Element 4: Room Pressure
*Updated in August 2018 from original 2014 post date.
One way AMCBP is “Constructing an Intelligent Tomorrow” is through the work of our Cleanroom Performance Solutions group. In this 4-part series, our very own Emil Bordelon, a NEBB Certified Professional, outlines the four main elements of a cleanroom that are considered during the design, certification and maintenance phases.
The 4 elements are:
- HEPA/ ULPA filters
- Cleanroom Classification
- Room Pressure
This week Mr. Bordelon will explain the importance of room pressure.
The main function of room pressure is to mitigate infiltration of particles from a less clean cleanroom to a cleaner cleanroom i.e. ISO Class 5 to ISO Class 4. The cleanest area in the fab should have the highest pressure cascading out to the less clean areas. In drawing 1, the cleanest room would be the Dry Lab ISO Class 5 then the Wet Lab ISO Class 6 and finally the Gown Room ISO Class 7 then outside of the cleanroom facility.
In Fed. Std. 209B published in 1973 the minimum pressure requirement was 0.05 inches of water gauge (12 Pascal (Pa)). This requirement was removed from the later Fed. Std. 209 standards. The 0.05” w. g. (12 Pa) requirement was found to be on the high side due to the cascading requirement (see drawing 1) with a 3 room cascade the pressure from the cleanest room to the exterior would be 0.15” w. g. or higher (37 Pa) . This higher pressure causes some issues i.e. doors not closing correctly or hard to open, higher air leakage and high demands on the HVAC system. The requirement now was modified to 0.03” to 0.05” w. g. (7 to 12 Pa). With the lower pressure and the same 3 room cascade the pressure from the cleanest room to the exterior would be .09’ w. g. (22 Pa) (see drawing 2). This is a reductionof the differential pressure by 0.06” w. g. (14 Pa). ISO 14644-4 has a recommendation of 5 to 20 Pascal (0.02” to 0.08” w. g.).
Constructing a cleanroom facility is like constructing a building inside of a building. A cleanroom needs to have a separate HVAC system, lighting system, flooring and wall system and a differential pressure to the interior of the building. An exterior wall of a building should not be used as a cleanroom wall. Also there should never be a door from the cleanroom to the outside of the building. This is due to wind pressure – a 10 mile per hour wind has a wind pressure of .05 “w. g. (12.9 Pa). A 30 mile per hour wind has a wind pressure of .47 “w. g. (116.2 Pa). A 50 mile per hour wind has a wind pressure of 1.3” w. g. (322 Pa). Of course, the wind pressure can vary due to conditions such as temperature, air density, and altitude.
Pressurizing a cleanroom is like keeping a balloon inflated that has a leak in it. There should be a constant airflow with constricted air return to build the pressure. For example, the Wet Lab ISO Class 6 is 600 sq. ft. with a 10 foot ceiling equals a room volume of 6,000 cubic feet. The recommended air change rates for ISO Class 6 is 70 to 160 changes per hour mid-range would be 115 changes per hour. To meet this air change rate the airflow requirement is 11,700 cubic feet per minute (CFM). An air return flow rate of 450 feet per minute (FPM) through an 80% open grill with a 30% grill deflection will create a 0.062” w. g. (15.4 Pa) pressure. Dividing 11,700 CFM by 450 FPM will equal a return area of 26 Sq. Ft. To calculate the running feet of air return divide the return area required (26 sq. ft.) by the height of the air return(1.5’) times the gill opening (80%) equals 22 running feet ( 21.6 feet rounded up). It is recommended not to exceed 600 FPM air return flow rate as this will create higher noise levels and vibration in the return space.
A cleanroom should be built tight with very little air leakage but there are always leakages that cannot be stopped i.e. gaps around doors, loose fitting ceiling tiles, penetrations though walls. Also, every time a door opens in a positive room there is airflow from that room causing a loss of air. Also, there is the air loss through the exhaust system if one is installed. To combat these air losses, makeup air is supplied to the cleanroom. It also supplies fresh air to the cleanroom. The makeup air is filtered and usually supplied to the return side of the recirculation air handler which feeds the filters in the cleanroom.
With today’s advance control systems and pressure transducers, the monitoring and controlling of the cleanroom room pressures have been simplified. Gone are the days of an employee walking around checking magnehelic gauges and documenting the room pressures. Today room pressures are monitored on a computer screen with the data stored for later use with low and high pressure alarms. With makeup air handlers (MAHU) being equipped with variable frequency drives (VFD) pressure control is also mostly automatic. The one major problem is with a lot of traffic going in and out of the cleanroom, the MAHU may start to hunt (ramp up and down rapid). One fix is to program a delay into the VFD and another is to have a wider range of set points.
For this test, an Air Data Meter (Electronic Micro Manometer) is used to measure pressure across a door or pass throughs. The meter has two ports – one positive and one negative. A testing tube is placed under the door or across the pass throughs to the other room. Connect the testing tube to the negative port of the meter. Take a reading – if the number is positive, the air is flowing away from you. If the number is negative, the air is flowing to you. Take the average of the four readings and document the result. The four readings should be similar – if there is a major difference, check for door openings or the MAHU hunting.
Putting this to use
For over twenty years, our Cleanroom Performance Solutions team has provided cleanroom testing, certification, reporting, cleanroom construction and consultation services across multiple markets, such as Semiconductor, Life Sciences, Technology and Research.
With over 30 years of experience, and the NEBB Certification, the AM Cleanroom Build and Performance Engineers and Technicians can identify and analyze cleanroom performance problems; and suggest the most effective solutions or self-perform the corrections. Our Engineers and Technicians are experienced in the most sensitive environments such as ISO 14644, Class 1 through Class 9, and have helped develop the industry’s standard protocols and testing specifications, as well as some of the key instrumentation, testing media, and standard documentation in use today.
For more information on our Cleanroom Performance Solutions, please contact us.