Ultra-low-temperature cascade systems

Why a single stage runs out

Three things happen together as evaporating temperature falls. Vapour density drops, so the compressor moves less mass per revolution and capacity collapses. The compression ratio rises, which pushes discharge temperature up until the oil is no longer doing its job. And suction pressure approaches or falls below atmospheric, so any leak draws air and moisture inwards rather than pushing gas out.

A cascade sidesteps all three by letting each circuit work over a sensible pressure range. The two circuits do not mix. They meet at a heat exchanger where the high stage condenses the low stage.

R-23

R-23 is trifluoromethane, a single-component HFC with a normal boiling point around minus 82 °C. Being a single component, it does not shift composition, which makes charging and topping up more forgiving than a blend.

Its GWP is about 14,800 on IPCC AR4, among the highest of any refrigerant in use. That is not a reason to avoid it, because there is no mainstream alternative for this duty, but it is a strong reason not to vent it. A small leak on an R-23 circuit carries a disproportionate CO₂-equivalent, which matters for any site reporting Scope 1 emissions.

We stock R-23 in 4.5 kg and 30 kg cylinders, subject to current stock.

R-508B

R-508B is an azeotropic blend of R-23 and R-116, with a boiling point near minus 87 °C. Azeotropic means it behaves as a single fluid: no glide, no composition shift, so it charges like a pure substance despite being a mixture.

It was developed as a replacement for R-13 and R-503, both of which are gone under the Montreal Protocol, so a machine originally built for one of those is the typical R-508B application. Its GWP is about 13,396.

We stock R-508B in 1 kg and 2 kg cylinders, subject to current stock. The small formats reflect how it is used: low-stage charges on these machines are measured in kilograms, not tens of kilograms.

Practical handling

Both fluids sit at high pressure at room temperature, which is why cylinders and low-stage circuits carry pressure relief. A cascade low stage that warms to ambient while shut down can reach pressures the circuit was not designed to hold, and an expansion vessel is normal on these systems for exactly that reason.

Recover rather than vent. Beyond the environmental case, the quantities are small and the gas is expensive enough that recovery pays for itself.

Both are A1: non-flammable, lower toxicity. As with any fluorocarbon, they displace oxygen in a confined space and decompose into hydrogen fluoride if they meet a flame.

Who buys these

Pharmaceutical plants running stability chambers, testing laboratories, ULT freezer fleets, and process plant with a genuine low-temperature stage. In Hyderabad that mostly means the pharma belt.

These are the lines most dealers do not hold, and the enquiry usually arrives when a chamber is already down. If you run this equipment, it is worth knowing where the gas comes from before you need it.

Related: R-23 supply · R-508B supply · Pharmaceutical plants · Zeotropic vs azeotropic