An article of W. Whyte (1), K. Agricola (2) and M. Derks (3)
(1) School of Engineering, University of Glasgow, UK; (2) VCCN, Dutch Contamination Control Society, Leusden, the Netherlands;
(3) Lighthouse Benelux BV, Boven-Leeuwen, the Netherlands.
Abstract
“This article discusses the mechanisms of particle deposition onto cleanroom surfaces. The main mechanism for particles above about 0.5 µm is gravitational settling. Turbulent deposition and electrostatic attraction can also occur at all particle sizes, and for particles below 0.5 µm Brownian diffusion is important.
Measurements of particle deposition rates (PDRs) were made of particles ≥ 10 µm on witness plates orientated in different directions and exposed in different ventilation conditions, and it was concluded that over 80% of particles were deposited by gravitational sedimentation, and probably more than half of the remainder by turbulent deposition.”
Discussions and conclusions
“The mechanisms of airborne deposition of particles onto surfaces have been reported in the scientific literature and reviewed in the introduction to this article. It was concluded that in cleanrooms the most important mechanisms were gravitational settling, turbulent deposition and, in certain circumstances, electrostatic attraction. Brownian diffusion was also important, but only for particles of a size less than about 0.5µm. Measurements of the PDR on witness plates orientated in different directions and in three air movement conditions were carried out to help to resolve the question of the relative importance of these deposition mechanisms.
The particle deposition rates (PDRs) of particles ≥ 10 µm were measured on witness plates exposed in four different directions in a cleanroom. Most of the deposition occurred on the upward- facing plates, and gravitational deposition account for 82% of the overall deposition. The deposition mechanisms of the remaining 18% of particles deposited by non-gravitational means were likely to be turbulent deposition or electrostatic attraction, and these possibilities were investigated.
Experiments carried out into the deposition of particles ≥ 10 µm onto witness plates orientated in different directions and airflow conditions suggested that at least half of the 18% of the non- gravitational deposition was caused by turbulent deposition. This finding was supported by previously-reported experiments carried out on microbe-carrying particles using nutrient agar plates, and therefore depositing onto surfaces free of electrostatic charge.
In that situation only 6% was non- gravitational but differences in the size distribution could be a contributing cause. The electrostatic field charge on the glass witness plates used in the particle experiments was measured at 20mm from the surface and found to range from -300v to +500v. This charge would attract particles and could account for some of the non-gravitational deposition, although the exact proportion was uncertain.
Experiments carried out on the deposition of particles on parallel plates gave an additional insight into particle deposition in cleanrooms. As gravitational deposition was the dominant mechanism in these experiments, it might be expected that a plate located directly above another plate, and 10 cm apart, would protect the lower plate from particle deposition. This method of protection is used in cleanrooms to minimise surface contamination but it is clear that it cannot be relied upon. The result was a little surprising but can be explained. Air turbulence above the top and bottom plates should be similar and give a similar amount of turbulent deposition. Any electrostatic deposition should also be the same. Cleanroom air passing between the two plates will have a turbulent movement in which the particles will move up and down but these movements should balance each other out, and the downward gravitational sedimentation of particles should largely determine the PDR. It should, therefore, be expected that the two parallel plates will have similar PDRs. It can be anticipated that in cleanroom areas where deposition might not be thought to occur but room air can flow in and out, deposition will occur and the PDR will be similar to that found in the general cleanroom area.
Further investigations into the PDR in a cleanroom will be reported in a further article, along with the relationship between the PDR and airborne particle concentration. Methods that can be used to calculate airborne particle contamination of products, and the cleanliness class of cleanroom required for an acceptable amount of product contamination, will also be discussed.”
Read full article here: “Airborne particle deposition in cleanrooms: Deposition mechanisms” by W.Whyte, K.Agricola and M.Derks
