Transient behavior of a vertical plate evaporator for ammonia

The paper analyses the transient behavior of a vertical plate evaporator for ammonia, this piece of equipment belonging to a refrigeration machine or heat pump. The evaporator consists of vertical wavy metal plates, tighten together by welding points. The liquid ammonia is fed at the upper side of the plates and flows film-wise downwards under the force of gravity. The secondary heating fluid is a thermal oil and flows upwards (in counter-current to the refrigerant) at full section. The mathematical model of this heat exchanger takes into account the fact that dry boiling may occur, so that the equations for normal elements, for the last wetted element and for the dry elements have particular forms. For the last wetted element, the heat passing from the heating fluid to the refrigerant film is not computed by means of the thermal resistance between the oil and the upper surface of the film, but by means of the total vaporized liquid quantity coming from the previous element. The vapor mass flow rate leaving this element is equal to the liquid mass flow rate that leaves the previous element. A special procedure for computing the total wetted length of the evaporator is developed. The program built for simulating the transient response of the evaporator allows to find out the values of the outlet parameters according to the values of the inlet parameters and their type of time variation.
A few graphical representations for some outlet parameters at different time and space steps for stepwise time variations of some inlet parameters are also provided. The start of the vaporizing process in analysed too and the results are plotted. Comments on the physical meaning of the results of the simulation are provided for each analyzed case.