Sheet temperature

The value of the coefficient of heat transfer from steam to sheet is determined by the conditions prevailiug on the inside and on the surface of the dryers. Low coefficients may be caused by (1) poor removal of air or other noncoudeusables from the steam in the cyhn-ders, (2) poor removal of condensate, (3) accumulation of oil or rust on the interior of the drums, and (4) accumulation of a fiber lint on the outer surface of the drums. In a test reported by Lewis et al. [Pulp Pap. Mag. Can., 22 (Februaiy 1927)] on a sulfite-paper diyer, in which the actual sheet temperatures were measured, a value of 187 W/(m °C) [33 Btu/(h ft" °F)j was obtained for the coefficient of heat flow between the steam and the paper sheet. 

Two causes seem available to explain the efficiency differences between GA and CEA flow sheets temperatures and couplings. 

The heat-transfer performance capacity of cylinder dryers is not easy to estimate without a knowledge of the sheet temperature, which, in turn, is difficult to predict. According to published data, steam temperature is the largest single factor affecting capacity. Overall evaporation rates based on the total surface area ofthe dryers cover a range of 3.4 to 23 kg water/(h-m ) [0.7 to 4.8 lb water/(h ft )]. 

The near-field analysis provides expressions for the wrinkled-flame motion and the reaction-sheet temperature in terms of the flame shape and the gas velocities at the edge of the wrinkled flame. To the first order in the small parameter bj, the equation for the flame motion may be written, in the nondimensional notation of Section 9.5.1, as... 

FIGURE 18.41a Multimode heating with an indirectly fired furnace. Shown is (a) predicted sheet temperature distributions. 

The drying rate in turn depends on the sheet temperature and hence the steam pressure in the drum, the sheet material, and to a lesser extent the thickness of the sheet. The thickness of the sheet depends on the relative speeds of rotation of the drums, the depth of the boiling pool at the nip, the nip width, and the rheological properties of the liquid. 

Moisture transport in the sheet is predominantly driven by the large temperature gradient and the subsequently large pressure gradient within the sheet (Figure 11.11). Excess surface water in the wet sheet flashes or boils off the sheet and the temperatures of the drum surface continue to fall whereas the product sheet temperature remains constant. The evaporation flux at this point is given by ... 

To obtain the moisture profile in the machine direction, Nissan et al. [90] have made several assumptions about the drying process, principally that the drying was a first-order process (linear falling rate period) and that the pressing felt in phase 2 of the drying cycle reduced the evaporation rate to one tenth that in the sheet s free traverse between cylinders under similar temperature driving forces. Over each of the periods when the sheet touches the cylinder, it is assumed that the sheet temperature is constant (or linearly varying about an arithmetic mean), both in the plane of the sheet and normal to it. These concepts lead to a relatively simple relationship for the sheet temperature after a time interval t ... 

Tq is the initial sheet temperature at the beginning of the considered phase Ts is the steam temperature U is the overall heat transfer coefficient Mg is the amount of dry solids per unit sheet area Cs is the specific heat of dry fiber Cl is the specific heat of moisture X is the dry basis moisture content... 

There are two main factors that affect the rate of mass transfer (1) the sheet temperature, which determines the vapor pressure of the evaporating water at the surface and (2) the partial pressure of water vapor in the air near the sheet, which is kept at a low value by replacing the air, i.e pocket ventilation. 

Shear forming is typically performed cold, but in case of high-strength materials and thick components, heating is applied to reduce the roller forces. In industry, manual heating is used widely sheet is heated by an oxyacetylene flame as it is formed. However, this improvised approach has its disadvantages sheet temperature cannot be controlled... 

Thereafter, a series of studies on heat transfer effects on viscoelastic fluid have been made by many authors under different physical situations including (Abel et al., 2002, Bhattacharya et al., 1998, Datti et al., 2004, Idrees Abel, 1996, Lawrence Rao, 1992, Prasad et al., 2000, 2002). (Khan Sanjayanand, 2005) have derived similarity solution of viscoelastic boundary layer flow and heat transfer over an exponential stretching surface. (Cortell, 2006) have studied flow and heat transfer of a viscoelastic fluid over stretching surface considering both constant sheet temperature and prescribed sheet temperature. (Abel et al., 2007) carried out a study of viscoelastic boundary layer flow and heat transfer over a stretching surface in the presence of non-uniform heat source and viscous dissipation considering prescribed surface temperature and prescribed surface heat flux. 

Two steel plates 0.15 m apart are used to heat large lacquered aluminum sheets. If the steel plates are respectively at 300°C and 25°C, what is the heat transferred and the lacquered sheet temperature Emissivities of the steel and lacquered sheets are 0.56 and 1.0. 

Polymer Organic Sheet Temperature [°C] Mold Temperature [ C] Thermoforming Pressure [bar]... 

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