Wednesday, May 6, 2009

The FFU: A Cleanroom Design Alternative

The FFU: A Cleanroom Design Alternative

We didn’t know what to call them. A fan attached to a 2’x4’ sheet metal enclosure containing a HEPA filter. Fan-Filter? (FF, maybe); Integrated Blower Module? (IBM, that’s been done); Integral Blower Filter? (IBF, that might work), So we went to market with the IBF, a look-alike to a competitor version, it was our effort to capture a new, small, but interesting segment of the cleanroom market.

Twenty years ago the IBF offered cleanroom designers an alternative to the positive pressure plenum and designs employing the ducted lay-in HEPA filter module. More importantly, or so we thought, it offered our company an opportunity to differentiate ourselves from our competitors by demonstrating that we were innovative, on the cutting edge, and able to leap tall buildings in a single bound. A marketer’s dream. “What? You need a 2’x2’ version? We can do that!” “What’s that? Oh, a 2’x3’ model? We can do that too!” “ Say what? OK, we’ll fabricate it to fit a 2’ grid system.” “Too noisy? We’re working on it.” “ Too heavy? The aluminum version is coming.” “Variable airflow? Naturally! and controllable from the room side or the top side.” “Energy reduction? Not a problem!” “Room side filter replacement? Of course!” “ DDC compatibility? Certainly!” And so it began, and grew, and evolved.


The IBF, and its many cousins, morphed into what is generically known today as the filter fan unit (FFU). It is used around the world in a wide variety of applications never thought of by its developers twenty odd years ago. We see it installed as the primary filtration unit on laboratory workstations, replacing more cumbersome built-up cabinetry and fixed filter/blower arrangements. We see it installed on a framework supporting lights and sidewall curtains to provide a portable or low cost entry-level cleanroom. It has been applied to ballroom cleanrooms covering tens of thousands of square feet of area with cleanliness ratings as low as ISO 3 (Class 1 in an earlier era).

Searching the Internet will result in a list of more than 50 manufacturers of FFUs. The units offered vary in size, though generally around the 2’x2’, 2’x4’, or 4’x4’ module, in low profile and “regular” height. They vary in airflow, some being two speed, some being infinitely variable. They vary in construction, galvanized steel and aluminum being most popular, although stainless and powder coated in decorator colors can be had, generally at a premium price. They are available in a wide variety of power configurations to accommodate electrical services around the world. They can be equipped with any efficiency of HEPA or ULPA filter. Some units are designed as “throw-aways,” others with room side or topside replaceable filters. They can be provided with an inlet duct fitting, round or square, with or without a pre-filter. They can come with a protective screen on the room side in any color or finish. They can have a face mounted control knob or be integrated into an elaborate DDC control system to vary airflow based on room particle count, time of day, room usage or other parameter. They can be as simple or as complex as the application and the ingenuity of the designer require.

Food For Thought

All products have limitations and the wise designer becomes familiar with a product before specifying. The fact is that caveat emptor is advice that designers and end users ignore at their peril. While many manufacturers can be found, more than a few are small specialty companies with perhaps a limited list of installations. It makes sense for the designer and buyer to be comfortable with the FFU supplier. In fact size may not matter. But questions as to company size, production capability, years manufacturing the FFU, customer references, third party verification of technical data and even a plant tour are not out of line in establishing credentials.

There is no specific certification or labeling program in place that provides third party safety testing of the FFU. Having a UL approved line cord does not convey UL approval to the unit. Laboratory test, labeling, or field inspection of the unit is typically required for the FFU to comply with building codes. In some cases this has not been an issue because local authorities are either unaware of a small project or are not familiar enough with the FFU application to question its use. In other cases the local code official may very well ask for UL approval for hundreds of FFUs installed on a new project. “Surprise!” This surprise, adding time and cost to the project, will not be welcome.

Manufacturers performance data may be suspect. It may be based on calculations, or competitive pressures, or best guess, or even, hopefully, based on third party testing. It is prudent to question the basis of data upon which a design or purchasing decision is to be based. Current available data is rarely comparable between manufacturers due to the variable test conditions employed. This is not to suggest that a given FFU will not perform as advertised in a specific application but rather that comparisons between vendor data may not yield accurate results. Work by Lawrence Livermore Laboratories and other international organizations related to FFU test standardization may find its way into common practice, perhaps through an IEST Recommended Practice, permitting selection of this equipment with a greater degree of confidence in performance claims. Certainly comparative data on airflow, sound level, and vibration would be welcome.

A Classic Application

The FFU has found application in a wide variety of cleanroom shapes and sizes serving a range of industries. It brings a special benefit to applications with limited overhead clearance. A low profile FFU, on the order of 13 inches high, can often be installed in a cleanroom, the ceiling of which is only three feet below the roof/floor above. A number of factors affect this decision, such as support of the floor/roof above, the foot print of the room, the density of the FFU placement based on cleanliness class, space required for air conditioning ductwork and process utility piping, sprinkler piping, etc. It is an engineering exercise to fit all the components together in a rational manner, but applying FFUs frequently helps meet design goals in restricted spaces.

Moving air is a major challenge in cleanroom design. The volume of recirculated HEPA filtered air, including conditioned air to handle high cooling loads typical of many cleanrooms, can range from less than twenty to more than five or six hundred air changes per hour. Traditionally ducting this large volumetric airflow has proven to be the driver of cleanroom support space size. Using FFUs without ductwork for cleanroom recirculation air reduces the required ducted airflow to perhaps 6-10 air changes per hour for air-conditioning only. The duct design challenge is considerably simplified by using FFUs. As an example, Figure 1 shows a 32-foot wide by 68 foot long cleanroom having a 10-foot high ceiling and 8-foot wide service chases on either side. The number of FFUs installed in the ceiling provides airflow adequate to achieve an ISO 7 (Class 10,000) cleanliness rating. Also shown in this view is an air handling unit (AHU) delivering conditioned air to the plenum above the ceiling. This air is discharged into the plenum space near the intake of each FFU. The FFU draws some conditioned air and some recirculation air from the plenum and delivers it through the HEPA filter to the cleanroom. In this example the AHU has a 5-ton capacity and draws approximately 2000 cfm of recirculation air from the service chase (and some ventilation air from outside the cleanroom). The AHU conditions the air, and delivers the air to the plenum space. The system shown is typical of three additional systems installed in the service chase to provide a total of 20 tons of cooling to the entire cleanroom.

Figure 2 shows a section of the cleanroom and AHU system. The available height is 13 feet. The cleanroom ceiling is 10 feet high. The scaled drawing shows room for the ductwork in the three-foot available plenum space. Upgrading to a more stringent cleanliness class merely requires additional FFUs installed in the cleanroom ceiling. As cooling load increases, additional AHU capacity (either more or larger systems) are installed. This illustrates one of the major benefits of utilizing the FFU, namely more cleanroom, in less space. Perhaps not an ideal design from the perspective of the cleanroom purist, but it is a practical approach to a difficult site situation.

There are as many design variations,as there are applications and designers. The FFU has proven its flexibility in meeting the needs of facility owners and designers around the world.

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