Refrigerant Pressure And Boiling Point

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The boiling point given in tables for a liquid is always referenced to an atmospheric pressure of 1 bar (14.5 psi). If the pressure acting on a fluid changes, its boiling point also changes.

Pressure is measured in different units: 1 MPa (mega Pascal) corresponds to 10 bar (145.04 psi) positive pressure or 145 psi. 1 bar (14.5 psi) absolute pressure corresponds to 0 bar (0 psi) positive pressure and thus to the ambient pressure (atmospheric pressure).

For example, water boils at a lower temperature the lower the pressure.

The vapor pressure curves for water and refrigerant R744 show that, at constant pressure, reducing the temperature changes vapor to liquid (in the condenser) or that reducing the pressure causes the refrigerant to change from liquid to vapor (evaporator).

Vapor Pressure Curve of Water

Fig 1: Identifying Vapor Pressure Curve Of Water Graph

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

A - Pressure and temperature range where water is liquid (also at a temperature greater than 100 °C (212 °F))

B - Pressure and temperature range where water is vapor or a gas (also at a temperature less than 100 °C (212 °F))

C - Vapor pressure curve of water

1 - Pressure acting on liquid in bar (absolute)

2 - Temperature in °C

Vapor Pressure Curve of Refrigerant R134a/Refrigerant R1234yf

Fig 2: Identifying Vapor Pressure Curve Of Refrigerant R134a Graph

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

A - Liquid

B - Gaseous

D - Vapor Pressure Curve of Refrigerant R134a/R1234yf

1 - Pressure acting on liquid in bar (absolute)

2 - Temperature in °C

The vapor pressure curves of both refrigerant R1234yf and R134a is similar across a large temperature range. The pressure difference between the two refrigerants within the 0 °C to +50 °C (32 °F to 122 °F) temperature range is only approximately 0.2 bar (2.9 psi), for example; therefore, it is not possible to differentiate between the two refrigerants. Refer to REFRIGERANT VAPOR PRESSURE TABLE and A/C TECHNOLOGY BASIC PRINCIPLES . Differentiating between refrigerant R134a and R1234yf is only possible using sensors that can analyze the chemical composition of the refrigerants.

Refrigerant R744 Vapor Pressure Curve (Static Pressure, Pressure with A/C System Off), Depending on Density

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

A - Pressure and temperature range where refrigerant R744 is a gas

B - Pressure and temperature range where there is no longer a boundary between the liquid and gas portions of refrigerant R744 (supercritical temperature range)

C - Pressure and temperature range where refrigerant R744 is a solid (dry ice)

D - Pressure and temperature range where refrigerant R744 is a liquid and a gas

E - Pressure range that is not permitted in a vehicle refrigerant circuit (density greater than 250 kg (551.16 lbs)/m³ (or g/Liter)

F - Refrigerant R744 vapor pressure curve with a density of 750 kg (1, 653.47 lbs)/m³ (or g/Liter), equivalent to the density of a fully filled container

G - Refrigerant R744 vapor pressure curve with a density of 250 kg (551.16 lbs)/m³ (or g/Liter), equivalent to the density of a refrigerant circuit filled according to specifications. Refer to REFRIGERANT VAPOR PRESSURE TABLE .

K - Critical point; above this temperature both liquid and gas can coexist. The pressure changes depending on the density

T - Triple point; at this pressure and temperature point, refrigerant R744 can exist as a solid (as dry ice), as a liquid, and as a gas at the same time.

1 - Refrigerant circuit pressure in bar (absolute)

2 - Ambient and refrigerant temperature, temperature in °C

If the refrigerant circuit pressure is in the -E- range, either too much refrigerant R744 was added, or the wrong refrigerant with a higher vapor pressure is in the refrigerant circuit.
The correlation between the pressure and temperature that is shown in this table only exists when all refrigerant circuit components have the same temperature.
The density specifies how many kilograms of refrigerant R744 per m³ (or g/Liter) of available internal refrigerant circuit volume are added. The capacity for refrigerant R744 was calculated for a particular available internal refrigerant circuit volume (approximately 250 kg (551.16 lbs)/m³ ).
If the density is exceeded, the static pressure (pressure when the A/C system is off) in the refrigerant circuit increases sharply and the pressure relief valve opens until a permissible static pressure in the refrigerant circuit is reached (at a density of 400 kg (881.85 lbs)/m³ , for example, the static pressure will already start to increase to 144 bar (2, 088.55 psi) at an ambient temperature of 80 °C (176 °F)).
The vapor pressure curves for refrigerants R744, R1234yf and R134a are very different. The pressure difference between refrigerants R744 and R1234yf / R134a in the 0 °C to +50 °C (32 °F to 122 °F) temperature range is more than 30 bar (435.11 psi); therefore, it is not possible to confuse the refrigerant R744 with R1234yf / R134a. Refer to REFRIGERANT VAPOR PRESSURE TABLE and A/C TECHNOLOGY BASIC PRINCIPLES .