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Evaporation-A Wastewater Treatment Alternative Page 2 of 5 <br /> The use of each additional effect increases the system's energy efficiency. For example, a <br /> double-effect evaporator requires approximately 50 percent of the steam consumed by a <br /> single effect unit, and has a theoretical economy of 2. The number of effects can be <br /> increased to the point where the capital cost of the next effect exceeds the savings in <br /> energy costs. <br /> The use of vapor compression is another proven technique for reducing energy input. In <br /> this approach (Fig 3), vapor discharged from the evaporator chamber is compressed to <br /> the pressure/temperature values required in the heat exchanger. <br /> Mechanical compressors are used most frequently for accomplishing vapor compression. <br /> Compressors may be of the positive displacement, centrifugal, or axial type. An <br /> evaporator system using mechanical vapor compression often will require only an outside <br /> steam source to initiate operation. This usually can be supplied by a small boiler or <br /> resistance heater in the evaporator feed tank. A steam jet thermal compressor using high <br /> pressure steam also may be considered. The use of a thermal compressor is <br /> approximately equivalent to adding an additional evaporator effect. <br /> When available, waste heat from other process streams also may be captured to lower <br /> evaporation costs. For example, hot process fluids may be pumped through the heating <br /> tubes instead of steam, recovering heat and transferring it to the fluid to be evaporated, or <br /> energy from hot flue gases can be converted to steam in a reboiler and subsequently used <br /> in an evaporator. <br /> Types of Evaporators <br /> Evaporators can be categorized according to the arrangement of their heat transfer <br /> surface and the method used to impart energy (heat) to the solution. Some common types <br /> of evaporators include <br /> Vertical tube falling film: Recirculating liquid is introduced at the top of a vertical tube <br /> bundle and falls in a thin film down the inside of the tubes. The liquid absorbs heat from <br /> steam condensing on the outside of the tubes and the water in the liquid is vaporized. This <br /> type of evaporator usually is selected for higher viscosity liquids and for concentrating <br /> heat-sensitive solutions that require low residence times (Fig 4). <br /> Horizontal tube spray film: Recirculating liquor is heated and sprayed over the outside of a <br /> horizontal tube bundle carrying low pressure steam, condensing water vapor inside the <br /> tube. Vapor from the evaporator chamber can be used as steam in a subsequent effect, or <br /> mechanically compressed and reused as the heating medium in the stage where it was <br /> generated (Fig 5). <br /> Scale forming on the outside of the tubes can be removed periodically through chemical <br /> cleaning. Horizontal tube designs can be applied in locations with low headroom <br /> requirements, and are especially beneficial in indoor installations. <br /> Forced circulation: Recirculating liquor is pumped through a heat exchanger under <br /> pressure to prevent boiling and subsequent scale formation in the tubes. The liquor then <br /> enters a separator chamber operating at a slightly lower pressure or partial vacuum, <br /> causing flash evaporation of water, and formation of insoluble crystals in the liquor (Fig 6). <br /> http://www.estormwater.com/print/4458 11/28/2012 <br />