Secondary Refrigerant (Glycol/Brine) Systems

A specialist reference on indirect, secondary-loop refrigeration for cold rooms — how it works, which heat-transfer fluid to choose, the efficiency penalties to engineer around, and when an indirect system beats direct expansion. Written for refrigeration engineers, consultants, facility managers, and contractors.

ㅤ secondary refrigerant system (also called an indirect lub secondary-loop system) keeps the primary refrigerant bottled up in a central plant and circulates a benign heat-transfer fluid — typically glycol, brine, or volatile CO₂ — out to the cold rooms. It is the architecture of choice when refrigerant charge must be minimised, when one plant serves many rooms, or when the primary refrigerant is too hazardous to pipe into occupied space.

Krótka odpowiedź: In an indirect system, a central chiller cools a secondary fluid (most often a water/propylene-glycol mix, a salt brine, albo pumped CO₂), and only that fluid enters the cold room. This slashes the primary-refrigerant charge — a decisive safety and F-gas-compliance advantage with ammonia or high-GWP HFCs — and lets one plant serve multiple rooms and setpoints. The trade-offs are a small thermodynamic penalty from the extra heat exchanger (typically the secondary loop runs 2–5 K colder than the room to drive that exchange), parasitic pump power, and higher capital cost. Indirect systems suit large, multi-zone, or charge-sensitive sites; a single small room is usually better served by bezpośrednie odparowanie (DX) — zobacz Wybór układu chłodniczego.

How an indirect system works

In a bezpośrednie odparowanie (DX) system, the primary refrigerant itself boils inside the cold-room evaporator — the working fluid is piped all the way into the conditioned space. A secondary system breaks that loop in two:

Primary loop (the "machine room"). A central chiller runs the vapour-compression cycle on a primary refrigerant — commonly amoniak (R-717), CO₂ (R-744), or an HFC/HFO. Instead of cooling room air, its evaporator chills the secondary fluid across a plate heat exchanger (the chiller barrel or "evaporator-chiller").
Secondary loop (the "distribution"). A pump pushes the now-cold secondary fluid through insulated pipework out to one or more cold rooms. Inside each room, the fluid passes through a finned-tube cooling coil (the room's "evaporator"), absorbs heat from the air, and returns — slightly warmer — to the chiller to be re-cooled.

The key consequence: the only fluid that ever enters the cold room is the secondary fluid. The hazardous, high-pressure, or high-GWP primary charge stays concentrated in one controlled location.

Two-loop schematic — a plant room with a small chiller keeps the refrigerant in one place, and a glycol loop runs from the chiller through two cold rooms connected in series and back; the hazardous refrigerant never enters the rooms. — Secondary (indirect) refrigerant system

Sensible vs. volatile secondary fluids

Secondary systems split into two families by how the secondary fluid carries heat:

Single-phase (sensible) secondaries — glycol, brine, water, or synthetic fluids that stay liquid and warm up as they absorb heat. Simple, robust, but they need higher flow rates and bigger pumps because they carry heat only as a temperature rise.
Two-phase (volatile) secondaries — most commonly pumped CO₂, which partially boils in the room coil and carries heat as latent heat. Far higher heat-carrying capacity per kilogram pumped, so pipes and pumps shrink dramatically — at the cost of a more complex, pressurised circuit.

Choosing the secondary fluid

The secondary fluid is the most consequential design decision in an indirect system. It sets the achievable temperature, the pump size, the pipe diameter, the corrosion regime, and the food-safety profile. The four common families:

Secondary fluid	Practical low-temp limit	Heat transfer	Key strengths	Key weaknesses
Propylene glycol (water mix)	≈ −10 to −15 °C usable	Umiarkowana	Non-toxic, food-safe (FDA/EU food-contact); the default for breweries, dairies, food plants	Wysoki viscosity when cold → bigger pump power, lower heat-transfer coefficient
Ethylene glycol (water mix)	≈ −20 to −25 °C usable	Better than PG	Lower viscosity → less pump penalty; cheaper	Toxic — barred from any food-contact-risk application
Brines (CaCl₂, NaCl, K-formate / K-acetate)	down to ≈ −40 °C (CaCl₂); acetates/formates lower	Dobra	Reach genuine mroźnia temperatures; potassium-formate/acetate brines are low-toxicity and low-corrosion	Salt brines are corrosive — demand inhibitors and compatible metallurgy
Volatile CO₂ (pumped, two-phase)	well below −40 °C	Doskonała (latent heat)	Tiny pipe/pump sizes; superb at low temperature; GWP 1	Pressurised circuit, specialised components, higher design skill

Glycol concentration and the freeze-point margin

For glycol and brine, you do nie powinien dose to the room temperature — you dose so the fluid stays comfortably liquid below the coldest point in the loop. A practical rule from ASHRAE practice:

Keep the fluid's freeze point at least 5–7 K below the lowest fluid temperature in the circuit (some practitioners use the burst point for the truly cold leg). Running a glycol close to its freeze point lets it turn slushy, which destroys flow and heat transfer.

Concentration is a balancing act. More glycol means deeper freeze protection but steeply rising viscosity in the cold: a 40 % propylene-glycol mix at 20 °C is already around 3.5× the viscosity of water, and the gap widens as it cools. Higher viscosity directly increases pumping power and reduces the in-coil heat-transfer coefficient — so over-dosing "to be safe" quietly inflates running cost. Dose to the duty, not beyond it.

Corrosion, inhibitors and fluid life

Glycols are not "fill and forget". As they age and oxidise they turn acidic and form glycolic acids that attack metals; brines are inherently corrosive. Industry practice:

Use an inhibited, refrigeration-grade glycol — never automotive antifreeze (its silicate/borate inhibitors foul plate heat exchangers and are not food-safe).
Test annually for concentration (refractometer) and inhibitor reserve/pH; top up or replace before the inhibitor is exhausted.
Match metallurgy and gaskets to the chosen fluid — chloride brines in particular require careful material selection.

Why choose indirect: advantages

Zaleta	Dlaczego to ma znaczenie
Minimised refrigerant charge	Only the central plant holds primary refrigerant. With ammonia this can shrink the inventory from hundreds of kg to a contained packaged charge; with HFCs it cuts both leak liability and F-gas quota exposure.
Safety / charge containment	Toxic ammonia or high-pressure CO₂ never reaches occupied rooms — satisfying machinery-room, ventilation, and leak-detection requirements far more easily.
Scalability & multi-zone service	One fluid loop serves many rooms at different setpoints; adding a room is a pipe-and-coil tap, not a new refrigerant circuit.
Thermal stability & cold reserve	The secondary fluid mass buffers the load — temperatures ride through door openings and short plant stops more smoothly than a snappy DX coil.
Simplified field service	Cold-room technicians work on a benign water-glycol loop; the refrigerant plant is isolated and serviced separately by specialists.
Future-proofing	Decouples the room from the refrigerant: swapping the primary (e.g. HFC → CO₂ or low-charge ammonia) leaves the distribution loop and room coils untouched.

The charge-containment benefit is increasingly a compliance lever, not just a safety one. Under Rozporządzenie (UE) 2024/573 (the revised F-gas Regulation, in force 11 March 2024), HFC supply is being cut sharply and high-GWP gases face service bans — so minimising and centralising charge directly reduces both quota cost and stranded-asset risk. See Dobór układu chłodniczego for the regulatory detail.

The costs: disadvantages and penalties

Indirect systems are not free efficiency. Three penalties must be designed around:

Penalty	Mechanizm	Ograniczanie ryzyka
Thermodynamic (ΔT) penalty	The extra plate heat exchanger means the chiller must make fluid 2–5 K colder than the room — a lower evaporating temperature, which lowers COP. Each extra kelvin of lift costs roughly 2–4 % more compressor energy.	Generously sized plate exchangers and room coils; keep approach temperatures tight; choose a low-viscosity fluid.
Parasitic pump power	Circulating viscous fluid through long runs consumes pump energy that DX simply doesn't have.	Right-size pipes (avoid over-velocity), use variable-speed pumps, dose glycol to duty, prefer volatile-CO₂ secondaries for low-temp duty (latent heat → tiny flow).
Higher capital cost	Central chiller + plate exchanger + distribution pipework + pumps + expansion/buffer tanks exceed a simple condensing unit.	Justified only at scale or where charge/safety rules demand it — not for a single small room.
Control complexity	Coordinating many zones at different setpoints needs more sophisticated controls and balancing.	Modern BMS/zone controllers; hydraulic balancing at commissioning.

A useful reframing of the pump penalty: in a single-phase glycol loop the pump can be a meaningful parasitic load, but in a two-phase pumped-CO₂ loop the pump work is typically well under 1 % of total system energy, because latent-heat transport moves far more heat per unit of fluid pumped. This is a large part of why volatile CO₂ has displaced glycol for the low-temperature leg of many modern installations.

DX vs. secondary loop: when to choose which

Czynnik	Skłania się ku Direct Expansion (DX)	Skłania się ku Secondary loop (indirect)
Scale	Single room, small–medium	Multi-room, multi-zone, large site
Refrigerant charge / safety	Acceptable with low-GWP/low-charge fluids	Decisive when using ammonia, or to minimise HFC charge
Budżet inwestycyjny	Najniższy	Higher (central plant + distribution)
Peak efficiency (small load)	Higher (no HX penalty)	Slightly lower (ΔT + pump penalty)
Pipe runs	Limited (pressure drop, oil return)	Long runs are easy (pump the fluid, not the refrigerant)
Adding rooms later	New circuit each time	Tap the existing loop
Maintenance model	One technician, whole circuit	Plant specialists + benign field loop

Zasady ogólne:

One small or medium cold room → DX, charged with a natural or ultra-low-GWP refrigerant. The HX/pump penalty of going indirect rarely pays back at this scale.
Many rooms, mixed setpoints, around-the-clock → secondary loop with ammonia or CO₂ primary. Higher CapEx repaid through contained charge, scalable distribution, and isolated maintenance.
Genuine freezer temperatures (≤ −18 °C) on a secondary loop → prefer volatile pumped CO₂ or a low-temperature brine (K-formate/CaCl₂); standard water/glycol runs out of usable range and viscosity becomes punishing.
Food-contact-risk environment (brewery, dairy, open processing) → propylene glycol if single-phase; never ethylene glycol.

Worked example: large food-processing facility

A meat-processing plant runs ammonia in a central engine room to chill propylene glycol (non-toxic, mandated by the food environment). The glycol is pumped through insulated mains to each processing line and CAM-LOCK cold store. This keeps the ammonia charge out of production and public areas — satisfying the machinery-room, ventilation, and leak-detection rules that large ammonia inventories trigger under EN 378 / ISO 5149 — while one plant serves very high, distributed cooling loads.

For the plant's −25 °C freezer tunnels, the same site would typically switch the cold leg to pumped CO₂ rather than push glycol below its practical limit: the latent-heat transport keeps pipe and pump sizes modest where a glycol loop would be unworkably viscous.

Integration with CAM-LOCK construction

A modular CAM-LOCK envelope makes indirect systems easier to build and maintain — relevant because secondary loops involve more in-wall pipework than DX:

Fabryczne przepusty. Panels can be factory-cut for secondary-fluid mains, condensate drains, and coil mounts, preserving insulation integrity and cutting on-site labour.
Removable access panels. Glycol or CO₂ runs within wall cavities, and the room coil itself, can be reached bez demontażu — important for the annual fluid testing and inhibitor top-ups that secondary loops require.
Tight envelope amplifies the win. A high-R-value, well-sealed box lowers the load the secondary loop must carry, shrinking pump flow and the ΔT penalty alike. See Nauka o izolacji i Specyfikacja paneli.
Decoupled future upgrades. Because the room sees only the secondary fluid, swapping the central primary refrigerant later (HFC → CO₂ / low-charge ammonia) touches the plant room, not the cold room.

To size the box itself before you size the plant, use the Kalkulator doboru komory, albo Poproś o wycenę for a configured room.

Standardowa oferta Frostbox. Frostbox supplies the insulated CAM-LOCK envelope and matched refrigeration units (Daikin families such as GS, SP-O, DB-O, SAR, SAS) — predominantly direct-expansion packages sized to the room. Central secondary-loop plant (chiller barrels, glycol mains, pumps) is a site-engineered, multi-room installation; talk to us about projects where an indirect architecture is the right fit.

Źródła i literatura uzupełniająca

Rozporządzenie (UE) nr 2024/573 w sprawie fluorowanych gazów cieplarnianych — Działania UE na rzecz klimatu / EUR-Lex — climate.ec.europa.eu/eu-action/fluorinated-greenhouse-gases
Podręcznik ASHRAE — Chłodnictwo (2022), Chapter 4 "Secondary Coolants in Refrigeration Systems" and Chapter 31 "Secondary Coolants (Brines)" — ASHRAE — ashrae.org
Å. Melinder, Properties of Secondary Working Fluids for Indirect Systems — International Institute of Refrigeration (IIR) — iifiir.org
"Secondary Refrigerant Systems" — Institute of Refrigeration (IOR) / cool-info technical notes — cool-info.co.uk/brines_steam/secondary_refrigerants
"A Performance Comparison of Secondary Refrigerants" — Purdue International Refrigeration and Air Conditioning Conference — docs.lib.purdue.edu
"Advanced Refrigeration Technologies — secondary loop & cascade systems" — US EPA GreenChill — epa.gov/greenchill/advanced-refrigeration-technologies
"Experimental investigation of thermophysical properties of propylene-glycol secondary fluids" — International Journal of Refrigeration (ScienceDirect) — sciencedirect.com
Refrigeration safety standards: EN 378 / ISO 5149 (refrigerating systems — safety and environmental requirements, machinery rooms for ammonia); ANSI/IIAR 2 i ASHRAE Standard 15 (ammonia systems, US)
"CO₂ as a Refrigerant — pumped volatile secondary & cascade systems" — Copeland / Emerson technical library — copeland.com

Frostbox supplies CAM-LOCK cold rooms and matched refrigeration packages; for multi-room or secondary-loop projects, or non-standard configurations, contact [email protected].