8.2 Saturated Properties, psia vs psig, and Saturation Temperature

Once a state is on the dome, ammonia has only one independent property: pick temperature and saturation pressure is fixed, or pick pressure and saturation temperature is fixed. That pairing is the saturated-properties (PT) table RETA puts on screen. CIRO does not require you to recite every row. It does require you to convert gauge to absolute, to recognize a handful of landmarks, and to interpolate between two printed rows without inventing a geometric mean in psig.

psia versus psig

Gauge pressure (psig) is pressure above the surrounding atmosphere. Absolute pressure (psia) is pressure above a perfect vacuum.

psia = psig + atmospheric pressure

Unless the problem or the on-screen data gives a local barometer, use 14.7 psi as atmospheric pressure:

psia = psig + 14.7

Examples you will use constantly:

• 0 psig = 14.7 psia (atmospheric boiling).
• 32 psig = 32 + 14.7 = 46.7 psia.
• 154 psig = 154 + 14.7 = 168.7 psia.
• A compound-gauge reading of 4.3 psi vacuum is 14.7 − 4.3 = 10.4 psia (about −40°F saturated ammonia), not “−4.3 psig” used as if it were a positive head.

Some tables print both columns. If you look up psig, do not add 14.7 again. If you look up psia, do not treat the number as a gauge reading. Mixing the columns is the single most common arithmetic failure on heat-flow items.

Why the conversion is not optional

Compression ratio is discharge absolute pressure divided by suction absolute pressure:

CR = Pdis,psia / Psuc,psia

Geometric-mean interstage pressure for a two-stage machine (later chapter) is

Pint ≈ √(Psuc,psia × Pdis,psia)

also in psia. Both formulas are meaningless in gauge. A 5°F evaporator at 19.6 psig (34.3 psia) lifting to an 86°F condenser at 154.5 psig (169.2 psia) has

CR = 169.2 / 34.3 ≈ 4.9

The gauge ratio 154.5 / 19.6 ≈ 7.9 is a different, wrong machine. The geometric mean of the gauge numbers is equally wrong. If an item asks for compression ratio or interstage pressure and you have only psig, add 14.7 first.

Landmark saturated ammonia (R-717)

Values below are consistent with ASHRAE Fundamentals / NIST-style saturation data. Enthalpies use the English-unit convention hf = 0 at −40°F. Pressures are rounded to the tenth that operators actually quote. Use the on-screen table as the authority on test day; use these landmarks to catch a slipped row.

Read the table the way a plant reads it:

• −28°F at 0 psig is ammonia’s normal boiling point. An atmospheric liquid-drain or an open-to-atmosphere receiver is not “zero pressure ammonia at 0°F.”
• 0°F is about 15.7 psig (30.4 psia), not 15.7 psia. 15.7 psia would be almost atmospheric—near −28°F, not 0°F. That swap fails a freezer-room item.
• 5°F is about 19.6 psig (34.3 psia), not 19.6 psia. 19.6 psia is only 4.9 psig, which is colder than 5°F (near −18°F). The number 19.6 is famous; the unit is what people drop.
• 20°F is about 33.5 psig (48.2 psia). It is not 18.3 psig. 18.3 psia / 3.6 psig is −20°F. Mixing 20°F with −20°F is the other classic row error.
• 86°F is about 154 psig (169 psia). RETA sample-style operating screens often show a condensing pressure near 154 psig. That is a ~86°F saturated condensing temperature, not a mystery number.

hf, hg, and hfg travel with the same rows. You need them for flash-gas and NRE work in the next section. Notice hfg falls as Tsat rises: 570 Btu/lb at 0°F versus 501 Btu/lb at 86°F. High-side liquid is also warmer, so hf is larger (138.9 Btu/lb at 86°F versus 48.4 at 5°F). Both effects worsen flash and NRE when head pressure is high.

How to interpolate (do this on paper, not in your head)

Suppose the table prints 15°F at 28.4 psig (43.14 psia) and 20°F at 33.5 psig (48.21 psia), and a suction gauge reads 32 psig.

Step 1. Stay in one pressure unit. 32 psig already matches the psig column, or convert: 32 + 14.7 = 46.7 psia.

Step 2. Confirm 32 psig sits between 28.4 and 33.5 psig. Do not jump to 20°F just because 32 is “near 33.”

Step 3. Linear interpolation is good enough between 5°F rows:

(32 − 28.4) / (33.5 − 28.4) = 3.6 / 5.1 ≈ 0.71 of the way from 15°F toward 20°F.

Tsat ≈ 15 + 0.71 × 5 ≈ 18.5°F.

The same fraction on psia: (46.7 − 43.14) / (48.21 − 43.14) = 3.56 / 5.07 ≈ 0.70, Tsat ≈ 18.5°F. If your two conversions disagree by more than about 0.5°F, you mixed columns.

What 32 psig is not: it is not 32°F (a leftover PT memory from another refrigerant). Saturated ammonia at 32°F is about 48 psig. It is not 20°F exactly (that is 33.5 psig). It is not 17°F or 22°F unless your printed rows interpolate there. On exam day, the on-screen table wins.

Interpolating the 154 psig condenser

85°F: 151.7 psig (166.4 psia). 90°F: 165.9 psig (180.6 psia).

154 psig is (154 − 151.7) / (165.9 − 151.7) = 2.3 / 14.2 ≈ 0.16 of the way from 85 toward 90.

Tsat ≈ 85 + 0.16 × 5 ≈ 85.8°F, which everyone in the trade calls 86°F. That is why a 154 psig high-side reading is the usual teaching condensing point: it is a round gauge number that lands on a round saturation temperature.

Using the table the way CIRO uses it

• Do not memorize the whole chart. Memorize the conversion, the −28°F / 0°F / 5°F / 20°F / 86°F landmarks, and interpolation.
• Match the column. If the screen says 19.6 psig, you are at 5°F, not at 19.6 psia.
• Watch the sign. −20°F is 3.6 psig; 20°F is 33.5 psig. A missing minus sign is a 30 psig error.
• Vacuum is still a pressure. Below −28°F the compound gauge reads inches of mercury or psi vacuum. Convert to psia before you compute a ratio.
• Gauges have error; the physics does not. A 1 psi error near 5°F is about 1°F of Tsat error (dP/dT ≈ 0.8–1.0 psi/°F in that band). Near 86°F, dP/dT is about 2.8 psi/°F, so the same 1 psi error is only about 0.4°F. Interpolation still matters more on the low side.

Worked compression-ratio check (5°F / 86°F)

A single-stage freezer plant shows suction 19.6 psig and discharge 154.5 psig. Convert first:

• Psuc = 19.6 + 14.7 = 34.3 psia (5°F sat).
• Pdis = 154.5 + 14.7 = 169.2 psia (86°F sat).
• CR = 169.2 / 34.3 ≈ 4.93, call it 4.9.

Wrong paths:

• 154.5 / 19.6 ≈ 7.9 (gauge ratio).
• 86 / 5 = 17 (temperature ratio is not compression ratio).
• √(19.6 × 154.5) ≈ 55 psig as an “interstage” (geometric mean of gauges). The legal geometric mean is √(34.3 × 169.2) ≈ 76 psia ≈ 61 psig. Two-stage selection is its own chapter; the only point here is absolute pressure.

A +20°F cold-room plant at 33.5 psig suction and the same 154.5 psig discharge has CR = 169.2 / 48.2 ≈ 3.5. Same condenser, lower lift, because suction psia is higher. That is why you cannot compare two plants’ “150 pound head / 30 pound suction” stories without converting.

Exam traps

• Trap: 0°F = 15.7 psia. 0°F is 15.7 psig / 30.4 psia.
• Trap: 5°F = 19.6 psia. 5°F is 19.6 psig / 34.3 psia.
• Trap: 20°F ≈ 18 psig. That is −20°F. 20°F is 33.5 psig.
• Trap: 154 psig is 154°F or 154 psia. 154 psig ≈ 86°F ≈ 169 psia.
• Trap: using 14.7 twice, or using 15 psi “because it is round,” then arguing with the table’s tenth. Stick with 14.7 unless the stem gives a barometer.
• Trap: borrowing an R-22 or R-410A PT memory. Ammonia at 32 psig is about 18.5°F, not 32°F.