Passive Aerosol Generator

1.2 Aerosol Recovery and Filtration System
The aerosol recovery and filtration system recovers aerosol that is discharged from the process area. It also provides final filtration prior to exhausting into the environment.

Recirculating Recovery System
The recirculating spray recovery system uses a spray system to recover the aerosol. The aerosol enters the recovery chamber in the lower half of the recovery chamber. There the aerosol is exposed to a spray bath. The spray saturates the air volume and causes the droplet size to increase. As the droplet size increases, the mechanical force of the spray and gravity cause the aerosol to " precipitate" out of the air. Distilled water is normally used for the spray, although non-water based solutions may also be used, depending on the coating selected. The recirculating spray recovery system is started with a known volume of liquid, such that the increase in volume can be measured. The increase in the recovery system volume subtracted from the decrease in the liquid volume in the PAG, allows for a mass balance calculation. This is particularly useful when encapsulating large volumes with multiple surfaces (piping, ventilation ducts, process hoods, work spaces, equipment, etc.). By estimating the amount of material that remains in the process area, an approximation of the thickness of the coatings can be made.

Filtration System
The exhaust train of the aerosol recovery system is filtered twice. First the exhaust train is pulled through a moisture separator, thereby removing free liquid. Next, the exhaust train is pulled through a HEPA filter; this HEPA filtration is the final processing filtration of the exhaust. After the exhaust is pulled through the HEPA filter it is exhausted from the system by an exhaust fan.

Exhaust System
The exhaust system operates at a slightly higher volume than the PAG delivers the aerosol. This provides for a slight negative pressure within the system at all times. A differential pressure indication system (dP) is used to insure that the aerosol flow is from the PAG, to the delivery system, into the process area, out to the recirculation spray recovery and filtration system, and then exhausted to the environment.

Differential Pressure Indication System
Pressure sensing instruments are located in the PAG pressurization chamber and in the recirculating spray recovery and filtration system. A differential pressure circuit indication provides information to the operator to allow them to control both the pressurization fan and the exhaust fan speed. A slight negative differential pressure (> -0.5" H2O)is maintained in the recirculating spray and recovery system. This insures that the aerosol flow is always maintained from the PAG, to the process area, and out to the recirculation spray and recovery system. In addition, by utilizing the dP indication, an experienced PAG operator can modify the flow characteristics of the encapsulation aerosol, thus helping to insure even and through coating in the process area.

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