29.1 Pressure-Enthalpy Diagrams & Operating Pressures

29.1 Pressure-Enthalpy Diagrams & Operating Pressures

Trade Area E (Maintenance Analysis) is 9 percent of Class A and 10 percent of Class B. After you can hang the machine (Chapters 20–28), the CILB outline asks you to analyze it: pressure-enthalpy diagrams for refrigerants, operating pressures, and the superheat/subcooling arithmetic that turns two gauges and two thermometers into a diagnosis. The 2026 Air Conditioning CBT book that carries this cluster is Refrigeration & Air Conditioning Technology, 9th Edition (2021). Florida Building Code — Mechanical, 2023 Chapter 11 still names design pressure, relief, and the nitrogen test you already ran in Section 25.4. F.S. 489.105 still fences Class B at 25 tons cooling and 500,000 Btu heating in any one system — a 40-ton rooftop has the same P-h shape, but Class B does not contract that box.

Quick Answer: Plot the cycle on a P-h diagram: evaporator and condenser nearly horizontal, expansion vertical ((h_4 = h_3)), compressor up and to the right. Superheat = suction-line °F − SST. Subcooling = SCT − liquid-line °F. Typical P-T chart (not an exam key): R-22 about 68–70 psig at 40°F and 260 psig at 120°F; R-410A about 118 psig at 40°F and 418 psig at 120°F. High SH + low SC = undercharge. High SH + high SC = restriction.

Why the diagram is a diagnostic tool, not wallpaper

Chapter 17 used the P-h chart to design refrigerating effect. Trade E uses it to see what the running machine is actually doing. A manifold set without a P-T chart is two numbers with no temperature. A thermometer without a saturation temperature is a weather report. Pearson VUE will hand you a suction pressure, a liquid-line temperature, and a refrigerant name and ask whether the coil is starved, flooded, or sitting on a restriction.

Class A diagnoses without a statutory capacity cap. Class B uses the same gauges through 25 tons / 500,000 Btu per system. The Class B outline drops 25–100 ton, over-100 ton, and centrifugal install clusters; it does not drop the idea that a 40-ton circuit still has a condensing pressure. You still owe the theory.

Field gauges read psig. Many textbook P-h axes are psia ((\text{psia} = \text{psig} + 14.7) at sea level). Convert before you plot, or use a gauge-pressure P-T pocket chart for that refrigerant. Never put an R-22 manifold on an R-410A high side — 410A discharge on a 95°F Florida afternoon lives near 400 psig, and the hose and gauge must be rated for it.

Plotting compressor, condenser, metering device, evaporator

A simple vapor-compression cycle on the pressure-enthalpy (P-h or Mollier) diagram is four state points:

Evaporator (4→1) is a nearly horizontal line at evaporating pressure (P_e) through the dome, then a short run to the right of the saturated-vapor curve — that extra enthalpy is superheat. Refrigerating effect is (h_1 - h_4) Btu/lb of refrigerant. Condenser (2→3) is a nearly horizontal line at condensing pressure (P_c): desuperheat, condensing, then a short run to the left of the saturated-liquid curve — subcooling. Metering (3→4) is vertical (isenthalpic). A TXV, piston, or cap tube does not change enthalpy; it drops pressure so point 4 sits in the two-phase region. Compressor (1→2) is the only process that adds enthalpy (shaft work).

Dirty condenser, recirculated discharge air in a Florida screen well, or a 95°F outdoor on a black roof raises (P_c). The 2–3 line moves up. Compressor work (h_2 - h_1) grows, refrigerating effect shrinks, and COP falls. A starved evaporator drops (P_e); the 4–1 line moves down, mass flow falls, and the compressor runs hot because compression ratio rose.

Superheat and subcooling — formulas with worked numbers

Superheat (SH)

(\text{SH} = T_{\text{suction line}} - T_{\text{SST}})

Read suction pressure. Convert to saturated suction temperature (SST) on the P-T chart for that refrigerant. Subtract from the actual suction-line temperature at the evaporator outlet (TXV bulb location, on the pipe metal, insulated from attic air).

Subcooling (SC)

(\text{SC} = T_{\text{SCT}} - T_{\text{liquid line}})

Read liquid-line (or high-side) pressure. Convert to saturated condensing temperature (SCT). Subtract the actual liquid-line temperature leaving the condenser.

Worked superheat (R-410A). Suction 118 psig. A typical P-T chart (not an exam key) lists R-410A at 40°F ≈ 118–119 psig, so SST ≈ 40°F. Clamp probe on the suction line leaving the A-coil reads 52°F.

(\text{SH} = 52 - 40 = 12°F)

That is a normal air-conditioning TXV target (often 8–12°F; follow the listing). 0–2°F is flooding. 20–25°F is a starved coil.

Worked superheat (R-22). Suction 70 psig. Typical chart: R-22 at 40°F ≈ 68.5 psig, so SST ≈ 41°F. Suction line 53°F. SH = 53 − 41 = 12°F.

Worked subcooling (R-410A). Liquid-line pressure 418 psig. Typical chart: R-410A at 120°F ≈ 418–419 psig, so SCT ≈ 120°F. Liquid line leaving the condensing unit reads 110°F.

(\text{SC} = 120 - 110 = 10°F)

A typical TXV charging target is about 8–15°F subcooling on the manufacturer’s chart (often near 10°F). Near 0°F with bubbles in a sight glass is missing liquid (undercharge or a condenser that cannot condense). 20°F+ with a flooded evaporator is often overcharge.

Worked refrigerating effect (typical R-22 chart enthalpies, ASHRAE-style −40°F reference). Evaporator 40°F, 10°F superheat; condenser 100°F, 10°F subcooling. Saturated vapor at 40°F is about 108.2 Btu/lb; add vapor specific heat (~0.16 Btu/lb·°F) × 10°F → (h_1 \approx 109.8) Btu/lb. Saturated liquid at 100°F is about 39.3 Btu/lb; subtract liquid specific heat (~0.30) × 10°F → (h_3 \approx 36.3) Btu/lb. Expansion is isenthalpic, so (h_4 = 36.3). Refrigerating effect (h_1 - h_4 \approx 73.5) Btu/lb. Those enthalpies are typical chart values for teaching the plot, not numbers to memorize as CILB keys. On CBT you read (h) off the diagram you are given.

Charge a TXV system by subcooling (and verify SH). Charge a piston by superheat at the published outdoor condition. Charge a cap tube by weigh-in. Mixing those three methods is how a 3-ton Fort Myers split slugs a compressor.

Typical operating pressures — R-410A vs R-22 (P-T chart, not exam keys)

Use a published P-T chart in the reference stack. The round numbers below are typical saturation pressures at sea level so you can see that 410A is not “22 plus a little.” They are not secret exam answers and they shift with altitude, glide (on blends), and whether the chart is bubble or dew.

Running comfort cooling on a ~95°F Florida outdoor, indoor near 80°F DB / 67°F WB, a healthy split often shows:

• R-22: suction on the order of 60–80 psig, discharge on the order of 200–280 psig (condensing 15–30°F above outdoor).
• R-410A: suction on the order of 115–140 psig, discharge on the order of 350–450 psig under the same idea.

Those running bands are typical, not nameplate. Always convert this machine’s gauges through this refrigerant’s chart, then compute SH and SC. A 410A system standing at 75°F that equalizes at 100 psig is not “a little low” — it is far below ~218 psig saturation and is undercharged, still in a partial vacuum, or on the wrong refrigerant.

R-454B / R-32 replacements are not R-410A on the chart. Use the cylinder and the manufacturer’s P-T table. R-22 gauges and recovery cylinders are the wrong pressure class for 410A.

High superheat vs low superheat — the diagnostic table

High superheat means the vapor at the evaporator outlet is hotter than it should be — the coil ran out of liquid: undercharge or a restriction that will not feed. Low superheat means liquid is still boiling at the outlet — overcharge, a TXV overfeeding, or the air is not delivering enough heat (low load / low CFM). Do not add charge to a low-airflow coil because SH is low; you will slug the compressor when someone finally changes the filter.

Worked restriction vs undercharge. Both can show 24°F superheat. Measure subcooling. If SC is 2°F and the sight glass flashes, weigh the charge — undercharge. If SC is 22°F, the condenser is stacked with liquid and the evaporator is starving — find the restriction (often a plugged liquid-line drier after a burnout). Adding pounds to a restriction only raises head further.

Florida scenario

Palm Coast Comfort, certified Class B in Volusia, is called to a 3-ton R-410A split on a 95°F Daytona afternoon. Helper A reads 118 psig suction and 418 psig liquid, then “adds 410A until the suction is 70 like the old R-22 truck.” That 70 psig is an R-22 40°F number. On 410A, 70 psig is a deep freeze, not a 40°F coil — he just diagnosed the wrong chart. Helper B converts correctly: 118 psig ≈ 40°F SST, suction line 62°F, SH = 22°F; liquid 418 psig ≈ 120°F SCT, liquid line 118°F, SC = 2°F. That is undercharge, not a TXV to replace. They weigh in the missing charge to the nameplate plus line-set allowance, SH falls to 11°F, SC rises to 10°F. Same afternoon, a 30-ton 410A RTU on the strip center shows the same pattern — Class B may know the plot but may not contract the 30-ton system under 489.105.

Traps: (1) Using an R-22 “70 and 250” memory on R-410A. (2) Charging a TXV by superheat as if it were a piston. (3) Calling every high SH an undercharge without reading subcooling (restriction vs starve). (4) Adding charge to a low-SH coil that is really a dirty filter. (5) Plotting expansion as a pressure rise.