Case study: Limited charge shell and tube ammonia spray evaporator with enhanced tubes

A low charge ammonia spray evaporator was designed for a process cooling application at a chemical plant. At off-peak hours the chilled water is stored in a 2500 m3 insulated tank that is part of a thermal storage system to reduce peak demand. With the existing chillers the water temperature was at its best never lower than 4.4 °C. Therefore, the minimum storage tank temperature achieved was between 5.6 °C and 7.2 °C. Design requirement was to cool water to 2.2 °C without risking tube freeze-up. To meet this requirement a shell and tube spray evaporator was proposed, designed and fabricated. Doubly enhanced carbon steel tubes were employed. Field results showed good agreement with the design criterion. Appropriate refrigerant distribution system was designed. A comparative study is presented here to show various aspects of spray evaporator versus a flooded evaporator of equal capacity. Field results showed exceptional performance under different seasonal conditions. Water outlet temperature as low as 0.6 °C has been recorded and there are confirmed reports of storage tank temperature as low as 1.7 °C.