Intelligent and adaptive control in transcritical CO₂ refrigeration

Monday, October 8, 2018

Intelligent control solutions are important for achieving reliable, optimized operation and realizing financial savings and environmental benefits in your transcritical CO2 refrigeration solution. Using controllers that constantly monitor system status and respond intelligently
to condition changes is crucial. Danfoss has long been the industry standard for CO2 pack and case controllers, and we are continuously updating our offering with new software that allows them to control and optimize CO2 refrigeration solutions. The newest generation of controllers is able to handle CO2 transcritical systems with Danfoss gas and liquid ejectors.

AK-PC 782A pack controller 

Developed specifically for transcritical CO2 refrigeration applications, the AK-PC 782A manages the refrigeration pack. It integrates with the other system components such as sensors, valves, System Manager, and case controllers, using network communication. It offers high-pressure control and control of Danfoss gas and liquid ejectors. The AK-PC782A also includes features such as oil management, floating receiver pressure control, heat recovery functions, and failsafe functions for ensuring robustness. The controller provides monitoring and overrides capabilities such as the forced closing of expansion
valves, coordination between pack components (e.g. compressors and valves), fail-safe operation, alarm messaging and enabling of Adaptive Liquid Control. The AK-PC 782A allows you to control both the medium temperature (MT) and low temperature (LT) suction groups as well as parallel (IT) compressors, so a single controller can manage the entire transcritical pack. 

Built-in scalability and easy setup

The AK-PC 782A, like other Danfoss pack controllers, can be extended to match the size of your plant by adding extension modules. Developed for multiple CO2 refrigeration applications, the controller gives you a great degree of flexibility and scalability. The AK-PC 782A is easily set up and supervised using Danfoss AK-ST 500 Service Tool software. You install the software on a PC, connect the PC to the pack controller using the USB connection, and go through the setup process using the intuitive menu system. Once network communication is established a graphical representation of the pack application is automatically made available on the System Manager.

Features and benefits (AK-PC 782A)

Scalable to control large booster packs with up to 12 compressors

Speed control of 2 compressors per suction group

Handles mixed compressor sizes

Easy first-time use with wizard-like set-up

Robust algorithms ensure safe initial start-up and reliable operation

World class refrigeration system efficiency with Danfoss Multi Ejector

Receiver pressure optimization for maximum utilization of IT (parallel) compressors

Optimization of suction pressure according to the most loaded refrigeration area for maximum utilization of MT and LT compressors

Gas Cooler control for maximum efficiency at all times (summer, winter, heat recovery etc.)

AK-CC 550A and AK-CC 750A case controllers

Danfoss AK-CC 550A and AK-CC 750A case controllers manage refrigerated display cases, cold rooms, or refrigeration cabinets. Controlling Electronic Expansion Valves (EEV), they optimize evaporator utilization and energy consumption using the Minimum Stable Superheat (MSS) or new Adaptive Liquid Control (ALC) algorithm, giving you the best possible refrigeration capacity from the energy consumed and ensuring food safety. The AK-CC 550A and AK-CC 750A give you a great degree of flexibility with predefined application types that allow you to quickly adapt the controllers to a different display case or cold room setups. With features such as day/night thermostat with on/off or modulating control, product sensor with separate alarm limits, thermostat setting switching via a digital input, and adaptive superheat control, they
provide all the control features you need. The Adaptive Defrost mode on both case controller models gives you the option of skipping a scheduled defrost if it is not necessary, based on evaporator performance. This can save substantial energy and preserve food quality with minimum temperature cycling on the product.

Advantages with connected, intelligent controls

Danfoss pack and case controllers are designed to communicate and interact with each other through the System Manager, controlling and adjusting your transcritical CO2 refrigeration system to deliver reliable, effective, and energy-efficient refrigeration. Here are a couple of examples of how the system components interact to help you optimize your processes through monitoring, communication, and calculations.

Intelligent evaporator control

The case controller carefully optimizes evaporator performance by adaptively regulating the refrigerant injection. Danfoss offers two adaptive control algorithms: Minimum Stable Superheat (MSS) for dry expansion and Adaptive Liquid Control (ALC) for systems with suction accumulators. Both algorithms adjust the injection to the evaporator based on factors such as operating conditions and load variations, optimizing the refrigeration capacity of the evaporator.
When using the ALC algorithm, in which larger amounts of refrigerant are injected into the evaporator, droplets of refrigerant may still be present in the gas leaving the evaporator. These droplets of liquid are collected in a small suction accumulator to protect the compressors. A Danfoss Liquid Ejector can be used to pump the excess liquid from the accumulator and back to the liquid receiver. For more information about the MSS and ALC algorithms, please see our article Selecting the right evaporator injection algorithm makes a world of difference.

Features and benefits (AK-CC 550A/AK-CC 750A)

Control of 1 evaporator (AK-CC 550A) or up to 4 evaporators (AK-CC 750A)

Electronic evaporator injection control using MSS and ALC algorithms

Adaptive Defrost based on evaporator performance

Case cleaning function

Integrated networking and  communication options

Compressor protection for system reliability

The pack controller constantly monitors the suction accumulator to detect if the liquid level is becoming dangerously high, or if the Liquid Ejector is unable to remove the liquid fast enough. In this case, the pack controller sends a signal via the System Manager to the case controller, instructing it to switch from the ALC algorithm to the MSS algorithm (dry expansion mode). This algorithm switching feature protects your compressors, greatly improving system reliability. The case controller will use the supremely energy-efficient ALC algorithm for as long as it is safe to do so, switching to the MSS fallback mode if there is a risk of liquid slugging. The controllers have been factory set for complete system safety; in case communication between the pack controller and case controllers fails, the case controller(s) will switch to MSS, providing reliable refrigeration operation at no risk to the compressors until communication can be re-established.

Suction pressure optimization

The System Manager monitors the load of each case controller and computes the optimum suction pressure target. Based
on this real-time control data, the System Manager calculates whether the pack controller can increase the suction pressure
(P0), thereby saving energy. If so, the System Manager informs the pack controller how much the pressure can be adjusted. This results in energy savings while ensuring that food is always stored at the required temperature. It also generates statistical data about the case or cold room load, enabling you to schedule service and maintenance based on actual system performance.

Conclusion

Danfoss has decades of refrigeration application experience and uses this expert insight to continuously develop products that deliver the performance you need through proven technology. With features such as factory defined smart settings and adaptive algorithms, our intelligent products (pack controller, case controllers, and System Manager) help you get the most from your transcritical CO2 refrigeration system. The products continuously monitor and adjust system performance to maximize refrigeration capacity, minimize energy costs, and keep your system running reliably. Despite their advanced features, the controllers are very easy to set up, commission, maintain, and service, enabling easy and highly automated control of complex refrigeration systems such as parallel compression systems with ejector technology. The result? Energy-efficient, economical, reliable, and climate-friendly CO2 refrigeration performance for your food retail application, with intelligent and adaptive control in all climates. 

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