As an eight-store food retailer based in Carol Stream, Ill., Angelo Caputo's Fresh Markets prides itself on family tradition. Founded in 1958 by Angelo and his wife, Romana, the stores retain their family flavor. Shelves are stocked with cakes, sauces and sausages made from recipes brought from Italy nearly 70 years ago.
The fresh-food brand is so popular that the store recently expanded its Carol Stream facility. That's when Caputo's went with another fresh idea -- using transcritical carbon dioxide (CO2) for its new refrigeration system. Usually associated with larger stores with major technical resources, sources report that the transcritical CO2 technology was no problem for Caputo's "locally grown" technical talent, thanks to smart Danfoss refrigeration control technology.
(Ed. note: transcritical means the refrigerant cycles between the subcritical and supercritical phases.)
"Our expanded, 300,000-sq.ft. Carol Stream facility is a commissary, food store and warehouse," says Luigi Addante, Caputo's information technology director who oversaw the refrigeration system's installation. He says the decision to move forward with transcritical CO2 was based on the expected future of formulated refrigerants.
"When you hear how synthetic refrigerants affect the environment and are headed for phaseout, I didn't want the typical HFC direct expansion (DX) system," Addante says. "Fortunately, we had expert refrigeration partners who supported a move to using transcritical CO2."
CO2 Refrigeration Systems Come in Different Flavors
"We already had experience with CO2 at our Elmwood Park location," says Addante. "It was a secondary CO2 system that cut the HFC charge from more than 2,000 pounds to 300 pounds. That gave us a positive experience to keep moving with CO2.”
With the secondary system, the Elmwood Park store is divided into mid-temperature (MT) and low-temperature (LT) loops. Each circuit employs its own compressors and condensers. The MT compressors employ an HFC refrigerant, and the LT compressors use CO2. The refrigerated cabinets in the MT loop are cooled by glycol pumped through a heat exchanger cooled by the HFC refrigerant. The LT cabinets and freezers are cooled by CO2 that has passed through a heat exchanger filled with low-temperature CO2 gas.
Unlike Elmwood Park, the Carol Stream facility uses a transcritical CO2 system employing CO2 as the only refrigerant in both the MT and LT circuits. The LT compressors serve as boosters, discharging CO2 gas into the MT compressors. If the CO2 gas is maintained above the critical pressure of 1067 psia and above 88F, it takes the form of a high-density vapor. Consequently, the condenser serves as a gas cooler that simply reduces the temperature of the CO2 gas, keeping it at a transcritical pressure that prevents the phase change into a liquid. After passing through a high-pressure reducing valve, some of the gas condenses into a liquid. Gas and liquid are then separated in a flash tank, also known as a "receiver". After leaving the receiver, the dense gas goes through an expansion device and is returned to the MT compressors. The liquid CO2 is distributed through expansion valves and heat exchangers in MT and LT cabinets to provide the required cooling.
According to Chris Brown, Danfoss applications engineer who assisted Addante's team, the energy efficiency of CO2 is only fully realized in a transcritical refrigeration cycle. "As a high-density vapor, CO2 is in a high-energy state. That means it holds a lot of heat that can be reclaimed for space heating and for hot water for cooking and cleaning. When you utilize heat reclaim, the efficiency of a transcritical CO2 refrigeration system is very high."
With Illinois’ weather, heat reclaim comes in handy. Springs and falls are cool, and the winters are long and cold. The average annual temperature is 49F — a temperature zone that enables heat reclaim to boost the total efficiency of a transcritical CO2 system over a direct expansion (DX) system.
Addante planned to take advantage of heat reclaim to produce hot water for lavatories, food prep and cleaning, as well as under-floor heating in the walk-in freezer rooms.
But in moving to a transcritical system at Carol Stream, Addante's technician team had several questions his refrigeration partners had to answer.
Answering Technicians' Questions about Tanscritical CO2
One of the big questions was about the high pressure of a transcritical CO2 system.
According to Brown, “Transcritical CO2 systems are built to withstand pressures that can reach 1700 psia on the high-pressure side of the system. Accordingly, the components and system design are much more robust than those in a DX system. That adds upfront cost to the system price. But the robust components are less prone to failure, which reduces repair and maintenance costs over the long term. Consequently, from a life-cycle perspective, a transcritical CO2 system is cost-competitive with DX systems.”
Brown adds that “CO2 costs much less than synthetic refrigerants. By standardizing on a refrigerant that won't be phased out, the cost of retrofitting or replacing equipment for new refrigerants in the future is also eliminated.”
Addante installed the system using Caputo's own in-house technicians. Throughout the project, his team worked closely with the Danfoss team.
“Some of their technicians were concerned about the complexity of the system,” says Brown. “We trained their team to show how Danfoss control technology makes their life easy.”
To control the flow of CO2 to all the cabinets, Danfoss supplied AK-CC 550A case controllers.
“The AK-CC 550A case controller regulates temperatures in display cases and freezers,” says Brown. “It receives inputs from one or two temperature sensors located in the case's airflow. Then it adjusts the amount of CO2 flowing through the evaporator, which affects the temperature of the air passing through the evaporator coil. For Caputo's, we installed 165 case controllers with setpoints of -5F for freezers, 32F for meat cases, 36F for dairy cases and 38F for produce cases.”
The case controllers use LON communication protocol to talk to Danfoss AK-SC 355 System Managers.
“The Danfoss AK-SC 355 System Manager was developed specifically for food retailers,” explains Brown. “It is the brains at the front end of the refrigeration system. It allows technicians to set up, to schedule, to defrost, to monitor and to log events and alarms of all the case controllers from the mechanical room or over the Internet. A graphical color display makes it easy to see all the devices on the network.”
Four AK-SC 355 System Managers are employed to handle the refrigeration loops. A fifth handles lighting and HVAC functions.
At the back end of the system are four transcritical compressor racks, with three LT compressors and five or six MT compressors per rack. Furthermore, every rack employs a variable frequency drive -- namely, the Danfoss VLT® Refrigeration Drive FC 103 -- on the first compressor of each suction group, totaling two 460V drives per rack. Designed for refrigeration applications, the VLT drives help maintain stable suction pressure and temperature. They also minimize compressor cycling, which otherwise causes high power consumption and stresses compressor mechanical parts.
Each rack also employs a pair of Danfoss AK-PC 781 rack controllers, one for LT compressors and the other for MT compressors.
“The AK-PC 781 rack controller provides complete capacity control of compressors, condensers and receivers,” says Brown. “Their main function is ensuring the compressors and condensers are operating at optimum pressure conditions. In other words, the rack controllers keep the CO2 gas in the high-pressure side of the system within the transcritical area as it passes from the compressor through the gas coolers. The operator doesn't have to worry about any of the variables."
Danfoss Support Ensures Success
Addante was really happy with the support Danfoss provided. “They were with us every step of the way -- from charging the system to startup to training.”
When Addante's technicians charged the system, Brown's team provided useful advice. “CO2 is touchier with water than HFC refrigerants, so filter driers have to be installed. Plus, system startup must occur in steps. When the rack is running, cases must be isolated and brought online one by one.”
In the end, Addante and his team saw how Danfoss control technology was indeed making transcritical CO2 refrigeration very easy. The Danfoss rack controllers were also able to handle oil management and the heat-recovery function.
“My most skeptical technician is now a believer," boasts Addante. "He'll tell you that it is a really good system. The CO2 didn't exceed 88F all summer, so we were able to take advantage of transcritical operation all year long.
“And we love how it controls temperatures," Addante adds. "In our warehouse coolers where employees are constantly going in and out, we aren't seeing the temperature spikes we used to get with the old system. And after defrosting frozen food cases, we see the temperature drop below zero much faster than in our DX stores.”
As it turned out, the installation in Angelo Caputo's Carol Stream store made history – it is believed to be the first transcritical CO2 refrigerated supermarket warehouse in the U.S.
"Danfoss pulled together all the control ingredients for a transcritical CO2 system that serves all areas of our Carol Stream facility," says Addante. "For us, CO2 refrigeration is a natural that fits our reputation for fresh thinking and wholesome goodness."