Steam tables are reference charts that list the thermodynamic properties of water at different temperatures and pressures. They tell you things like how much energy water holds, how much space it takes up, and at what point it changes phase. Learning to use them comes down to three skills: knowing which table to open, finding your values in the grid, and interpolating when your exact number isn’t listed.
What Steam Tables Actually Contain
Each row or entry in a steam table gives you a set of properties for water at a specific condition. The most common properties you’ll look up are:
- Temperature in °C (or °F in Imperial tables)
- Pressure in kPa or MPa (or psi in Imperial tables)
- Specific volume in m³/kg, which tells you how much space a kilogram of water or steam occupies
- Enthalpy in kJ/kg, representing the total energy content including pressure-related work
- Internal energy in kJ/kg, the energy stored in the substance itself
- Entropy in kJ/(kg·K), which tracks energy dispersal and is essential for analyzing efficiency
For saturated conditions (where liquid and vapor coexist), you’ll see subscripts: “f” for the liquid value, “g” for the vapor value, and “fg” for the difference between them. So “hf” is the enthalpy of saturated liquid, “hg” is the enthalpy of saturated vapor, and “hfg” is the energy required to vaporize the liquid at that pressure.
The Three Types of Steam Tables
Most textbooks and reference sheets organize steam data into three separate tables, each designed for a different situation.
Saturated Tables by Temperature
These list properties at every degree (or every few degrees) of temperature along the saturation line. If you know the temperature and you know the water is at its boiling point (saturated), this is where you start. You look up your temperature in the left column, then read across to find the corresponding saturation pressure, specific volume, enthalpy, and entropy for both the liquid and vapor phases. Use this table when temperature is given in whole, convenient values.
Saturated Tables by Pressure
These contain the same data as the temperature-based table but are organized with pressure as the entry point in the left column, using even increments of pressure. If a problem gives you a pressure and tells you the steam is saturated, this table gets you to the answer faster. The layout is identical otherwise: you read across the row to find saturation temperature, specific volumes, enthalpies, and entropies for liquid and vapor.
Superheated Steam Tables
Once steam is heated beyond its saturation temperature at a given pressure, it becomes superheated, and the saturated tables no longer apply. Superheated tables use a grid layout where pressure values run across the top of the page in blocks, and temperature values run down the left column within each block. You first find the correct pressure block, then scan down to your temperature to read off the properties. There are no “f” and “g” columns here because superheated steam exists entirely as vapor.
Deciding Which Table to Use
The first step in any steam table problem is figuring out the phase of the water. This determines which table you open. If you’re given both a temperature and a pressure, compare them to the saturation values. Look up the given pressure in the saturated table and check the corresponding saturation temperature. If your actual temperature is lower than the saturation temperature, the water is a compressed (subcooled) liquid. If it matches the saturation temperature, you’re in the two-phase region and should use the saturated tables. If your actual temperature is higher, the steam is superheated, and you move to the superheated table.
For example, if you’re told water is at 200 kPa and 200°C, you’d first check the saturated table at 200 kPa. The saturation temperature there is about 120°C. Since 200°C is well above 120°C, the steam is superheated, and you’d use the superheated table at 200 kPa to find your properties.
Reading Values in the Two-Phase Region
When water exists as a mixture of liquid and vapor (inside a boiler during heating, for instance), you need one extra piece of information: the quality, often labeled “x.” Quality is the fraction of the total mass that exists as vapor. A quality of 0 means all liquid; a quality of 1 means all vapor.
To find any property in the two-phase region, use this pattern:
property = (value for liquid) + quality × (difference between vapor and liquid values)
So for enthalpy: h = hf + x × hfg. The saturated tables give you hf, hg, and hfg directly. This same formula works for specific volume, internal energy, and entropy. Just swap in the corresponding f, g, and fg values.
How to Interpolate Between Listed Values
Steam tables are printed at discrete intervals. Your value will often fall between two rows. When it does, you use linear interpolation, which assumes the property changes in a straight line between the two nearest listed points. It’s an approximation, but a very good one for the small gaps in most tables.
Say you need the enthalpy at a temperature of 153°C, and your table lists values at 150°C and 155°C. The formula is:
unknown value = lower value + [(your temperature − lower temperature) / (upper temperature − lower temperature)] × (upper value − lower value)
In concrete terms: if enthalpy at 150°C is 2746 kJ/kg and enthalpy at 155°C is 2763 kJ/kg, then at 153°C you’d calculate 2746 + [(153 − 150) / (155 − 150)] × (2763 − 2746) = 2746 + 0.6 × 17 = 2756.2 kJ/kg.
The same approach works for any pair of properties. If you’re in the superheated table and your pressure falls between two listed pressure blocks, you’d interpolate between the values from each block at the same temperature. Double interpolation (interpolating in both pressure and temperature) follows the same logic applied in two steps: first interpolate along one variable, then along the other.
The Upper Limit: Water’s Critical Point
Steam tables for saturated conditions stop at a specific ceiling: the critical point. For water, this occurs at 22.09 MPa and 374.14°C. Above this point, there is no distinct boundary between liquid and vapor. The physical differences between phases, such as density and energy content, shrink as you approach the critical point and become zero when you reach it. The energy required to vaporize water also drops to zero at this threshold.
If your conditions exceed the critical pressure and temperature, standard saturated tables won’t help. You’d need supercritical water property data, which is organized differently and used primarily in advanced power plant design.
SI vs. Imperial Units
Steam tables come in two unit systems. SI tables use kPa or MPa for pressure, °C for temperature, m³/kg for specific volume, and kJ/kg for energy properties. Imperial (English) tables use psi for pressure, °F for temperature, ft³/lb for specific volume, and BTU/lb for energy. The structure and reading method are identical across both systems. Just make sure you’re consistent: mixing units from SI and Imperial tables is one of the most common errors in thermodynamics coursework.
Practical Tips for Avoiding Mistakes
Always determine the phase first. Jumping straight to a table without confirming whether the water is subcooled, saturated, or superheated is the single most common source of wrong answers. Write down the saturation temperature or pressure for your given condition before doing anything else.
Watch the units on pressure carefully. Some tables use kPa, others use MPa, and mixing them up by a factor of 1,000 is easy to do under time pressure. Check the column header before reading any value.
When interpolating, keep track of which property is the “known” one (what you’re entering the table with) and which is the “unknown” (what you’re solving for). Setting up the fraction backwards will give you a nonsensical answer. A quick sanity check: your interpolated result should always fall between the two bracketing values in the table. If it doesn’t, you’ve made an arithmetic or setup error.