Liquid or Vapor Pressure

Have you ever looked at a pressure-temperature (PT) chart and wondered why some refrigerants have just one column for pressures and others have two that are labelled: liquid and vapor pressure, or sometimes bubble and dew pressure? You are not alone! It’s a common question our team receives and there is a slightly complicated answer. Scroll below to find out more.

 

Why are there two pressure columns for one refrigerant on my PT Chart?

 

A refrigerant will have a liquid and a vapor pressure column on a PT chart if it is a zeotropic blend (sometimes called non-azeotropic) of two or more components with significantly different boiling points.   Refrigerants with a “400” number, such as R-407C, are zeotropic blends and are likely to fractionate, that is separate into its individual components as they evaporate.  Refrigerants with a “500” number, such as R-507A, are azeotropic blends and much less prone to fractionation and thus behave more as a single component refrigerant.

 

When inside a cylinder or tank, the actual composition of the liquid and the vapor phase of the refrigerant may be different, even though they are at the same overall pressure and temperature.  The liquid composition is more likely to be within the intended composition range of the blend, while the vapor composition may be slightly different.  In this case, for the temperature of the refrigerant, the saturation pressure will be the liquid pressure.

 

However, even with a 400-numbered blend, it is possible that the effects of fractionation are so small that the difference in value between the vapor and liquid pressures is practically insignificant.  In that case, you will only see one column on the PT chart for the liquid pressure.  An example of this would be R-410A, which is a zeotropic blend, but only has one column for liquid pressure on the PT chart.

 

Which pressure column do I need to use for SH and SC calculations?

 

In general, the calculation of the superheat (SH) or subcooling (SC) of a zeotropic blend with only one column on the PT chart, an azeotropic blend, or a single component refrigerant at a particular point in a refrigerating circuit is straightforward.  There is one pressure that corresponds to one temperature and by performing a simple subtraction the SH or SC can be determined.  This is the case for refrigerants like R-134a or blends like R-410A or R-507A.  When there are two pressure columns, such as R-407C, use the vapor (dew) pressure for superheat and the liquid (bubble) pressure for subcooling calculations.