Modelling, validation and control of an evaporative cooling system for scraped surface heat exchangers

Abstract

Scraped surface heat exchangers (SSHE) are key unit operations in the production of food products. These unit operations are used for both heating and cooling/freezing the food product. In this contribution we will focus on the cooling/freezing side of the scraped surface unit operation. Boiling ammonia is very often used as the refrigeration medium due to its high evaporation enthalpy and specific heat compared to other media. The amount of cooling/freezing applied by the refrigerant is determined by the level of liquid ammonia in the jacket and the temperature of the ammonia. A constant amount of cooling is important for the quality of the product being cooled/frozen, whereas too much cooling could result in a freeze-up and eventually mechanical damage to the system. In order to evaluate and optimise the performance of the evaporative refrigerant system, a dynamic model of this system will be designed. One complicating factor is to obtain a suitable description of the liquid-vapour equilibrium. After validating, the model can be used to evaluate various design and operational options for the refrigerant system.