Passive Aerosol Generator

1.1.2 Passive Delivery System Using Pressure Chamber and Aerosol Directional Delivery Device
Using a passive delivery system facilitates treatment of all surfaces within a process area simultaneously without resuspending contaminants. Additionally, the delivery system maintains a high concentration of the aerosol in the process area further mitigating airborne contaminants.

Aerosol Flow Rate to Substrate Coating Requirements
Once the transducers have generated the encapsulating aerosol, the PAG has to deliver the aerosol to the process area at a rate that allows the aerosol to coat the surfaces within the area. The coatings used in the PAG model PK-2000 vary in chemical composition, so the following description will focus on a water based polyurethane suspended in a two part organic solution. Further information on the various coatings is presented in Section 1.3 of this manual.

As a rule, the slower the flow rate of the aerosol within the process area, the more efficiently this process encapsulates contaminants. The flow rate into a given treatment area will be determined by a variety of factors. These include, (1) the coating material, (2) the substrate(s) being coated, (3) the frequency of the transducers, (4) the temperature of the coating liquid, (5) the distance from the PAG to the process area, (6) the temperature and relative humidity of the process area, (7) if multiple coatings are being used, which coating is being applied, and (8) relative elevation of the process area to the PAG.

When the coating material is changed, a different frequency of transducers may be required. Generally, the larger the organic molecules and the more "binder" or "solid" molecules present, the lower the frequency that needs to be used. As the frequency of the transducers decreases, the flow rate into the process area needs to increase. This is due to the larger aerosol droplet size generated at the lower frequencies. The coating materials affect flow rate by requiring an increase in flow rate as the molecule size increases, as the viscosity increases, and as surface tension passes through the optimum point as defined as the slope of the differential of the function being equal to zero. The flow rates will also vary depending upon the number of previous coatings in the process area and the shape and material being coated. The following table shows the numerous interacting variables relative to flow rate. For most applications, flow rates will vary from < 2 standard cubic feet per minute (scfm) to ~ 18 scfm.

The desired effect for varying the flow rate is to allow the aerosol droplets to contact a surface and coalesce with other aerosol droplets to form an encapsulating film.

The concentration of the aerosol reaches dynamic equilibrium between the material entering the process area and the sum of the material deposited on the surfaces and the material being discharged from the process area. Small uniform droplets of ~ 8-12 mils in diameter form over all surfaces within the process area. If the flow rate is increased too high, the "virgin" material contacting the surface will create drops large enough to allow streaking of the surface or puddling of the surface.

In summary, the following guidelines should be followed. Insert the aerosol at the slowest flow rate practical for a given process area and establish a maximum concentration of the aerosol within the process area. Once maximum concentration of the aerosol is reached, maintain this concentration by a feed and bleed process until the ideal droplet size forms on the substrate. Then secure aerosol generation until the area dries to the desired consistency.

Thermal Control of Delivery System
In addition to the passive delivery system discussed above, the PAG uses a heating system to control the temperature of the aerosol. This is done by maintaining specified temperatures in the liquid volume used to create the aerosol. Heating of the coating liquid 10 to 15 degrees above the ambient temperature in the process area will cause the aerosol to fill the void from the top down. Since the process of generating the aerosol removes heat from the liquid, securing the heating system allows the temperature of the aerosol to drop below ambient temperature and therefore, fill the void from the bottom up. These characteristics are useful when delivering the aerosol in certain applications.

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