Start With How Any Chiller Works
Every mechanical chiller runs the same basic loop. A refrigerant absorbs heat from your process fluid as it boils (in the evaporator), a compressor squeezes that low-pressure vapor into a hot high-pressure gas, the gas dumps its heat to the outside air or water (in the condenser) and condenses back to liquid, and an expansion valve drops it back to low pressure to start again. Heat gets picked up cold and dropped off hot.
The catch is the compressor. It has to bridge the gap between the low pressure on the cold side and the high pressure on the hot side. The colder you want the evaporator, the lower that suction pressure gets — and the bigger the pressure ratio the compressor has to overcome.
Single-Stage: Great Until It Isn’t
A single-stage system — one compressor, one refrigerant — handles the vast majority of process cooling. It’s simple, efficient, and reliable from room temperature down into the sub-zero range. AG Chill’s Standard (40°F), LT (10°F), and ELT (-22°F) series are single-stage machines.
But as you push the evaporator colder, that pressure ratio climbs. The compressor works harder for less output, discharge temperatures rise, efficiency falls, and eventually you hit a wall where a single stage simply can’t maintain a stable, efficient cycle. For most refrigerants that practical wall shows up somewhere in the -30°C to -40°C neighborhood.
The key idea: you don’t hit a hard cliff, you hit rising cost and falling efficiency — until adding hardware becomes the smarter way to go colder than pushing one compressor past its comfortable range.
Two-Stage: Splitting the Work
The first answer is a two-stage system. Instead of one compressor spanning the whole pressure gap, two compression stages split it — each handles a smaller, more manageable ratio. The vapor is compressed partway, cooled between stages, then compressed the rest of the way. That keeps discharge temperatures sane and efficiency reasonable at temperatures a single stage would struggle with.
AG Chill’s SLT Series uses a Bitzer two-stage reciprocating compressor to reach -58°F (-50°C) — the range where hydrocarbon extraction condensing and deep ethanol pre-cooling live. Two stages, but still one refrigerant in one circuit.
Cascade: Two Circuits, Stacked
To reach ultra-low temperatures — think -80°C — even two stages of one refrigerant isn’t enough, because no single refrigerant behaves well across that entire span. At -80°C the refrigerants that thrive up high have effectively stopped working, and the ones that work down low would sit at impractical pressures up top.
The solution is a cascade: two complete, independent refrigeration circuits stacked on top of each other, each using a refrigerant suited to its own temperature band. They meet at a single shared heat exchanger:
- The high-stage circuit uses a refrigerant that runs efficiently at higher temperatures. Its job is not to cool your process — it’s to cool the second circuit.
- The low-stage circuit uses a refrigerant that stays effective at extreme cold. It rejects its heat into the high stage (instead of to warm ambient air), which is the only reason it can operate that low.
In other words, the first circuit chills the second, and the second chills your process. AG Chill’s ULT Series is a two-stage cascade pairing an R-448A high stage with an R-508B low stage, using Copeland Scroll and Discus compressors to reach -112°F (-80°C).
Why R508b down low: it’s one of the few refrigerants that still boils and moves heat usefully at ultra-low temperatures — but it needs the high stage’s help to reject its heat, which is exactly what the cascade provides.
Putting It Together
| Architecture | Reaches | AG Chill series | Why |
|---|---|---|---|
| Single-stage | 40°F to -22°F | Standard, LT, ELT | Simple, efficient, reliable in the common range |
| Two-stage | to -58°F (-50°C) | SLT | Splits the pressure gap to go deeper on one refrigerant |
| Cascade (two circuits) | to -112°F (-80°C) | ULT | Each circuit uses a refrigerant matched to its temperature band |
This is also why you can’t just “add tonnage” to a warm chiller to make it cold. Temperature capability is baked into the architecture — a topic we cover in fluid temperature vs. tonnage. Choosing correctly starts with the coldest temperature you actually need.
What This Means When You Specify
- Match the architecture to the temperature, not the other way around. If your process needs -70°C, you need a cascade — no single-stage machine, however large, will do it.
- Don’t over-buy cold. A cascade system is more complex and more expensive to run than a single-stage unit. If -30°C does your job, a cascade is money and energy you don’t need to spend.
- Ask about the refrigerant. Ultra-low and cascade systems have fewer low-GWP options; make sure you understand what’s in the machine and how it fits the AIM Act transition.
The right way to choose is to fix the coldest temperature your process needs first — that selects the architecture and the series — then size the capacity. Compare the full lineup on the series comparison page.
Related: Fluid Temperature vs. Tonnage: The Spec That Actually Decides Your Chiller ›
Need to Go Below -50°C?
Ultra-low and cascade systems are our specialty. Tell us your target temperature and load — we’ll spec the architecture that reaches it reliably.