Why Steam?
Steam is the most widely used heat transfer medium in industrial plants because it:
- Carries large amounts of latent heat (2,257 kJ/kg at 100°C)
- Transports through pipes without pumps (pressure-driven flow)
- Provides uniform, predictable temperature at a given pressure
- Is clean, non-toxic, and cheap (water is the raw material)
- Releases heat at constant temperature (condensation)
Steam Properties
Saturated Steam Table (Key Points)
| Gauge Pressure | Temperature | Latent Heat (kJ/kg) | Specific Volume (m³/kg) |
|---|---|---|---|
| 0 bar (atmospheric) | 100°C | 2257 | 1.673 |
| 1 barg | 120°C | 2201 | 0.881 |
| 3 barg | 144°C | 2133 | 0.461 |
| 5 barg | 159°C | 2085 | 0.315 |
| 7 barg | 170°C | 2047 | 0.240 |
| 10 barg | 184°C | 1999 | 0.177 |
| 15 barg | 201°C | 1947 | 0.132 |
Steam System Components
Boiler
Generates steam by heating water. Common types:
- Firetube: Hot gases through tubes in water; low pressure (<17 barg); for heating
- Watertube: Water through tubes in hot gas; high pressure/temperature; power generation
- Electric: Clean, small-scale; expensive to operate
- Package boiler: Factory-assembled watertube/firetube, 1-50 ton/h
Typical boiler efficiency: 75-85% (fuel-to-steam).
Steam Distribution Piping
Design considerations:
- Size steam lines for 15-40 m/s velocity (saturated)
- Slope 1:100 in direction of flow to drain condensate
- Install drip legs and steam traps at 30-50m intervals, all low points, before risers
- Insulate all lines (mineral wool or calcium silicate)
- Use separators near turbines/engines to remove entrained water
Pressure Reducing Valves (PRVs)
Steam is generated at high pressure (smaller distribution pipes, higher enthalpy) then reduced at point of use:
- Higher pressure = smaller pipes, lower capital cost
- Lower pressure = lower temperature, safer, better heat transfer control
- Use pilot-operated PRVs for accurate pressure control
- Install safety valve downstream of PRV (in case PRV fails open)
Steam Traps
Steam traps discharge condensate while preventing live steam from passing:
| Trap Type | Operation | Best For |
|---|---|---|
| Inverted bucket | Mechanical (buoyancy) | General process, robust, handles dirt |
| Float & thermostatic (F&T) | Float + thermostatic air vent | Best for heat exchangers, high condensate loads |
| Thermodynamic (disc) | Flash steam dynamics | Drip legs, tracing, high pressure; simple, cheap |
| Thermostatic (bellows) | Temperature difference | Radiators, low-pressure heating |
| Bimetallic | Thermal expansion | High temp, superheat |
Trap sizing: select for 2-3× expected condensate load at operating pressure differential.
Condensate Return
Condensate is valuable:
- It's hot (90-150°C) — returning it saves 15-20% of fuel energy
- It's pure distilled water — reduces boiler water treatment cost/chemicals
- It reduces makeup water requirements (and blowdown)
Condensate return system:
- Traps discharge into condensate header (2-5m/s)
- Flash steam forms when high-pressure condensate enters lower-pressure return
- Flash tanks recover flash steam for low-pressure heating
- Condensate pumped back to deaerator/boiler feed tank
Typical target: return 70-90% of condensate.
Basic Calculations
Heating Load to Steam Flow
Worked Example
A process heater needs 500 kW of heating. Steam at 5 barg (hfg = 2085 kJ/kg).
m = 500 × 3600 / 2085 = 863 kg/h steam flow
Pipe Sizing for Steam
Size for velocity (not pressure drop alone):
- Saturated steam: 15-25 m/s (short runs can go to 40 m/s)
- Superheated steam: 30-60 m/s
- Exhaust steam: 30-50 m/s
- Vacuum steam: up to 100 m/s
Use steam tables for specific volume at operating pressure:
Deaeration and Feedwater
Dissolved gases (O₂, CO₂) cause severe corrosion in boilers and condensate systems. Deaerators remove dissolved gases by heating feedwater to saturation:
- Spray-type or tray-type deaerators
- Heat water to within 1-2°C of saturation
- Reduces O₂ to < 7 ppb
- Vent non-condensable gases
Boiler feedwater treatment: chemical oxygen scavenger (sodium sulfite or hydrazine), pH control (9.0-10.0), phosphate treatment.
Flash Steam Recovery
When hot condensate at high pressure is discharged to a lower pressure, some percentage "flashes" to steam:
Worked Example
Condensate at 10 barg (hf = 781 kJ/kg) discharges to atmospheric (hf = 419, hfg = 2257 kJ/kg):
% Flash = (781 − 419) / 2257 × 100 = 16%
That 16% can be recovered for low-pressure heating instead of being vented.
Energy Efficiency Best Practices
- Maximize condensate return — 80% return vs 0% saves ~15% fuel
- Insulate everything — uninsulated 100mm steam line loses ~1.5 kW/m
- Fix failed steam traps — annual ultrasonic survey
- Recover flash steam — use for LP heating or deaerator
- Optimize boiler pressure — generate at lowest practical pressure
- Recover boiler blowdown heat — blowdown heat exchanger preheats makeup
- Preheat combustion air — economizer/air preheater adds 3-5% efficiency
- Use pressure regulators at point of use — match steam pressure to process need
Steam System Layout Best Practices
- Drip leg at every 30-50m and every low point
- Strainer before every PRV, trap, and control valve
- Separator before any equipment sensitive to water carryover (turbines)
- Pressure gauge and thermometer on each header and at equipment
- Safety valve at boiler and downstream of each PRV
- Air vents at end of mains (air is a major insulator and cause of slow heating)
- Slope mains in direction of flow
- Provide expansion loops/bellows for thermal growth
Summary
Steam is an efficient heat transport medium using latent heat of vaporization. Saturated steam temperature depends only on pressure — select pressure based on process temperature needs. Key components: boiler, distribution piping with drip traps, PRVs, steam traps, and condensate return system. Returning condensate and fixing failed traps are the highest-return energy improvements. Design steam lines for 15-40 m/s velocity and slope them to drain condensate safely. Always warm steam lines gradually to avoid water hammer.