8.1 Reading the Ammonia P-h Diagram

The pressure-enthalpy (P-h) diagram is the working map of an industrial ammonia plant. CIRO Heat Flow items, and many Monitoring screens, ask you to recognize where a state sits on that map and which process just moved it. You are not being tested on artistic chart-reading. You are being tested on whether you know that pressure is vertical, enthalpy is horizontal, that the dome is saturation, and that the four legs of the vapor-compression cycle are four different kinds of line.

English-unit ammonia charts (R-717) plot specific enthalpy h in Btu/lb. Many U.S. industrial charts still use the ASHRAE convention in which saturated-liquid enthalpy is 0 Btu/lb at −40°F. SI charts use kJ/kg and a different datum. The shape of the cycle does not change with the datum; every enthalpy difference (work, heat, net refrigerating effect) is the same. If the on-screen CIRO table uses a different zero, ignore the absolute numbers’ origin and work with the printed hf, hg, and hfg.

Axes

• Vertical axis: pressure. Plant gauges read psig. The chart is in absolute pressure (psia). Logarithmic spacing is common so 10 psia and 200 psia both fit. A point at “154 psig condensing” is not 154 on the chart; it is 154 + 14.7 = 168.7 psia (the next section drills the conversion).
• Horizontal axis: specific enthalpy. Moving right means the refrigerant gained energy (evaporator heat in, or compressor work in). Moving left means it rejected energy (condenser / oil cooler / subcooler).

A thermodynamic state on this plot is a pressure-enthalpy pair. Temperature, entropy, quality, and specific volume appear as overlay families. For CIRO you must know what the overlays mean; you do not need to sketch a freehand Mollier chart from memory.

The saturation dome and five regions

The thick curve that looks like an inverted U is the saturation dome.

The critical point sits at the top of the dome. For ammonia it is about 270°F and about 1,646 psia. Every industrial condenser you will ever run is far below that. Do not treat “we are near critical” as an ammonia-plant explanation for high head pressure.

Under the dome, T and P are not independent. That is why a pressure-temperature (PT) chart works: pick a saturation pressure and you have picked the boiling/condensing temperature, and vice versa. Off the dome you need two properties—usually measured pressure and measured temperature—to know the state. Superheat and subcooling (Section 8.3) are exactly that second-property problem.

Overlay lines you will actually use

• Isobars (constant pressure) are horizontal. Condensers and evaporators are drawn as isobars.
• Isenthalps (constant enthalpy) are vertical. A throttle is an isenthalp.
• Isotherms (constant temperature) coincide with isobars inside the dome (because Tsat is fixed at a given P). In the superheat region they fall away to the right; in the liquid region they are steep.
• Isentropes (constant entropy, s) slope up and to the right through the superheat region. Isentropic compression follows one of those lines.

If you remember only one geometric fact for the exam, remember this contrast: vertical = throttling (h constant); horizontal = heat exchanger (P roughly constant); up-and-right = compressor (P and h both rise).

The four cycle processes

Label a single-stage dry-suction cycle 1–2–3–4.

1 → 2 Compression. Suction vapor at evaporator pressure is raised to condensing pressure. Enthalpy increases. The compressor work per pound is h2 − h1. Reversible adiabatic compression is isentropic: the path follows a constant-s overlay to discharge pressure, ending at h2s. A real screw or reciprocating machine has isentropic efficiency

ηs = (h2s − h1) / (h2,actual − h1)

less than 1. Industrial ammonia screws often live in a roughly 70–85% band depending on built-in volume ratio, slide-valve position, oil temperature, and how far the compression ratio sits from the Vi the rotor was cut for. You do not need a manufacturer’s map on CIRO. You do need the direction: actual h2 is larger than h2s (the discharge point sits farther right), actual discharge temperature is hotter, and you buy more kWh per pound of ammonia. Thermosiphon oil coolers exist because that extra enthalpy has to leave the machine somehow.

2 → 3 High-side heat rejection. Pressure is approximately constant (pipe and condenser drops are real but small compared with the lift). Draw a horizontal line to the left. Three sub-steps live on that isobar: (1) desuperheat of discharge gas down to saturated vapor, (2) condensation across the dome at condensing pressure, (3) optional subcooling left of the saturated-liquid line. Heat rejected per pound is h2 − h3. That is why a dirty condenser, a high wet-bulb, or non-condensables—anything that raises the isobar—moves state 2 up and usually to the right, and the motor pays for it.

3 → 4 Expansion (throttling). Hand expansion valves, TXVs, EEVs, and high-side floats all throttle high-pressure liquid down to evaporator pressure. A throttle is isenthalpic: h4 = h3. On the P-h diagram that is a vertical line down. Temperature falls to the new Tsat; part of the liquid vaporizes as flash gas. The valve does not produce compressor work and does not reject condenser heat. The enthalpy number on the leaving two-phase stream is the same number the liquid had going in.

4 → 1 Evaporation. Pressure is approximately constant. Draw a horizontal line to the right. The two-phase mixture that entered at 4 absorbs heat until it reaches the compressor suction state. Net refrigerating effect (NRE) per pound is h1 − h4. In a DX coil, state 1 is usually superheated (right of the dome) so liquid does not return to the compressor. In a flooded or liquid-overfeed plant, vapor leaving the separator is very near saturated vapor on the right dome; the extra circulated liquid is not “superheat insurance,” it is a heat-transfer and oil-return strategy (later chapters).

“Roughly isobaric” is the exam language for condensers and evaporators because a healthy ammonia heat exchanger does not drop tens of psi. A condenser stacked with non-condensables is the important exception: compressor discharge pressure sits above the ammonia saturation pressure that matches the coil temperature. On the chart it still looks like a higher isobar; the extra pressure is air (or hydrogen), not ammonia. That diagnosis belongs with approach and purging. For this chapter, know that the process is still drawn horizontal.

Placing a plant point

1. Convert the gauge reading to psia (or use the table’s psig column consistently).
2. Find that pressure on the vertical axis or the matching table row.
3. Compare measured temperature to Tsat at that pressure: equal → on the dome; lower (liquid) → subcooled; higher (vapor) → superheated.
4. Read h from the chart or from hf / hg / a superheat or subcooling correction.

You will not memorize the whole ammonia chart. RETA’s on-screen references exist so you can look values up. What you must bring is the geometry of the processes and the discipline to convert gauge to absolute before you talk about compression ratio or interstage pressure.

Worked region identification (use these as screen drills)

The numbers below are ASHRAE/NIST-style saturated ammonia (R-717) properties, English units, liquid enthalpy 0 at −40°F. On-screen CIRO tables may round a tenth of a psi or a degree; the method is what is scored.

Screen A — DX suction. Suction pressure 19.6 psig, suction temperature 20°F. Convert: 19.6 + 14.7 = 34.3 psia, which is saturation at 5°F. The vapor is at 20°F, so it is 15°F superheated, sitting to the right of the dome on the 34.3 psia isobar. Compression starts from that superheated point, not from hg at 5°F.

Screen B — sample-style high side. Condensing pressure 154 psig (168.7 psia). Saturation temperature at that pressure is about 86°F (Section 8.2). Liquid line 76°F is 10°F subcooled, left of the saturated-liquid line on the 169 psia isobar. Discharge temperature 190°F at the same 154 psig is superheated discharge, right of the dome, before the condenser has done its job.

Screen C — flooded recirculator suction. Separator pressure 15.7 psig (30.4 psia) and vapor at 0°F. That pair is saturated (0°F sat P is 15.7 psig). State 1 sits on the right dome, not out in superheat. Do not “add superheat” that the plant is not running.

Operator traps on the chart

• Trap: plotting psig on a psia axis. 154 psig is 169 psia. Compression ratios and geometric-mean interstage pressures use absolute pressure only.
• Trap: drawing compression as a vertical line. Vertical is throttling. Compression raises enthalpy.
• Trap: drawing condensation as a temperature drop at constant h. Condensation is constant pressure, falling h.
• Trap: walking the compressor up the saturated-vapor curve. Discharge of a real machine is superheated, well to the right of the dome. Oil cooling and the condenser’s desuperheating zone exist because of that.
• Trap: treating ηs = 100% as normal. Actual h2 is to the right of h2s. Higher discharge temperature is not proof the PT chart is wrong; it is proof the compressor is not isentropic.
• Trap: memorizing the whole chart instead of using the on-screen table. Know the landmarks (−28°F at 0 psig, 0°F near 16 psig, 86°F near 154 psig) so you can sanity-check a lookup. Interpolate the rest.

Latent heat hfg shrinks as saturation temperature rises. That is visible as a narrower dome at high pressure. It is one reason high condensing pressure hurts capacity: you throttle from a higher hf, you flash more, and each remaining pound of liquid has less hfg left to use in the coil.