Evaporator temperature profile

T. Cam, K. CuUen, and S. L. Baldus, Running Eoss Temperature Profile, SAE 930078, Society of Automotive Engineers, Warrendale, Pa., 1993 H. M. Haskew, W. R. Cadman, and T. E. Liberty, Evaporative Emissions UnderReal-Time Conditions, SAE 891121, Society of Automotive Engineers, Warrendale, Pa., 1989. 

In a water cooling tower, the temperature profiles depend on whether the air is cooler or hotter than the surface of the water. Near the top, hot water makes contact with the exit air which is at a tower temperature, and sensible heat is therefore transferred both from the water to the interface and from the interface to the air. The air in contact with the water is saturated at the interface temperature and humidity therefore falls from the interface to the air. Evaporation followed by mass transfer of water vapour therefore takes place and latent heat is carried away from the interface in the vapour. The sensible heal removed from the water is then equal to the sum of the latent and sensible heats transferred to the air. Temperature and humidity gradients are then as shown in Figure 13.18 . 

The temperature distribution along the micro-channel axis is not monotonic. It has a maximum that is located within the liquid domain. An extraordinary form of the temperature profile is a result of the influence of two opposite factors, namely, absorbs heat from the wall and heat transfer from liquid to the front in order to establish the evaporation process. An increase of heat flux on the wall leads to displacement of the point corresponding to maximum temperature towards the inlet cross-section. 

The approach is readily extended to problems involving multiple profiles. For example, in a batch crystallization process, the temperature profile and evaporation profile... 

Let us examine methanol. Its flashpoint temperature is 12 to 16 °C (285-289 K) or, say, 15 °C. If this is in an open cup, then the concentration near the surface is Xl = 6.7 %. Performed under normal room temperatures of, say, 25 °C, the temperature profile would be as in Figure 6.2. This must be the case because heat must be added from the air to cause this evaporated fuel vapor at the surface. This decrease in temperature of an evaporating surface below its environment is sometimes referred to as evaporative cooling. If the convective heat transfer coefficient, typical of natural convection, is,... 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

The composition and temperature profiles in the RDC are shown in Figure 7. The ester product with traces of methanol is the bottom product, whereas a mixture of water and fatty acid is the top product. This mixture is then separated in the additional distillation column and the acid is refluxed back to the RDC. The ester is further purified in a small evaporator and methanol is recycled back to the RDC. 

Returning to the Nissan and Hansen model, they use a finite difference numerical analysis model to determine both the temperature profile of a sheet material and the subsequent water removal as it passes over the cylinder. Their experimental results match well with the predicted values. However, their experiments were limited to a cylinder surface temperature of 93.3°C. Accordingly, the maximum vapor pressure of the evaporated water is less than one atmosphere. The diffusion model advanced by Hartley and Richards is in close agreement with experimental work of Dreshfield(12l. However, the boundary conditions are still relatively uncertain since the convective flow region outside the sheet is relatively unknown. Later work by this author studies this convective flow. 

FIGURE 14J3 Schematic diagram of evaporation in a porous network (a) geometry of pore and boundary layer, (b) liquid partial pressure proMe, and (c) temperature profile. Taken from Castro et ol. [5]. Reprinted by pennissian of the American Ceramic Socieiy. 

Infrared measurements of extent of cure under conditions similar to the TICA experiments were conducted. ATS was cast from methylene chloride solution onto KBr windows and, after vacuum evaporation of all solvent, the KBr windows were put into the Rheometrics RMS environmental chamber and were subjected to a temperature profile under nitrogen identical to the mechanical measurement experiments (2 C/min). The windows were removed one at a time at various temperatures and IR spectra were taken at room temperature. 

The close correspondence of the measured wall temperature profiles and exit-gas compositions to those of Bernstein and Churchill (3) for the combustion of premixed propane vapor and air suggests that combustion in a refractory tube is relatively insensitive to the composition and state of the fuel as long as evaporation precedes combustion. 

In this process, porous particles are easily formed. The porosity is controlled by changing the precursor concentration in the droplets or by adjusting temperature profile in the furnace. Hollow particles can also be prepared when the solute concentration gradient is created during evaporation of solvent. Easy scaling up is a major advantage of this method, too. 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

In the case of 44 % moisture there is liquid water present in the structure and also the permeability of liquid influences the conversion time of the sample. Figure 6 shows the measured temperature profiles of Figure 1 and the two simulated Cases 6 and 7 in Table 1, simulated for liquid axial permeabilities of 10 and 10 respectively. In both cases the radial permeability is assumed to be 10 lower. It is seen from Figure 6 that the intrinsic permeability of liquid has a large influence on the pyrolysis time. A higher permeability leads to a larger transport of water through the wood and less water evaporates inside the sample, which reduces the time of conversion. 

Performing a series of droplet tracking calculations using the results from the CFD simulation, a representative droplet drying experience can be developed from the example shown in Figure 15. The early droplet temperature profile with time is shown in Figure 16 in which the initial decline, warm-up and constant rate equilibrium are evident. The falling rate period was not modeled hence the particle temperature is seen to rise to the local gas field temperature once evaporation has ceased. Note the time scale for this series of events is less then 6 milliseconds. 

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