Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 13

To adsorption device
or atmosphere

Drying
tunnel

Part travel

Hood
Monorail

Degreased
part

Spray
nozzles

Condensing
coil

Free
board

Vapor level

Water jacket

Liquid
level

Condensate
trough

Solvent spray
pump

Boiling sump Solvent spray

reservoir Heat source

FIGURE 1.2 Coni guration of typical offset vapor degreaser. (Adapted from University of Tennessee Center
for Industrial Services, 1995.)

The cleaning action of a vapor degreasing system is dependent on the temperature differential
between the vapor and the work. The efi ciency of the process decreases as the work becomes
warmer. Some degreaser operations use cool liquid solvent to presoak the work before bringing it
into contact with solvent vapor; thereby extending the cleaning time, and promoting good solvent
penetration from vapor condensation on the work (Petering and Aitchison, 1945). Some degreas-
ing operations use multiple dips, where the work is moved from a dirty tank to subsequently
cleaner tanks, followed by a i nal cleaning in clean solvent vapor (Dow Chemical Company,
1999b). The higher boiling point of perchloroethylene prolongs the condensation effect in a con-
tinuous degreaser because it takes more time to bring the cold work to perchloroethylene's higher
vapor temperature. Compared to the solvents that have lower boiling points, perchloroethylene is
therefore considered to have better cleaning power per unit of time that the work spends in a con-
tinuous vapor degreaser (Johnson, 1973).
Some degreaser systems are equipped with a wand that can be used to spray warm solvent onto
the work to complete the cleaning process. The physical action of the spray pressure knocks off
the strongly bound soils (AFCEE, 1999; Pro-ACT, 1999; Dow Chemical Company, 1999b).
However, high-pressure sprays can disrupt the vapor blanket, leading to solvent vapor losses,
unless care is taken to spray the work beneath the vapor blanket or inside an enclosed compart-
ment of a vapor degreaser. A diagram portraying the features of a typical vapor degreaser is
shown in Figure 1.2.
Solvent vapors usually remain below the level of the condensing coils; nevertheless, open-top
degreasers routinely incurred substantial solvent losses. Solvent vapors diffuse from the dense
solvent vapor blanket into the overhead air mass at a rate dependent on solvent vapor density and
boiling point and the temperature differential with the ambient air. Tests have shown that vapor loss