Particle Deposition Rate

In a cleanroom, clean zone or clean controlled environment clean air is applied to remove airborne particles. Particles that cannot be removed will deposit on surfaces. This phenomenon is expressed as particle deposition rate or as particle deposition rate level.

The particle deposition rate parameter can be used in 5 ways:

1. Particle deposition rate limit to be able to make a product or perform a process
2. Determination of air cleanliness requirements for a contamination control solution
3. Monitoring the effectiveness of the established control solution
4. Predicting surface cleanliness of various surfaces in a controlled environment
5. Providing information to improve the contamination control solution

Particle deposition rate R_D

The quantity of particle deposition rate parameter is expressed as particle deposition rate in number of particles ≥ D µm per m² per hour. It is denoted by R_D.

R_D = airborne concentration of particles ≥ D µm [number/m³] × deposition velocity [m/h]

The deposition velocity depends on the particle size, particle shape and material (specific density). For spherical particle the deposition velocity can be calculated using Stokes’ law.

R_D = increase of surface concentration of particles ≥ D µm [number/m²] / time of exposure [h].

Particle deposition rate level L

Particle deposition rate L is expressed as the equivalent number of particles ≥ 10 µm per m²·h.

L = maximum R_D × D/10              for D ≥ 5 µm  to ≥ 500 µm  or the particle size range of interest

Particle deposition rate limit to be able to make a product or perform a process

For a product or process that is vulnerable to macroparticles (≥ 5 µm ) the particle deposition rate determines the expected contamination by critical particles during exposure.

Contamination by particles ≥ critical Di µm N_D depends on vulnerable product area A, time of exposure T and the particle deposition rate R_D.

N_Di = R_Di × A × T

By a product analysis the limit for the particle deposition rate or particle deposition rate level can be determined by the maximum accepted number of one or more critical particles sizes. For microorganism 20 or 25 µm can be used as critical particle size.

R_Di limit = N_Di divided by A × T

L limit = minimum N_Di × Di divided by 10 × A × T

In the APMON 2 user interphase there is an INSIGHT app to calculate the L limit.

Determination of air cleanliness requirements for a contamination control solution

A contamination control solution needs a clean controlled environment, clean zone or cleanroom to keep the particle deposition rate below the limit. The expected air cleanliness is determined by the clean air supply, ventilation efficiency and source strength of potential particle sources. The number of personnel in the cleanroom and their type of cleanroom garment determine the source strength determine the airborne concentration of particles ≥ 5 µm. The entry procedures and surface cleaning program determine the particle deposition rate of particles  ≥  25 µm.

For the contamination control solution both a adequate cleanroom installation and cleanroom operation program are required.

In the APMON 2 user interphase there is an INSIGHT app to calculate the expected air cleanliness level in a given cleanroom.

Monitoring the effectiveness of the established control solution

Monitoring the particle deposition rate at locations where the vulnerable product or process is exposed to the environment shows what particle deposition rate level has been reached. The real time data show when particle deposition events occur. This information can be used to determine what activities cause particle depositions.

Predicting surface cleanliness of various surfaces in a controlled environment

The particle deposition rate during operation will predict the increase of surface concentration of surrounding surfaces. Together with information of the cleaning program the surface cleanliness can be predicted. The information can also be used to determine the optimal cleaning frequency. For evaluation of the efficiency of the cleaning method additional surface cleanliness measurements with a SUMON can be used.

Providing information to improve the contamination control solution

The cumulative particle size distribution  shows what type of control should be improved to reduce the particle deposition rate level: cleanroom installation, cleanroom garment use or cleaning.