Properties of Steam

• Saturated steam properties — enter pressure or temperature, get Tsat/Psat, vf, vg, hf, hg, hfg, sf, sg
• Superheated steam properties — enter pressure + temperature, get interpolated enthalpy
• Dryness fraction (wet steam) calculator — enter pressure/temperature + known enthalpy, get quality x, or enter x to get mixture properties

The data is built from standard steam-table reference values with linear/bilinear interpolation between table points — good for practical engineering estimates.

 

Calculation of Steam Properties at Saturated T and P

• Steam tables are one of the most important references used in boiler design, thermal engineering, power plants, process industries, and energy audits. They provide the thermodynamic properties of water and steam at different pressures and temperatures.
• For a given saturated pressure, the corresponding saturation temperature and other steam properties can be obtained from the saturated steam tables. Similarly, when the saturation temperature is known, the corresponding saturation pressure and steam properties can be determined.

 

1. What Is Saturated Steam?

Saturated steam is steam that exists at the saturation temperature corresponding to its pressure. At this condition, water and steam can coexist in equilibrium.

For example, approximately:

Saturation Pressure	Saturation Temperature
1 bar(a)	99.6 °C
2 bar(a)	120.2 °C
5 bar(a)	151.8 °C
10 bar(a)	179.9 °C
20 bar(a)	212.4 °C
40 bar(a)	250.4 °C
60 bar(a)	275.6 °C
100 bar(a)	311.0 °C

Values are approximate; engineering calculations should use the applicable steam table/standard.

Important Steam Properties

The main properties normally obtained from steam tables are:

• Saturation temperature, Tsat
• Specific volume, vv
• Specific enthalpy, hh
• Specific entropy, ss
• Internal energy, uu

For saturated water and saturated steam, the following notation is commonly used:

Symbol	Description
hfh_f	Enthalpy of saturated water
hfgh_{fg}	Latent heat of vaporization
hgh_g	Enthalpy of saturated steam
sfs_f	Entropy of saturated water
sfgs_{fg}	Entropy of vaporization
sgs_g	Entropy of saturated steam
vfv_f	Specific volume of saturated water
vgv_g	Specific volume of saturated steam
ufu_f	Internal energy of saturated water
ugu_g	Internal energy of saturated steam

The relationships are:

hg=hf+hfgh_g

. Calculation When Saturated Pressure Is Given

Suppose the saturated steam pressure is:

P=10  bar(a)P=10\;bar(a)

Step 1 – Find saturation temperature

From the saturated steam pressure table:

P=10  bar(a)P=10\;bar(a)

corresponds approximately to:

Tsat=179.9∘CT_{sat}=179.9^\circ C

Therefore, the steam temperature at saturation is approximately 180°C.

Step 2 – Read steam properties from the table

At 10 bar(a), the saturated steam table gives approximately:

Property	Symbol	Approximate value
Saturation temperature	TsatT_{sat}	179.9 °C
Enthalpy of saturated water	hfh_f	763 kJ/kg
Latent heat	hfgh_{fg}	2,014 kJ/kg
Enthalpy of saturated steam	hgh_g	2,777 kJ/kg
Entropy of saturated water	sfs_f	2.14 kJ/kg·K
Entropy of saturated steam	sgs_g	6.59 kJ/kg·K
Specific volume of saturated steam	vgv_g	0.195 m³/kg

Thus, if the boiler produces dry saturated steam at 10 bar(a), its specific enthalpy is approximately:

hg≈2777  kJ/kg

Calculation When Saturated Temperature Is Given

Now suppose the saturation temperature is known:

Tsat=180∘C

The first step is to determine the corresponding saturation pressure.

From the saturated temperature steam table:

Tsat≈180∘C

corresponds approximately to:

Psat≈10.0  bar(a)

The remaining properties can then be obtained from the same table.

Therefore:

Tsat=180∘C⇒Psat≈10  bar

 Saturated Water vs Saturated Steam

A steam table normally provides two limiting conditions:

Saturated water

At the beginning of vaporization:

x=0

and the properties are represented by:

h=

Saturated dry steam

At the completion of vaporization:

x=1

and:

h=hgh

Here xx is called the dryness fraction or steam quality.

Wet Steam Calculation

If the steam contains both water and steam, it is called wet steam.

For wet steam with dryness fraction xx:

h=hf+x*hfg=h_f+xh_{fg}

Similarly:

s=sf+x*sfgs=

and:

v=vf+x(vg−vf)

For most engineering calculations, it is very small compared with vg, so:

v≈xv

Example

Consider steam at 10 bar(a) with:

x=0.90

Using approximate steam-table values:

hf=763  kJ/kg; hfg=2014  kJ/kg        hfg=2014\;kJ/kg

Therefore:

h=hf+x hfg =2576 Kj/kg

Thus, 90% dry steam at 10 bar(a) has an enthalpy of approximately 2576 kJ/kg.

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7. Why Absolute Pressure Must Be Used

One of the most common mistakes in steam calculations is confusing gauge pressure with absolute pressure.

The relationship is:

Pabs =Pgauge+Patm

At approximately atmospheric pressure:

Patm ≈1 .013  bar

Therefore, if the boiler pressure gauge reads:

Pg=10  bar(g)

then:

Pab =10+1.013P_{abs}

The steam-table calculation must normally be performed using 11.013 bar(a), not 10 bar(a).

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8. Pressure-to-Temperature Calculation

For a saturated steam system, pressure and temperature are not independent variables.

If pressure is known:

P→Tsat

If temperature is known:

Tsat→Psat

This relationship can be represented as:

Known pressure → Steam table → Saturation temperature + properties

or:

Known temperature → Steam table → Saturation pressure + properties

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9. Interpolation Between Steam-Table Values

Sometimes the required pressure or temperature does not appear exactly in the steam table.

For example, suppose the required pressure is:

P=7.5  bar(a)

but the table contains values at 7 and 8 bar(a).

Linear interpolation can be used:

Y=Y1+P−P1P2−P1(Y2−Y1)Y=Y_1+ \frac{P-P_1}{P_2-P_1}(Y_2-Y_1)

where:

• YY = required steam property
• Y1Y_1 = property at lower pressure
• Y2Y_2 = property at higher pressure
• P1P_1 = lower pressure
• P2P_2 = higher pressure

The same method can be used for temperature, enthalpy, entropy, specific volume, etc., when appropriate.

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10. Steam Property Calculation Procedure

A practical calculation procedure is:

Case A – Saturated pressure is given

1. Identify whether pressure is absolute or gauge.
2. Convert gauge pressure to absolute pressure if required.
3. Locate the pressure in the saturated steam table.
4. Read:
   • Tsat
   • hf
   • hfg}
   • hg
   • sf
   • sg
   • vf
   • vg
5. If the exact pressure is unavailable, interpolate between adjacent values.
6. Use the appropriate equations for wet steam if dryness fraction is known.

Case B – Saturated temperature is given

1. Identify the saturation temperature.
2. Locate it in the saturated-temperature steam table.
3. Read corresponding saturation pressure.
4. Obtain the remaining thermodynamic properties.
5. Interpolate if necessary.

 Application in Boiler and Power-Plant Calculations

Steam properties obtained from steam tables are used extensively for:

• Boiler efficiency calculations
• Steam generation calculations
• Turbine performance
• Feedwater heating
• Deaerator calculations
• Condensate calculations
• Steam piping design
• Heat exchanger calculations
• Energy audits
• Cogeneration plants
• Process steam calculations
• Boiler heat balance
• Steam consumption calculations

For example, boiler steam generation can be estimated using:

Q=m˙(h_steam  −h_feedwater)  

 

Accurate values of h_steam and h_feedwater are therefore essential.

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12. Saturated Steam Property Calculation – Quick Reference

Given condition	Steam-table section	Main property obtained
Saturated pressure	Pressure table	Tsat
Saturated temperature	Temperature table	Psa
Dry saturated steam	x=1	hg,sg,
Saturated water	x=0	hf,sf,vfh_f,s_f,v_f
Wet steam	0<x<10<x<1	h,s,vh,s,v using quality
Gauge pressure	Convert to absolute	Pabs=Pg+Patm

Conclusion

• Steam tables provide a reliable and standardized method for calculating the thermodynamic properties of saturated water and steam.
• For a given saturated pressure, the saturation temperature and properties such as enthalpy, entropy, specific volume, and latent heat can be directly obtained from the table.
• Conversely, a known saturation temperature can be used to determine the corresponding saturation pressure.
• For practical boiler and thermal-system calculations, it is particularly important to use absolute pressure, distinguish between saturated, wet, and superheated steam, and interpolate carefully when the exact operating condition is not listed in the steam table.