The medium-temperature (MT) suction gauge on an R-744 system reads 396 psi g. What is the saturation temperature there, in °C?
CO₂ (R-744) practice: pressures, states and the critical point
For UEERA0066 Repair and service carbon dioxide refrigeration systems and the pressure–temperature side of UEERA0006 Apply safety awareness and legal requirements for carbon dioxide refrigerant, electives of UEE32225 Certificate III in Air Conditioning and Refrigeration, and for technicians moving onto CO₂ plant. Every figure is worked out by Kelvin on R-744’s public table computed with CoolProp, and every answer shows the working.
1The ideas
The ideas
R-744 · CO₂
- High pressures
R-744 works at far higher pressures than the HFCs. With liquid in it at 20 °C, R-744 stands at 5,729.1 kPa abs, 56.28 bar g; R-134a at the same temperature stands at 571.7 kPa abs, 4.70 bar g.
Both are their tables’ 20 °C rows.
- Critical point
30.98 °C and 7,377.3 kPa abs (72.76 bar g). Above it liquid and vapour are no longer distinct: nothing boils or condenses, and there is no saturation temperature to read.
Air warmer than 30.98 °C can’t cool R-744 below its critical temperature, so an air-cooled high side can’t condense then.
- Triple point
−56.56 °C and 517.964 kPa abs (4.17 bar g). Below that pressure CO₂ can’t be liquid: it is solid (dry ice) or vapour. Liquid let down below it turns to dry ice and vapour.
The table starts just above it, at −56 °C.
- At atmospheric pressure
Most refrigerants have a boiling point at atmospheric pressure. R-744 has none: atmospheric pressure is below its triple point, so there is no liquid CO₂ there at all, and dry ice turns straight to vapour.
- Subcritical and transcritical
A system whose high side is below the critical pressure is subcritical: it condenses, at a saturation temperature read off a P–T chart as for any refrigerant. One whose high side is above it is transcritical: a gas cooler cools the gas without condensing it, and its pressure isn’t tied to a temperature.
- Standing pressure
With the plant off, a vessel with liquid in it settles at the saturation pressure of the temperature around it: at 28 °C, the table’s last row, 6,891.8 kPa abs (67.90 bar g).
Above 30.98 °C no liquid is left, and the pressure depends on how much CO₂ is in the vessel for its volume, not on the temperature alone.
- Superheat and subcooling
Read as for any single-component refrigerant, from the saturation temperature at the line’s pressure, and only below the critical point: above it there is no saturation temperature to read them from.
- Try it
R-744’s table and P–h diagram, its printable P–T chart (PDF), and the superheat and subcooling calculator set to R-744. For a transcritical system worked through on R-744’s P–h diagram, see the worked example Transcritical CO₂ supermarket pack, cycle to issued report.
2Worked example: a cascade’s R-744 low stage
R-744 is a single-component refrigerant, so its superheat and its subcooling are each read from the one saturation temperature at the line’s pressure. This low stage evaporates at about −30 °C and condenses at about −5 °C in the cascade heat exchanger, well below the critical point. Its gauges read in bar g, so the working starts by turning bar g into kPa abs. Every figure is the R-744 table’s, the same the calculator reads.
Superheat
Suction line · saturation temperature
- Suction line gauge
- 13.3bar g
- Suction line temperature
- −24.0°C
Step 1: Absolute pressure
gives 1,431.3 kPa abs
13.30 bar g × 100 = 1,330.0 kPa g
1,330.0 kPa g + 101.325 kPa = 1,431.3 kPa abs
A gauge reads above the atmosphere. The table is in absolute pressure.
Subcooling
Liquid line · saturation temperature
- Liquid line gauge
- 29.5bar g
- Liquid line temperature
- −9.0°C
Step 1: Absolute pressure
gives 3,051.3 kPa abs
29.50 bar g × 100 = 2,950.0 kPa g
2,950.0 kPa g + 101.325 kPa = 3,051.3 kPa abs
A gauge reads above the atmosphere. The table is in absolute pressure.
3Guided exercise
Work the readings through yourself: the absolute pressure, then the saturation temperature, then the answer. Each step opens once the one before is checked. A figure within a few tenths counts, since a printed chart is read to about that.
Guided exercise
Work each step; Kelvin checks it
On an R-744 system the medium-temperature (MT) suction gauge reads 435 psi g, and the suction line, at the evaporator outlet, reads −0.1 °C. What is the superheat, in K?
01The absolute pressure
02Saturation temperature
03Superheat
4Practice set
Every kind of question in the module, 11 in all, with a new set of figures each time. Check each one to see Kelvin’s answer and the working.
0 right of 0 checked, out of 11. Only each question’s first check counts.
A subcritical R-744 system is to condense at 26 °C. What should its gauge read, in psi g?
An R-744 receiver with liquid in it has stood with the plant off at 20 °C, long enough to settle at that temperature. What does its gauge read, in psi g?
An R-744 receiver with liquid in it stands at 16 °C with the plant off. The room warms to 24 °C and the receiver settles at that temperature. How far does its pressure rise, in kPa?
An R-744 receiver held liquid when the plant was stopped. It has since stood at 42 °C, long enough to settle at that temperature. What sets its pressure now?
R-744's critical point is 7,377.3 kPa abs, at 30.98 °C: above it liquid and vapour are no longer distinct. What is that pressure in psi g?
A line holding R-744 is let down until its gauge reads 40.3 bar g. Can liquid CO₂ exist in it at that pressure?
The high-side (discharge) gauge on an R-744 system reads 80.2 bar g. Is the system running subcritical or transcritical?
On an R-744 system a gauge reads 26.0 bar g where a probe on the pipe reads 4.2 °C. What state is the refrigerant in there?
On an R-744 system the medium-temperature (MT) suction gauge reads 24.4 bar g, and the suction line, at the evaporator outlet, reads −5.0 °C. What is the superheat, in K?
On a subcritical R-744 system the liquid line gauge reads 42.0 bar g, and the liquid line, at the condenser outlet, reads 1.5 °C. What is the subcooling, in K?
5Check a worksheet
Each printed worksheet carries its own code. Type it here to open the same questions and check your answers, with the working.
Note
For teachers
Practice and teaching material mapped to the unit's code. It isn't an RTO assessment tool: your assessment tools stay yours, and whether you use any of this as evidence is your call. Worked out in the browser. Nothing a student types is sent anywhere, and there are no accounts to make. This module practises the pressure–temperature side of UEERA0006 only. Its safety and legal requirements aren’t covered here. See Refrigeration training with Kelvin for the worksheets and the units this maps to.
Note
Verify before relying on these figures
The saturation pressures and temperatures, and the critical and triple points, are computed with CoolProp from published equations of state; a temperature between its 2 K rows is interpolated. They must be verified by a qualified professional before they’re relied on for a real system.