Steam Properties

Steam properties govern the design and operation of steam heating systems, humidification equipment, and power generation cycles. Understanding the thermodynamic behavior of water in its vapor phase is essential for accurate system sizing, energy analysis, and performance prediction.

Saturation Temperature-Pressure Relationship

The saturation state represents equilibrium between liquid and vapor phases. At any given pressure, water boils at a specific saturation temperature (Tsat), and conversely, at any temperature, a corresponding saturation pressure (Psat) exists. This relationship is nonlinear and described by the Clausius-Clapeyron equation:

dP/dT = hfg / (Tsat × vfg)

Where:

• hfg = latent heat of vaporization (Btu/lbm or kJ/kg)
• Tsat = absolute saturation temperature (°R or K)
• vfg = difference in specific volume between vapor and liquid (ft³/lbm or m³/kg)

Representative Saturation Data:

Steam heating systems typically operate between 2-15 psig (16.7-29.7 psia), corresponding to saturation temperatures of 219-250°F. Higher pressure systems (50-150 psig) are used in industrial applications and district heating.

Latent Heat of Vaporization

The latent heat of vaporization (hfg) represents the energy required to convert saturated liquid to saturated vapor at constant temperature and pressure. This property decreases with increasing pressure, reaching zero at the critical point (3206.2 psia, 705.4°F).

hfg = hg - hf

At atmospheric pressure (14.696 psia), hfg = 970.3 Btu/lbm. This high latent heat makes steam an effective heat transfer medium, delivering substantial thermal energy during condensation without temperature change.

For HVAC applications, the decrease in hfg at elevated pressures affects heat exchanger sizing. At 50 psig (64.7 psia), hfg = 915.5 Btu/lbm, approximately 6% less than atmospheric pressure.

Specific Volume and Density

Specific volume (v) is the volume occupied per unit mass, expressed in ft³/lbm (SI: m³/kg). For saturated steam, specific volume decreases dramatically with increasing pressure:

Specific Volume Variation:

• 1 psia: vg = 333.6 ft³/lbm (density = 0.00300 lbm/ft³)
• 14.696 psia: vg = 26.80 ft³/lbm (density = 0.0373 lbm/ft³)
• 100 psia: vg = 4.432 ft³/lbm (density = 0.226 lbm/ft³)

This property directly impacts piping sizing and steam velocity calculations. Maximum recommended steam velocities range from 6000-10000 ft/min depending on pressure to limit erosion and noise.

For superheated steam, specific volume increases with temperature at constant pressure, following the ideal gas law approximation for moderate superheat:

v = (RT) / P

Where R = 85.78 ft·lbf/(lbm·°R) for water vapor.

Enthalpy

Steam enthalpy (h) represents the total thermal energy content per unit mass. Three enthalpy values define the saturation state:

• hf: Enthalpy of saturated liquid (sensible heat from 32°F reference)
• hfg: Latent heat of vaporization (energy to convert liquid to vapor)
• hg: Enthalpy of saturated vapor (hg = hf + hfg)

For superheated steam, enthalpy exceeds hg by the superheat energy:

hsup = hg + cp,sup × ΔTsup

Where cp,sup ≈ 0.45-0.50 Btu/(lbm·°F) for typical superheat conditions.

Enthalpy is the primary property used in energy balance calculations for steam heating equipment, condensers, and heat exchangers. The energy delivered by condensing steam is:

Q = ṁ × (hsup - hf,condensate)

Steam Quality

Quality (x) quantifies the vapor fraction in a two-phase liquid-vapor mixture, ranging from 0 (saturated liquid) to 1 (saturated vapor):

x = mvapor / (mvapor + mliquid)

Properties of wet steam are calculated using quality:

• h = hf + x × hfg
• v = vf + x × vfg
• s = sf + x × sfg

Steam quality below 0.96-0.97 causes erosion in piping and turbine blades due to entrained liquid droplets. Proper steam separator design maintains quality above this threshold. In HVAC applications, steam traps remove condensate to maintain dry saturated or superheated conditions.

Superheat

Superheat occurs when steam temperature exceeds the saturation temperature at a given pressure. The degree of superheat is:

ΔTsup = Tactual - Tsat

Superheated steam behaves as a compressible gas, with properties determined by both pressure and temperature. Unlike saturated steam, superheated conditions do not maintain constant temperature during sensible heat transfer.

Applications of Superheat:

• Prevents condensation in distribution piping and reduces water hammer risk
• Improves turbine efficiency and blade longevity in power generation
• Eliminates two-phase flow complications in control valves
• Required for accurate steam flow measurement with differential pressure devices

Typical superheat in HVAC systems ranges from 10-50°F above saturation. Greater superheat reduces energy delivery per pound of steam, as the superheat heat capacity (cp ≈ 0.47 Btu/lbm·°F) is significantly lower than the latent heat.

Steam Tables and Property Evaluation

ASHRAE Fundamentals Chapter 1 provides comprehensive steam tables in both I-P and SI units. Modern practice employs IAPWS-IF97 formulations for precise property calculations across all states.

Table Types:

• Saturation tables (temperature basis): Properties tabulated at discrete temperatures
• Saturation tables (pressure basis): Properties tabulated at discrete pressures
• Superheated steam tables: Properties at specified pressure-temperature combinations

Linear interpolation between table entries introduces minimal error for most HVAC calculations. For precise work, use steam property software based on IAPWS-IF97.

Critical Point Properties:

• Pressure: 3206.2 psia (22.064 MPa)
• Temperature: 705.4°F (374.1°C)
• Specific volume: 0.0503 ft³/lbm (0.00317 m³/kg)

Beyond the critical point, no phase distinction exists between liquid and vapor.

Practical Considerations:

Steam property accuracy directly affects system performance predictions. A 5% error in enthalpy translates to equivalent errors in calculated steam flow rates, heat transfer, and energy consumption. Always verify:

• Pressure measurement reference (gauge vs. absolute)
• Temperature measurement location relative to pressure taps
• Steam quality at measurement points
• Subcooling of condensate returns

Reference steam tables assume pure water. Real steam systems contain dissolved gases and minerals that slightly alter properties, particularly affecting condensate pH and corrosion potential.

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