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Basics of Steam

# How to Read a Steam Table

Just as a map (or GPS navigation system) is necessary when driving in a new area or a flight timetable is indispensable when taking the plane, steam tables are essential to steam users in industry. This article will introduce steam tables, pointing out the different types and offering an overview of the different elements found within them.

#### Saturated Steam Tables

A saturated steam table is an indispensable tool for any engineer working with steam. It's typically used to determine saturated steam temperature from steam pressure, or the opposite: pressure from saturated steam temperature. In addition to pressure and temperature, these tables usually include other related values such as specific enthalpy (h) and specific volume (v).

The data found in a saturated steam table always refers to steam at a particular saturation point, also known as the boiling point. This is the point where water (liquid) and steam (gas) can coexist at the same temperature and pressure. Because H2O can be either liquid or gas at its saturation point, two sets of data are required: data for saturated water (liquid), which is typically marked with an "f" in subscript, and data for saturated steam (gas), which is typically marked using a "g" in subscript.

Example of Saturated Steam Table

Legend:

• P = Pressure of the steam/water
• T = Saturation point of steam/water (boiling point)
• vf = Specific volume of saturated water (liquid).
• vg = Specific volume of saturated steam (gas).
• hf = Specific enthalpy of saturated water (energy required to heat water from 0°C (32°F) to the boiling point)
• hfg = Latent heat of evaporation (energy required to transform saturated water into dry saturated steam)
• hg = Specific enthalpy of saturated steam (total energy required to generate steam from water at 0°C (32°F)).

Heating processes using steam generally use the latent heat of evaporation (Hfg) to heat the product. As seen in the table, this latent heat of evaporation is greatest at lower pressures. As saturated steam pressure rises, the latent heat of evaporation gradually decreases until it reaches 0 at supercritical pressure, i.e. 22.06 MPa (3200 psi).

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#### Two Formats: Pressure Based and Temperature Based

Since saturated steam pressure and saturated steam temperature are directly related to one another, saturated steam tables are generally available in two different formats: based on pressure and based on temperature. Both types contain the same data that is simply sorted differently.

#### Pressure Based Saturated Steam Table

Press.
(Gauge)
Temp. Specific Volume Specific Enthalpy
kPaG °C m3/kg kJ/kg
P T Vf Vg Hf Hfg Hg
0 99.97 0.0010434 1.673 419.0 2257 2676
20 105.10 0.0010475 1.414 440.6 2243 2684
50 111.61 0.0010529 1.150 468.2 2225 2694
100 120.42 0.0010607 0.8803 505.6 2201 2707

#### Temperature Based Saturated Steam Table

Temp. Press.
(Gauge)
Specific Volume Specific Enthalpy
°C kPaG m3/kg kJ/kg
T P Vf Vg Hf Hfg Hg
100 0.093 0.0010435 1.672 419.1 2256 2676
110 42.051 0.0010516 1.209 461.4 2230 2691
120 97.340 0.0010603 0.8913 503.8 2202 2706
130 168.93 0.0010697 0.6681 546.4 2174 2720
140 260.18 0.0010798 0.5085 589.2 2144 2733
150 374.78 0.0010905 0.39250 632.3 2114 2746

#### Different Units: Gauge Pressure and Absolute Pressure

Saturated steam tables can also use two different types of pressure: absolute pressure and gauge pressure.

• Absolute pressure is zero-referenced against a perfect vacuum.
• Gauge pressure is zero-referenced against atmospheric pressure (101.3 kPa, or 14.7 psi).

Saturated Steam Table using Absolute Pressure

Press
(Abs.)
Temp. Specific Volume Specific Enthalpy
kPa °C m3/kg kJ/kg
P T Vf Vg Hf Hfg Hg
0 -- -- -- -- -- --
20 60.06 0.0010103 7.648 251.4 2358 2609
50 81.32 0.0010299 3.240 340.5 2305 2645
100 99.61 0.0010432 1.694 417.4 2258 2675

Saturated Steam Table using Gauge Pressure

Press.
(Gauge)
Temp. Specific Volume Specific Enthalpy
kPaG °C m3/kg kJ/kg
P T Vf Vg Hf Hfg Hg
0 99.97 0.0010434 1.673 419.0 2257 2676
20 105.10 0.0010475 1.414 440.6 2243 2684
50 111.61 0.0010529 1.150 468.2 2225 2694
100 120.42 0.0010607 0.8803 505.6 2201 2707

Gauge pressure was created because it is often easier to reference measured pressure against the pressure we normally experience.

Steam tables based on gauge pressure indicate atmospheric pressure as 0, while steam tables based on absolute pressure indicate it as 101.3 kPa (14.7 psi). Also, to distinguish gauge pressure from absolute pressure, a "g" is typically added to the end of the pressure unit, for example kPaG or psig.

Converting Gauge Units to Absolute Units

#### For SI Units

Steam Pressure [kPa abs] = Steam Pressure [kPaG] + 101.3 kPa

#### For Imperial Units

Steam Pressure [psi abs] = Steam Pressure [psiG] + 14.7 psi

Important note: Problems can easily occur when absolute pressure is mistaken for gauge pressure (or vice versa), so it is always extremely important to pay close attention to the pressure units used in the table.

Summary Table

#### Gauge pressure:

• Zero-referenced against Atmospheric Pressure*
• Zero pressure = Atmospheric Pressure

#### Absolute pressure:

• Zero-referenced against Absolute Pressure
• Zero pressure = Perfect Vacuum

*Atmospheric pressure is 101.3 kPa (14.7 psi)

#### Superheated Steam Tables

Values related to superheated steam cannot be obtained through a regular saturated steam table, but rather require the use of a Superheated Steam Table. This is because the temperature of superheated steam, unlike saturated steam, can vary considerably for a same pressure.

In fact, the number of possible temperature-pressure combinations is so great that it would be virtually impossible to gather them all in a single table. As a result, a large number of superheated steam tables use representative pressure-temperature values to form a summary table.

Example of Superheated Steam Table

The above superheated steam table contains data about Specific Volume (Vg), Specific Enthalpy (Hg) and Specific Heat (Sg) at typical values of pressure and temperature.