Casting tube

First results are, however, rather positive. It was possible to coat silica layers with a selectivity, which was somewhat above the Knudsen selectivity. This shows the presence of at least some degree of microporosity with, fortunately, still a relatively high N2 permeance compared to that of the centrifugal cast tubes described in chapter 4. Coating with just 1 silica layer from an undiluted silica coating solution showed to give best results. Coating with a second layer, did not improve selectivity, it did only reduce the N2 permeance of the membrane with... 

Large effort has been put in the preparation of centrifugal cast tubes that are superior in homogeneity, roundness, strength and smoothness to any commercially available product (chapter 4). 

Possibly perform lab-scale stability tests on tubular membranes (centrifugal cast tubes). Test on (changes in) ... 

Figure 6-24. Removal of acrylamide gels from their casting tubes using a 2 in., 26 gage hypodermic needle.

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

Further support for the Chilton-Colbum analogy. particularly for Sh oc Sc1 1. Is fonnd from the data on dissolution of cast tubes of various soluble solids. although Davies suggests that dissolution data for high Sc are better correlated by... 

In practice, tubes,with inside diameter from 65 mm (Stone and Webster) to 120 mm [Lummus, Selas) and 7 to 5 mm thick are employed for naphtha steam cracking Tbe gases flow within these tubes at a linear velocity of about 300 m/s. The pressure drop recorded between the furnace inlet and outlet may reach 0.4 to 0.7.10s Pa in these conditions. The tubes are fabricated by extrusion or centrifugal casting Cast tubes are made of medium alloy steels (25 to 30 per cent chromium and 20 to 30 per cent nickel) containing 0.5 per cent carbon. If they are positioned vertically, their sagging strength allows for metal skin temperatures of 1000 to 1100 C. 

Alternatively, in the double-bubble process, a cast tube is blown into a large bubble with simultaneous stretching in both directions to produce a balanced film. This process is used to make heat-shrink films as well as standard BOPP films. 

Seal one end of the Bio-Rad casting tube with several layers of Parafilm and insert the second glass tube. 

Materials for catalyst tubes are selected in combination with the process conditions employed. Alloys with high chromium and nickel content are used for the reactor tubes in a steam-reforming furnace. The first centrifugally cast tubes such as HK 40 contained 25% Chromium and 25% Nickel. Today, tube material containing 25% Chromium and 35% nickel, niobium, and traces of zirconium and titanium are used (so called HP alloys) (50). The HP alloys are more expensive but allow a higher tube design temperature and have a better creep strength and oxidation as well as carburization resistance. 

Heaters will soften the tube and the tube is inflated with air. The initial filling of the tube with air requires good timing by the operator. As air is pumped into the expanding tube, the operator pulls the tube away faster than the top nip supplies cast tube. Once the inflated bubble reaches the bottom of the stretch tower, a second nip closes. This second nip seals the air in the tube. The second nip runs at a speed greater than the first and provides the machine direction orientation. The amount of air pumped into the tube before the second nip closes is one of the primary factors in the transverse direction stretch ratio. Other process variables that contribute to the transverse stretch ratio are the web temperature and machine direction stretch ratio. Pressure in the tube may be increased by narrowing the frame used to collapse the bubble. 

Centrifugal Casting. Tubes and other rotationally synmetric shapes are cast on the inside of moulds spun at high speed to produce large forces which increase consolidation rates. A dense cast results. No central core is needed to cast hollow articles. The moulds may be of plastics or metal and need not be porous. Cone-shaped glassware such as television tubes is made in this way. 

A considerable effort is often made in the tubing industry to produce on the interior wall of the tube an initial scale that has optimal properties. Centrifugally cast tubing is often bored to remove porosity near the inner surface, honed to provide a smooth surface profile, and then steamed heavily to produce a thin, continuous, dense, resistant oxide layer on the interior with a remarkably increased life of tubing sections for particular services. 

More recently, however, a hi er strength 25% chromium and 35% nickel-niobium (HP modified) cast tube is being used. The newer tube material is stronger (as evidenced by greatly improved stress-to-rupture properties) and can result in thinner tubes containing less net metal for the same design tube life. 

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