Introduction to Atmospherically Controlled Processing and Laboratory Environments

Demand for environmentally controlled environments in manufacturing, food production, research, laboratory work, and medical applications is on the rise. While the reasons vary, production and results reliability are the bottom line goals. Controlled environments reduce product contamination, lower rejection rates, and prevent spoilage. They insure the quality and purity of products and processes.

In order to assist Morris Kurtzon Inc. sales representatives, lighting fixture specifiers and customers, Kurtzon Lighting has prepared this technical information bulletin describing the basics of clean space construction and lighting. A clean space is a compartment, area or room specifically designed to reduce the availability of airborne particulates to an acceptable level. While not meant to be an exhaustive manual on the subject, this bulletin is an introduction to cleanroom concepts, components and terminology. It will help those involved in choosing lighting fixtures to ask the right questions and make informed recommendations and decisions.

Use and Needs

Controlled environments are becoming part of many manufacturers' plans for research and production construction. These businesses know the importance of preventing product and laboratory contamination by using high tech controlled environments.

The pharmaceutical industry works with precise chemical compounds, worth hundreds of thousands of dollars per kilogram. Material contamination could gravely endanger patients and trace amounts of previously manufactured compounds or other matter can easily destroy an entire production run. Therefore, controlled clean space environments are normal in this industry.

Precision manufacturers know that the control of air cleanliness is essential to consistent production of such equipment as aeronautical instruments, hydraulic valves and optical components. Controlled environments in the electronics industry have increased the yield of semiconductors by several hundred percent. Even during the last decade, great increases have been made in the production of VLSI chips.

Clean Space Standards

Although each industry or manufacturer determines the acceptable particulate levels for their processes, construction industry architects and engineers, along with regulating agencies, requested that the General Services Administration (GSA) establish a definitive universally applied standard for cleanroom applications. FED-STD-209E (which supersedes FED-STD-209D) is the standard used to determine the level of environmental particulates for a work space defined as a cleanroom. The intent and scope of FED-STD-209E is to establish standard classes for specific concentrations of airborne particulates. (A copy of this standard is available on disk from the office of Kurtzon's Managing Engineer.)

The standard classes for cleanrooms are named for the maximum quantity of .5 micron or larger partides found within a cubic meter or cubic foot of air. A .5 micron particle is about 500 times smaller than the width of a human hair, and is invisible to the unaided eye. The class name is written as M3.5(log 10) or Class 100 where 3,530 particles per cubic meter, or 100 particles per cubic foot, are found. Each class name is defined as follows:

Clean Space Basic Configurations

There are three types of basic clean spaces: work stations, clean zones, and cleanrooms.

Work stations are self-contained stand-alone controlled environments where air cleanliness is maintained using a single fan forced High Efficiency Particulate Air (HEPA) filter unit. The filter unit is located in the rear or on top of an enclosure that allows limited access. Entrance into the workstation is through an open side wall or glove fitted panel similar to newborn incubators. These environments are small, usually having a work surface of less than 10 sqft.

A clean zone is a modular or temporary construction having free standing clear polymer soft walls. The walls support a sealed ceiling system that is fitted with fan forced HEPA filter units. Clean zones are usually installed within a higher class (less clean) clean space staging area holding work pieces to be processed within the clean zone. The area is positively pressurized by the HEPA units to keep the space cleaner than the surrounding environment.

Cleanrooms are permanent constructions using some of the same materials and techniques used for work stations and clean zones. They are usually installed within a superstructure common to modern construction but this is where the similarities end. Cleanrooms vary from basic stud and drywall construction, all the way to deluxe fabrications using high tech prefab mating-metal walls. Specialized gel-filled grid ceiling systems and exhaust air recapture mechanisms are some times required for high end users.

Basic Operation

The basic operation of the cleanroom is to isolate airborne particles such as lint, dust, pollen, dry skin and process compounds from the clean area, and remove such partides from the cleanroom environment. This is done with various methods using one common component, the filter. The filter is the last stage before the air enters the cleanroom. In order to sustain the cleanliness of the cleanroom every component involved in the construction of the space, as well as the equipment and supplies used thereafter, must be carefully chosen. Materials such as wood, paint plastic, shop towels, and paper are common sources of continuous particulate matter. To remove these, as well as work process particulates, specially designed filters are used. These filters are similar to automobile air filters with continuous rows of folded material except the elements are made of glass fibers and are pleated/folded to increase their strength and surface area. The element is combined with other gaskets and is sealed into an aluminum frame or housing. The housing may contain the blower fan or ducting system or just an open top. The HEPA type filter is designed to filter out or capture 99.99% of all particles .3 microns and larger The Ultra Low Penetration Air (ULPA) filter removes 99.9995% of particle matter .12 microns and larger.

The heart of every cleanroom is the ceiling/plenum system. By providing a source of clean air emanating from the ceiling and traveling to the floor, the air and product/specimen can be kept at their cleanest. There are several reasons for this airflow method.

1. The shortest path through the space is from ceiling to floor.
2. Gravity will aid in removing the contaminates from the space.
3. An uninterrupted and undisturbed flow of dean air is desired at the work surface.

The class of a cleanroom is the major issue that determines the amount of clean filtered air needed. This also will, with other factors, determine the ceiling construction. For the typical class 100,000 cleanroom, (a high class/low requirement level clean space environment), simple drywall ceiling construction can be used because the required volume of filtered air can be supplied with relatively few filter units. In this construction, standard Kurtzon Lighting Klean-Lock KL-F-2x2 and KL-F-2x4 fixtures can be used. As the classes get cleaner (lower number/higher requirements) the materials will move away from drywall construction to grid ceilings and vinyl covered walls.

The availability of clean filtered air throughout the room is what governs the quantity, size and configuration of light fixtures. As the demand for ceiling surface and plenum space increases with the lower class/higher requirement rooms, there is less space left for lighting fixtures.

The clean air supply can be provided from several different sources. Based on the type of work done within the cleanroom, the cleaning methods used, and the construction budget, a choice is made among one of the following filter systems:

1. Motorized Fan Modules: Typically, a module is placed within the ceiling. The unit is supplied with temperature and humidity conditioned air. This air is internally pulled through a removable pre-filter by a 1/4 to 1/3 HP fan motor and then forced through the HEPA filter just before entering the room. The fan module front panel is made of perforated metal and resembles a standard air diffuser. The fan speed (air velocity) is user adjustable from a few to more than 150 cubic feet per minute.

2. Ducted Modules: Very similar in form to the fan module above except that air is forced through the filters by a remote air handler The air handler pre-filters and conditions the air, and provides the positive pressure needed by the cleanroom. Air from the handler is fed through the plenum space using sheet metal ducts and 12" diameter tubes directly to terminal HEPA filter units. The plenum space is sealed from the room space and is only accessible through a removable access panel.

3. Pressurized Plenum: Air handlers pre-filter the air supply that feeds directly into the plenum space. The air escapes into the cleanroom only through the filters, providing a unidirectional (lam-inar) airflow. This is the method most commonly used where the entire ceiling is filled with HEPA or ULPA filter assemblies. This arrangement of filters makes it difficult to use ducted systems, and costly to use and operate motorized fan modules.

There are several design considerations to be aware of when applying products to this space. The pressure used to force the air through the filters is significantly high. This pressure can force leaks through all but the best seals. Some designs use a special grid that contains a small channel. Fixtures and filters are manufactured with a vertical knife edge so that they can be laid into the channel that is filled with a wet gel that later becomes semi-solid. This system is specifically designed to maintain the seal of the plenum space. Other systems use neoprene closed-cell gaskets that interface between the top of the inverted T-grid and the filters or fixtures.

Fixture Selection

There are many designs of cleanrooms in use today. As mentioned earlier, each client has its own process needs and requires special considerations as equipment and materials are chosen. If it is your job to help choose lighting components for these environments, it is important to keep in mind the following criteria.

Clean-ability

Each exposed surface within the cleanroom will be cleaned at some interval with some method. The lighting fixture must be able to withstand the abuse of cleaning. The cleaning schedule, the chemicals used, and the extent of cleanliness required will dictate the design and durability of the lighting fixtures. The fixture design should not have any exposed (room side) joints that are not sealed or welded. All surfaces are required to be smooth and easy to clean. The ideal fixture housing should be completely covered by a single-surface door frame to eliminate hard-to-clean crevices. The typical extruded aluminum lighting fixture door frame is a good example of a hard-to-clean design because each corner has a mitered joint with a deep slit that is virtually inaccessible for cleaning. The prismatic lens in a clean space fixture is always inverted so that the smooth surface is faced into the cleanroom.

Each surface will be hand cleaned using expensive supplies and labor, so there may be a delayed, hidden cost to a cheaper, poorly designed fixture. The Cleanroom industry's favorite material is stainless steel. Though costly, it has benefits that are impossible to get from other materials. It is the most durable fixture construction material, and is required by the National Sanitation Foundation (NSF) for all exposed surfaces. Although many fixture manufacturers claim their products are "Made to NSF standards" only a few companies, including Kurtzon Lighting, actually test with the NSF and can affix the NSF component listing mark to our products. This can be very important because a customer may require such a quality standard if that facility will be subject to inspections by regulatory agencies that insist on this mark. There are good reasons to specify that the fixture bear the NSF listing mark:

• The customer may expect the mark, especially if the process involves food, pharmaceuticals or medical research.
• The fixture construction and materials are inspected to strict standards within the NSF laboratory and at unannounced factory visits throughout the year to insure quality and consistency of manufacturing.
• An NSF certified fixture will withstand the abuse of cleaning and will reliably maintain a quality seal under the worst of conditions.

Laminar Flow:

The nemesis of stringent cleanroom requirements is a particle that will not wash away with the current of air. The cleanest cleanrooms exchange air as many as 15 times per minute. This process, done at considerable expense, is of no effect if the particles themselves will not cooperate with the air flows. When air is moving it acts in the same ways as water does. Imagine a fast moving stream, the water seems to move as one body if the riverbed is smooth. As water passes over an imperfection far below the surface, the water will churn at the surface. The same thing happens to air where it encounters a less than smooth surface, or other air traveling at another velocity. Radical changes in direction of flow caused by barriers are responsible for turbulent flow. Turbulence is responsible for suspension of dirt in the river and in the cleanroom. The shape and size of that barrier is key to maintaining "laminar" or unidirectional flow. In a cleanroom, the ceiling is the source of air and light, and each must not interfere with the function of the other. The lighting fixtures used within the flow of air (when at significant velocities) must be shaped so that no turbulence is made. The industry calls this type of fixture "the tear-drop", and is installed on the T-grid lower surface. Please see Kurtzon Lighting Klean-Lock catalog page KL01, KL-AFV.

Sealing the Plenum

The cleanroom function is only possible where there is separation of the space from less clean areas including the plenum space. Since this space can be positively pressurized, (deliberately or incidentally), it is necessary that any device that traverses the ceiling will stop all unintended air flow through the ceiling. Fixtures must be designed and tested for this condition. Gaskets, gels or caulks are used to maintain the seal. The fixture design must lend itself to easy and reliable sealing of the plenum. The fixture door frame should cover all the fixture components, including flanges, and should create a tight seal at the ceiling (or T-Grid) and at the housing. This arrangement also seals the electrical conduit, which is a common source of unfiltered air, from the plenum and cleanroom. The door frame gaskets must be made of high resilience materials that "bounce back" after compression to reseal the housing and plenum after re-lamping. If a fixture has any other seals subject to disturbance after installation, such as plenum access plates, they should be of this quality too. See below for gasket material properties.

Particle Generation

Fixture construction materials themselves can contribute to the cleanroom particle count. For instance, a common transformer-type ballast failure can result in a completely contaminated cleanroom costing the customer untold thousands of dollars in destroyed product as well as lost usage. Use of electronic ballasts is widespread in the industry for energy conservation reasons, but knowledgeable cleanroom managers appreciate their mode of failure, which is environmentally harmless. Other sources of contamination are from painted components and gaskets.

Gasket and Seal Materials

Outgassing, which is the slow release of chemical vapors or particles from an apparent solid, is another area of concern because of the choice of lighting fixture gasket and seal materials. Solvent based parts eventually stop outgassing but are affected by some typical solvents used in cleaning. Thermoplastic gasket materials are cheap to buy, but they are common sources of outgassing. These vapors or particles are usually essential to the life of the thermoplastic material but add to the total particle counts for the cleanroom space. While the outgassing materials contaminate the cleanroom, the seals and gaskets disintegrate and fail to perform their critical functions, increasing contamination. A better choice in materials is a thermoset or cross-linked polymer variety such as polyurethane powder coatings and Kurtzon Klean-Lock SealPro (TM) gaskets. A simple test of these products reveal that they don't shrink or melt with heat as do the thermoplastic varieties.

Fixture Types

The lighting fixture types used in cleanrooms vary, based on the ceiling construction and other engineering considerations. The typical plaster or drywall ceiling system will use flanged fixtures. Flanged fixtures are commonly used where rigorous liquid cleaning of the ceiling is required and airflow is relatively slow. The flanged fixture is inserted through an opening cut in the ceiling from below. The fixture is pulled up tight to the lower surface of the ceiling using provided hardware. The plenum is sealed by a bead of caulk and by the fixture door frame.

Fixtures come in all shapes and sizes, from one foot square to two by four feet. Kurtzon, a quality manufacturer emphasizing customer service, provides ceiling cutout templates, upon request, that will insure properly sized and shaped ceiling openings.

The Continuous Row Mounting (CRM) system is a new approach to sealed lighting installations. This system allows for a single ceiling cutout to be filled with several fixtures that butt and seal to each other, providing the most efficient use of ceiling and plenum space. In this type of installation, foot-candle levels can be higher than single fixture designs allow.

T-Grid Recessed Fixture Systems

This variety of fixture is installed from above and sits atop the grid. T-grid recessed fixtures are the most popular in cleanrooms today. Grid fixtures can be found in all classes of cleanrooms, though they are usually not used in class 1 and class 10 spaces. There are four different types of cleanroom grid systems and each will require a different fixture. The types are 1", 1.5", 2", and 2.6" and refers tO the width of the grid material cross-section. There are standard fixture sizes to fit each of the grid types available. Remember to specify the grid type when ordering fixtures. To seal the plenum, the fixture uses a gasket at the lower surface that interfaces with the grid. Gel-seal grid systems are far less common today than standard gasketed grid systems, and require special fixtures.

Surface fixtures are reserved for only two categories of installation. The first is the class 100,000 space, where airflow is very low and cleaning is infrequent. This environment is where the typical rectangular fixture is installed. Adding surface mounted fixtures within existing clean spaces is not recommended because there is always one surface that cannot be cleaned.

The other application is the class 10 and cleaner spaces where the air velocity is very high and the need for a special fixture shape is required. This fixture is what the industry calls "teardrop" or "airflow," such as the Kurtzon Lighting Klean-Lock KL-AFV (See Fig. 5). Airflow fixtures are mounted directly to the bottom surface of the T-grid. Using this system provides the needed light without using up valuable ceiling space occupied by filters. Other systems are available for more extraordinary installations.

Special Requirement Fixtures

Kurtzon Lighting provides a system of fixtures employed where the user or engineer does not want to disturb equipment or production for fixture or building maintenance. The system is based on two types of fixtures, a plenum access fixture and a top access fixture such as Kurtzon Lighting Klean-Lock KL-F/TR and KL-F/PA fixtures. The plenum access fixture is installed in a special location separate from the work areas of the cleanroom. The rest of the fixtures in the space are of the top access type and are permanently sealed in the ceiling with a special caulk or surface membrane. Access to the plenum space is gained through the first fixture so that service for the other fixtures is performed without disturbing the environment below.

Figure 11 is a simple guide to the clean space environment and the associated velocities for typical cleanroom classes.

If additional assistance is desired in specifying lighting products for cleanroom environments please contact the Kurtzon Lighting Customer Service or Application Engineering departments.