Maximising Filter Efficiency: Gasketing and Recombination of Flow

Transcription

1 Technical Note Ref: TN002/01 May 2001 Contents Introduction Experimental Results and discussion Conclusions Maximising Filter Efficiency: Gasketing and Recombination of Flow Introduction Ceramic Foam Filters (CFF) s, are widely used throughout the industry for both bulk metal transfer and in-mould filtration. They have pore sizes in the range microns and are characterized by a tortuous metal flow path. They operate predominantly as a cake filter and rely essentially on the bridging of inclusions at their surface. They also trap a number of inclusions in the body of the filter. The bridging mechanism means that their efficiency is very dependent on both the type and size of inclusions present in the metal. High oxide film loadings, for example, lead to a more stable inclusion bridge and a resultant higher measured efficiency. Filters are incorporated into the running systems of castings in two main ways. By using the KALPUR (Foseco) type combined filter, feeder and pouring cup or location directly into a mould. For optimum filter efficiency the sealing of the filter to prevent metal by-passing the filter element is very important. Recombination of the metal stream after leaving the filter is also critical, to prevent ingress of air and the consequent generation of new oxide films. In this study filter efficiency has been quantified using the Prefil technique. More information about Prefil may be found in the MetalHealth building at MetalGate. Experimental The experimental flow cell used is shown in Figure 1. The metal used was LM24 ingot, (A380) (EN AC TO BS EN 1706:1998). The filters were 20ppi CFFs, which were combined with a ceramic sleeve in the

2 form of a KALPUR. In some experiments the CFF was sealed to the ceramic sleeve by means of alumino-silicate wool, to reduce the possible leakage and ingress of air around the filter. Metal Flow Kalpur Constant Head Filter Elements Prefil Crucible Metal Flow (controlled exit hole size) Metal Collection Mould Time Load Cell Figure 1 Experimental set-up Re-combination of the flow after the filter was achieved by mounting the KALPUR into a crucible with a controlled hole in the bottom. Unrestrained flow experiments had no constricting hole after the filter. The flow cell was constructed to enable metal flow parameters to be controlled as follows: - Flow densities 4-80 g/cm 2 /s Flow times 0-60 s Total weight filtered 0-15 kg Metallostatic head mm Metal was melted in either a small induction coil or a 50gg electric bale out furnace. Control samples of the as-melted metal were evaluated using the Prefil technique to establish the base line metal quality. During flow experiments up to 15kg of metal was poured through the flow cell and collected in a weigh pan below. The pouring times were recorded. Prefil samples were taken from the metal stream at various times during a pour

3 to measure the inclusions passing through the filter elements and to determine the efficiency of the filter. A higher quantity of metal filtered in a given timeie steeper gradient on a weight versus time plot, indicates a cleaner metal. Flow densities were calculated from a knowledge of the flow times and the cross sectional areas of each element. Results and Discussion Effect of Sealing the Filter Figure 2 shows the difference between a loose fitted CFF in a sleeve and a filter sealed with a refractory wool gasket. The metal quality is degraded significantly by a loose fitting filter due to metal leakage around the filter causing aspiration of air at the outlet side of the filter. This is a common observation in filtration studies which is exacerbated if the flow does not properly recombine downstream of the filter element. In this series of tests there was no constraint to metal flow at the filter outlet and metal was essentially in free fall W eight ( gm) LM24 Control 20ppi CFF 20ppi sealed CFF Time (secs) Figure 2. Effect of sealing the filter on metal quality Flow Recombination (Constrained Flow) Figure 3 shows the beneficial effect of fully re-combining flow downstream of the filter element. Control experiments were run before each filter trial to allow for variation in the starting metal. The effect of the filter arrangement in each test needs to be judged against the relevant control sample. Highest filtration efficiencies are obtained with constrained and sealed CFF giving the steepest (cleanest) flow characteristic.

4 W eight ( gm) Time (secs) Control 1 CFF Unconstrained Unsealed CFF Unconstrained Sealed Control 2 CFF 15mm Exit Sealed Control 3 CFF 20mm Exit Sealed Figure 3. The effect of re-combined flow on metal quality An unconstrained unsealed CFF made the metal quality worse, and sealing the filter improved the metal quality. The amount of constraint on the flow depends on the size of the exit hole through which metal passes; a 15mm diameter hole gives more constraint and re-combination of flow than the 20mm diameter exit hole. A 20mm exit hole with a sealed filter produced a slight improvement in metal quality. A 15mm exit hole with a sealed filter gave a large improvement in metal quality. Conclusions m Sealing of a filter to its mounting improves filtration efficiency. m Constraining the flow to allow re-combination of the metal stream after the filter also improves filtration efficiency. m Repeatable and efficient filtration requires both efficient filter sealing and re-combination of the metal stream. For more information regarding anything in this report, please contact MetalGate via the Members Forum in MetalPlex. More information about Prefil and the Prefil Footprint may be obtained by visiting the MetalHealth Building.

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