Open a PT chart for R-407C and there are two temperatures at every pressure. That is not a misprint. Zeotropic blends do not boil at one temperature, and handling them correctly comes down to three habits.
What is temperature glide?
A blend is a mixture of refrigerants. In a zeotropic blend, each component boils at its own rate, so as the mixture boils at constant pressure its temperature changes. The span between where boiling starts and where it ends is the glide. Blends in the R-400 series are zeotropic; the amount of glide varies from almost none (R-410A) to several degrees (R-407C).
What are dew point and bubble point?
At a given pressure, the bubble point is the temperature where the first bubble of vapour forms in the liquid. The dew point is where the last drop of liquid disappears into vapour. For a blend with glide, the dew point is higher than the bubble point at the same pressure.
Which value do you use for superheat and subcooling?
- Superheat: use the dew point. Superheat is measured from where the last liquid boils off, which is the dew point.
- Subcooling: use the bubble point. Subcooling is measured from where the liquid is fully condensed, which is the bubble point.
Digital manifolds usually apply this automatically once the refrigerant is selected. When you use a chart, pick the right column.
Why must blends be charged as liquid?
Vapour drawn off the top of a cylinder is richer in the components that boil most easily. Charge from vapour and the blend that enters the system does not match its design, a separation called fractionation. Remove blends from the cylinder as liquid. When adding to the suction side of a running system, meter the liquid so it flashes before reaching the compressor.
What happens to a blend after a leak?
A vapour leak can shift composition the same way. For many common blends, manufacturers permit topping up after a leak is repaired; for others, or after large losses, recovering the remaining charge and recharging fresh is the recommended route. Follow the refrigerant manufacturer's guidance for the specific blend.
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Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.