Steam heated cylinders

Sheet Drying. At a water content of ca 1.2—1.9 parts of water per part of fiber, additional water removal by mechanical means is not feasible and evaporative drying must be employed. This is at best an efficient but cosdy process and often is the production botdeneck of papermaking. The dryer section most commonly consists of a series of steam-heated cylinders. Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely against the surface of the dryers by fabrics of carefuUy controUed permeabiHty to steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water evaporates. The water vapor is removed by way of elaborate air systems. Most dryer sections are covered with hoods for coUection and handling of the air, and heat recovery is practiced in cold climates. The final moisture content of the dry sheet usually is 4—10 wt %. 

Other types of dryers may be employed for special products or situations. For example, the Yankee dryer, a steam-heated cylinder, 3.7—6.1 m dia, dries the sheet from one side only. It is used extensively for tissues, particularly where creping is accompHshed as the sheet leaves the dryer, and to produce machine-gla2ed papers where intimate contact with the poHshed dryer surface produces a high gloss finish on the contact side. 

These cylinder units are applicable to almost any form of sheet material that is not injuriously affected by contact with steam-heated metal surfaces. They are used chiefly when the sheet possesses certain properties such as a tendency to shrink or lacks the mechanical strength necessary for most types of continuous-sheeting air diyers. Applications are to diy films of various sorts, paper pulp in sheet form, paper sheets, paperboard, textile piece goods and fibers, etc. In some cases, imparting a special finish to the surface of the sheet may be an objective. 

Indirect-Heat Rotary Steam-Tube Dryers Probably the most common type of indirec t-heat rotaiy diyer is the steam-tube diyer (Fig. 12-69). Steam-heated tubes running the full length of the cylinder are fastened symmetrically in one, two, or three concentric rows inside the cylinder and rotate with it. Tubes may be simple pipe with condensate draining by gravity into the discharge manifold or bayonet-type. Bayonet-type tubes are also employed when units are used... 

The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

Pascal [2] describes it as follows. Phenol, which is supplied in iron drums, is melted by placing the drums, alter removing their lids and bottoms, into steam heated cylinders (I) (Fig. 111). Molten hot (80°C) phenol is collected in the heated tank (2) which also acts as a proportioner. Adjacent to it, is a tank (3) for 92% sulphuric acid. 

When the operation started, the iron tank and the individual boxes were filled with saturated solution of sodium (or potassium) chloride while solid salt was put into the stoneware cylinders. The level of electrolyte in tho anode boxes was always somewhat higher than in the tank. During electrolysis the temperature was kept at 85 °C by steam heating steam entered the heating element of the tank through piping H. The voltage across the electrolyzer was 3.5 to 4.0 V. The total current load was about 2500 A which corresponded to a current density of some 2 A/sq. dm. 

All evidence indicated that the cylinder ruptured due to the hydrostatic pressure of expanding liquid ammonia due to excessive heat on the cylinder. Fragments of the cylinder were sent to a testing laboratory for failure analysis. The ruptured cylinder was a full cylinder just put in service. According to the company s literature search, a full cylinder would become hydrostatically full of liquid ammonia at about 130° F (54.4° C). If the liquid was further heated, tremendous pressures would be developed. An alternate possibility was that the cylinder was heated to 250° F (121° C) (the steam supply temperature). Raising the temperature that high is unlikely with the crude heating methods and the expected heat losses, but if they were, the internal cylinder pressure could reach... 

Cylindor dryors for continuous sheets such os poper, celiophone, textile piece goods. Cylinders ore generoily steam-heated, and rotate... 

There are three heat transfer mechanisms used to dry textiles. Conduction methods involve direct contact of the wet textile with heated surfaces. These are the most efficient heat transfer methods, but do not allow for control of fabric width during drying. Steam heated cylinders are examples of conduction drying methods... 

Towards the end of the wire the water content is below 85%. The material leaving the wire is called the felt . This felt is pressed through speed-synchronised cold rollers (the presses) to reduce the water content to 65-70%. This is followed by the drying section, which is a series of steam-heated cylinders over which the felt winds its way with alternate sides being dried. 

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