Thermal Cleaning Process of Metal Parts

Epcon Industrial Systems, recently designed and built an evaporative deoiling system for cleaning aluminum fins. Thermal deoiling is a state-of-the-art degreasing system, apt to replace solvent-based cleaning technologies wherever appropriate. Thermal deoiling is pollution-free, energy efficient and cost-effective. Epcon has taken a lead in bringing this technology to the forefront.

Small aluminum parts of various geometry and weight are used in building the evaporator of an automobile. During the stamping process oil is used as a lubricant, and the usual practice is to use a solvent such as tetrachloroethylene to deoil the parts, so that there is no interference with braze joint quality performed in a Vacuum or Controlled Atmosphere furnace. The evaporator parts pass through the oven on an I-beam conveyor monorail located inside the oven. The oily evaporator fins nestled tightly together are placed into stainless steel rod and wire mesh baskets, which are placed into a basket holder attached to the monorail. The oily evaporator refrigerant plates with cups and beads are nestled together and placed on stainless steel flat rods, which are placed on rod carriers attached to the monorail.

Even the state-of-the art vapor degreasing systems that are based on the use of a solvent have inherent problems associated with its use. Apart from the fact that EPA has currently regulated the emissions of these solvents, the capital and maintenance costs associated with the washing line, as well as disposal costs are considerable. In the pipe-coating industry it is common practice to burn-off the old coatings. Transfer of this idea in the field of emission control essentially was the inception of thermal deoiling technology. Here the oil in the parts is evaporated in an oven by heating the parts at the appropriate temperature. In the current system the emitted oil is sent to an oxidizer where it is destroyed by thermal oxidation; the heat generated in turn supplies the heat required in the oven. This in a nutshell was the basis behind designing and building the system.

System Design

In the thermal deoiling system clean hot air is used to vaporize (and subsequently burn) all residual process oil present in the evaporator parts so that there is no interference with braze joint quality performed in a Vacuum or Controlled Atmosphere Furnace.This innovative new system has a decided advantage against other degreasing systems. Since no cleaning agent is required, and the evaporated oil is destroyed in an oxidizer, there is no emission of hazardous substances in the atmosphere. At the same time the exhaust is recovered to feature indirect heating and a unique air recirculation system designed to produce uniform heating and part movement, and to prevent oil re-condensation. The oven is completely enclosed so that smoke (oily air) does not leak out of the system into the open atmosphere.

Design Constraints

The design of this system was particularly challenging because of the associated constraints.  On one hand there was a lot of research and development work where the actual operating parameters were not known beforehand. On the other hand, the equipment being a production piece of equipment switching from one technology to another, there was very limited scope for taking any chances or modifying the process. Following is a run-down of the variety of constraints that had to be worked with.

The thermal deoiling system had to be incorporated within the existing operation as essentially a one-to-one direct replacement of the wash line. This implied not only staying within the space limitations, but that the system has to fit in using the conveyor system, holding baskets and carriers, and patterns of loading already in place and on top of it maintain the production rate. The parts were so close to each other and sticking together that it was not possible to get the air through in between parts which is essential to achieve efficient deoiling. This resulted in design of a special air circulation system to shake the parts in order to achieve sufficient separation between the parts. Moreover, since the process is an integral component of the mass production system, very little opportunity was available for on-line testing.  In addition, neither the operating temperature nor the amount of air flow, nor the residence time necessary to achieve efficient deoiling was known with certainty. Time available was also limited because of an EPA deadline.

Oven

Central to an oven performance is the design of the air circulation system. The air temperature inside the oven is maintained by recirculating air flow.  Hot air exhausted must be replaced with fresh air. The incoming fresh air (preheated by the oxidizer exhaust) provides the heat necessary for the parts to attain the desired temperature and the heat required to offset the loss through the insulated walls and exhaust.

Apart from the traditional function of maintaining the temperature uniformity, the air flow system in this case was designed to ensure that the parts are adequately separated from one-another.  This involved blowing relatively high-pressure air through nozzles in a large pipe from the bottom as well as a delivery system of holes in pipes closely resembling the carrier geometry. This innovative air distribution assured that the parts vibrate enough so that the air comes in contact with each part.  At the same time the pressure was low enough to prevent any part from lifting off the carrier. The air being distributed close to the carrier was helpful in ensuring that all portions of stacked parts come in contact with the heated air.

In addition, care was taken to ensure that after flash-off no smoke bypasses the oxidizer and escapes in the atmosphere. This was achieved through an air-curtain and careful design and placement of the return plenum. In order to provide sufficient residence time for the parts inside the oven, while honoring the space restrictions the oven had to be designed with a U section. Moreover, the parts needed to be cooled for handling upon exit. A separate cooling section was dedicated for this purpose.

As discussed before, the thermal deoiling system being described was in essence a pilot system; however, unlike traditional pilot systems, the finished equipment had to go in the production line.  Hence there was no opportunity for major design modifications based on the initial test results and the system had to be built with a lot of flexibility.  For example, the recirculation as well as the “high pressure” blowers and the supply blower, were all equipped with variable frequency drives to allow control over air-flows. A dump-stack was provided to dump the excess heat from the oxidizer in case the oven temperature needs to remain at a lower value.

Oxidizer

In the deoiling system the oxidizer serves as the pollution control device as well as the heat source.  Time, temperature and turbulence (known as three T’s) are the three necessary ingredients for successful oxidation. The design ensured that of all three are sufficiently present. Destruction Efficiency in excess of 95% was achieved by using a residence time of 1 second at 1400ºF.  The exhaust coming out of the oxidizer was first used to heat the supply air (primary heat exchange) and the remaining heat was used to preheat the oven exhaust (secondary heat exchange) entering the oxidizer. The ductwork between the oven and the oxidizer was insulated to prevent any additional heat loss.

Conclusion

The parts coming out of the thermal deoiler were clean and the switch-over from the vapor degreasing to thermal degreasing went quite smoothly. Extensive use of thermal deoiling in automotive and other industries will result not only in cleaner environment but also millions of dollars in savings in capital, operating and maintenance costs.