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36 / JULY.AUGUST.2015 RINKMAGAZINE.COM Oil Management System Once the discharge gas leaves the compressors, it passes through oil separators with coalescing filters that remove contaminates from the oil. An external reservoir stores the oil that is separated from the discharge gas. Oil is fed from the oil separators to the reservoir through a solenoid valve that opens when the oil reaches a preset level in the separators. From the reser- voir, the oil is fed back to the compressors. Each compres- sor has an oil level switch that sends a signal to the controller when the level drops below a minimum level. The controller operates an oil line solenoid to feed oil to the compressors when needed. Pressure in the oil reservoir is maintained approximately 30 psig above compressor suction pressure. Maintaining this pressure difference between the reservoir and the compressor crankcases keeps oil flowing to the compressors. As with all oil separation devices, the separators are not 100 percent efficient and mixes with liquid CO 2 in the receiver. An oil return valve is used to meter a mixture of liquid CO2 and oil from the main supply line to the suction return line based on the super- heat in the suction line. Condenser/Gas Cooler In ambient conditions below 80°F, the condenser/gas cooler works the same way a conventional condenser does in a DX system. Discharge gas enters it and rejects heat to the outside air as it passes through the coils of the unit. But when the ambient tem- perature rises above 80°F, the system begins operating in the tran- scritical range. At this point the discharge gas passing through the system can no longer undergo any further state change. It instead becomes what is called a supercritical fluid. Under transcritical conditions, the discharge gas enters the condenser/gas cooler as a supercritical fluid and stays that way all the way through the con- denser/gas cooler to the high pressure control valve. No condens- ing of the gas takes place as in a regular condenser. Under 80°F, however, the unit then works just like a condenser. High-Pressure Control Valve Like the condenser/gas cooler, the high-pressure control valve works under two modes of operation. It usually controls subcooling in the condenser/gas cooler when that unit operates as a condenser. But when the unit works as a gas cooler (above 80°F, ambient), the valve controls pressure in the gas cooler. When the condenser is being bypassed in heat reclaim mode, a second transducer located in front of the condenser provides input to the valve. The high pressure control valve operates to maintain optimal COP through an algorithm within the controller. The optimal pres- sure is calculated from the condenser/gas cooler outlet temperature. CO2 Receiver The CO2 receiver stores liquid CO2 and is designed to facilitate the separation of vapor and liquid CO2. This specially designed vessel has a pressure rating of 650 psig and a typical operating pressure of 375 to 400 psig. When CO2 returns from the condenser/gas cooler under high ambient conditions, it drops in pressure across the high pressure control valve and enters the receiver as a liquid and vapor mixture. CO 2 returning from the ice rink floor also enters the receiver as a liquid-vapor mixture. CO2 vapor is then pulled out of the receiver as suction gas going to the compressors, which oper- ate to maintain the desired saturated pressure in the receiver. The receiver is typically sized to hold the entire charge of the system with three sight glasses that indicate liquid levels of 5, 20 and 80 percent. Other Components: Piping One of the benefits of CO2 is its high volumetric capacity. This allows for smaller diameter piping to be used. Smaller diameters decrease the refrigerant charge and handle higher pressures. Piping and elbows to and from the condenser/gas cooler should be carbon or stainless steel and should be installed to comply with appropri- ate standards. During operation, the pipes can get hot (i.e., 200° F) and insulation is recommended anywhere they might be touched. Results The first U.S. ice rink to use sustainable CO2 refrigeration is ben- efiting from lower operational costs and reduced environmental impact just two months after opening. "We looked at all the potential non-Freon refrigerant solutions, but sooner or later, they all were likely to be affected by environ- mental concerns," said John Rodda, Parks and Recreation Director for Anchorage, Alaska. "We decided CO 2 had the most benefits. We're anticipating energy savings of 25 percent to 40 percent when all the results are in." J Hillphoenix, the manufacturer of the Advansor Ice Rink System, contributed to this article. , Continued from page 35 , The receiver is designed to store both liquid and vapor CO2 and provides cold liquid CO2 to the piping under the ice surface. "WE LOOKED AT ALL THE POTENTIAL NON-FREON REFRIGERANT SOLUTIONS, BUT SOONER OR LATER, THEY ALL WERE LIKELY TO BE AFFECTED BY ENVIRONMENTAL CONCERNS. WE DECIDED CO2 HAD THE MOST BENEFITS. WE'RE ANTICIPATING ENERGY SAVINGS OF 25 PERCENT TO 40 PERCENT WHEN ALL THE RESULTS ARE IN." – JOHN RODDA, PARKS AND RECREATION DIRECTOR FOR ANCHORAGE, ALASKA PHOTOS: COURTESY OF RINK AND HILLPHOENIX