The suction gauge on an R-404A system reads 222 kPa g. What is the absolute pressure, in kPa abs?
UEERA0036 practice: saturation, superheat and subcooling
For UEERA0036 Establish the basic operating conditions of vapour compression systems, a core unit of UEE32225 Certificate III in Air Conditioning and Refrigeration. Every figure is worked out by Kelvin on the public refrigerant tables computed with CoolProp, and every answer shows the working.
1The ideas
The ideas
UEERA0036
- Gauge and absolute
A gauge reads against the air around it: gauge pressure. Absolute pressure is measured from a perfect vacuum. Kelvin takes the air as a standard atmosphere, so absolute = gauge + 101.325 kPa.
Saturation tables are in absolute pressure; a printed P–T chart may give either, so check which.
- Saturation
Where liquid and vapour sit together, the pressure fixes the temperature: the saturation temperature. Raise the pressure and it boils hotter. That is the pressure–temperature relationship a P–T chart prints.
- Superheat
The suction line temperature less the saturation temperature at the suction pressure: how far the vapour leaving the evaporator is above the temperature its last liquid boiled off at.
- Subcooling
The saturation temperature at the liquid line pressure less the liquid line temperature: how far the liquid leaving the condenser is below the temperature it would start to boil at.
- Dew, bubble and glide
A single-component refrigerant, such as R-32 or R-134a, boils at one temperature at a given pressure. A zeotropic blend, such as R-454B or R-407C, boils across a range: its liquid starts to boil at the bubble point, and the last of it boils off at the dew point. The difference is its glide.
So superheat is read from the dew point (only above it is the refrigerant all vapour) and subcooling from the bubble point (only below it is it all liquid).
- Try it
The superheat and subcooling calculator does all of this for 28 refrigerants and shows its working; the printable P–T charts are the same figures on paper.
2Worked example: a split system on R-454B
R-454B is a zeotropic blend with about a kelvin of glide, so its superheat is read from the dew point and its subcooling from the bubble point. Step through each half; every figure is the R-454B table’s, the same the calculator reads.
Superheat
Suction line · dew point
- Suction line gauge
- 720kPa g
- Suction line temperature
- 12.0°C
Step 1: Absolute pressure
720.0 kPa g + 101.325 kPa = 821.3 kPa abs
A gauge reads against the air around it, taken as a standard atmosphere, 101.325 kPa. The table is in absolute pressure.
Subcooling
Liquid line · bubble point
- Liquid line gauge
- 2,350kPa g
- Liquid line temperature
- 37.5°C
Step 1: Absolute pressure
2,350.0 kPa g + 101.325 kPa = 2,451.3 kPa abs
A gauge reads against the air around it, taken as a standard atmosphere, 101.325 kPa. The table is in absolute pressure.
3Guided exercise
Work the readings through yourself: the absolute pressure, then the saturation temperature (or the dew or bubble point for a blend), 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
An R-449A system: the suction gauge reads 76 psi g and the suction line, at the evaporator outlet, reads 16.3 °C. What is the superheat, in K?
01The absolute pressure
02Dew point
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.
R-1234yf is a single-component refrigerant. Its gauge reads 131 kPa g. What is its saturation temperature, in °C?
An idle R-134a system has stood at 4 °C long enough to settle. What should its gauge read, in kPa g?
R-454B is a blend. Its suction gauge reads 357 kPa g. What is its dew point, in °C?
An R-744 system: the suction gauge reads 1,269 kPa g and the suction line, at the evaporator outlet, reads −19.1 °C. What is the superheat, in K?
An R-407C system: the liquid line gauge reads 19.5 bar g and the liquid line, at the condenser outlet, reads 36.0 °C. What is the subcooling, in K?
R-407C at 39 psi g: what is its glide (the dew point less the bubble point), in K?
R-454C is a zeotropic blend. At the suction pressure, which temperature is its superheat measured from?
R-449A is a zeotropic blend. At the liquid line pressure, which temperature is its subcooling measured from?
The suction line reads colder than the dew point at the suction pressure, so the superheat works out negative. What does that point to?
A refrigeration gauge reads gauge pressure (kPa g). What is gauge pressure measured against?
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. See Refrigeration training with Kelvin for the worksheets and the units this maps to.
Note
Verify before relying on these figures
The saturation temperatures are computed with CoolProp from published equations of state and interpolated between its 2 K rows. They must be verified by a qualified professional before they’re relied on for a real system.