Stacked exchangers in series improve cooling while lowering maintenance costs.
A mechanical contractor for a Chicago-based liquid-foods packager had an interesting and challenging cooling application. An important client planned to award the foods packager a large contract for packaging soup if they could satisfy one stipulation: The company had to guarantee that the product would be cooled from 198°F (92°C) to precisely 77°F (25°C) before packaging.
In addition to the tight temperature requirements, process flexibility was required. The contract was for various types of soups, so the packager had to be able to cool products having different thermal properties. At the same time, the packager needed to be able to clean the system easily between batches to avoid any carry-over from different soup types. The cooling point had to be met precisely. If the soups were too warm when packaged, spoilage potentially could occur. If they were too cool during packaging, the containers could sweat, and the labels would not properly adhere to the packages.
In addition, the packager had to accomplish this within a physical area with space limitations. The entire cooling system had to fit within a 14-by-6’ footprint and fit under a 12’ ceiling. Additionally, in order to minimize utility costs, the packager wanted to take advantage of ambient water from their cooling tower to perform the bulk of the cooling. A glycol/water mix through a chiller would be used for final cooling. Another factor considered in the design was the requirement for a sanitary food-grade system that met the 3-A sanitary standards for polished surface finishes and cleanability.
Dual-Stage Heat Exchanger Design Selected
After exploring the options, the food packager selected a designer of shell-and-tube heat exchanger systems. Often when designing shell-and-tube heat exchangers, multiple configurations can perform the duty requested. The best design is selected based on surface area, utility service provided, regulatory preferences and customer priorities. Working together in a collaborative process, the heat exchanger designer and food packager pursued the best option balancing all of the conditions.
In order to provide a fairly simple solution, the first design presented was for a pair of 24” by 10’ long BEM-style straight tube exchangers in series. The soup product would flow through the tubes of the first exchanger while being cooled by cooling tower water in the shell. After the first exchanger, the soup would flow through the tubes of the second exchanger while being cooled by 45°F (7°C) glycol/water mix in the second shell. Both exchangers were inclined to allow the units to drain out when not in use between batches. The two heat exchangers were designed with davit swing-arm assemblies to help facilitate removal of the bonnets for periodic inspection and manual cleaning when needed.
This dual exchanger approach, despite the advantage of simplicity, had several drawbacks. First, the cooling performed in the first exchanger was limited to the temperature that the cooling tower water was being heated to on the shell side. In other words, when the soup entering the exchanger at 198°F (92°C) met the cooling tower water entering the shell at 70°F (21°C), it heated up the cooling tower water to around 120°F (49°C). The soup could not be cooled below this level of 120°F (49°C), which is known as the temperature cross or temperature pinch. This would then put most of the burden on the glycol/water chiller to perform the bulk of the cooling, requiring a larger and more expensive chiller unit.
The other issue that presented itself was the ability to completely clean the unit between batches of product. Although the exchanger could be cleaned by backflushing the tubes with water and cleaning solution, there was no way to accelerate the wash water to the preferred velocity (5 ft/sec) needed for adequate cleaning. This was limited by the size of the onsite cleaning clean-in-place (CIP) system (200 gal/min). With the sheer size of the exchangers and number of tubes, it would have taken a CIP system using 1,500 gal/min to reach the proper cleaning velocity. These factors led to a redesign to a more complex yet more effective solution.
In order to allow for a smaller tube field that would provide the 5 ft/sec velocity for cleaning, the exchanger diameter was reduced from 24” to 6”. Because of the reduction in surface area per heat exchanger, it was necessary to add more exchangers to the set. The first, large unit being cooled by the cooling tower water was replaced by six smaller exchangers. For the final cooling utilizing the chiller, the larger exchanger was replaced by two of the smaller units.
As the design simulations unfolded, other benefits started to show themselves. By restricting the flow of the product to a smaller number of tubes, the velocity of the product also increased. This improved the heat transfer when cooling the soup product. It also allowed the cooling tower water to be split into a fresh stream flowing into each of the six shells, avoiding the temperature cross experienced in the larger unit. During winter months, when their cooling water was colder than 70°F (21°C), it was possible to shut down the glycol/water chiller and perform all of the cooling with just cooling tower water, saving utility costs.
The smaller diameter exchangers were easier to construct and polish to meet the 3-A sanitary requirements. They were efficient to clean using the onsite CIP system, and they were simpler to take apart to inspect due to the smaller, lighter bonnets on each exchanger.
The eight exchangers were stacked on a custom rack with all of the interconnecting product jumpers, utility piping and the contractor’s manual valves. During design, it was decided to leave enough space to mount two more of the same size exchangers on the rack, allowing for future growth in batch size, or for tough-to-cool products requiring more surface area. The units were all pitched slightly to allow for full draining of the product and cleaning fluids from the tubes. Another advantage was that spare parts like gaskets and tri-clamps were less expensive and more readily available for the smaller exchangers than with the two original, large exchangers. An added bonus is that the parts are interchangeable between the eight exchangers in the set.
The new system of stacked heat exchangers in series still fit within the packager’s space limitations, and it ended up costing about the same as the two larger custom units. This allowed the food package to stay within the budget and timeline for the project. The stacked set approach of smaller heat exchangers in series performed consistently. This allowed the packager to win the contract while enjoying the benefits of lower utility costs, increased regulatory compliance and automation of the maintenance process for the system.