Saturated Steam Table: Pressure and Temperature

At 15 psig (about 29.7 psia) saturated steam sits at 249.7°F / 121.0°C. That is why 15 psig is the classic autoclave and SIP set point — it is the pressure that delivers the 121°C condition the sterilization cycle is built around. Run the header at 12 psig instead and the steam is 117.6°C, which is not the same cycle.

Saturated steam is steam in equilibrium with its own liquid. While both phases are present, pressure and temperature are locked together — you cannot set them independently. Add heat and you make more steam at the same temperature; raise the pressure and the temperature follows it up the saturation curve. That single degree of freedom is what makes a steam table useful: measure one property and you know the other.

The physics is identical. Saturated clean steam at 15 psig is 121°C just as plant steam is. What differs is what the steam carries: plant steam contains boiler treatment chemicals (amines, phosphates) and pipe-scale debris, and clean steam is generated from pretreated water in a stainless generator so its condensate meets WFI or purified-water chemistry. The table tells you the temperature; it says nothing about whether the condensate is acceptable to touch product.

Superheated steam is steam heated above its saturation temperature at that pressure, so it is off the table entirely. It sterilizes poorly because sterilization depends on condensation: steam gives up its latent heat when it condenses on a cold surface, and that energy transfer is what kills organisms. Superheated steam behaves more like a hot dry gas, so it heats without condensing and the lethality the cycle assumes never arrives.

Air and other non-condensables do not condense, so they collect at the coldest points and in dead legs, forming pockets that steam cannot displace. Inside that pocket the temperature is below saturation even though the gauge reads the right pressure. The gauge says the vessel is at 121°C; the pocket is not. This is the single most common reason a validated SIP cycle fails at one thermocouple location while passing everywhere else.

Yes, if the table is in gauge pressure (psig) as this one is, and you are near sea level. The conversion assumes a standard atmosphere of 14.696 psia. At meaningful altitude the absolute pressure for a given gauge reading is lower, so the true saturation temperature is lower than the table shows. For validated cycles the correct practice is to work in absolute pressure and verify with calibrated thermocouples rather than trusting a gauge.

About 15.1 psig at sea level. The values are steep enough that small pressure errors matter: 14.5 psig gives 120.4°C and 15.5 psig gives 121.5°C. A cycle written to hold 121°C with a gauge tolerance of plus or minus 1 psi is implicitly allowing roughly a 1°C band, which is why SIP cycles are controlled and documented on temperature, not pressure.

The saturation temperatures are computed from the IAPWS-IF97 Region 4 backward equation, the international standard formulation for the thermodynamic properties of water and steam. Spot values agree with published steam tables to within 0.05°F: 14.696 psia gives 211.95°F against a reference of 212°F, and 100 psia gives 327.82°F against 327.8°F.

The energy absorbed turning boiling water into steam at constant temperature, and released again when the steam condenses. At atmospheric pressure it is roughly 970 BTU per pound, against about 180 BTU to heat that same pound from room temperature to boiling. Almost all the useful energy in steam is latent, which is why steam heats by condensing rather than by cooling.

Because of that latent heat. A pound of condensing steam releases around 970 BTU at one temperature; a pound of hot water gives up only about one BTU per degree it falls. Steam therefore delivers a large, isothermal heat input through a small pipe, which is exactly what a jacket or a sterilisation cycle wants.

The mass proportion that is actually vapour rather than entrained water droplets. Quality of 0.95 means five percent of the mass is liquid water being carried along. Wet steam carries less latent heat per pound than the table implies and erodes valve trim, so specifications for clean steam commonly call for a minimum dryness.

Heat loss from uninsulated distribution, carryover from a boiler or generator operating above its rated load, and pressure drops that flash condensate back into the line. The remedy is upstream: insulate, drain properly at low points, and separate the entrained water before the point of use rather than trying to correct quality at the equipment.

Steam that forms when hot condensate is released to a lower pressure. The condensate is above the saturation temperature of the lower pressure, so part of it instantly boils. It is not a fault; it is thermodynamics. It matters because flash steam in a condensate return line occupies far more volume than the liquid did and can pressurise the return system.

A trap removes condensate while holding back live steam. If it fails closed the condensate floods the space, the heat transfer surface is blanketed and the equipment under-performs. If it fails open it passes live steam continuously, which is a direct and often invisible energy loss. Trap survey is the routine maintenance item with the fastest payback on most steam systems.

Its heat transfer collapses, because condensate is a poor conductor compared with condensing steam. The symptom is an exchanger that cannot hold setpoint at high load while the steam pressure reads correctly. It is usually blamed on undersizing, and it is very often a drainage or trap problem instead.

Meaningfully, because the table is in gauge pressure and assumes 14.696 psia of atmosphere. Atmospheric pressure falls roughly half a psi per thousand feet, so a gauge reading at altitude corresponds to a lower absolute pressure and therefore a lower saturation temperature than the sea-level table shows. Validated cycles work in absolute pressure for exactly this reason.

Gauge pressure is measured relative to local atmosphere; absolute pressure is measured from a vacuum. At sea level they differ by 14.696 psi. Gauges read gauge, thermodynamic equations need absolute, and mixing the two is one of the most common arithmetic errors in steam calculations.

Multiply psi by 0.0689 to get bar. Fifteen psig is about 1.03 barg. Take care with the same gauge-versus-absolute distinction, since European datasheets frequently quote bar absolute, which is a full atmosphere different from bar gauge.

F0 is the lethality of a sterilisation cycle expressed as the equivalent number of minutes at 121 degrees C. It is accumulated over the whole cycle, including heat-up and cool-down, by integrating the measured temperature against a reference z-value of 10 degrees C. Because lethality is exponential in temperature, time spent slightly below 121 contributes very little while time slightly above contributes a great deal.

There is no single answer, because the requirement is an F0 value rather than a clock time, and the cycle must deliver it at the slowest-heating location. A commonly cited reference condition is 15 minutes at 121 degrees C, but the real acceptance criterion is the accumulated F0 at the worst-case thermocouple, which is what the validation establishes.

Because filtration removes particles and microorganisms but not dissolved volatiles. Boiler treatment chemicals such as amines carry over in the vapour phase and would pass straight through a filter into product contact. Clean steam is therefore raised from pretreated feedwater in a dedicated stainless generator so the condensate chemistry is controlled at source.

Culinary steam is plant steam filtered to a defined standard for direct food contact, typically through a fine filter close to the point of use. Clean steam is generated from pretreated water so the condensate itself meets purified water or WFI chemistry. Culinary steam is a filtration standard; clean steam is a generation standard, and pharmaceutical work generally requires the latter.

Because condensate pooling in a low point displaces steam from that volume, and an area full of water at less than saturation temperature does not sterilise even though the vessel gauge is correct. Sloped lines, drainable low points and correctly placed traps are part of the sterilisation design, not a plumbing afterthought.

Direct. An autoclave holds a set pressure and relies on the saturation relationship to deliver the corresponding temperature, which is why a jacketed autoclave is essentially a pressure vessel running on this curve. The same caveat applies as anywhere else: air trapped in the load creates cool pockets while the chamber gauge reads correctly, which is why air removal precedes the sterilisation phase.

For heating duty, yes, and it is the usual method: changing steam pressure moves the condensing temperature directly along this curve. For tight product-temperature control it is less suitable on its own, because the jacket surface sits at the steam temperature rather than at the product setpoint, which can overheat a shear-sensitive or heat-labile product at the wall.