Exhaust recirculation system installed at Warner-Robins Air Logistics Center

Published Oct. 23, 2007
By Mindy Cooper
Materials and Manufacturing Directorate

ROBINS AIR FORCE BASE, Ga. -- Scientists at the Air Force Research Laboratory's Materials and Manufacturing Directorate determined using exhaust recirculation during spray painting in a corrosion control facility here is safe and compliant with Occupational Safety and Health Administration regulations.

Materials and Manufacturing Directorate Scientists at Tyndall Air Force Base, Fla. conducted 15 years of research in this area and transitioned it into practice at Robins Air Force Base. There, they participated in the design of a corrosion control facility, or CCF for C-5 Galaxy aircraft that uses an exhaust recirculation system which provides important energy conservation benefits and large cost savings throughout its life cycle.

Dr. Joe Wander, a senior chemist who was the lead of Air Pollution Control Technology Research and Development for the directorate , served as a consultant on the project from concept to completion.

Wander explained that spray application of aircraft surface coatings generates air pollutant mists and vapors that are harmful both to the environment and to personnel working in CCFs, where aircraft coatings are applied. Painting crews must adhere to strict environmental and occupational safety guidelines concerning these pollutants. Present-generation CCFs employ ventilation technology that brings fresh air into the facility to remove pollutant solvent vapors and overspray particles from the atmosphere via a filtered exhaust system.

"In climates in which the temperature and humidity of the fresh air that is filtered must be adjusted to allow proper application of coatings, expensive, energy-intense process air conditioning is applied to the entire air stream," Dr. Wander said. "If exhaust emission control is applied to the ventilation system, that cost is also large and proportional to the volume of air exhausted."

At Warner Robins Air Logistics Center the Air Force conducts depot-level maintenance of C-5 aircraft in a hot, humid location. Stripping exterior coatings and repainting requires continuous movement of air across the entire surface of the aircraft. Due to the large size of the aircraft, large volumes of clean air--more than two million cubic feet per minute--would be brought into the painting bay of the new CCF and conditioned by a conventional ventilation system.

"Robins engineers were interested in employing a less costly method of ventilation and climate control. They consulted with AFRL and determined that they would use Materials and Manufacturing Directorate's published reports of 15 years of research projects--which include data showing that using this method would cause no increase in exposure of personnel to the harmful air pollutants during exhaust recirculation (ER)--as technical justification to secure a full set of approvals to install and use ER in the new CCF," Dr. Wander said.

Exhaust recirculation adds a return duct to the facility that carries most of the filtered (and conditioned) exhaust air back into the building in lieu of the same amount of fresh air. With this method, the same air movement is preserved inside the painting area, but the amount of energy required to maintain proper temperature and humidity is decreased by more than half, resulting in a substantial energy cost savings.

According to Dr. Wander, the pivotal element in the laboratory's 15 years of research proved to be a study conducted at Hill Air Force Base, in Ogden, Utah, to validate predictions that exhaust recirculation would cause no measurable increase in toxic exposure levels to painting personnel than the traditional method of ventilation. During this project, base bioenvironmental engineers measured the air concentrations at several places in the hangar and also the exposure levels of the personnel on the painting team during the painting of three C-130s. The results of this study showed that there was indeed no identifiable increase in exposure when painting in 80 percent recirculated air.

"The data gathered at Hill AFB inspired development of a predictive model as a research project by an AFRL scientist, and was used to validate this model, which emerged during the design campaign as the Air Force's de facto decision tool for ER designs," Dr. Wander explained.

"The model calculates the vapor concentration that accumulates in the workspace from an input of the dimensions of the workspace, the rate of ventilation, the percent recirculation, and the composition and amount of coating sprayed per minute. The result of this calculation lets engineers know if using an exhaust recirculation method in a particular facility would cause personnel to be exposed to a higher level of pollutants than they should be. If so, the engineers would need to change the design of the system."

The scientists educated the full design committee by sharing and interpreting the results of their research during a round table meeting and then cooperated intensively with the small team that designed the campaign and engaged the system using the research results and the concentration model to justify their exhaust recirculation system for the C-5 CCF. It resulted in a $10M reduction in construction costs and it is estimated that the method will continue to save $1.5M in annual energy costs.

The AFRL researchers continued to work with the Warner Robins Air Logistics Center team to document the decision and approval processes, and a technical paper that includes copies of significant precedent documents cited in the base's justifications was published during construction of the hangar.

After reading in an e-newsletter about the success Robins had securing concurrence to use ER and its economic impact, engineers at Elmendorf Air Force Base, Alaska, were referred to AFRL. Using information in this document and consulting with Materiels & Manufacturing Directorate scientists at Tyndall, along with Warner Robins Air Logistics Center staff, Elmendorf engineers were able to incorporate ER ventilation systems in two new CCF construction projects.

The final AFRL publication in this series will document the decision processes at Elmendorf and initial exposure measurements--which are even lower than expected--in the newly opened CCF at Robins AFB.