Comparison of R134a and HyChill Minus 30

I am considering replacing the R134a refrigerant in my car aircon system with a hydrocarbon refrigerant. The candidate is Hychill Minus 30 (HC-30).

This article is a limited comparison of the R134a and HC-30 from the point of view of pressure temperature behavior as it impact practical implementation and measurement.

Exploring HyChill Minus 30 laid down the basis of a CoolProp model of HC-30 for comparison with CoolProp model of R134a.

Fig 1

Above is a comparison of the pressure/temperature of HC-30 and R134a over the range of interest in a vehicle aircon. The typical high and low side HC-30 operating pressure bands are shaded.

In the case of my own vehicle using a TX valve and variable displacement compressor control, superheat and low side pressure is effectively regulated. This raises the question of suitability of the replacement refrigerant. Referring to Fig 1, the low side saturated vapor pressure / temperature relationship of HC-30 almost exactly coincides with R134a… no accident there, HC-30 was designed for compatibility in R134a systems. (On compatibility, HC-30 requires full barrier hoses for low hose leakage.)

Lets discuss methods of using existing instrumentation that has R134a scales with HC-30. They are different enough to require correction in some circumstances.

Subcooling

Subcooling is the cooling of the condenser liquid below the SLT at that pressure. Subcooling of around 5° gives some assurance that the restrictor is fed with liquid free of bubbles. (Note more subcooling may be needed for high friction tubing or high vertical rise.)

Subcooling might be the optimisation target for a refrigeration system using a non-fixed restrictor, such as a thermostatic expansion valve (TXV).

The characteristic of HC-30 may not be known by the instrumentation being used, but R134a may be and can be used with a correction.

In the case of HC-30 subcooling, we are interested in the SLT implied by the high side gauge compared to a measurement of the liquid line temperature (the difference being the subcooling).

Fig 2

Above, the error between R134a indicated temperature and HC-30 SLT. It ranges from about 3.5° @ 30° down to 0° at 55° (typical high side temperatures). The gauge indicated R134a temperature can be corrected to HC-30 SLT by subtracting the error from the graph above.

Fig 3

The error between R134a indicated temperature and HC-30 SLT (and calculated subcooling) is shown above against gauge pressure as that will often be simultaneously visible on A digital display (eg when displaying subcooling).

Superheat

Evaporator superheat is the heating of evaporator outlet gas above the SVT at that pressure. Evaporator superheat of around 5° gives some assurance that the compressor will not induct liquid refrigerant. (Note less superheat may be needed for long high friction return line to the compressor.)

Superheat might be the optimisation target for a refrigeration system using a fixed restrictor, such as a capillary tube or orifice tube.

The characteristic of HC-30 may not be known by the instrumentation being used, but R134a may be and can be used with a correction.

In the case of HC-30 evaporator superheat, we are interested in the SVT implied by the low side gauge compared to a measurement of the evaporator outlet temperature (the difference being the superheat).

Fig 4

Above, the error between R134a indicated temperature and HC-30 SVT. It ranges from about 0° @ -16° down to -2.3° at 10° (typical low side temperatures). The gauge indicated R134a temperature can be corrected to HC-30 SVT by subtracting the error from the graph above.

Fig 5

The error between R134a indicated temperature and HC-30 SVT (and calculated superheat) is shown above against gauge pressure as that will often be simultaneously visible on a digital display (eg when displaying subcooling).