Numerical analysis of flow maldistribution in large-scale liquid overfed finned-tube ammonia evaporators

The objective of the research is to analyze the effect of flow maldistribution in different evaporators for industrial refrigeration systems with ammonia. As the minimization of refrigerant charge has become increasingly important in these systems, refrigeration engineers and equipment manufacturers are developing new ways to control the evaporator overfeeding or circulation ratio close to unity. These efforts bring along challenges regarding flow maldistribution and possibly large superheated or unused heat exchanger areas in the evaporators. In the current research, we simulated the refrigerant flow distribution numerically, in order to understand the consequences in terms of cooling capacity for different tube circuitries as the recirculation ratio was reduced towards unity. The different tube circuitries involved two bottom-fed evaporators, (a traditional straight through and a crossover or interlaced circuitry) and one side-fed evaporator with and without inlet orifices to outbalance the liquid column in the inlet header. The results showed that the bottom fed evaporator with crossover circuitry revealed the best performance at low recirculation ratio and was robust towards airflow maldistribution. The side fed evaporator showed difficulties to handle low recirculation ratios and was sensitive towards airflow maldistribution.