Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 12

such as welding, heat treatment, or machining, also benei t from vapor degreasing between steps to
remove all traces of processing oils. Welding oily metal parts forms carbonaceous deposits because
of pyrolysis of organic matter at high temperatures; such deposits are extremely difi cult to remove.
Vapor degreasing oily metal parts before welding greatly reduces the subsequent cleaning require-
ments. Electron-beam welding requires that parts be completely free of oily wastes and is therefore
preceded by high-quality vapor degreasing (Reuter, 2002). Vapor degreasing is also a i nal step
before packing to deliver clean parts and before shipping copper, aluminum, and plated chromium,
zinc, and silver products in a clean, bright, and shiny condition [American Society of Testing and
Materials (ASTM), 1962].

1.1.1.3.1 Types of Vapor Degreasers
Three types of vapor degreasers are used for solvent cleaning: batch vapor, in-line machines, and
non-air-interface machines. Most of the degreasing machines operating since about 1960 fall into
the batch vapor category. Conveyor systems such as monorail degreasers are in-line machines.
Non-air-interface machines—vacuum degreasers that use subatmospheric pressure in the cleaning
chamber—were introduced in the late 1990s to eliminate emissions and worker exposure. Energy
costs in vacuum degreasers are reduced because parts can be dried at temperatures lower than the
boiling point of the solvent. With a solvent such as perchloroethylene that has a high boiling point,
it is possible to clean heat-sensitive parts at lower temperatures and still get them completely dry
under vacuum, but the cycle time is longer (Dow Chemical Company, 1999b).

1.1.1.3.2 Vapor Degreaser Operations
Vapor degreasing systems typically consist of an open tank that is partially i lled with solvent. A
heater in the bottom of the tank heats the solvent to its boiling point, which generates vapors.
Solvent vapors i ll the tank to a level controlled by the elevation of the degreaser's cooling or con-
denser coils, positioned around the top of the tank's perimeter. These coils circulate cold water or a
refrigerant, creating a layer of cold air that coni nes vapors to the tank and minimizes vapor escape.
The condenser coils chill the air in the freeboard zone above the desired solvent vapor elevation.
National Emission Standards for Hazardous Air Pollutants (NESHAP) require that refrigeration
must create a cool air zone that is 30% or less of the solvent's boiling point. Table 1.7 lists the mini-
mum temperatures needed for the major chlorinated solvents (Thomas, 1995).
The work requiring cleaning enters the degreaser at room temperature. When the work enters the
heated vapor zone inside the tank, cool surfaces cause condensation of the solvent on the soiled
metal surface. The solvent then dissolves the oil, grease, l ux, or rosin and loosens metal chips and
other debris, which drip off into the bottom of the tank. Solvent vapor condensed by cooling coils is
returned to a clean solvent reservoir, which overl ows back into the boiling solvent compartment
(Petering and Aitchison, 1945).

TABLE 1.7
NESHAP Temperature Requirements for Freeboard Refrigeration Devices in Vapor Degreasers
Solvent Boiling Temperature [°C (°F)] Required Air Blanket Temperature [°C (°F)]

Dichloromethane 40 (104) 12 (31)

Methyl chloroform 74 (165) 22 (50)

Trichloroethylene 87 (189) 26 (57)

Perchloroethylene 121 (250) 36 (75)

Sources: Thomas, T., 1995, Clean Air Act compliance for solvent degreasers—regulatory strategies for manufacturers
affected by Clean Air Act amendments NESHAP for halogenated solvent cleaners. University of Tennessee Center
for Industrial Services, Tennessee Department of Environment and Conservation.