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.
Frequently Asked Questions
What is the difference between saturated and superheated steam? Saturated steam is at its boiling point for a given pressure — temperature and pressure are directly linked, and it carries maximum latent heat. Superheated steam is heated above saturation temperature at the same pressure, carrying additional sensible heat; it is used in turbines to avoid condensation damage.
How much steam do I need for a given heating load? Steam flow (kg/h) = Q (kW) × 3600 / h_fg (kJ/kg). For example, a 500 kW heater using 5 barg steam (h_fg = 2085 kJ/kg) needs 500 × 3600 / 2085 = 863 kg/h.
Why is condensate return important? Condensate is hot (90-150°C) distilled water. Returning it saves 15-20% of fuel energy, reduces boiler water treatment chemicals, and cuts makeup water and blowdown. Target 70-90% condensate return.
What is flash steam and how is it recovered? When hot condensate at high pressure discharges to lower pressure, a percentage flashes to steam. For example, 10 barg condensate flashing to atmosphere produces ~16% flash steam, which can be recovered in a flash tank for low-pressure heating instead of being vented.
How often should steam traps be checked? 20-30% of steam traps fail open in typical plants. Implement an annual trap survey using ultrasonic testing. A single failed 10mm trap at 7 barg can waste ~$5,000/year in steam.
Related Engineering Tools
- Steam Trap Selection — Choose the right trap type
- Condensate Recovery Systems — Design condensate return
- Heat Transfer Calculator — Sizing heat exchangers
- Heat Exchanger Selection — Select process heat exchangers
- Insulation Selection Guide — Insulate steam lines
- Pressure Drop Calculator — Steam line pressure loss
- Pipe Velocity Calculator — Steam line sizing
- Water Hammer Analysis — Prevent steam line water hammer