Unit
- Steam as a working fluid: Water in its vapor phase carries high latent heat (approximately 2,257 kJ/kg at atmospheric pressure) that makes it ideal for transferring thermal energy in industrial processes. The ability to store and release heat during phase change allows steam to deliver uniform temperatures at constant pressure.
- Phase change diagram: The pressure-temperature (P-T) diagram shows the triple point (where solid, liquid, and vapor coexist) and the critical point (beyond which liquid and vapor phases become indistinguishable). Below the triple point, water sublimes directly from solid to vapor.
- Saturated steam vs. superheated steam: Saturated steam exists at the boiling point for a given pressure and contains both liquid and vapor phases in equilibrium. Superheated steam is heated above the saturation temperature, providing a dry gas that eliminates condensation concerns in turbine applications.
- Dryness fraction: Defined as the mass fraction of vapor in a saturated mixture; a dryness fraction of 0.9 means 90% vapor and 10% liquid by mass. Low dryness fraction reduces heat transfer efficiency and can cause water hammer in pipes.
- Steam tables: These reference tables list specific volume, enthalpy, entropy, and internal energy for saturated and superheated steam at various pressures and temperatures. Engineers use steam tables to calculate energy balances, process heating requirements, and turbine performance.
- Enthalpy of vaporization: The energy required to convert 1 kg of saturated liquid to saturated vapor at constant pressure. This value decreases as pressure increases until reaching zero at the critical pressure (22.06 MPa for water).
- Quality measurement: Industrial methods to determine steam quality include calorimeter tests (throttling calorimeter), electrical conductivity measurement, and temperature drop across a known heat load. Throttling calorimeters superheat the sample and compare actual temperature with theoretical saturation temperature.
- Steam as a heat transfer medium: Because condensation occurs at constant temperature, steam provides uniform heat flux to process equipment. This characteristic makes steam preferable to hot water for many heating applications where temperature uniformity is critical.
- Pressure-enthalpy diagram: The Mollier diagram plots enthalpy against entropy and is used to visualize expansion processes in steam turbines and throttling processes in pressure reducing stations. Engineers use it to determine steam conditions after a pressure drop.
Process Flow
- Step 1 - Feedwater heating: Cold feedwater enters the economizer where exhaust gases preheat it to near-saturation temperature, recovering waste heat.
- Step 2 - Boiling in the evaporator: Preheated water enters the boiler drum where it absorbs latent heat from combustion gases, forming saturated steam with a typical dryness fraction of 0.97-0.99.
- Step 3 - Separation and drying: Steam passes through chevron separators and scrubbers inside the drum to remove entrained moisture droplets, raising dryness fraction above 0.995.
- Step 4 - Superheating (optional): For high-temperature applications, saturated steam passes through a superheater section where its temperature rises 50-150°C above saturation at constant pressure.
- Step 5 - Distribution to load: Steam flows through insulated mains to process users, experiencing pressure drops due to friction and heat losses that cause partial condensation.
Quick Revision
- Steam carries high latent heat, making it an efficient thermal energy carrier for industrial processes.
- Phase change behavior is governed by pressure-temperature relationships shown in steam tables.
- Dryness fraction measures the vapor-to-liquid ratio; low quality reduces heat transfer and risks water hammer.
- The Mollier diagram helps visualize expansion and throttling processes in steam systems.