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  • How Booths Work
  • Crossdraft
  • Semi-Downdraft
  • Downdraft Side Exhaust
  • Downdraft

1.1.5.a How Booths Work

By design, a spray booth collects solids known as particulate emissions. Filtration media, which can be filter pads or water, and moving air, are the primary tools in this collection. A spray gun or similar device, with either human or robotic assistance, applies the coating material. The airstream moving through the booth gathers the solids and transports them to the filtration medium. The force and direction of the air, the efficiency of the filtration, and the characteristics of the coating equipment — these three elements — determine not only the overall efficiency of the coating operation, but also the quality of the finish.

Prep workstations and mix room systems work in similar fashion. Moving air transports solids to filtration. In a mix room, the moving air stream also transports harmful and dangerous evaporants out of the working area, minimizing worker injury and hazard of explosion.

1.1.5.b How Booths Work

 

Crossdraft or Horizontal Flow

Booths are also classified by the method of draft. In the simplest configuration of all, a crossdraft or horizontal flow spray booth uses its powerful exhaust fan to pull in air at one end of the booth from the shop or plant area immediately surrounding the spray booth. The air may pass through a filter door before entering the working chamber, enter the working chamber unfiltered through an open face, or be pushed in through a pressurized input plenum. The air flows parallel to the floor, passes through the working chamber and picks up the particulate. Then the air passes through a filter bank to capture the particulate, and is 100% exhausted through ducting direct into the atmosphere. The majority of booths are designed as crossdraft.

Because the zone of particulate capture is the same as the worker zone, this airflow is considered by OSHA to be the one with the most adverse worker exposure. It is also hardest in this airflow to attain a superb finish.

1.1.5.c How Booths Work

 

Semi-Downdraft

 

The semi-downdraft booth is a "hybrid", combining features of both cross- and downdraft booths. The method of inputting the air to the booth makes it a semi-downdraft. Air is introduced to the booth through the ceiling in the first 25 to 30% of booth length. This input air may be introduced by relying on the suction of the exhaust fan or it can be pressurized. For the best results, air make-up should be used and booth positively pressurized. The exhaust fan continues to pull the air through the working chamber, causing the air to change directions and become parallel with the floor. The air then is drawn through the exhaust chamber at the booth rear and vented to the atmosphere through ducting.

Because the zone of particulate capture is the same as the worker zone, this airflow is considered by OSHA to be one with adverse worker exposure. It is also difficult in this airflow to attain a superb finish.

1.1.5.d How Booths Work

 

Downdraft Side Exhaust

 

Air is pulled into a downdraft side exhaust configuration through a full-length ceiling plenum. The air passes through the working chamber parallel to the walls and perpendicular to the floor. When the air reaches the floor it is pulled into and through floor level filtered exhaust plenums down each side of the booth and out into the atmosphere through ducting. No pit or elevated platform is required for this booth.

Because the captured particulates are rapidly removed from the worker zone, downdraft airflow is considered by OSHA to be safer than other airflow patterns. Downdraft airflow is the best choice when a superb finish is required.

1.1.5.e How Booths Work

 

Downdraft

 

 

Downdraft Airflow, Spray Cycle

 

Downdraft Airflow, Recirculating Bake Cycle


In the downdraft booth, the air flow is from overhead and moves down towards the building floor. The building floor normally has a sunken pit to accept either dry filter or water wash exhaust. A bar-type grating is laid over the pit opening. The booth can also be placed on an elevated platform in order to avoid pit construction. The top of booth may be open or enclosed with a filter input plenum. Most downdraft booths have overhead, filtered input plenums. A booth with a filter plenum is normally used in conjunction with a heated air make-up unit. This is considered a must for a clean paint job.

The graphic above shows a recirculating system. During the spray cycle, fresh air is pulled into the air system, heated to 70°F, then filtered and 100% exhausted.During the cure cycle, once the air has passed through the working chamber, 90% of the air is pulled through filters and back to the air make up unit. There it is combined with 10% fresh new air, reheated and returned to the working chamber. The 10% air not returned to the air make up unit is filtered and exhausted directly into the atmosphere through ducting. The value of a recirculating system lies in the preservation of resources: the working air is used over before its heat is lost.

Because the captured particulates are rapidly removed from the worker zone, downdraft airflow is considered by OSHA to be safer than other airflow patterns. Downdraft airflow is the best choice when a superb finish is required.

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