Mathematical modelling of heat exchange in flash tank heat exchanger cascades

Abstract

Flash tank evaporation combined with a condensing heat exchanger can be used when heat exchange is required between two streams, where at least one of these streams is difficult to handle (tends severely to scale, foul, causing blockages). To increase the efficiency of heat exchange, a cascade of these units in series can be used. Heat transfer relationships in such a cascade are very complex due to their interconnectivity; thus the impact of any changes proposed is difficult to predict. Moreover, the distribution of loads and driving forces at different stages and the number of designed stages faces tradeoffs which require fundamental understanding and balances. This paper addresses these problems, offering a mathematical model of a single unit composed of a flash tank evaporator combined with a condensing heat exchanger. This model is then extended for a chain of such units. The purpose of this model is an accurate study of the factors influencing efficiency of the system (maximizing heat recovery) and evaluation of the impact of an alteration of the system, thus allowing for guided design of new or redesign of existing systems.