A secondary refrigerant is a liquid (or sometimes a gas) that carries cold from a central refrigeration unit to the place where cooling is actually needed, without itself going through the compression cycle that generates that cold. It acts as a middleman: a primary refrigerant does the heavy thermodynamic lifting in a sealed, compact unit, and the secondary fluid simply absorbs that cold and delivers it elsewhere through a separate piping loop.
This two-loop design is increasingly common in supermarkets, ice rinks, industrial food processing, and large HVAC systems. Understanding why it exists, what fluids are used, and what tradeoffs come with it helps make sense of modern refrigeration design.
How a Two-Loop System Works
In a conventional (direct expansion) refrigeration system, the primary refrigerant travels through long pipes all the way to each cooling point. In a supermarket, for example, that means refrigerant lines running from a central compressor rack to every single display case on the floor. The refrigerant evaporates inside each case, absorbs heat, then returns as a gas to be compressed again.
A secondary loop system splits this into two separate circuits. The primary loop is a traditional compressor-based refrigeration cycle, but it’s confined to a small, sealed mechanical room. Instead of sending refrigerant out to every display case, it cools a secondary fluid in a heat exchanger. That chilled secondary fluid is then pumped through a second set of pipes to wherever cooling is needed. After picking up heat at the cooling points, the secondary fluid returns to the heat exchanger to be re-chilled, and the cycle repeats.
The key distinction is what happens physically to each fluid. The primary refrigerant undergoes phase changes, evaporating and condensing as it absorbs and releases heat. Most secondary refrigerants stay liquid the entire time, transferring heat purely through temperature change (sensible heat). Some newer secondary systems use fluids like carbon dioxide that do change phase, which brings efficiency advantages covered below.
Why Use a Secondary Loop at All?
The biggest motivation is reducing how much primary refrigerant you need. Many primary refrigerants are potent greenhouse gases or carry safety concerns (flammability, toxicity). By confining the primary system to a compact sealed unit in a mechanical room, you eliminate the long interconnecting pipes that would otherwise carry refrigerant throughout a building. This dramatically cuts refrigerant charge and leak rates.
In practice, switching to a secondary loop system can reduce the primary refrigerant charge by roughly 90% compared to a flooded direct expansion system. That’s a significant change for facilities that might otherwise need hundreds of kilograms of refrigerant circulating through aging pipe joints, valves, and fittings spread across a large building. Fewer joints and shorter primary piping mean fewer opportunities for leaks.
Secondary loops also simplify maintenance. If the secondary fluid develops a leak in a display case or piping run, you’re dealing with a glycol solution or salt brine rather than a regulated refrigerant. Repairs are simpler, cheaper, and don’t require specialized refrigerant recovery equipment.
Common Secondary Refrigerant Fluids
The most widely used secondary refrigerants are water-based solutions with a freezing point depressant mixed in. The choice depends on the target temperature, toxicity requirements, cost, and how easily the fluid flows at low temperatures.
Glycol Solutions
Ethylene glycol and propylene glycol mixed with water are the workhorses of secondary refrigeration. Both are highly miscible with water, and adjusting the concentration lowers the freezing point for sub-zero applications. Ethylene glycol delivers better thermal performance (it conducts heat more effectively and is less viscous at low temperatures), but it’s mildly toxic. Propylene glycol is considered non-toxic, making it the default choice in food processing and anywhere the fluid might contact consumable products. The tradeoff is that propylene glycol is more viscous, which increases pumping energy, and it costs more.
Salt Brines
Calcium chloride and sodium chloride brines have been used for decades, particularly in ice rinks and industrial cold storage. They’re inexpensive and effective at low temperatures, but they’re corrosive to metals and require careful pH management and corrosion inhibitors.
Newer salt-based options like potassium formate have gained attention. Potassium formate is biodegradable, non-corrosive, and highly soluble in water (up to 80% concentration at room temperature). Its viscosity is 4 to 30 times higher than pure water depending on concentration and temperature, which matters for pump sizing. Below about 70% concentration, viscosity stays relatively stable across temperatures, but at higher concentrations it becomes much more sensitive to temperature changes.
Carbon Dioxide as a Volatile Secondary
CO2 is a fundamentally different kind of secondary refrigerant. Rather than staying liquid and absorbing heat through a temperature rise, liquid CO2 evaporates as it picks up heat, then returns to a vessel where it’s re-condensed by the primary system. This phase-change approach (sometimes called a “volatile brine”) transfers far more energy per kilogram of fluid, which means the pipes can be much smaller and the pumps use less power.
A CO2 secondary system paired with ammonia as the primary refrigerant is a common industrial configuration. Analysis of these systems has shown they can be about 15% more energy efficient annually compared to an ammonia/glycol setup, with roughly 5% lower installation costs. The efficiency gain comes partly from being able to run at a warmer evaporating temperature in the primary system (around minus 8°C instead of minus 10°C in one documented comparison), since CO2 handles small pressure drops with minimal temperature penalty. A 0.5 bar pressure drop in CO2 at minus 40°C translates to only about a 1.5°C temperature loss, while the same pressure drop in ammonia would cost roughly 11°C.
The practical operating range for CO2 secondary systems tops out at about 10°C and bottoms out around minus 30°C before a cascade design becomes more efficient. The pressures involved are higher than glycol systems (CO2 is a high-pressure fluid), which requires appropriately rated piping and components, though the smaller pipe diameters partially offset material costs.
Energy and Efficiency Tradeoffs
Adding a secondary loop introduces an extra step in the heat transfer chain, which always comes with some thermodynamic penalty. The secondary fluid has to be cooled a few degrees below the target temperature to account for the temperature difference across the heat exchanger, and the primary system has to work slightly harder to reach that lower temperature. On top of that, the pumps circulating the secondary fluid consume electricity that a direct expansion system wouldn’t need.
For liquid secondary systems (glycols, brines), this penalty is real but manageable. The energy cost of pumping depends heavily on the fluid’s viscosity, which is why fluid selection matters so much at low temperatures. Propylene glycol, for instance, gets significantly thicker as it gets colder, increasing pumping power. Ethylene glycol handles cold temperatures better in this regard.
Volatile secondary systems like CO2 largely sidestep the viscosity problem. Because CO2 transfers heat through phase change rather than temperature rise, you need far less fluid flow to move the same amount of cooling energy. The pumps are smaller, the pipes are smaller, and the overall energy picture can actually improve compared to a glycol-based secondary loop.
Maintaining a Secondary Loop
Secondary fluids don’t last forever without attention. Glycol solutions degrade over time, becoming more acidic and losing their corrosion-inhibiting properties. Regular testing of pH, freeze point, and inhibitor concentration is standard practice. Most manufacturers recommend annual fluid analysis.
Corrosion is the main enemy in any water-based secondary system. Metal piping and heat exchanger surfaces can deteriorate if the fluid chemistry drifts, and biological growth (bacteria, algae, fungi) can form biofilms on internal surfaces that insulate against heat transfer and accelerate corrosion underneath. Systems typically include biocides and corrosion inhibitors in the fluid mixture to manage this. Keeping the system sealed and pressurized also helps prevent oxygen ingress, which accelerates both corrosion and microbial growth.
CO2 secondary systems avoid most of these concerns since there’s no water in the loop, but they require attention to system tightness (CO2 will leak through any gap) and moisture exclusion to prevent internal ice formation or carbonic acid corrosion.
Where Secondary Systems Are Most Common
Supermarkets are the highest-profile application. A typical supermarket direct expansion system circulates refrigerant through hundreds of meters of piping across the sales floor, with annual leak rates that can reach 15 to 25% of the total charge. Secondary loop systems confine the refrigerant to the machine room and send chilled glycol or CO2 to the cases instead.
Ice rinks use secondary loops almost universally. Calcium chloride brine or glycol circulates through the floor slab under the ice, chilled by a primary refrigeration plant in a separate mechanical room. Industrial cold storage, pharmaceutical manufacturing, and chemical processing also rely heavily on secondary systems wherever it’s impractical or unsafe to route primary refrigerant through an entire facility.
District cooling systems, which chill water centrally and pipe it to multiple buildings, are essentially large-scale secondary refrigeration. The concept scales from a single supermarket to an entire campus or city block.