4.4 Pressure–Enthalpy Diagrams, Compression Ratio, and Cycle Analysis
Key Takeaways
- Plotting the refrigeration cycle on a pressure-enthalpy chart is an explicit competency on five separate HVAC Excellence task-list sheets.
- The P-H chart's saturated liquid line and saturated vapor line meet at the critical point, dividing the chart into subcooled liquid, the saturated mixture region, and superheated vapor.
- Net refrigerating effect is the enthalpy gained across the evaporator, and heat of compression is the enthalpy added by the compressor.
- Compression ratio equals absolute discharge pressure divided by absolute suction pressure, using psia rather than psig; ratios above about 10:1 sharply reduce volumetric efficiency.
- Coefficient of performance from the P-H chart equals net refrigerating effect divided by heat of compression, so a cycle with 65 BTU/lb of effect and 20 BTU/lb of compression work has a COP of 3.25.
4.4 Pressure–Enthalpy Diagrams, Compression Ratio, and Cycle Analysis
"Plotting the refrigeration cycle on a pressure enthalpy chart" appears verbatim on the Residential Air Conditioning, Commercial Air Conditioning, Commercial Refrigeration, Mini-Splits, and Low-GWP sheets of the HVAC Excellence Competency and Task List. It is one of the most heavily repeated competencies in the entire document, and it is the tool that turns two gauge readings and two temperatures into a statement about energy.
Section 4.3 covered pressure–temperature relationships, superheat, and subcooling. The P-H chart adds the third axis of the story: enthalpy, the total heat content per pound of refrigerant.
1. Reading the Chart
A pressure–enthalpy (Mollier) diagram is refrigerant-specific — an R-410A chart cannot be used for R-454B.
- Vertical axis: absolute pressure (psia), on a logarithmic scale. Logarithmic spacing compresses the enormous pressure range into readable space and makes the constant-pressure processes plot as straight horizontal lines.
- Horizontal axis: enthalpy (BTU per pound), linear. Moving right means the refrigerant contains more heat.
- The saturation dome: the saturated liquid line on the left and the saturated vapor line on the right meet at the critical point at the top. Above the critical point no amount of pressure will condense the refrigerant.
- Three regions: left of the liquid line is subcooled liquid; inside the dome is a saturated liquid-vapor mixture; right of the vapor line is superheated vapor.
- Quality lines inside the dome mark the percentage of vapor by weight — the 0.2 line means 20% of the mass has flashed to vapor.
- Constant temperature lines run vertically in the subcooled region, horizontally inside the dome (because temperature and pressure are locked together during a phase change), and drop steeply to the right in the superheat region.
- Constant entropy lines run steeply up and to the right in the superheat region; ideal compression follows one of them.
2. Plotting the Four Processes
Consider an R-410A system with a 40°F evaporator (118 psig / 132.7 psia), a 120°F condenser (418 psig / 432.7 psia), 10°F of superheat, and 12°F of subcooling.
| Point | Location | State | Enthalpy |
|---|---|---|---|
| 1 | Compressor suction | 50°F vapor at 132.7 psia (40°F sat + 10°F SH) | $h_1 = 121\text{ BTU/lb}$ |
| 2 | Compressor discharge | Superheated vapor at 432.7 psia | $h_2 = 141\text{ BTU/lb}$ |
| 3 | Liquid line, metering-device inlet | 108°F liquid at 432.7 psia (120°F sat − 12°F SC) | $h_3 = 56\text{ BTU/lb}$ |
| 4 | Evaporator inlet | 40°F mixture at 132.7 psia, about 22% flash gas | $h_4 = 56\text{ BTU/lb}$ |
1 → 2: Compression. Up and to the right, following (approximately) a constant-entropy line. Pressure and enthalpy both rise. 2 → 3: Condensation. Straight left at constant pressure. The line crosses the vapor line (desuperheating ends), traverses the dome (latent condensation), crosses the liquid line, and continues into the subcooled region. 3 → 4: Expansion. Straight down, vertically. Expansion through a fixed orifice or TXV is isenthalpic — constant enthalpy — because no heat and no work cross the boundary. The refrigerant does not lose heat in the metering device; some of its liquid flashes to vapor, and that flash gas cools the remaining liquid to the new saturation temperature. 4 → 1: Evaporation. Straight right at constant pressure, crossing the vapor line and continuing into superheat.
3. The Numbers the Chart Gives You
Net Refrigerating Effect (NRE) — the useful cooling each pound performs:
Heat of Compression — work the compressor adds:
Total Heat of Rejection (THOR) — everything the condenser must remove: This is why a condenser is always larger than the evaporator: it rejects both the heat absorbed indoors and the work the compressor added.
Coefficient of Performance (theoretical):
Mass flow rate required for a given capacity: A 3-ton system (36,000 BTU/hr) needs $36{,}000 \div 65 = 554\text{ lb/hr}$, or 9.2 lb/min.
Why subcooling matters, quantitatively. Increase subcooling from 12°F to 22°F and $h_3$ falls to roughly 51 BTU/lb. NRE becomes $121 - 51 = 70\text{ BTU/lb}$ — a 7.7% capacity increase for the same compressor work, and flash gas at the metering device drops from about 22% to about 17%. This is the entire argument for liquid-line subcooling and for head-pressure control on low-ambient systems.
4. Compression Ratio
Absolute, not gauge. Add 14.7 to each gauge reading. Using psig produces a wildly wrong answer, and that error is a favorite exam distractor.
For the example above: $\text{CR} = 432.7 \div 132.7 = 3.26{:}1$ — comfortable for an air conditioner.
| Application | Typical CR |
|---|---|
| Residential/commercial air conditioning | 2.5:1 to 4:1 |
| Medium-temperature refrigeration | 4:1 to 8:1 |
| Low-temperature refrigeration (single stage) | 8:1 to 12:1 |
| Above 12:1 | Two-stage or cascade required |
Consequences of a high compression ratio
- Volumetric efficiency collapses. Vapor trapped in the compressor's clearance volume re-expands on the down-stroke and blocks the suction valve from opening until pressure falls below suction. The higher the ratio, the longer that re-expansion takes and the less fresh vapor enters. At 10:1 a reciprocating compressor may move less than half its displacement.
- Discharge temperature rises. Less mass flow means less returning suction vapor to cool the motor and less oil circulation, so windings and discharge valves overheat. Sustained discharge temperatures above roughly 225°F at the line (about 300°F internally) begin breaking down oil, forming acid and carbon.
- Diagnostic value. A low-temperature freezer running at 14:1 is not a compressor problem — it is a symptom. Look for a dirty or undersized condenser (high head), a starved evaporator or restricted suction line (low suction), or non-condensables raising head pressure.
5. Using the Chart to Diagnose
| Chart symptom | Physical cause |
|---|---|
| Point 1 far right of the vapor line (high superheat) with low suction pressure | Undercharge, restriction, or starved metering device |
| Point 1 on or left of the vapor line (no superheat) | Flooding — overcharge, oversized/hunting TXV, low airflow; liquid is reaching the compressor |
| Point 3 on the liquid line (zero subcooling) with a sight glass full of bubbles | Undercharge, or a restricted liquid-line drier |
| Point 3 far left of the liquid line (very high subcooling) with high head | Overcharge, or liquid backed up in the condenser from a restriction downstream |
| The 2 → 3 pressure line sitting abnormally high | Dirty condenser, low condenser airflow, high ambient, or non-condensables |
| $h_2 - h_1$ abnormally small with normal pressures | Compressor not compressing — worn rings, leaking valves, or a broken scroll |
A worked field example. A walk-in freezer using R-404A reads 8 psig suction and 235 psig discharge. That is at the practical single-stage limit. The technician cleans the condenser and head pressure falls to 190 psig: $\text{CR} = 204.7 \div 22.7 = 9.0{:}1$. Volumetric efficiency recovers, the box pulls down, discharge temperature drops, and the oil stops degrading — all diagnosed from two gauges and a division.
An R-410A system plots with h1 = 122 BTU/lb at the compressor suction, h2 = 143 BTU/lb at discharge, and h3 = h4 = 54 BTU/lb entering the evaporator. What are the net refrigerating effect and the theoretical coefficient of performance?
A low-temperature R-404A system reads 5 psig suction and 250 psig discharge. What is the compression ratio, and what is the most likely consequence?
Why does the expansion process from the liquid line through a TXV plot as a vertical line on a pressure-enthalpy diagram?