R407C Fractionation Guide: Why Liquid Charging Matters for UK Engineers

R407C Fractionation Guide: Why Liquid Charging Matters for UK Engineers

R407C is a zeotropic blend — a mixture of R32, R125 and R134a — that behaves differently from pure refrigerants and from near-azeotropic blends like R410A. Understanding fractionation is the most important technical concept for UK engineers working with R407C, particularly when charging or topping up systems.

What Is Fractionation?

In a zeotropic blend like R407C, the three component refrigerants have different volatilities. When vapour is drawn from a cylinder, the most volatile component (R32) evaporates preferentially. This leaves the remaining liquid depleted of R32 and enriched in the heavier components (R134a in particular). The result is a cylinder that no longer contains R407C at its specified composition — it becomes a different, unspecified blend.

A fractionated R407C charge causes: incorrect operating pressures, reduced cooling capacity, compressor oil compatibility issues and incorrect superheat and subcooling values. In severe cases it can cause compressor failure.

How to Charge R407C Correctly

Always charge R407C as liquid:

• Use an inverted cylinder so liquid exits from the valve, or use a siphon cylinder (marked with a yellow band) that has an internal dip tube to draw liquid from the bottom
• On the liquid line side: open the service valve slowly and use a flow restrictor to control the charge rate
• On the suction side: if you must add refrigerant via the suction port, use a liquid charging device (flash chamber or flow restrictor) to vaporise the liquid before it enters the compressor
• Never open a liquid cylinder valve directly into a suction port without a liquid-to-vapour conversion device

R407C Temperature Glide

R407C has a temperature glide of approximately 6–7°C — meaning the bubble point (start of evaporation) and dew point (complete evaporation) occur at different temperatures at the same pressure. When reading PT data for R407C:

• Use dew point temperature for the suction/vapour side
• Use bubble point temperature for the liquid/condensing side
• Superheat calculation uses dew point; subcooling uses bubble point

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Frequently Asked Questions

What is fractionation and why does it affect R407C?

Fractionation occurs when individual components of a zeotropic blend separate during vapour withdrawal. For R407C, drawing vapour preferentially removes R32, leaving an off-spec mixture. This changes operating pressures and causes poor system performance.

How should R407C be charged to avoid fractionation?

Always charge as liquid — use an inverted or siphon cylinder. Never draw vapour from the cylinder. Charge via the liquid line or use a liquid-to-vapour device on the suction side.

Can you top up R407C if a system has lost charge?

Not recommended if the remaining charge may be fractionated. Best practice is full recovery and recharge with fresh R407C to specification weight, charged as liquid.