Evaporator variable heat transfer surface

Mg. 6.2. Evaporator with variable heat transfer surface. 

Liquid level in the vapor body is an important variable affecting operation of natural circulation calandrias. Normally units are operated with the evaporator liquid level at the top tubesheet of the calandria. For non-fouling fluids, the liquid level can be lowered to the optimum value in order to minimize heat transfer surface or maximize performance. The optimum value is approximately half the distance between the top and bottom tubesheets of the calandria, and will vary with each system. The liquid level should not be appreciably above the top tubesheet and certainly should not be maintained above the caiandria outlet nozzle. Liquid levels above the vapor return will limit the performance of the calandria and may result in damage to the evaporator. Flow instabilities may also be experienced. 

Capital costs for vapor compression systems are usually higher than for multiple-effect systems, because they are usually designed for a lower temperature difference across the evaporator. As a result, greater heat transfer surface is required. In addition, the compressor and drive are relatively expensive. Equipment costs are affected by variables such as feed properties, materials of construction, available energy supply, and calandria type. Expected maintenance and necessary spare parts must be also evaluated. Heat exchangers must be provided to exchange heat between feed and condensate and between feed and product. Other costs that must be considered, but are less easily defined, are the penalty for interrupted operation during repairs and the part load response. 

The evaporator with variable heat exchanging surface has the same behavioral model as shown in Fig, 4.1. The energy balance, however, has to be adapted to accommodate the changing heat transfer area ... 

From the particle formation mechanism it is seen that the major cooling effect of the droplet is in the fast temperature drop and the CO2 evaporation due to expansion. At low melt temperature, 60°C, 70 bar, a surface crust of the particle is formed because the melt temperature is near the solid point. Because of this fact no spherical particles are formed. At high temperature, 70-80°C, and 70 bar, formation of spherical particle are observed because the droplet has to remove more heat to reach the solid point, which gives more time for the formation of spherical particle. At 80 C and 140 bar agglomeration is obtained because of the high pressure and heat transfer, which leads to high droplet velocities. The flexibility of the droplet surface is variable with changes in the CO2 concentration and the melt temperature. 

The sample to be analyzed, say C60 fullerene, is mixed with an appropriate amount of KBr in an agate mortar and then transferred into a press and compressed at 4,000 Kg into a pellet with a diameter of 1.2 cm and a thickness of 0.2 cm. The pellet was mounted into the sample holder of the Specac variable temperature cell and inserted into the cell. The cell was then evacuated with the aid of a pump to a vacuum of 0.1 torr and then heated gradually at 120°C in order to permit the humidity absorbed on the internal surfaces of the cell and in the KBr pellet to evaporate. The sample was then cooled to the desired temperature to record the infrared spectrum. In order to go below room temperature, use was made of liquid nitrogen, added cautiously and in small amount in the cavity present inside the cell. Such cavity is connected with the sample holder and permits to cool the sample to the desired temperature. The temperature of the sample was monitored with adequate thermocouples. The lowest temperature reached with this apparatus was -180°C (93K) while the highest temperature was +250°C. Heating is provided by the Joule effect and an external thermal control unit. 

It consisted of a single-pass, five-plate, water-cooled condenser capable of independent flow and temperature control. Heat was supplied to the evaporator reservoir, so that it could be operated at selected temperature levels. The rotating assembly, consisting of a shaft with spacers capable of accepting six disks 12 inches in diameter, of varying thickness and composition, was equipped with a variable speed drive. With the arrangement shown, approximately 2 sq. feet of disk-condenser surface area was available for mass transfer. Tests were performed on this unit at atmospheric pressure, using city water (125 p.p.m. of dissolved solids) as feed, to determine the effect on distillation rate of (1) reservoir tem-... 

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