Integrating HVAC on a grocery store’s CO2 (R744) refrigeration rack provides parallel compression to stabilize medium-temperature (MT) compressor demand during high ambient temperatures, according to a Montreal case study conducted by Canadian-based OEM Evapco.
Details of the installation and operation of the integrated system, which included system heat and gray water reclaim, were presented at the Food Industry Association (FMI) Energy and Store Development Conference by François Aucoin, Director of Engineering at Evapco, and Nicholas Torres, Commercial Refrigeration Sales Manager at Evapco. The event was held in Baltimore, Maryland, October 2‒4.
“Because we integrated the systems, we had better control over all the conditions ‒ temperature for the cases, store humidity and less defrosting,” Aucoin said. “If you are in a warm climate and want to do parallel compression, the additional cost of adding the air conditioning [AC] with the suction group is minimal,” he added.
According to Aucoin, the AC coil connected to the high-temperature (HT) suction group improved compressor efficiency to maintain subcritical operation. “It also helped regulate the flash tank.”
The Montreal retailer did not share the energy performance data with Evapco. “However, they said we were comparable to any other store with any other system for the same size and type of store,” Aucoin reported.
As part of the study, Evapco integrated existing technologies in a single CO2 rack for a full-service 30,000ft2 (2,787m2) supermarket owned by a “major” Canadian retail chain operating 1,500 stores. The rack consists of three suction groups, with each group using one variable frequency drive (VFD):
- Low-temperature (LT), with four compressors ‒ one at -30°F (-34.4°C) and three at -20°F (-28.9°C) suction ‒ providing 300,000BTU/hr (25TR/87.9kW) capacity;
- MT, with six compressors at 20°F (-6.7°C) suction, providing 1,600,000BTU/hr (133TR/468.8kW) capacity; and
- HT, with four compressors at 34°F (1.1°C) suction, providing 1,000,000BTU/hr (83TR/291.9kW) for AC and heat reclaim.
The discharge of the LT suction group feeds the MT suction group, while MT discharge funnels into a heat exchanger plate for the HT suction and, when needed, the adiabatic gas cooler.
“With 100% heat reclaim, the HT group operates the AC, the dehumidification coils, the flash gas bypass from the flash tank and the MT discharge heat,” Aucoin said, adding that the heat reclaim also provides hot water. “There is always a demand for that rejected heat rather than wasting it outside.”
Heat reclaim from the system provides 750,000BTU/hr (62.5TR/219.8kW) for air heating for the cash register, loading docks, entrance and warehouse areas; 800,000BTU/hr (66.7TR/234.6kW) for dehumidification for the servicing floor; and 250,000BTU/hr (20.8TR/73.4kW) for sanitation hot water.
Controls for the stores were also fully integrated using the Parker Micro Thermo platform, with remote access monitoring and control.
Adding in the AC requires a different design approach, “especially for the lift suction,” Aucoin said. “However, that store wanted to test the feasibility and benefits of such an integration.”
“With the CO2 HT compressors operating at a higher suction temperature, the AC coils never accumulate frost,” he noted, with the AC coils operating in two air handling units. Heat reclaim and the refrigeration system’s gas cooler cool the discharge from the AC compressors.
When the HT compressors do not have an AC load, heat from the MT group discharge is transferred to the HT group suction for heat reclaim, cooling the MT discharge and allowing for a better liquid-to-gas ratio.
A second heat rejection circuit supplies dehumidification using Evapco’s patented Purity Plus design. “With a couple of valves on the rack, we redirect the outlet from the gas cooler, used on warm summer days, to the heat reclaim coil,” Aucoin noted.
“When we activate the dehumidification system, the CO2 gas return temperature drops by roughly 20°F [-6.7°C] to stabilize compressor demand despite high outside temperatures,” he added. Moreover, this provides a “better liquid-to-gas ratio” and decreases the throttle valve opening by 20% and the flash gas bypass valve by 15%.
Treating the store humidity benefits the refrigeration cases, providing stable temperatures with less defrost, Aucoin remarked.
“We have also tested and validated this technology in another retail [facility] in California, U.S., with similar results,” Aucoin said, making the data applicable to multiple locations.
“There is always a demand for rejected heat rather than wasting it outside.”François Aucoin, Director of Engineering at Evapco
Gray water reclaim
Technically, according to Aucoin, gas cooling was not required in this application. However, with Montreal experiencing more than 400 hours annually with ambient temperatures above the CO2 system threshold and the observed warming temperature trends, Evapco decided to add it. “We used two adiabatic gas coolers piped in parallel with mist technology,” he noted.
The design funnels reclaimed rainwater from the store roof and condensate from the AC coils into a water collection system in the mechanical room. A float regulates the addition of city water for misting the coolers. “We tried to recoup all the wastewater to minimize the usage of treated city water,” Aucoin said.
Aucoin noted two challenges to the store integration. First, the store tried to use indoor and outdoor pressure sensors to control the negative pressure created by the cooking hoods. However, there were too many variables to control. “We went back to monitoring the status of each exhaust hood and opening louvers according to the total air displacement,” he said.
Additional coordination with the different disciplines involved was the other challenge. “As the rack manufacturer, I know refrigeration, but I had to coordinate with a lot of different trades that are not usually involved in refrigeration,” he said, adding that from the retailer’s point of view, it was probably a benefit.
“The CO2 technology and benefits are there; nothing is preventing the integration of the HVAC system into the refrigeration,” Aucoin concluded. “It is just a matter of adapting the design.”
Located in Laval, Quebec, Evapco specializes in manufacturing CO2 modular refrigeration equipment systems, pumping systems and electrical panels. “At this point, we have more than 432 CO2 installations in North America and one in Costa Rica,” Torres said.
“The CO2 technology and benefits are there; nothing is preventing the integration of the HVAC system into the refrigeration.”François Aucoin, Director of Engineering at Evapco