Cleaning with Solvents

Loss of Vapor Containment in Large Vapor Degreasers

The image of Figure 1.43 suggests a potential problem – that the cooling effect produced at the sidewalls of a vapor degreaser may not extend far enough into the maw of the freeboard zone above the cooling coils. That suggestion has come to fruition in the idealized image of a large vapor degreaser shown in Figure 1.44 , and in the outcome of a loss of containment of solvent vapor. • For it is the cooling coils that maintain a zone of solvent vapor, which is the primary barrier that keeps nearly all the cleaning solvent inside the open-top degreaser. The barrier rejects passage of additional solvent vapor upward through it by condensing the vapor and returning the condensed solvent to the sumps as liquid droplets (light blue circles in Figures 1.44 and 1.45 ). What if the flow of coolant to the coils is deliberately stopped, unintentionally or intentionally blocked or limited, while the boilup heater maintains its supply of heat? Why then, the vapor zone (the vapor barrier or blanket) starts to disappear, because the cooling effect which supports it is diminished or depleted.
Figure 1.42 Open-Top Vapor Degreaser with Integral Superheat Coils Another way that can happen is to increase the size of the vapor degreaser relative to the capability of the cooling coils to remove heat. This is shown in Figure 1.44. Note in this figure that the vapor barrier (blanket) has collapsed in the center portion of the vapor degreaser. This is because there was not adequate cooling capacity supplied from adjacent to the degreaser walls to maintain it.
Figure 1.43 Cooling Overpowers Solvent Vapor
Figure 1.44 Solvent Vapor Overpowers Cooling
Figure 1.45 Closeup View of Figure 1.44 Showing How Inadequate Cooling Allows Hot Vapor to Penetrate Solvent Vapor Cloud Figure 1.45 is a cutaway section from Figure 1.44. In the former one can see that without the cooling capability supplied from adjacent to the walls, there is nothing to stop vaporized solvent from rising to the top of the freeboard zone, and almost certainly escaping into the work area. Note also in Figure 1.44 that where the vapor barrier has collapsed, there is no return of liquid droplets (light blue circles) to the degreaser sumps through the condensate trays59 . There are two causes for the inadequacy of the cooling effect: (1) reduction of coolant supply for whatever reason, or (2) overlong separation of the center area of the vapor degreaser from the cooling coils adjacent to the degreaser walls. The former cause is an operational affair; the latter cause is that the cooling unit is undersized for the combined width of the two sumps of liquid solvent. The latter cause is a real limitation on the implementation of vapor degreasing technology. Simply, one generally can’t clean in a vapor degreaser parts of some large width dimension – such as airplane wings. Transfer of heat through vapor by convection and radiation is inefficient and ineffective over distances measured in feet. The design width of the vapor degreaser is the key parameter, as shown in Figure 1.46 . It’s the shorter distance between opposite cooling coils. Cooling must be effectively implemented over a maximum of one half of that distance. Widths of degreasers seldom exceed 30 inches; a degreaser width of 48 inches would probably exceed that of most commercial units.
Figure 1.46 The white center area of Figure 1.46 represents inadequate cooling relative to the width of the degreaser; it represents an area through which solvent vapor can escape upward from the vapor degreaser. The light blue peripheral area of Figure 1.46 represents adequate cooling relative to the width of the degreaser; it represents the area where the vapor barrier is established. One can test the integrity of the vapor barrier by measuring the temperature in the center of the degreaser at the level of the freeboard areaBB (above the vapor barrier). It should be significantly lower than the normal boiling point of the solvent – with the understanding that it will not be as low as the coolant temperature. Should it approach the former high value, the vapor barrier has been destroyed; should it approach the latter low value, the vapor barrier is sufficiently intact. • A metric recommended by the US EPACC is that the measured temperature in the centerline of the degreaser at the level of the freeboard zone (above the cooling coils) should not exceed 30% of the solvent normal boiling point. Values of the maximum allowable (per EPA NESHAP) temperature at the centerline of the degreaser (Figure 1.45) are given in the second column from the right (column head in pink) in Table 1.11 . The cooling coils at the wall must keep the vapor temperature in the center of the degreaser below this value (26°C for trichloroethylene). • This goal will be more achievable the more narrow is the degreaser and the colder is the cooling coil. So the width of the hypothetical degreaser can be no more than whatever width causes the vapor temperature at the centerline to rise from the freezing point of the coolant to the maximum allowable temperature (?21°C and 26°C respectively for trichloroethylene – a 47°C rise). With no exceptions in Table 1.11, the width of the degreaser can be expanded for every solvent until the temperature in the center of the vapor barrier has increased at the degreaser wall by about 45°C (column head in yellow) from the freezing point of the coolant. It is impossible to know what this width is in units of length because the character of the parts and their basket are not known. • The point of this calculation is that a large degreaser used with one degreasing solvent can probably be used with another solvent and another coolant with the same loading of parts without violating the EPA guidance about centerline temperature. Obviously, the two solvents may impose other concerns. If a coolant with a lower freezing point had been chosen, such as calcium chloride (FP = ? 52°C), the width of the degreaser could be increased without violation of the EPA guideline about emission control so as to accommodate larger baskets of parts. Obviously, that would substantially increase the energy costs for providing chilled coolant.

Table 1.11

Limitation on Temperature at Center of Vapor Barrier (Blanket)

Trichloroethylene Sodium Chloride ?21 26 ? (?21) = 47 26.0 = 86.7 x 30% 86.7 Methylene Chloride Propylene Glycol ?28.9 11.9 ? (?28.9) = 40.8 11.9 39.8 Perchloroethylene Sodium Chloride ?21 36.3 ? (?21) = 57.3 36.3 121.1 n-Propyl Bromide Sodium Chloride ?21 21.3 ? (?21) = 42.3 21.3 71.0 Acetone Sodium Chloride ?21 16.9 ? (?21) = 37.9 16.9 56.2 Methyl Acetate Sodium Chloride ?21 17 ? (?21) = 38 17.0 56.9 t-Butyl Acetate Sodium Chloride ?21 29.4 ? (?21) = 50.4 29.4 98.0 Dimethyl Carbonate Sodium Chloride ?21 27 ? (?21) = 48 27.0 90.0 Methyl Formate Ethylene Glycol ?40 (9.6 ? (?40) = 49.6 9.6 32.0 Parachlorobenzo-trifluoride CTW 0.0 41.8 ? (0) = 41.8 41.8 139.2 One can see from this discussion, and the associated images of Figures 1.44 to 1.46, and Table 1.11, that low temperature refrigeration is critical to successful vapor degreasing in open-top machines. Operating cost spent on electricity to provide refrigeration is well-justified (usually). Low temperature refrigeration: keeps the solvent in the tank to save cost, protect employees, and the environment; enables rapid drying of parts; and more than occasionally...