Quenching baths

emission capture and reduction essentially involves the capture of fiime at the quenching baths, especially at the oil quenching baths. Roof ventilation, extraction domes and edge extractors are used. Here similar difficulties arise as with shake-out grids. The baths must be frequently loaded by a crane, so the extraction hoods need to be installed high above the hall floor. 

Abatement systems are not widely applied at this pointin general, but if any abatement is performed, electrostatic filters are mostly used. 

The applicable techniques for different types of baths are characterised in Table 4.55. 

Edge extractors are only effective up to a certain size and lose their effectiveness almost completely at the time of the highest emissions, i. e. when a hot workpiece is submerged in the bath. In spite of this, edge extraction is the most sensible capture measure, utilising big baths combined with a blast veil and a roof dome. 

The technique is used in several foundries around Europe. 

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The simplest form of melt extmsion is the use of a slot die to form the molten polymer into a thin flat profile which is then quenched immediately to a solid state (Fig. 1). This is usually done by contacting the hot web very quickly on a chilled roU or dmm. A liquid quenching bath may be used ia place of or contiguous to the chill roU. Depending on the polymer type or formulation, the quenched web is generally substantially amorphous. In some cases, the web may be drawn down ia thickness by overdriving the quenching roU relative to the extmsion velocity. 

Fig. 6. Schematic of dry-jet wet spinning employing tube-in-orifice spinneret A, bore injection medium (liquid, gas, or suspended soHds) B, pump C, spinneret D, polymer spinning solution E, micrometer ( -lm) "dope" filter F, coagulation or cooling bath G, quench bath and H, collection spool.

Other important operating parameters include plasticizer changes, gas type and pressure, gap length between spinneret and quench bath, line speed, and rope tension. All variables must be carefully controlled to obtain a hoUow fiber of desired characteristics. 

The copolymers are used in the manufacture of filaments. These may be extruded from steam-heated extruders with a screw compression ratio of 5 1 and a length/diameter of 10 1. The filaments are extruded downwards (about 40 at a time) into a quench bath and then round drawing rollers which cause a three- to four-fold extension of the filaments and an increase in strength from about 10000 to 36 000 Ibf/in (70-250 MPa). The filaments are used for deck chair fabrics, car... 

Enclosure takes the concept of isolation still further. In this approach, the complete operation is physically enclosed by an impervious barrier, perhaps metal or plastic, with access allowed only occasionally by suitably protected maintenance workers or in emergencies. This approach might be found, for example, in sand-blasting operations, in conditions where there is a severe fume hazard, or perhaps, as seen in the cautionary tale about Jake Whitman, in a continuous lead-quenching bath operation. It should be noted that in most instances, exhaust ventilation of the enclosed area should also be undertaken. 

Hot Surfaces. The incomplete immersion of hot metal in quenching baths, the contact of flammable vapors and hot combustion chambers, hot dryers, ovens, boilers, ducts and steam lines all are frequent causes of flammable vapor fires. Care should be taken that material whose auto-ignition point is lower than the temperature sometimes reached by operating equipment be kept at a safe distance from such equipment. 

Figures 7.18(b) and 7.18(c) show the breakup into droplets of an extended filament of high density polyethylene in a polystyrene matrix. In Fig. 7.18(b) the distance between the extruder die and the quenching bath is short and the fiber freezes before breaking up, whereas in Fig. 7.18(c) the distance was increased, giving the filaments sufficient time for breakup. As the filament extends, its diameter is reduced until shear forces no longer dominate the surface tension cohesive forces and the filaments breaks into droplets, just like a stream of water from a faucet breaks up into droplets.

It is advisable to use Dl water in the gel tank. The author has successfully cast many large batches of UF membranes using tap water in the quench bath. However, this often results in membrane product which is discolored (iron oxide) and variable with the seasons as the tap water quality changes. Dl water with an 18Mfi resistivity is the best standard. However, it must be borne in mind that high resistivity water will result in faster leach rates of solvent from the gelling casting solution which may affect the pore size of the final membrane. 

Water in the gel bath must be replaced periodically or better yet, continuously to minimize the buildup of solvents or other contaminants in the quench bath. If bacteria slime or turbid water is noted, the tank should be drained and cleaned. 

Quenching bath sludge from oil baths from metal heat treating operations (R T)... 

Generally, quenching is a batch process. For this reason emission levels alternate substantially. Emissions from the quenching baths are regarded mostly to be irrelevant for the environment, although they may become more relevant with large scale production. Data about source strength (emission rates) and chemical compounds have not been found. 

Table 4.SS Applicability of fume collection techniques for quenching baths [32, CAEF, 1997]...

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