Economic and Technical Tradeoffs Between Open and Closed Cycle Vapor Compression Evaporators

Abstract

Evaporators are used extensively in the chemical, pulp and paper, food and beverage, and related industries. Mechanical vapor recompression (MVR) evaporators are a type using an open heat pump cycle with steam as the working fluid. The technology is widely used and time-proven. Closed cycle vapor compression is an emerging technology. A secondary working fluid such as a halocarbon is used in a closed heat pump cycle to transfer heat from the steam condenser to the hottest surface. Advantages of closed cycle evaporation include flexibility in working fluid selection, potential savings in compressor first cost, and isolation of the compressor from potentially corrosive or troublesome gases and liquid droplets in the suction vapor. A major disadvantage of closed cycle evaporation is that the driving force developed by the compressor (temperature difference) must be shared between two heat transfer surfaces, vs. one in the open cycle design. This paper explores the advantages and disadvantages of open and closed cycle evaporators. Factors considered include working fluid physical properties and thermodynamics, compressor operating limits and cost, and surface area sizing and cost.