This technique, also known as dry fluxing is gaining popularity as an alternative fluxing practice and therefore will be described here in some detail. Dry fluxing is a technology whereby the flux is electrostatically charged and applied to a grounded work piece, in our case a heat exchanger or individual heat exchanger components. The electrostatic attraction causes a layer of flux to be deposited on the work piece. A typical flux application system consists of a powder feed system, the electrostatic spray gun, the gun control unit, the grounded work piece and finally the flux recovery system.
The advantages of such a system over conventional wet fluxing are evident. Since the flux is applied dry, there is no need to prepare flux slurries, hence no need to monitor flux slurry concentrations. There is also no wastewater generated therefore more environmentally friendly. The dehydration or dry-off section of the furnace may be eliminated since the heat exchangers enter the furnace already dry. However, one must keep in mind that this is a relatively new fluxing technique and there are some minor drawbacks. Flux adhesion is not as good compared to that of wet fluxing. The flux also tends to accumulate on the leading edges of the heat exchanger and because of the Faraday cage effect, may have some difficulties in coating into corners or more specifically, in tube to header joints.
Powder Feed Systems
Presently, there are two types of powder feed systems on the market. The first type begins with the flux being fluidized in a hopper. Dry compressed air is fed through a porous membrane in the bottom of the hopper. The air rising through the volume of flux makes it behave like a fluid since the powder is essentially diluted with air. A pick up tube attached to an air pump is extended in the fluidized flux. Powder flow is then regulated by controlling the air-flow to the pump which is then delivered through the feed system to the spray gun. This type of feed system works perfectly well for powder paints. However, the flux has very different physical characteristics than powder paints (particle size, morphology) and so is difficult to fluidize. This must be taken into consideration when the manufacturer designs a powder feed system that relies on fluidization.
The second type of powder feed system works on the principle of mechanical delivery or positive displacement. This means that the powder feed rate to the air pump is controlled by a screw or auger. The flux is contained in a main feed hopper and delivered mechanically at a controlled feed rate to the air pump. Powder flow is thus regulated by controlling the auger feed rate. This powder feed system does not rely on the flux being fluidized. Nonetheless, modifications over conventional mechanical powder feed systems are still necessary to overcome the differences between the flux characteristics and conventional powder paints.
Japanese heat exchanger manufacturers have used the technique of dry fluxing for many years now. Within the last few years, North American and European manufacturers have also installed electrostatic fluxing stations. Experience with this technique is being accumulated at a rapid rate, given that the equipment manufacturers and flux suppliers are taking an active role in improving the technology.