This checklist, which has been prepared by the NC Office of Waste Reduction, is designed to help operators of solvent vapor degreasers determine problem areas and minimize air emissions. Outlined below are pollution prevention practices that can help reduce evaporative losses of cleaning solvents.
- Install lids/silhouettes on tanks. All tanks should be covered when not in use. Covers that can be used during cleaning (silhouette entries) permit even greater vapor loss reductions of up to 55 percent. All covers should be designed to slide horizontally over the tank top. This movement disturbs the vapor zone less than hinged covers.
- Increase freeboard space on tanks to reduce solvent emissions by up to 46 percent. An increase in the the freeboard space from 50 percent (one-half tank width) to 75 percent can reduce solvent emissions by up to 46 percent. An increase of 100 percent in the freeboard space can provide an additional 39-percent reduction when air turbulence is a problem.
- Avoid drafts over the degreaser. Fans, air conditioners, heaters, windows, doors, general plant air movement, and equipment movement can blow the vapor/air mixture out of the degreaser. Locate the degreaser to minimize natural drafts or use baffles to prevent vapors from being disturbed. Solvent loss reductions of up to 30 percent can be realized.
- Adjust cooling. Regulate the cooling level by adjusting the temperature of the cooling water or by altering its flow rate. Since a fluctuating vapor level pumps the vapor/air mixture out of the tank, the vapor level should balance at the midpoint of the condensing coil.
- Check the water jacket for proper water flow and temperature. Proper water flow and temperature can prevent not only migration of hot vapor up the side walls but convection currents as well.
- Install freeboard chillers in addition to cooling jackets to reduce solvent use by as much as 60 percent. A second set of refrigerated coils above the condenser coils creates an additional barrier to vapor loss. Reductions in solvent use of 60 percent have been realized by this technique. However, because solvent contamination can occur if condensation accumulates on the coils, air inside the tank vapor zone should be dehumidified. Special water collection equipment may also be necessary.
- Avoid spraying parts above the vapor zone or cooling jacket. Spraying above the vapor zone generates a vapor/air mixture (which is immediately lost), and falling droplets of solvent (which disrupt the vapor interface) cause additional vapor/air mixing.
- Move the work slowly. Rapid movement of the parts or basket disrupts the vapor zone and causes air to mix in with the vapor. Control the hoist speed to less than 11 feet per minute of vertical travel. Consider installing programmable transporters to ensure proper conveyor speed.
- Avoid solvent carry-out. Solvent that is not properly drained from parts is immediately lost to evaporation outside the degreaser. Adjust the positioning of baskets or racks for easy drainage. If necessary, rotate parts to promote drainage.
- Bring parts up to temperature before removal. The cleaning cycle is not complete until the parts have reached the temperature of the vapor and condensation has ceased. If condensation is still forming, solvent drag-out will increase.
- De-water the solvent. A water separator should be able to reduce dissolved water in the solvent. The surface can also be skimmed to reduce the water content of the solution.
- Do not overload the degreaser. Avoid inserting oversized items or large baskets into the tank. As a general rule, the cross-sectional area of the work load should not exceed 50 percent of the tank's open area.
- Repair leaks. Leaks are difficult to detect because of rapid evaporation of solvent losses. Careful inspection should routinely be performed, especially in hidden spots.
- Consolidate cold cleaning operations into a centralized vapor degreasing operation. Cold cleaning solvents typically must be discarded when contamination levels reach 10 percent; however, vapor degreasers can operate with up to 25- to 30-percent contamination. In addition, vapor degreasers provide much better cleaning, and the parts leave the unit dry.
- Locate the cold cleaning tanks away from heat sources. Additional heat will substantially increase vapor loss.
NOTE: Under new air quality regulations for vapor degreasers that utilize trichloroethylene, perchloroethylene, 1,1,1-trichloroethane, carbon tetrachloride, chloroform, and methylene chloride, more stringent operating practices will be required. To eliminate or at least minimize the use of these solvents in vapor degreasing, consider the following steps: (1) eliminate the need to clean by removing upstream contamination or by changing cleanliness requirements, (2) modify the part or contaminant so that a less hazardous cleaning material or method can be used, and (3) select the least hazardous cleaning material or method that will achieve the desired level of cleaning.
For assistance in selecting the most appropriate solvent alternative, contact the NC Office of Waste Reduction at (919) 715-6500 or E-Mail.
The Office of Waste Reduction provides free, non-regulatory technical assistance on methods to eliminate, reduce, or recycle wastes before they become pollutants or require disposal.