Vapor Degreasing Equipment

All vapor degreaser designs provide for an inventory of solvent, a heating system to boil the solvent, and a condenser system to prevent loss of solvent vapors and control the upper level of the vapor zone within the equipment. Heating the degreaser is usually accomplished by steam. However, electrical resistance (< 3.0 W/cm2 or < 20 W/in.2) heaters, gas combustion tubes, and hot water can be used. Gas combustion heaters with open flames located below the vapor degreaser are not recommended and are prohibited by OSHA regulations. Specialized degreasers are designed to use a heat pump principle for both heating and vapor condensation. In this instance, the compressed gases from the heat pump are used for heating the vapor degreasing solvent, and the expanded refrigeration gases are used for vapor condensation. Such a degreaser offers mobility that permits movement without having to be connected to water, steam, or gas for operation.

Normal vapor control is achieved with plant water circulation through the condensing coils. Refrigeration-cooled water or direct expansion of the refrigeration gases in the condenser coils are effective means of vapor control. Where a sufficient cool water supply is not available, or where plant water is excessively warm, a low boiling vapor degreasing solvent, such as methylene chloride or fluorocarbon 113, is chosen. Refrigerated cooling coils above the normal condenser coils (also called a cold trap) can reduce solvent losses.

For safety, economy, and in some cases, to comply with regulations, degreasers are usually equipped with a number of auxiliary devices:

• Water separator: a chamber designed to separate and remove water contamination from the degreaser.

Solvent and water condensate collected by the condenser coils are carried by the condensate collection trough and exterior plumbing to the water separator. The water separator is designed to hold 5 to 6 min of solvent and water condensate flow. This provides for nonturbulent flow and flotation of the insoluble water. This water is discharged from the equipment while the solvent condensate is returned to the degreasing equipment.

• Vapor safety thermostat: located just above the condensing coils, detects the heat of solvent vapors if they rise above the designed level in the equipment. This could occur with inadequately cool condensing water or condenser water flow interruption. The purpose of this device is to prevent massive solvent vapor escape into the plant atmosphere. When solvent vapors are detected, the heat input to the degreaser is turned off automatically. Manual resetting is preferred and used, because this demands attention and alerts the operator to a malfunction.

• Boiling sump thermostat: In the cleaning operation, high boiling oils and greases are removed and collect in the boiling chamber. These contaminants elevate the boiling temperature of the solvent and could cause solvent decomposition if left to accumulate without control. The boiling sump thermostat is located in the boiling chamber solvent and, like the vapor safety thermostat, turns off the heat to the degreaser if it senses temperatures higher than those appropriate for the solvent being used.

• Condenser water thermostats and/or flow switches: The water flow switch will not allow heat to be turned on unless condensing water is flowing into degreaser coils, and it will turn off the heat source if flow stops during operation. The condenser water thermostat shuts off the heat source if condensing water leaving the degreaser is too warm, indicating that the water flow through the condenser system is inadequate or that the water temperature is insufficiently cool to control the solvent vapors in the degreaser.

• Solvent spray thermostat: a temperature-sensing device, located just below the vapor-air interface in the degreaser and designed to prevent manual or automatic spraying if the vapor zone is not at or above the thermostat level. This device has been required by some regulations. Spraying above the vapor zone can exaggerate solvent losses by causing air and solvent vapor mixing.

• Liquid level control: This control shuts the heat off if the liquid level in the boiling chamber drops to within 50 mm (2 in.) of heaters. This control protects the heaters and reduces the possibility of thermal breakdown of solvent.

Modifications in this basic vapor degreaser are designed to permit various cleaning cycles, including spraying of the workpieces or immersion of the workpieces in boiling or cool solvent. Further, vapor degreaser designs are available to provide various conveyor and transport means through the cleaning cycles. Common conveyor systems include the monorail vapor degreaser, the crossrod vapor degreaser, the vibratory conveyorized degreaser, and the elevator degreaser. Open-top degreasers constitute over 80% of the vapor degreasers used in industry. Their sizes range from benchtop models with perhaps 0.2 m2 (2 ft2) of open-top area to tanks over 30 m (100 ft) long. The most common sizes range between 1.2 to 2.4 m (4 to 8 ft) long and 0.6 to 1.2 m (2 to 4 ft) wide. The most frequently used cleaning cycle is vapor-solvent spray-vapor. Among the conveyorized vapor degreasers, the monorail is the most prevalent. Generally, open-top degreasers are much lower in cost, permit greater flexibility in cleaning different workloads, occupy much less floor space, and are adaptable to both maintenance and production cleaning. Because of their relatively low cost and minimum space requirements, they are preferred for intermittent operations and for decentralized cleaning where transport of parts to be cleaned to a centralized location adds substantially to the cleaning cost.

Emerging technology combines vacuum autoclave with solvent cleaning. This system cleans in a sealed chamber, using either solvent spray or immersion to clean the parts. The solvent can be perchloroethylene, trichloroethylene, or HFC. After the parts are placed in the chamber to be cleaned, it is dried by evacuating the chamber to 29 mm/Hg. The vacuum reduces the boiling temperature of the residual solvent, flashing it off. The solvent vapors from the chamber are condensed (Fig. 4).

Air diffuser

Air diffuser

Vapor Degreaser Equipment

chill tank Condenser tank

Fig. 4 Vacuum cleaning system. Courtesy of Baron-Blakeslee Company chill tank Condenser tank

Fig. 4 Vacuum cleaning system. Courtesy of Baron-Blakeslee Company

Installation of degreasing equipment should be supervised by a qualified individual. Some important considerations relating to installation are:

• A degreaser should never be installed in a location that is subjected to drafts from ventilators, unit heaters, fans, doors, or windows. When units cannot be ideally located, such drafts should be reduced by the installation of baffles.

• No degreaser should be installed near open flames unless the combustion products of these flames are exhausted outside the building. Location near welding or other operations using high temperatures must be avoided, because exposure of solvent vapors to high temperatures and high-intensity ultraviolet light results in decomposition to toxic and corrosive substances such as phosgene and hydrogen chloride.

• The flue from the combustion chamber of a gas-fired unit should conform with local laws or ordinances. All exhausts should be discharged outside the building at an adequate distance from air intakes.

• Water outlets from condenser jackets or coils should not be connected directly to sewer lines, but instead should drain freely into a funnel or other open-to-view collecting device that is connected to sewer lines. This prevents back pressure and ensures maximum efficiency of the condensing coils. As water and sewage treatment costs continue to escalate, recirculating condenser water systems such as water chillers and cooling towers are being used. Many degreasers using low-temperature boiling solvents incorporate direct refrigeration. Several manufacturers offer heat recovery of heat recycling systems for use with low-boiling-temperature solvents.

• All degreaser containers should have a legible, highly durable sign attached to them that bears solvent label information (see ASTM D 3698) and operating procedures, as required by most state environmental protection agencies.

Baskets and racks should be constructed of open-mesh, nonporous material. When baskets are completely filled with closely packed small items, basket size should not exceed more than 50% of the work area of the degreaser. For baskets handling large parts with generous open spaces, however, the 50% maximum may be exceeded slightly. Baskets that are too large may act as pistons as they enter the tank and displace the vapor level, thus forcing the vapor from the unit into the atmosphere.

The placement of work in the basket is critical, particularly when the parts have blind holes, which may entrap solvent. Precautions must be taken to ensure that entrapped air does not prevent liquid solvent or vapor from reaching all surfaces. After cleaning, the solvent must be completely drained from the parts to reduce dragout. To satisfy these requirements, specially designed racks or rotating baskets may be necessary.

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