By Win Sabatka
Finishing Equipment, Inc.
St. Paul, MN

Solvent vapor degreasing is a parts cleaning process that has been used in many applications for some years. Virtually every communications or electronics system and aircraft flying today has components that have been vapor degreased. Also, surgical tools and medical implants often are cleaned using solvent vapor degreasing. In fact, solvent vapor degreasing is almost limitless in its possible uses. There are five reasons for its widespread use:

1.The halogenated solvents used are non-flammable and will not burn or explode.
2.Solvent soluble oils, greases and other materials are removed effectively and economically.
3.Solvent vapor degreasing requires a minimum amount of labor, energy and floor space.
4.The self-distilling nature of the equipment allows cleaning to be done with pure, distilled solvent.
5.The cleaning solvents can be recovered from the soils and recycled.

Solvents
Solvents are effective cleaners because of their ability to dissolve grease and oil. Therefore, they remove contaminants more easily than other cleaning methods.

Two solvents widely used in the past for vapor degreasing were 1,1,1-trichloroethane and CFC 113. According to the Montreal Protocol, these solvents have been phased out due to their reported damaging of upper atmosphere ozone levels.

In the SNAP (Significant New Alternatives Policy) program, EPA lists acceptable alternatives to 1,1,1-trichloroethane and CFC 113. The most common alternatives used today are trichloroethylene, methylene chloride and perchloroethylene. These solvents, however, are listed as hazardous air pollutants under the 1992 Clean Air Act.

Much effort has been devoted to find and develop new solvents, and a few are available for vapor degreaser use (see Table I). This table shows whether the solvent is a VOC implicated in smog and related air pollution or if the solvent is an ozone depletor. Solvent power is an important consideration when choosing a solvent as well. While Table I is based on published data and believed correct, the latest data should be obtained when considering solvents.

Choosing the appropriate solvent is very complex and depends on a number of factors. It is best to consult with solvent and equipment manufacturers before making a decision. State and federal regulations will have a significant effect too.

The Process
Vapor degreasing is a relatively simple process. A heat source raises the liquid solvent to its boiling point. When the solvent boils, it produces hot, heavy vapors that rise to an established vapor line. At this point, the vapors are condensed on cold circumferential condenser coils, and the vapors rise no higher. Because the solvent vapors are heavier than air, they push the air above the vapor line. Parts at ambient temperature are then introduced into the solvent vapor, and the solvent vapor condenses on the part’s surface. The liquid solvent produced as a result of this condensation dissolves the greases and oils on the part and flushes them away. As the parts are cleaned, more vapors are produced in the boiling sump to replace those that were condensed.

To clean parts only by pure vapor condensation from the vapor zone, enough vapors must condense into liquid on the parts to remove the contaminants. This makes the solvent’s boiling point an important consideration. As the solvent’s boiling point increases, more solvent will condense on a part (see Table II).

Vapor Degreasing Equipment
A diagram of a basic vapor degreaser can be seen in Figure 1 (see previous section to read how the degreaser works).

Sometimes it is necessary to enhance the cleaning capability of the degreaser. In these cases, a spray feature is added to the degreaser. The addition of a condensate spray system allows flushing and cleaning of many complex-shaped parts that vapor condensation alone could not clean. Spraying must be done with care, below the vapor line and never deflected into the freeboard air. Heat input into this degreaser must be adequate to produce condensate for the production load and spray volume required.

Spray degreasers are similar to standard vapor degreasers except for the spraying system, which consists of solvent pump, filter canister and spray wand and/or a fixed spray header. The system also has a condensate tank where clean distillate is collected and stored for spray use.

Effectively sprayed or where an accumulation of parts in a basket obstructs the spray from impinging all the parts, liquid immersion de-greasing is desirable (see Figure 2). In an immersion degreaser, parts are lowered into the boil sump, or sump No.1, for 15-60 sec then transferred beneath the vapor line to the condensate sump, or sump No. 2, for another 15-60 sec. The load is then held in the vapor zone until the solvent has completely drained off the part.

The cleaning quality of immersion degreasers depends on a number of factors, including the manner in which the load is handled. The most important factor though is the cleanliness of sump No. 2, which directly affects the cleanliness of the finished work. Sump No. 2 is kept clean by the internal distillation of the degreaser. Solvent boils from sump No. 1, condenses on the condensing coil, travels through the water separator and into sump No. 2. Sump No. 2 then overflows to sump No. 1.

If no work is processed through it, sump No. 2 becomes full of pure condensed solvent. However, the soil being removed in sump No. 1 contaminated that sump. Some of this contaminated solvent is carried into sump No. 2.

One way to improve the cleanliness of an immersion degreaser is to add a third sump (see Figure 3). The distilled clean solvent circulation is counter current to the workflow. Results are similar to the great advantage counter current rinsing provides in an electroplating process.

For example, 1 gal of oil is deposited into the degreaser per hour. At the same time, we have heaters providing enough vapor generation from sump No. 1 to put 50 gal of condensate into sump No. 3. This amount of flow also occurs from sump No. 3 to sump No. 2 to No. 1. If we introduce 1 gal of oil into sump No. 1 along with the 50 gal of solvent overflow, sump No. 1 will have an oil concentration at equllibrium of about 2%. If we assume that the workload carries over about the same volume, or 1 gal of 2% oil and solvent into sump No. 2, sump No. 3 becomes contaminated to a level of about 0.04% oil (400 ppm). By the same reasoning, sump No. 3’s oil level content is reduced to approximately 8 ppm. This is virtually the equal of new solvent as to cleanliness and better than DI water.

If we add a still of 50 gph capacity to take dirty solvent from sump No. 1 and return clean solvent to sump No. 3 we would have

Clean pure solvent to sump No. 3 = 100 gph
Oil to sump No. 1 = 1 gph
Oil concentration in sump No. 1 = 1%
Oil concentration in sump No. 2 = 0.001%
Oil concentration in sump No. 3 = 0.00001%

While this purity will not be totally realized in such an operation, it is proven in production machines that extremely high cleanliness is easily and consistently achievable.

Vacuum degreasers have been in use for about 35 years. With these systems, the workload is placed in a vacuum chamber. The chamber is evacuated and then solvent is introduced to clean the parts. The solvent is then removed and the chamber evacuated again, with solvent vapors recovered by refrigeration. Final solvent residues are generally recovered by carbon adsorption. Air is then reintroduced into the chamber so it can be opened and the parts removed. The smaller the diameter and volume of the vacuum chamber, the more practical these systems become.

The advantage to vacuum degreasers is that they have very low solvent emissions. However, there are many factors against their use, especially their high cost, low production of one to two loads per hour and complex operation.

Parts can be cleaned using a sealed chamber degreaser, which contains air. Solvent is introduced as suited to clean the parts, and excessive solvent is drained off. The air in the chamber is circulated through a carbon adsorption system or refrigeration system until the solvent level is low enough for the chamber to be opened and the work removed.

For vacuum and sealed-chamber systems, perchloroethylene has been the preferred solvent because it is most effectively recovered with carbon adsorption or refrigeration.

In the past, open top degreasers were commonly exposed to undesirable air movement, resulting in excessive solvent losses. Therefore, automatic handling of the loads through the degreaser is very desirable. This permits total enclosure of the degreasers and insures a proper cleaning cycle.

Such degreasers eliminate solvent losses caused by air movement. Also, human errors in work handling are eliminated in an automatic system. In cleaning applications where production is significant, the enclosed automated system can provide sufficient solvent and labor savings to economically justify the cost. In addition, it improves worker safety by removing the operator or other personnel from the operation.

Part Handling

Good cleaning and efficient solvent use requires proper work handling:

1. Parts must be handled so they fill and drain readily.
2. Sufficient time in sumps must be provided to remove soils and to heat the load.
3. Parts transfers must always be below the vapor line.
4. It is desirable to minimize the amount of extraneous material that goes along with the work through the vapor solvent sumps. Chains, conveyors, baskets and racks all increase solvent loss.

Solvent Losses
Vapor degreasing can be a highly efficient method of cleaning, using a fraction of the energy of other processes with no water pollution. However, one must use the right equipment to minimize the amount of solvent loss from the cleaning process. There are four main causes of solvent loss: dragout; air movement; diffusion; and convection. Dragout and air movement represent 80-90% of solvent losses while diffusion and convection make up the balance.

Dragout. Dragout of solvents on parts has been a major cause of solvent loss in most degreaser operations in the past. All parts from an ordinary degreaser carry out liquid solvent. This could not be prevented in the past, but new technology can eliminate it.

Additional heat is required to provide the heat of vaporization to remove liquid solvent residue from the part. That heat is not available from a solvent vapor at a boiling point that is the same temperature as the part and remaining liquid solvent. By super heating the vapor, the heat necessary to vaporize all liquid on the part is now available. The part will come out perfectly dry with no liquid solvent present. This applies to conveyorized and in-line degreasers as well as spray or dip systems.

Air Movement. It has been long recognized that drafts around and above degreasers have a significant effect on increasing solvent losses. Open windows, doors, air intakes, fans and heaters are typical sources of air movement and corresponding solvent losses. What was not realized was how small an amount of air movement can impact the amount of solvent loss.

Solvent losses are extremely low from air movement of 0-30 fpm across the degreaser while losses increase rapidly at higher rates of air movement. The EPA selected 50 fpm, a breeze of 0.57 mph, as a low rate of air movement. This is virtually a dead calm and is not detectable to a person. Therefore, if air movement is detectable, it is too much. Simple instruments are available to measure air movement.

Diffusion. Diffusion is the process by which two gasses tend to mix with each other by the motion of their molecules. It is the movement of solvent molecules through air without air movement. Diffusion results in a mixing that occurs regardless of the relative densities of the gasses. Diffusion is greater with low boiling solvents. Diffusion is a minor cause of solvent loss from a vapor degreaser.

Convection. Convection is air movement caused by heated air rising in the freeboard area because of its contact with hot parts, hot vapor or equipment surfaces. Convection losses are higher with high boiling solvents.

Tests shows that convection losses are typically larger than diffusion losses and are more difficult to control. However, the use of vapor line covers is most effective. Such covers close just above the primary condensing coil, effectively reducing diffusion and convection losses. Using fixed sprays mounted and operated below a closed vapor line cover can eliminate the use of an operator and a manual spray lance. This helps provide maximum efficiency.

Proper consideration and evaluation of all the factors presented here can result in a highly efficient and economical vapor degreaser. Also, because of the low energy use of vapor degreasing when compared to other cleaning processes, it may become the choice to reduce global warming.