A controlled flash evaporator operated by a vapour compressor

Abstract

The idea of operating a distillation process by a vapour compressor is well-known. The new successfully operating serially produced controlled flash evaporators, as installed e.g. in Aruba, offer the possibility of combining the advantages of this special distillation process with the advantages of vapour compression. In the first part, the paper gives a summary of the features of controlled flash evaporation based on R & D work which resulted, among others, in modules operating perfectly in sea water desalination plants of about 10,000 m 3/d capacity. Its main features are: quick start-up and short control periods; small temperature differences can also be obtained; practically no nonequilibrium allowance in boiling; etc. It allows the calculation of investment costs and operating expenses of modules with different temperature differences between heat input and output. This temeprature difference is the common feature between the distillation process and vapour compression. The relationship between this temperature difference and the cost factors is the first topic of this paper. In the second part of the paper, compressor investment and operating costs are indicated, again as a function of the temperature difference between heat input and output. An investigation will be made on how this relationship is affected by the variation in the temperature level of the process, and how it changes if different types of compressors are used. It will be shown which type of compressors (roots, centrifugal, axial, etc.) are useful in different applications. In the third part of the paper, conclusions of the first two chapters will be combined and a program for a TI 59 Texas Instrument calculator will be presented. The program takes into consideration as input, the number of stages in CFE and the TD between heat input and output, the boiling point elevation, the heat transfer coefficient in CFE, the quantity of brine, the compressor efficiency, the properties of vapour, etc. - all in all, about twenty significant determining features of the process. The output of the program provides about twenty important bits of information: The main equipment, sizes of the energy consumption of the compressor and of the pumps, the quantity of the distilled water produced, and so on. Finally, the paper concludes that the combination of CFE with vapour compression offers excellent possibilities if the right temperature levels, stage numbers, and compressors are selected and used.