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Refrigeration

Superheat Calculator

Computes refrigerant superheat as the measured suction-line temperature minus the dew-line saturation temperature at the absolute pressure, reading T_sat by inverse interpolation on the committed refrigerant saturation tables.

Superheat is how far the refrigerant vapour leaving the evaporator sits above its saturation temperature at the measured pressure. This calculator reads the saturation temperature off the committed saturation table of the selected refrigerant — the DEW line for a zeotropic blend, a single line for a one-component fluid — and subtracts it from the measured suction-line temperature. 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.

Vapour leaving the evaporator. °F is the trade primary; °C accepted.

Superheat (K / °C)5.6 K
Superheat (°F difference)10.1 °F
dew-line saturation temperature4.37 °C
Measured line temperature10.00 °C
Absolute pressure914.91 kPa
Absolute pressure132.7 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 = 914.91 kPa (132.7 psia)
  • Dew-line saturation temperature = 4.368 °C (interpolated on the committed R-410A dew line, ln-P linear)
  • Superheat = T_line − T_sat,dew = 5.632 K = 10.138 °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
Superheat (against the dew line for blends)
ΔT_sh = T_line − T_sat,dew(P_abs)
Saturation temperature
T_sat(P_abs) by inverse interpolation, linear in ln(P) vs T, on the committed table

Diagram

TPdew lineP_absT_satT_linesuperheat

Worked example

R-410A. Atmospheric pressure 101.325 kPa. Measured gauge pressure 1341.605 kPa (≈ 194.6 psig); measured suction-line temperature 27.0 °C (80.6 °F). Find the superheat.

  1. 01Absolute pressure = 1341.605 + 101.325 = 1442.93 kPa — exactly the committed 20 °C node on the R-410A dew line
  2. 02Dew-line saturation temperature T_sat,dew = 20.0 °C (read directly at the node; no interpolation needed)
  3. 03Superheat = T_line − T_sat,dew = 27.0 − 20.0 = 7.0 K (= 7.0 °C)
  4. 04As a Fahrenheit difference: 7.0 K × 9/5 = 12.6 °F
Result

The superheat is 7.0 K (12.6 °F) above the dew-line saturation temperature of 20.0 °C.

FAQ

Why is superheat measured against the dew line?
For a zeotropic blend the refrigerant boils over a temperature glide: the dew line is the temperature at which the last liquid evaporates at a given pressure, so the saturated vapour leaving the evaporator is at the dew-line temperature. Superheat — the warming of that vapour above saturation — is therefore measured from the dew 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 superheat is correct?
No. It computes the superheat 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.
Why does R-744 (CO₂) get refused at high pressure?
The committed R-744 saturation table ends below the critical point (30.98 °C / 7377.3 kPa). Above the critical point R-744 is transcritical, with no saturation line, so the calculator refuses rather than returning a meaningless number.

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