Optimizing Heat Exchanger Designs for Refrigeration and Cooling Technology

Cooling an indoor ski slope, providing air conditioning to a prestigious old castle, or chilling and freezing consumer goods — these scenarios all require heat exchanger technology. thermofin GmbH ensures that their heat exchanger devices are optimized for a variety of client needs using multiphysics simulation.


By Rachel Keatley
March 2021

An estimated 93.4 million tonnes (103 million tons) of food went to waste in the United States alone in 2018 — a number greater than the weight of 600 thousand average-sized blue whales (Ref. 1). A majority of food waste ends up in landfills, where it decomposes and produces methane. The United States Food and Drug Administration (FDA) even reports that food waste accounts for the largest percentage of material in landfills (Ref. 2). Food can be wasted during any stage of its lifecycle, which is why it is important for consumers and the food industry alike to be aware of solutions to help alleviate this problem. One way to help reduce food waste on an industrial level is to ensure that consumer goods are being properly stored before they end up in customers' homes.

thermofin GmbH, a leading manufacturer of heat exchangers, designs technology to help make this solution a reality. Their heat exchangers are used in air conditioning and refrigeration systems in commercial and industrial buildings around the world. Their devices can be found in supermarkets, cold storage facilities, ice arenas, power plants, and more. Julius Heik, a thermodynamics development engineer at thermofin GmbH, performs simulations to ensure that their heat exchangers are optimized for specific use cases and client needs.

Heik's favorite part about working with simulation? You are able to gain knowledge before actual measurements are carried out.

Designing Optimized Heat Exchangers

Since its founding in 2002, thermofin GmbH has expanded from 6 employees to more than 500, with production sites on several continents. Their dependable heat exchangers have made them a popular choice in the refrigeration and air conditioning industry.

Heat exchangers sound like a simple concept, but they can actually be quite challenging to design. The essential task in cooling a product is to get rid of unwanted heat so that thermal energy from perishable goods is extracted. This is where the refrigerant of a refrigeration cycle comes into play. By changing the refrigerant phase from a liquid to a vapor state, the heat exchanger is removing heat from its ambient surroundings. This heat then has to be passed over to a second heat exchanger, which emits this energy to the outside environment.

In transcritical CO2 refrigeration cycles, a so-called gas cooler chills the refrigerant inside a heat exchanger. Often, people get confused by the name "gas cooler", as if it uses gas to chill its surroundings. Designing heat exchangers in general, and gas coolers in particular, presents a fair amount of difficulties, according to Heik. When striving for better, more energy-conserving refrigeration cycles, well-engineered heat exchanger designs serve as a main contribution.

Like many cooling systems, gas coolers are designed to have a minimal direct impact on the environment, so they use the natural refrigerant CO2. For example, in the supermarket sector, CO2 is now used almost exclusively because it is classified as a nonhazardous gas (safety group A1). Due to its properties, however, it must dissipate its heat at air temperatures above 20–25°C, in the so-called transcritical range. That is why these systems have a large temperature difference, consist of many different circuits, and are made up of a wide range of materials. Using simulation, Heik is able to efficiently and simultaneously analyze the airflow and material properties of these devices.

Figure 1. thermofin® heat exchangers are used in a variety of devices, such as blast freezers, hybrid condensers, and gas coolers (shown).

Designing the inner finned tubes presents another unique challenge when developing heat exchangers. These tubes are used in heat exchangers to transform a hot fluid into a cold fluid or vice versa. The arrangement, diameter, material (stainless steel is required if using ammonia), and fin spacing of these finned tubes all depend on the type of heat exchanger in which they are being used. "There is not a lot of measured data available on how these tubes work," said Heik. With simulation, he can get a better understanding of how finned tubes affect a heat exchanger design by modeling multiple tube geometries and investigating their inner and outer heat transfer capabilities. The finned tube geometries that offer the best performance are built and tested at an in-house experimental station. "We look to see if the calculations and results are the same or similar, and then we take the best tube for our industrial line," said Heik.

Figure 2. Left: Geometry of a thermofin® heat exchanger. The large arrows represent the airflow, while the small arrows represent the refrigerant flow. In addition, the red and blue colors indicate a temperature change. For example, airflow is hot at the inlet (red) and cold at the outlet (blue). Right: thermofin® heat exchangers contain slats or fins of varying material properties and spacing requirements. To better understand how these slats work, thermofin GmbH uses simulation to analyze the direction of flow.

Cold Storage Room Simulations

In addition to performing simulations of heat exchanger technology, thermofin GmbH also simulates their customers' cold storage warehouses. For one specific project, a customer asked for help designing a meat storage room, which would include several robotic machines that hold the meat. In this storage room, meat enters at room temperature and needs to be cooled before it can be brought into a different cold storage room. "It was important that the air velocity in the room wasn't too high so that the meat wouldn't fall off the robotic [machines], and on the other hand, it was really important that every area in the room gets the same or similar amount of air," said Heik.

Figure 3. Geometry of a cold storage warehouse from another project, where air is distributed via the cold lake principle, in which cold air is introduced toward the floor, spreads out due to density differences there to rise at the other end of the room, and is drawn back in at roof height. The model takes into account the high stacking density of the storage racks with forklift passages.
Figure 4. Simulation of the cold storage room's temperature distribution (left) and speed distribution of the airflow (center, right).

When performing cold storage simulations like this one, there are several criteria that need to be taken into account, including temperature distribution, airflow distribution, relative humidity, adjacent heat loads, and natural convection. At first, thermofin GmbH believed that their customer would need to use five heat exchangers in order to get an even amount of air distribution within the storage room.

After simulating a room with five devices, Heik noticed a problem. "The backflow of the air would partly bypass into the intermediate ceiling," said Heik. To fix this issue, Heik simulated some air-guiding veins in the room, which would help ensure a smooth backflow, ultimately reducing the amount of vortexes in the room. Following the advice of thermofin GmbH, the customer ended up using five thermofin® units and built their storage room with air-guiding veins. According to Heik, the customer is happy with the results and has thankfully not experienced any occurrences of falling meat.

The Future of Heat Exchanger Technology

As thermofin GmbH continues to expand globally, their plans for innovative simulation work continue to grow as well. "In our future research plans, we want to design [heat exchangers with a] new fin shape," said Heik. A change like this requires that the heat exchanger's tubes expand in diameter. To successfully implement this change, thermofin GmbH first needs to find the optimal way to space out these tubes. "For a new fin geometry, we would have to simulate it before we buy the tools to produce it ourselves," said Heik. A modification like this could help enhance the heat transfer capabilities of their heat exchanger designs.

References

  1. "2018 Wasted Food Report", United States Environmental Protection Agency, 2020, https://www.epa.gov/facts-and-figures-about-materials-waste-and-recycling/advancing-sustainable-materials-management.
  2. "Food Loss and Waste", United States Food and Drug Administration, 2020, https://www.fda.gov/food/consumers/food-loss-and-waste.