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Refrigeration

Subcooling Calculator

Computes refrigerant subcooling as the bubble-line saturation temperature at the absolute pressure minus the measured liquid-line temperature, reading T_sat by inverse interpolation on the committed refrigerant saturation tables.

Subcooling is how far the liquid refrigerant leaving the condenser sits below its saturation temperature at the measured pressure. This calculator reads the saturation temperature off the committed saturation table of the selected refrigerant — the BUBBLE line for a zeotropic blend, a single line for a one-component fluid — and subtracts the measured liquid-line temperature from it. Gauge pressure in psig and temperature in °F are the primary fields for the trade, with kPa/bar and °C accepted; the result is shown both as a kelvin/°C interval and a °F difference.

TypeInteractive engineering calculator

Calculator

Measured state

Measured at the gauge — psig is the trade primary; kPa/bar accepted.

kPa

Default 101.325 kPa (sea level); adjust for altitude. Absolute = gauge + atmospheric.

Liquid leaving the condenser. °F is the trade primary; °C accepted.

Subcooling (K / °C)14.7 K
Subcooling (°F difference)26.5 °F
bubble-line saturation temperature46.93 °C
Measured line temperature32.22 °C
Absolute pressure2859.23 kPa
Absolute pressure414.7 psia

R-410A is a zeotropic blend (R-32 / R-125 (50 / 50 wt%)); superheat is measured against the dew line and subcooling against the bubble line. Its committed temperature glide is 0.08 K, so at this pressure the bubble line lies about 0.08 K below the dew line.

Compare the result against the equipment manufacturer's specified target.

Audit trail
  • Absolute pressure = gauge + atmospheric = 2859.23 kPa (414.7 psia)
  • Bubble-line saturation temperature = 46.928 °C (interpolated on the committed R-410A bubble line, ln-P linear)
  • Subcooling = T_sat,bubble − T_line = 14.706 K = 26.471 °F difference
Copyable summary

On the committed R-410A dew line an absolute pressure of 1442.93 kPa is the 20 °C node, and on R-407C the 25 °C dew node sits at 1019.95 kPa — exact saturation-table values, read straight from the dataset, not interpolations.

T_sat is read by inverse interpolation (linear in ln P) on the committed refrigerant saturation tables, cross-checked against the refrigerant property hubs. Computes the value from the measured pair only — it states no target value and no service procedure.

Related: Superheat · Subcooling · R-410A properties

Formulas

Absolute pressure
P_abs = P_gauge + P_atm
Subcooling (against the bubble line for blends)
ΔT_sc = T_sat,bubble(P_abs) − T_line
Saturation temperature
T_sat(P_abs) by inverse interpolation, linear in ln(P) vs T, on the committed table

Diagram

TPbubble lineP_absT_satT_linesubcooling

Worked example

R-410A. Atmospheric pressure 101.325 kPa. Measured gauge pressure 1346.125 kPa (≈ 195.2 psig); measured liquid-line temperature 14.0 °C (57.2 °F). Find the subcooling.

  1. 01Absolute pressure = 1346.125 + 101.325 = 1447.45 kPa — exactly the committed 20 °C node on the R-410A bubble line
  2. 02Bubble-line saturation temperature T_sat,bubble = 20.0 °C (read directly at the node; no interpolation needed)
  3. 03Subcooling = T_sat,bubble − T_line = 20.0 − 14.0 = 6.0 K (= 6.0 °C)
  4. 04As a Fahrenheit difference: 6.0 K × 9/5 = 10.8 °F
Result

The subcooling is 6.0 K (10.8 °F) below the bubble-line saturation temperature of 20.0 °C.

FAQ

Why is subcooling measured against the bubble line?
For a zeotropic blend the refrigerant condenses over a temperature glide: the bubble line is the temperature at which the last vapour condenses at a given pressure, so the saturated liquid leaving the condenser is at the bubble-line temperature. Subcooling — the cooling of that liquid below saturation — is therefore measured from the bubble line. For a single-component fluid there is one saturation line and the distinction disappears.
What pressure should I enter — gauge or absolute?
Enter the gauge pressure your manifold reads (psig is the primary field; kPa or bar are accepted). The calculator adds the atmospheric pressure you set, default 101.325 kPa, to get the absolute pressure the saturation table is indexed on.
Does this tell me whether my subcooling is correct?
No. It computes the subcooling value from the pressure and temperature you measured. It states no target value and no service procedure — compare the result against the equipment manufacturer’s specified target.
How is a blend’s glide handled?
Superheat is read against the dew line and subcooling against the bubble line. The two lines differ by the blend’s temperature glide, which the page states from the committed blend record; for a near-azeotrope such as R-410A the glide is a fraction of a kelvin, while for R-407C it is several kelvin.

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