately controlled function. It is unrealistic to expect such a system to control to 45 +/5% RH, a fairly common process-related cleanroom humidity specification. If comfort is the goal and there are no considerations requiring a lower humidity then the 60%+ relative humidity achievable with standard equipment may be appropriate.
The existing standard commercial air-conditioning system may very well have been sized based on a calculated cooling load where a 72-75ºF indoor design temperature is considered adequate. If the people working in the cleanroom are going to be wearing cleanroom garments they will very likely be uncomfortably warm at such a temperature. It is not unusual to have a cleanroom temperature as low as 66-68ºF where full bunny suits are worn. This could add 20-25% to the cooling load that the “old” system is required to handle. The whole area of reusing existing HVAC equipment in a cleanroom upgrade should be reviewed before attempting to use the existing system.
Air patterns within the cleanroom can have a significant affect on particle counts. In a turbulent flow cleanroom, that is, a facility with less than 90-100% HEPA filter coverage, filtered air directed onto a work surface can yield particle counts close to zero. However, the particle counter probe, moved into an area out of the main clean air stream may show particle counts tens, even hundreds of times that value. Eddy currents at the juncture of wall and ceiling may trap particles and see them recirculate within the room, being removed only very slowly, if at all. Such pockets of particles can be disturbed by personnel or other movement within the cleanroom resulting in particles being deposited on critical surfaces.
All air entering a rated cleanroom should be HEPA filtered. The point of entry should be evaluated relative to the equipment, process, or personnel that the air will encounter and the point at which the air leaves the room should located to insure that the air can readily exit the room without depositing any entrained particles in critical areas. High entry, low return is still the preferred air flow scheme in most cleanrooms. Increasingly the use of computer software is being used to permit this flow pattern to be visualized. As a practical matter unidirectional cleanrooms with 100% HEPA filter coverage and raised floor plenum returns tend not to be affected much by airflow pattern issues unless process equipment within the room creates eddy currents. However, non-unidirectional cleanrooms may very well not meet expectations if airflow patterns are ignored.
Air change rate has traditionally been the criterion for establishing cleanroom cleanliness classification. However owner/operator expectations of high quality productivity in a comfortable worker environment may not be met without a detailed review of how the cleanroom will be equipped, staffed, and operated. Specific examples of the issues detailed above have, over the years, demonstrated this.
Ray Schneider is a consultant and on the faculty of Clemson University and can be reached at www.hvacforensics.com