MNTAP Fact Sheet: Reducing Solvent Emissions from Vapor Degreasers

References

• U.S. Environmental Pollution Control Agency. 1989. Alternative Control Technology Document - Halogenated Solvent Cleaners. U.S. EPA, EPA-450/3-89-030.
  
• Ramsey, R. 1991. Vapor Emission Control in Vapor Degreasing and Defluxing Equipment. Internal publication of DuPont, Inc.
  
• Branson Ultrasonics Corporation. 1989. Solvent Emissions Control: A Technical Guide. Internal publication of Branson Ultrasonics Corporation, Danbury, CT.
  
• Branson Ultrasonics Corporation. 1989. Solvent Emissions Audit. Internal publication of Branson Ultrasonics Corporation, Danbury, CT.
  
• U.S. Environmental Protection Agency. 1991. Conservation and Recycling Practices for CFC-113 and Methyl chloroform.

Fact Sheet

The following information identifies the numerous factors affecting solvent emissions, and suggests ways to reduce these emissions. In individual degreasing operations, these factors are likely to vary significantly in importance. This information is based on comparing two situations for each emission variable; therefore, individual estimates of emission reduction potential for different variables are not necessarily comparable.

Emissions Due to Drag-Out

Drag-out losses are generally three to eight times larger than diffusion losses. Therefore, if drag-out and diffusion losses are cut by an equal percentage, the drag-out reduction method will result in larger actual savings.

Reduction Techniques

1. Withdraw parts from the degreaser when they stop dripping. Withdrawing moist parts from the vapor zone results in a 60 percent reduction in solvent emissions compared to withdrawing dripping parts. Parts should be left in the vapor zone (below the cooling coils) at least until no drops are visible on the parts.
   Estimated % Reduction - 60%

   During the vapor zone hold, vapors condense on the cold part, completely coating it in liquid. As the part warms up and approaches the vapor temperature, liquid will drip off the part faster than it recondenses. Some liquid will remain on the part until it reaches the vapor temperature, which will take a long time. Therefore, residual liquid will remain when parts are removed from vapor. Try to minimize the solvent film thickness on parts.

2. Hold parts in the freeboard zone until all parts are completely dry. Holding parts for a short time just above the cooling coils in the freeboard zone can result in 40 percent less emissions than if the parts are immediately withdrawn from the vapor zone. When removing parts from the degreaser, hold parts in the vapor zone (below the cooling coils) at least until all dripping from the parts stops (one to two minutes, for smaller parts). Then leave parts in the freeboard zone until all parts are completely dry. If time allows, lengthen the hold time in the vapor zone.
   Estimated % Reduction - 40%

   If parts are taken immediately out of the degreaser, the residual liquid film will quickly evaporate into the unsaturated air and be lost. If there is a hold in the freeboard zone, the residual liquid will still evaporate quickly into the unsaturated air, but since the solvent vapor is heavier than air and is still directly above the unit, much of the vapor will fall back into the vapor zone and be recaptured. The best way to control drag-out emissions is through the use of a programmable transport system, since even with excellent training on the need and methods for controlling emissions, it is physically difficult, if not possible, for a person to hold heavy parts away from their bodies for any length of time. If programmable transport systems cannot be justified, set up a stand for hanging parts or baskets at appropriate levels, and provide timers to keep the hold time above the minimum required for your application.

3. Use parts-holding fixtures that promote better drainage. The effect of using better fixtures for holding parts (racks or baskets) varies widely with the parts design and fixturing method. Position parts so draining is promoted and liquid hold-up is minimized. Minimize the surface area, the weight, and the heat capacity of the fixture or basket to reduce drag-out caused by the parts holder.
   Estimated % Reduction - extremely variable

Emissions Due to External Drafts

Reduction Techniques

1. Avoid using lip vents, or keep vents turned off. A degreaser without lip vents (or vents turned off) will have 15 percent less solvent emission than a degreaser with a vent. If lip vents are required to meet workplace health rules, use covers on the degreaser and shut the vent off when the cover is closed or, at a minimum, locate the vent above the cover. Also try to limit air volumes in the vent to no more than six cubic-feet-per-minute (cfm) per square foot of degreaser opening.
   Estimated % Reduction -

   Odors or high concentrations of solvent in the workplace generally come from liquid solvent dragged out on parts, splashed out by sprays, or leaked from external fittings. The lip vent will never capture these emissions. First, correct your procedures, and then use a lip vent only if additional protection for workers is needed. Or use a lip vent in conjunction with a scrubber to limit emissions from your building. Then, if possible, compensate for the higher vapor losses by adding freeboard to your degreaser.

2. Eliminate external drafts around the degreaser. The effect on solvent emissions from a degreaser in a room with still air compared to a degreaser exposed to external drafts from fans and ventilators (possibly the chiller fan), varies widely with velocity. Keep the air velocity above the degreaser below 30 fpm.
   Estimated % Reduction - variable

   The effect will depend primarily on the speed and direction of the draft. Use the lip vent estimate described above as a point of comparison. Try to reduce this effect by: redirecting air away from degreasers, placing baffles around the degreaser, or placing baffles between the degreaser and the draft source.

Emissions Due to Internal Drafts

Reduction Techniques

1. Slow the speed at which parts are moved. Parts moved through the vapor zone at speeds of 10 fpm vertically will have emissions that are 30 percent less than parts moved at 20 fpm.
   Estimated % Reduction - 30%

   Moving parts by hand is typically in the 30-100 fpm range. If parts with large cross-sections are moved between sumps using a complex cleaning cycle, horizontal speeds also may be important in avoiding drafts. Slower speeds are recommended for very heavy loads to avoid vapor collapse.

2. Keep large cross-sections of parts 50 percent smaller than the corresponding degreaser section. The effect of emissions of parts with very large cross-sections in the direction of movement through the vapor zone was not measured. As a rule keep the cross-sectional areas of parts less than 50 percent of the corresponding degreaser section.
   Estimated % Reduction - unknown

3. Sliding covers may reduce drafts and turbulence. The effect of using a sliding cover compared with using a hinged or lifting cover varies widely. Each time a hinged cover is opened, induced drafts and turbulence will carry out some vapor. The magnitude will depend primarily on how often the degreaser is uncovered and how fast air moves above the degreaser. If lip vents are used, place the sliding cover between the vent and the vapor. Consider using a rotating basket to drain complex parts.

Emissions Due to Diffusion

During periods when the degreaser is idling, diffusion losses tend to be the most significant. Although diffusion losses occur when the degreaser is cleaning parts, these losses are much smaller than losses due to dragout or drafts. Address diffusion losses when:

1. the degreaser is running idle most of the time;
   
2. when all other sources of loss have been fully examined; and
   
3. when reducing diffusion loss can be done quickly, cheaply and easily.

Reduction Techniques

1. Add freeboard height to degreasers. Degreasers with 75 percent freeboard will have about 30 percent less solvent emissions than degreasers with 45 percent freeboard when both are idling and open. Degreasers with 100 percent freeboard will have about 15 percent less solvent emissions when idling than ones with 75 percent freeboard.
   Estimated % Reduction - 15-30%

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2. Keep an idling degreaser covered. A covered, idling degreaser will have about 15 percent less emissions than an uncovered degreaser.
   Estimated % Reduction - 15%

   Drafts or inadequate freeboard or chiller capacity would greatly increase the value of covers.

3. Reduce vapor displacement. The effect on emissions caused by high volume parts displacing vapors above the cooling coils varies widely with the size of the parts load. If the vapor level changes significantly when parts are put in the degreaser, try slowing the hoist speeds or using the stop-and-go technique. A withdrawal speed of less than 10 feet-per-minute (fpm) may be required for large loads.
   Estimated % Reduction - variable

   Vapor displacement works on the same principle that causes the level of water in a tub to rise when a solid object is placed in that tub. Vapors above the coils are more likely to diffuse out as they warm up, and they are more likely to be carried out by drafts or by parts being withdrawn.

4. Lower the condenser temperature. Decreasing the condenser temperature from 50=F to 30=F can reduce the diffusion losses from CFC 113 by 40 percent. Decreasing condenser temperatures from 50=F to 0=F can reduce diffusion losses by 80 percent.
   Estimated % Reduction - 40-80%

Emissions Due to the Use of Sprays

The use of sprays may indicate that vapor degreasing is inadequate for your cleaning task--review your options. Sprays can augment vapor degreasing by physically loosening or removing tough soils. However, sprays can also reduce or eliminate the rinsing of parts with clean solvent, which is the fundamental advantage of the vapor degreaser. Cold sprays temporarily destroy the vapor blanket causing the unit to stop functioning as a vapor degreaser. Hot sprays warm the parts quickly, so they stop condensing vapor sooner. The choice is between longer and more complex cleaning cycles, or the possibility that inadequate or variable cleaning may occur.

Reduction Techniques

1. Minimize the use of sprays, and keep sprays at temperatures near the boiling point. Eliminating solvent sprays inside a degreaser will result in 30-50 percent less solvent emissions than if sprays are used. Minimize spray use and assure that spray temperatures are near the solvent boiling point--avoid cold sprays in a vapor degreaser.
   Estimated % Reduction - 30-50%

   Sprays increase emissions by inducing drafts inside the degreaser. The spray stream itself carries air or vapor along with it. Cool or cold spray will condense vapors, thereby collapsing the vapor blanket and sucking air below the cooling coils. At a minimum, this air will carry some vapor out with it as the vapor blanket is reestablished and pushes the air out. Careless spraying can result in liquid solvent being splattered out of the degreaser entirely.

2. Keep the spray nozzle below the cooling coils. Holding the spray nozzle below the cooling coils will result in 30 percent less emissions than if the nozzle is kept above the vapor blanket. Consider positioning the spray nozzle permanently below the coils and manipulate the part for cleaning.
   Estimated % Reduction - 30%

   Drafts that are above the cooling coils in the degreaser are more likely to carry vapors out of the unit entirely.

3. Use short spray bursts. Keeping spray bursts short will result in 15 percent less solvent emissions than spraying for intervals longer than 10 seconds.
   Estimated % Reduction - 15%

   Long bursts of spray induce larger drafts and are more likely to collapse the vapor blanket.

Reducing Solvent Emissions by Purchasing New Equipment or Making Major Modifications to Existing Equipment

Reduction techniques

1. Superheat the vapor. Raising the vapor temperature above its normal boiling point (superheating) will reduce emissions by 90 percent compared with having the vapor temperature at the solvent's boiling point.
   Estimated % Reduction - 90%

   The elevated temperature allows parts to dry in the vapor zone completely and quickly. In a standard degreaser design, parts remain wet with solvent as long as they remain in the vapor zone. Drying reduces or eliminates the need for a freeboard hold. The superheated vapor temperature would typically be 150 percent of the normal boiling point. Retrofitting an existing degreaser with superheated vapor may be possible in some cases.

2. Install freeboard cooling coils. Adding freeboard cooling coils (0=F) near the top of the degreaser will reduce idling losses by 15 percent.
   Estimated % Reduction - 15%

   Freeboard coils are a second set of cooling coils placed in the freeboard zone, and are usually refrigerated. Freeboard coils decrease diffusion by keeping vapors cold and heavy; heavy vapors are less susceptible to drafts. However, these coils add cost and dehumidify air in the freeboard zone. This additional water can overwhelm standard water separators, which can lead to solvent acidification, equipment damage, and high waste disposal costs. Make sure your water separator is adequate for the task, or better provide a second, large, water separator for the Freeboard coils.

3. Install secondary condenser coils. For HCFC's (modelled using CFC-11 [a refrigerant]), adding secondary coils (0=F) just above the primary condenser coil (45=F) reduces CFC-11 idling losses by 70 percent.
   Estimated % Reduction - 70%

   The same result could be achieved by lowering the primary condenser temperature to 0=F. However, the higher temperature allows for using inexpensive cooling water as the primary cooling medium. The chiller for the secondary condenser can then be down-sized.

4. Install a third coil for dehumidification. Adding a third, dehumidification or freeboard coil (0=F) near the degreaser lip reduces CFC-11 idling losses by an additional 80 percent.
   Estimated % Reduction - 80%

From this it can be inferred that for higher boiling halogenated solvents (e.g., CFC-113 or trichloroethane), the best coil configuration would be a dehumidification coil operating at the same temperature as the main condenser coil to eliminate internal convection currents.

The information in this fact sheet is based on tests run by Branson, Inc. of Danbury, Connecticut; and E.I. DuPont De Nemours and Company of Wilmington, Delaware, as well as tests compiled by the U.S. EPA. Branson directly measured solvent loss under various conditions intended to simulate typical industrial situations. Tests were run on both CFC-113 and 1,1,1-trichloroethane. DuPont tests to measure idling losses were run using CFC-11 as a surrogate for HCFC 123.

Manufacturers of vapor degreasers and solvent are a good source of advice on emission reduction. The manufacture of your equipment may be able to provide assistance in reducing the emissions from your specific unit.

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