8.3 Superheat, Subcooling, and Flash Gas

Superheat and subcooling are not extra properties you guess from a gauge. Each is a temperature difference against saturation at the pressure you actually have. Flash gas is what that high-side enthalpy becomes when you throttle. The three ideas share one discipline: convert pressure, look up Tsat and the enthalpies, then subtract.

Superheat

Superheat = Tactual,vapor − Tsat(Pmeasured)

The vapor must be superheated—to the right of the dome—for this number to be positive. A flooded separator running on the saturation curve has ~0°F useful superheat at the drum, even if the machine-room suction line later picks up a few degrees.

Method (every time):

1. Read suction (or discharge) pressure and convert to the table’s column (psig or psia = psig + 14.7).
2. Look up or interpolate Tsat at that pressure. That is the boiling temperature of ammonia in the two-phase region at that P. It is not the pipe-surface temperature.
3. Read the actual vapor temperature with a well-placed sensor (true suction line at the compressor, or a dedicated well, not a sunny discharge tube).
4. Subtract: SH = Tactual − Tsat. Units are °F of superheat, not psig.

Worked example — 32 psig suction at 26°F

A DX suction screen shows 32 psig and 26°F vapor.

• Absolute pressure: 32 + 14.7 = 46.7 psia.
• Table brackets: 15°F at 28.4 psig (43.14 psia) and 20°F at 33.5 psig (48.21 psia).
• Interpolation: (32 − 28.4) / (33.5 − 28.4) ≈ 0.71, so Tsat ≈ 18.5°F.
• Superheat = 26 − 18.5 = 7.5°F.

That is a tight but plausible DX superheat, not “26 degrees of superheat” and not “32 degrees of superheat.” If you wrongly took Tsat as 20°F because 32 psig is near 33.5 psig, you would report 6°F SH—close, but you skipped the interpolation the table is there for. If you used 22°F because “32 psig feels like about 20–22,” you would report 4°F and might chase a TXV that is actually fine. If you used 17°F from a rounded memory, you would report 9°F. The table-and-subtract method is the exam skill; 18.5°F / 7.5°F is the arithmetic for these rows.

Discharge superheat uses the same definition at discharge pressure. At 154 psig, Tsat ≈ 86°F. A 190°F discharge temperature is 104°F of discharge superheat (190 − 86). That number is a compressor-health and oil-cooling clue, not a TXV setting.

What superheat is for

• DX coils: enough superheat that liquid does not return; not so much that the coil is starved and TD blows up. Typical industrial DX targets are often in a roughly 8–15°F band at the bulb, but follow the valve manufacturer and the coil design—CIRO will give you screen values to interpret, not a single sacred setpoint.
• Flooded / overfeed: separator vapor is near saturated. Superheat at the compressor may still appear because of heat pickup in a long dry suction, but you do not set a TXV on a recirculator the way you set one on a DX coil.
• Diagnostics: rising superheat at a fixed load often means underfeed (TXV, strainer, flash-gas in the liquid line). Falling toward zero means overfeed or a failing bulb. Compare SH to evaporator TD (air or fluid in minus Tsat), which is a later diagnostics chapter.

Subcooling

Subcooling = Tsat(Pliquid) − Tactual,liquid

Pressure is the condenser outlet / liquid-line pressure, not the compressor discharge if there is a large drop, and not the receiver pressure if you are measuring before a drop. Temperature is the liquid temperature, not the condensing temperature from a PT chart.

Worked example — 154 psig liquid at 76°F

• P = 154 psig → 168.7 psia → Tsat ≈ 86°F (Section 8.2 interpolation).
• Tliquid = 76°F.
• Subcooling = 86 − 76 = 10°F.

If the liquid were still at 86°F at 154 psig, subcooling would be 0°F: saturated liquid on the left dome. If someone reports “76 degrees of subcooling,” they used the thermometer as the answer.

Liquid enthalpy falls by roughly the liquid specific heat times the subcooling. Between 80°F and 90°F, hf rises about 1.16 Btu/lb per °F, so 10°F of subcooling lowers h by about 12 Btu/lb (138.9 → ~127 Btu/lb). That lower h is the whole point of the next paragraph.

Why subcooling pays

• Less flash gas at the expansion device (lower entering h).
• More net refrigerating effect per pound (NRE = h1 − h4, and h4 = h3, so lowering h3 raises NRE).
• Fuller liquid lines and better expansion-valve behavior; a saturated liquid line that loses pressure in a riser can flash before the valve and starve a DX coil.

Subcooling is produced by extra condenser surface, a dedicated subcooler, or a cold liquid-suction heat exchanger. It is not produced by raising head pressure. Raising head pressure raises Tsat; if liquid temperature does not fall, you have not gained subcooling, you have moved to a hotter isobar with a larger hf.

Flash gas and quality

Throttling is isenthalpic. High-side liquid at h3 enters the two-phase region at evaporator pressure with h4 = h3. At that low pressure the saturated liquid enthalpy hf is much smaller than h3, so part of the stream must vaporize to keep enthalpy constant.

x = (h − hf) / hfg

where h = h3 = h4, and hf, hfg are taken at evaporator saturation. Quality x is the mass fraction that is already vapor as the stream enters the evaporator. That vapor is flash gas. It occupies volume in the distributor and coil, and its enthalpy has already been “spent.” Only (1 − x) of the mass is liquid that can still absorb hfg in the coil. Equivalently:

NRE = h1 − h4 ≈ hg − h3 for saturated leaving vapor (flooded/overfeed), or hg + cp,v·SH − h3 for DX suction with superheat.

Flash gas that forms inside a properly fed evaporator is just the start of boiling. Flash gas that forms in the liquid line before the coil—from pressure drop, heat pickup, or zero subcooling plus a riser—does no useful refrigerating in that coil; it is parasitic vapor the expansion device and distributor have to swallow.

Worked example — 86°F saturated liquid into a 5°F evaporator

Use the table:

• High side, saturated 86°F liquid: h3 = hf = 138.9 Btu/lb (no subcooling).
• Low side, 5°F: hf = 48.4 Btu/lb, hg = 614.4 Btu/lb, hfg = 566.0 Btu/lb.

x = (138.9 − 48.4) / 566.0 = 90.5 / 566.0 = 0.160 → 16.0% flash gas.

NRE for saturated leaving vapor: 614.4 − 138.9 = 475.5 Btu/lb. Check: (1 − 0.160) × 566.0 = 475.4 Btu/lb. Same energy, two pictures: 16% of the mass is already vapor; 84% is liquid that can still boil.

Same evaporator with 10°F subcooling

Liquid at 76°F still at 154.5 psig. hf at 75°F is 126.3 Btu/lb and at 80°F is 131.9 Btu/lb, so 76°F liquid is about 127.4 Btu/lb.

x = (127.4 − 48.4) / 566.0 = 79.0 / 566.0 = 0.140 → 14.0% flash.

NRE = 614.4 − 127.4 = 487.0 Btu/lb, about 12 Btu/lb more than the saturated-liquid feed (~2.4% more refrigerating effect per pound) and two percentage points less flash. That is why subcooling is worth condenser surface: you did not change the low-side Tsat, you changed h3.

Same 86°F saturated liquid into a 0°F evaporator

0°F: hf = 42.9, hfg = 569.7, hg = 612.6.

x = (138.9 − 42.9) / 569.7 = 96.0 / 569.7 = 0.168 → 16.8% flash.

NRE = 612.6 − 138.9 = 473.7 Btu/lb. Colder evaporators flash slightly more from the same high-side liquid because hf on the low side is lower (the numerator grows). High head pressure is still the bigger lever: it raises h3.

If the high side were 20°F colder liquid at the same 5°F evaporator (66°F liquid, hf ≈ 116 Btu/lb from the 65–70°F rows), x would fall near 12%. Head pressure and subcooling both write the flash-gas story through h3.

Putting a screen together

A sample-style plant might show 32 psig / 26°F suction (7.5°F SH), 154 psig condensing, and 76°F liquid (10°F SC). You now know, without memorizing RETA’s copyrighted screens:

• Suction state is slightly superheated DX vapor at 46.7 psia, not flooded 18.5°F liquid.
• The condenser is sitting on the common ~86°F / 154 psig isobar.
• Liquid is subcooled, so flash at a 5°F load is nearer 14% than 16%.
• None of those conclusions used a geometric mean, a gauge-pressure compression ratio, or a memorized 400-row chart. They used +14.7, table lookup, interpolation, and subtraction.

Exam traps

• Trap: calling the suction temperature the superheat. 26°F is the vapor temperature. Superheat is 26 minus Tsat at that pressure.
• Trap: Tsat at 32 psig = 32°F. Ammonia at 32 psig is about 18.5°F. 32°F ammonia is about 48 psig.
• Trap: 4–6°F superheat because someone remembered “20°F sat at 32 psig.” 20°F sat is 33.5 psig. Interpolate; 32 psig is 18.5°F, so 26°F vapor is 7.5°F SH.
• Trap: subcooling = liquid temperature. Subcooling is Tsat − Tliquid.
• Trap: flash gas = useless boiling inside the evaporator. Flash from the valve is the two-phase mixture the coil is supposed to finish boiling. Flash before the evaporator (liquid-line vapor) is the parasitic part.
• Trap: quality x = hfg / h. The definition is (h − hf) / hfg at the low-side saturation row, with h equal to the high-side liquid enthalpy.
• Trap: raising head pressure to “make subcooling.” That raises Tsat. Unless liquid actually leaves colder relative to the new Tsat, you only raised hf and increased flash.