Film-release heat transfer process

Since the temperature of the emitter is generally known (preselected or readily determined in an actual operation), the absorptivity value Cr is the unknown. This absorptivity is partly a measure of the ability of radiant heat to penetrate the body of a solid particle (or a moisture film) instantly as compared with diffusional heat transfer by conduction. Such instant penetration greatly reduces processing time and case-hardening effects. Moisture release and other mass transfer, however, still progress by diffusional means. 

The second stage features the moisture sorption of fibers, which is relatively slow and takes a few minutes to a few hours to complete. In this period, water sorption into the fibers takes place as the water vapor diffuses into the fabric, which increases the relative humidity at the surfaces of fibers. After liquid water diffuses into the fabric, the surfaces of the fibers are saturated due to the film of water on them, which again will enhance the sorption process. During these two transient stages, heat transfer is coupled with the four different forms of liquid transfer due to the heat released or absorbed during sorption/desorption and evaporation/condensation. Sorption/ desorption and evaporation/condensation, in turn, are affected by the efficiency of the heat transfer. For instance, sorption and evaporation in thick cotton fabric take a longer time to reach steady states than in thin cotton fabrics. 

The macropore diffusion of nc adsorbates is described by the Maxwell-Stefan equation as learnt in Chapter 8 (Section 8.8). The micropore diffusion in crystal is activated and is described by eq. (10.6-11), and the adsorption process at the micropore mouth is assumed to be very fast compared to diffusion so that local equilibrium is established at the mouth. Adsorption and desorption of adsorbates are associated with heat release which in turn causes a rise or drop in temperature of the pellet. We shall assume that the thermal conductivity of the pellet is large such that the pellet temperature is uniform and all the heat transfer resistance is located at the thin film surrounding the pellet. How large the pellet temperature will change during the course of adsorption depends on the interplay between the rate of adsorption, the heat of adsorption and the rate of heat dissipation to the surrounding. But the rate of adsorption at any given time depends on the temperature. Thus the mass and heat balances are coupled and therefore their balance equations must be solved simultaneously for the proper description of concentration and temperature evolution. 

Many events occur in the MTZ during adsorption which render the analysis complex. First, one or more adsorbates transfer from the fluid bulk by convection or diffusion across the fluid film which is external to the solid surface. Secondly, these adsorbates penetrate the particle by Maxwell, Knudsen and surface diffusion mechanisms (see Chapter 4), and adsorb onto the internal surface where the heat of adsorption is released. Heat may then be transferred to the adsorbent, to the flowing process fluid, and, via the vessel wall, to the surrounding environment. Heat and mass transfer may occur in the MTZ by bulk and diffusive flows in both the radial and axial directions. An additional complexity is that the flow through a packed bed may not be uniform across its entire cross-sectional area. This may be because of channelling of fluid at the wall or because of temperature gradients created when the heat of adsorption is released. 

Note that although the aqueous-based formulations are well suited to decoration of ceramic ware by brushing and direct screen printing applications (onto flat ware such as tiles or glass), the decoration of ware by decal transfers is more complex for these systems because they use water to release the transfer from the decal paper. It is reported that cross-linking aqueous polymer systems can be utilized to produce water-resistant films suitable for water-slide decal use, but no spedlic examples of any such formulations are recorded [26]. Alternatively, the aqueous gold formulations can be printed onto heat release transfer paper and used to decorate ware using an automated application process. 

Sublimation (diffusion) printing is a textile process in which color patterns in dry die crystals are transferred from a release film to the fabric under high heat and pressure. The process has been adapted to plastics. The equipment used is very similar to that used for hot stamping. Under heat and pressure, the dye crystals sublime (go directly to the vapor phase from the solid phase without melting) and the vapor penetrates the plastic product. As a result, the decoration is very durable and wear resistant. It is also cost competitive against other processes such as two-step injection molding or silk screening. 

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