A General Mills yogurt processing plant in Murfreesboro, Tenn., needed to cool propylene glycol brine for use in different parts of the plant. The company evaluated several evaporator options, including shell-and-tube and shell-and-plate heat exchangers. The plant initially installed twin parallel shell-and-plate exchangers because of the relatively small amount of refrigerant required to charge the circuits. However, due to regulatory and operational issues, the facility later decided to replace the heat exchangers with shell-and-tube units. The decision presented a significant challenge: How could the company get shell-and-tube heat exchangers, which are typically much larger than shell-and-plate exchangers, to fit within the existing limited space?
Enhanced Tube DesignThe plant owner wanted a compact system that would perform efficiently and require a low ammonia charge. Conventional flooded shell-and-tube units with prime-surface carbon steel tubes were too large. After evaluating a number of different vendors and equipment options, the company chose Isotherm Inc., in Arlington, Texas, to develop an effective solution.
Isotherm designed a system that uses various kinds of enhanced tubes along the height of the tube bundle.
“Tubes with high-efficiency, strong nucleate boiling characteristics were used for the lower section of the tube bundle, followed by tubes with moderate nucleate boiling characteristics in the midsection, and predominantly convective-boiling enhanced tubes in the top section,” explained Zahid Ayub, Ph.D., P.E., and the president of Isotherm Inc. “This concept reduces the ’vapor blanket’ effect - a vapor-rich zone that results in a lower heat transfer coefficient - in the top section, resulting in an optimized evaporator with no parasitic losses.”
Engineers applied complex mathematical modeling to decide the final geometry of the tubes and the layout of the tube bundle. Figure 1 shows one of the four replacement shell-and-tube evaporators. The design parameters and physical characteristics of the system are shown in table 1. For comparison purposes, table 1 also includes data for a conventional shell-and-tube evaporator with plain surface tubes.
“The final design consisted of a three-pass tube sheet arrangement,” Ayub explained. Figure 2 shows the three-pass tube sheet arrangement. Ayub also noted that each pass has tubes similar to those shown in figure 3. The lower section (Section I) comprised 15 rows of highly structured outside surface tubes with strong nucleate boiling characteristics and internal grooves, as shown in figure 3a. The middle section comprised 10 rows of tubes with a slightly wider gap structure on the outside (figure 3b) to overcome strong convective effects.
“These tubes also have internal ’turbulators’ - twisted tape inserts with a specific pitch that are designed to enhance the inside of the tube,” Ayub said. “This type of tube has shown good nucleate boiling behavior in the presence of strong convective forces.”
The top section comprised 15 rows of tubes with predominantly convective-boiling characteristics and internal turbulators, as shown in figure 3c. Figure 4 shows one of the installed chiller packages with an enhanced shell-and-tube evaporator.
By using the enhanced tube design, engineers were able to create an efficient evaporator that fit within the plant’s designated confined space and required a low refrigerant charge, comparable to that of the two original shell-and-plate evaporators.
Isotherm Inc., Arlington, Texas, manufactures heat transfer equipment for refrigeration, petrochemical and industrial process applications. For more information, call (817) 472-9922; visit www.iso-therm.com; e-mail email@example.com.