Technology overview: hybrid and secondary loop CO2 refrigeration systems

Rusty Walker, Senior Corporate Trainer with Hill PHOENIX Learning Center, a one-stop source for up-to-date information and training in the commercial refrigeration industry, presented the latest GreenChill webinar on CO₂ refrigeration systems. The webinar provides a useful overview of both hybrid and secondary loop systems where CO₂ is used either as a direct expansion refrigerant or a secondary coolant.

In North America CO₂ refrigeration systems are increasing in popularity thanks to their ability to deliver superior refrigeration performance with a reduced refrigerant charge. Helping keep industry members up to date on CO₂ subcritical technologies, Rusty Walker’s talk looked at three systems in use today:

Cascade low temperature CO₂ and medium temperature glycol,
Secondary low temperature CO₂ and modular medium temperature glycol
Cascade low temperature CO₂ and secondary medium temperature CO₂.

Cascade low temperature CO₂ and medium temperature glycol

In a cascade low temperature CO₂ and medium temperature glycol system the glycol is pumped from the cases and coolers, where it removes heat, through a low pressure-piping network back to the machine room. This is the point where the heat is transferred through chiller heat exchangers connected to the lower cascade system, low temperature, direct expansion (DX) CO2 system, and the primary HFC, upper cascade system. Key system features include:

Upper cascade with R404A compressors with an air cooled condenser and a receiver;
Lower cascade with CO₂ compressors with receiver and electronic expansion valves on all receivers;
Glycol chiller with expansion valve, glycol heat exchangers with solenoid valves, and a glycol pump;
Suction line heat exchanger;
Parallel condenser evaporator heat exchangers with expansion valves.

Secondary low temperature CO₂ and modular medium temperature glycol

This secondary system is made up of a modular chiller CO₂ secondary system and compact chiller modules. The secondary loop system utilises inhibited glycols as the heat transfer fluid. Heat is absorbed in the display cases through fully flooded coils specifically designed for medium temperature system operation. Key system features inlcude:

Modular chiller CO₂ secondary system with R404A condensers, receivers, condenser-evaporators, a liquid-vapour separator, and a CO₂ pump
Compact chillers modules
High-side pump station and fluid cooler
Chilled glycol pump station

Cascade low temperature CO₂ and secondary medium temperature CO₂

This system shares a common, medium temperature primary system, combining a CO₂medium temperature secondary system with a CO₂ low temperature cascade system to further reduce refrigerant charge and consolidate duplicate components. The CO₂ removes heat from the display cases and walk-in freezers via copper piping that is also significantly smaller than typically required in traditional HFC-based, direct expansion systems. The system features:

A CO₂ pump, typically installed as a primary pump with a parallel backup pump, delivers liquid CO₂ through the supply distribution piping to both the low and medium-temperature evaporators.
Low-temperature evaporators designed specifically for CO₂ direct expansion operation.
CO₂ as a direct expansion refrigerant with better heat transfer properties compared to conventional HFCs.
Medium-temperature evaporators designed for fully-flooded CO₂ secondary operation.
Medium-temperature display cases and coolers are equipped with solenoid valves, which control the flow of CO₂ into the evaporators.

Background

Below are some useful definitions relating to CO₂ hybrid and secondary systems:

Direct Expansion: A refrigeration system that includes a compressor, condenser, evaporator coil, and an expansion device;
Primary refrigerant: A fluid used to lower the temperature of a secondary coolant (i.e. R-22, R-404a, R-507, R-410A, R-717, etc…);
Secondary Coolant (a.k.a Secondary Refrigerant, Secondary Fluid): A fluid used to transfer heat from a heat source (i.e. refrigerated space) to primary refrigerant.
Single-Phase Secondary Coolant (a.k.a Secondary Refrigerant, Secondary Fluid): A fluid used to transfer heat from a heat source (i.e. refrigerated space) to a primary refrigerant;
Two-Phase Secondary Coolant: a secondary fluid which absorbs heat by means of latent heat transfer resulting in a change in phase (i.e. carbon dioxide, ice-slurries)
Cascade System: A system having two (or more) refrigerant circuits, each with a compressor, condenser and evaporator, where the evaporator of one circuit cools the condenser of another circuit;
Upper Cascade: The refrigerant circuit in a cascade system that cools the condenser of the lower-cascade and transfers the heat to a heat sink, typically outdoor ambient;
Lower Cascade: The refrigerant circuit in a cascade system that removes heat from a refrigerated load and transfers the heat to the upper- cascade.