Spray Cleaning

Spray cleaning provides the advantages of power impingement, continuous flushing, and no redeposition of contaminants. The mechanical action of the spray tends to cut into soils to help break them away. The continuous flushing exposes the next dirt layer, allowing the emulsion to work through even heavy buildup quickly. The spray solution can be filtered in series with the wash pump prior to recirculating over parts. The rest of the contaminants are contained in the solution tank below the wash cabinet. Thus, it is possible to get an acceptably clean part in a single-stage spray machine, whereas this is unlikely in a single-stage immersion system.

The main drawback to spraying an emulsion is the increased exposure to VOC. Spraying releases more of the solvent to air and requires significantly more ventilation than an immersion application. In addition, like immersion, spray emulsion leaves an oil-like residue. If this is unacceptable, then subsequent alkaline wash and rinse stages are required, as discussed in the section "Immersion Cleaning" in this article.

Spraying can be done via two methods:

• Manual spray/flush over large parts in a vented tank at low pressure (only enough to deliver the emulsion to the work, approximately 35 kPa, or 5 psi). With the operator at the point of contact, there is still potential exposure, depending on the particular emulsion and the temperature. Fewer vapors are emitted at lower temperatures.

• By machine, either in-line or cabinet, usually at 100 to 700 kPa (15 to 100 psi). These pressures atomize the emulsion, which increases the flash potential, particularly at normal operating temperatures of 60 to 70 °C (140 to 160 °F). Explosion-proof cabinets and electrical controls should be critically analyzed before spray emulsion is attempted.

For very large parts, an emulsion can be flushed on manually at high concentrations, then rinsed off with a power spray. Processing Variables

Temperature. Some cleaning is accomplished at ambient temperature, but spray emulsion is more efficient and effective at elevated temperatures. Temperatures should be kept 15 to 20 °C (25 to 40 °F) below the flash point.

Agitation. The mechanical power of spray significantly reduces cleaning time and increases the ability to flush out cavities. Spray pressure should be kept to the minimum required. If pressures greater than 515 kPa (75 psi) are required to accomplish cleaning, a different process may be more appropriate.

Concentration. For spray emulsion, emulsion cleaners are typically used in concentrations of 1 to 5% (Table 3) because the spray adds power and exposure speed. These concentrations, 85% less than those used for immersion, make spray emulsion a very economical process. Also, a low volume of emulsion (diluted in 95 to 99% water) does reduce the risk of VOC exposure and flash, and it results in less residue on the parts. This is a second reason that spray emulsion can sometimes be used without secondary cleaning.

Time. Power spray in-line or cabinet equipment reduces cleaning time up to 75% compared to the time required for immersion, even at lower concentrations (Table 4).

0 0

Post a comment