Multi-objective optimization of low charge liquid overfeed ammonia evaporators for industrial refrigeration

Charge minimization in ammonia refrigeration systems is pertinent due to safety restrictions associated to these systems. The objective of the present research is to minimize the ammonia charge and the required heat transfer area of evaporators in large-scale ammonia refrigeration systems by numerical optimization. The studied evaporator is a traditional flooded inline finned tube evaporator operated with pumped liquid recirculation rated to 22 kW cooling capacity. The optimization involved the interaction between heat-exchanger design parameters such as transverse tube pitch, longitudinal tube pitch, tube diameter and number of tube circuits, and operation parameters such as ammonia circulation ratio. The boundary conditions were the given required cooling capacity, the air volume flow rate and the frontal area, and provided the same cooling load and air throw length in a given cold store for each solution of the optimization. Furthermore, the airside pressure drop was constraint to less than 55 Pa, and the fin spacing was fixed at 12 mm to accommodate freezing conditions. The results show that a significant reduction in refrigerant charge and heat transfer area is possible through smaller tube diameters, a higher number of tubes per row and more refrigerant circuits.