Next Generation Supermarket Refrigeration: CO2

In the latest webinar organised by archnews.com on 20 October Clay Rohrer, Sporlan Division Parker Hannifin Corporation, presented an overview of CO₂ supermarket applications, which are rapidly gaining ground in the commercial refrigeration sector. There are over 75 CO₂ installations in North America.

Clay Rohrer, Market Development Manager Food Preservation Climate and Industrial Controls Group, Sporlan Division Parker Hannifin Corporation, presented the webinar “Next Generation Supermarket Refrigeration: CO₂”, giving an overview of the properties of CO₂ and the different modes of operation of CO₂ supermarket installations: transcritical, subcritical liquid overfeed, and subcritical cascade systems. Parker produces various components designed for use with CO₂.

Transcritical installation overview

Transcritical operation occurs above the saturation dome, that is, operation occurs above the critical point, which is around 87°F (30.5°C). The same evaporation and compression occur as in DX operation but instead of condensing superheated gas into a liquid, the system cools a superheated, supercritical gas into a cooler supercritical gas. Moreover, the transcritical system DX cycle directly feeds the low temperature discharge into the suction of the medium temperature, helping to improve efficiency.

Advantages of transcritical CO₂ systems:

Zero HFC charge
Energy efficient where average temperature is below ~55°F (12.7°C) depending on the design
High heat reclaim effectiveness without typical heat exchanger losses
Supermarket can use one refrigerant used for entire system
No pumps required for operation helping to reduce costs

Parker components for CO₂ transcritical installations coming soon:

Transcritical CO₂ Gas Cooler
Flash Gas Bypass Valves
Transcritical CO₂ Solenoid Valves
Transcritical CO₂ Contaminant Control

CO₂ liquid overfeed system overview

Liquid overfeed systems use pumps to circulate liquid CO₂ throughout the display cases at the required case temperature. The systems can be used for both low and medium temperature applications. However, unlike DX systems overfeed systems are typically optimised to operate at a saturated state. This means the CO₂ evaporates in the evaporator, condenses at the plate type heat exchanger, and is then pumped back to the cases via the pump station all at around the same saturated evaporator temperature.

Some of the advantages of liquid overfeed systems are:

Fast pull down and stable temperature control, produced by the high heat transfer coefficient to CO₂ and the ability to flood the evaporator
Energy reduction because of the phase change in the evaporator
Lower pressure drop losses and lower required pump power
Overall energy consumption close to a DX HFC

Parker components for CO₂ liquid overfeed systems:

CO₂ Liquid Overfeed Solenoid Valves XSP-5 and XSP-15

CO₂ cascade system overview

CO₂ cascade systems use compressors to produce the desired saturated suction and reject heat to the DX high side rack. They are typically used only in low temperature applications. Like overfeed systems, cascade systems use a DX HFC system to cool the CO₂ side of the system. The CO2 side in this case operates like a typical DX system, except that it has an intermediate plate type heat exchanger, instead of a condenser.

CO₂ cascade systems have the following advantages:

More energy efficient than typical 4404a systems at high ambient conditions because of high volumetric efficiency of the cascade compressors, low pressure losses and high heat transfer coefficients
Refrigerant reduction to 600 lbs (272.7kg) or lower, compared to 2500lbs (1136.4kg) for HFC
Fast pull down and great temperature control because of the high heat transfer coefficient and optimised electronic expansion valves.

Parker components for CO₂ subcritical systems:

Electric expansion valves
Electric evaporator pressure regulators
CO₂ case controllers
Subcritical CO₂ solenoid valves E2-HP through to E42-HP
CO₂ contaminant control Catch-All Series, Catch-All HH Series, and See-All Moisture & Liquid Indicator

CO₂ Safety and Design Considerations

CO₂ leak detectors should be installed wherever CO₂ could be leaked into a closed, non-ventilated space. In addition, CO₂ relief valves should be piped in parallel from the liquid vapour separator and have no piping down stream in order to eliminate the risk of dry ice blockage at the outlet. Finally, piping should be carefully designed to ensure it does not trap liquid, in addition to which check valves need to be installed to make sure all the liquid CO₂ makes it back to the liquid vapour separator. It is important to pay close attention to the pressure rating when applying piping, keep in mind that CO₂ relief valves are typically set to to around 580 psi.