Fortunately a new evaporation technology using low cost, flexible, corrosion resistant polymeric materials as heat transfer surface has been developed by Windsor. Proportionally large heat transfer areas in combination with MVR-operation leads to exceptionally low energy use. In addition, the use of flexible heat transfer surfaces gives a new opportunity in the elimination of hard evaporator scale. Recycling of dilute industrial effluents by evaporation will now prove to be a profitable solution.
The unique innovation of the technology is the polymeric thin film heat transfer surface design. This is the "heart" of the system. The plastic heat transfer elements will be used instead of metallic heat exchangers.
Because of low heat conductivity for polymeric materials a thin wall thickness has to be used. Windsor's patented construction methods used in the manufacture of the high quality heat transfer elements make their use ideal for most applications. The flexibility of these materials greatly reduces the tendency of concentrated waste streams to scale or foul the surface.
The basic material of construction for Windsor's heat transfer surface is a high-tech polymer with a thickness of about 0.04mm. At this thickness, the heat transfer coefficients approach those achieved with metallic surfaces at a thickness of approximately 1.2 mm. To illustrate the relative thickness of the Windsor material, refer to Figure 2 below:
Fig. 2. Heat Transfer Coefficient of Stainless Steel and Polyethylene Polymer
Fig. 3 illustrates the minimal pressure difference required to be added by the fan at a temperature differential of 2,3 oC
Fig 3. Pressure Differential for a Temperature Differential of 2,3 oC at Various Effluent Boiling Temperatures
The amount of mechanical energy required to be added to the vapour by the fan depends on the design temperature difference across the heat transfer surface (i.e. the pressure ratio of the compressor). Since the Windsor system operates on a relatively small pressure differential, the operational cost of evaporation is the lowest in the industry. In full scale units, 1 kW of electrical energy typically generates evaporation corresponding to heat energy of 70 kW to 100 kW. Alternatively stated, this same 1 kW of electrical energy generates an evaporation quantity of 100 kg to 130 kg.
The low cost of the polymeric material in combination with efficient
production and assembly methods makes it possible to use of much larger
surface areas for evaporation. The greater the surface area, the lower
the temperature differential necessary to achieve the desired evaporation
capacity. This design feature allows the use of a low speed fan as
a vapour compressor, eliminating the need for costly, high maintenance
mechanical compressors. Operating costs are substantially lower than
conventional technology as illustrated in Figure 4 below.
Fig 4. Relative Power Usage Per Ton of Evaporation
The Windsor system operates most efficiently in waste streams where the total dissolved solids (TDS) content is relatively low. The low design temperature differential means that the maximum boiling point elevation (BPE) of the concentrated liquid must also remain low. In most applications, the maximum practical TDS of the incoming stream is 5%. Figure 5 illustrates the amount of clean condensate removed from the waste effluent stream at various TDS.
Fig 5. Flow is reduced as Evaporation increases Solid Content
Boiling Point Elevation (BPE) is defined as the difference between the boiling point of the concentrated effluent and pure water. Typically, the Windsor system is well suited for applications where the BPE of the final concentrate is 1,5 oC or less. Figure 6 illustrates typical BPE's of various liquids. If further concentration is needed, this has to be done by conventional evaporators and crystallisers at considerably higher costs. But, in many cases, the major part of the water has already been recovered at a low cost. The total cost of operation is still reasonable thanks to the new technology.
Fig 6. BPE (deg C) of Various Liquids
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