Direct Steam Injection for Tanks: Steam Demand, Sparge Pipes and Injectors

Direct steam injection uses steam both as the heating medium and as part of the final liquid inventory. Steam is discharged into a colder liquid as bubbles, where it condenses and transfers heat directly to the surrounding fluid.

The method is suitable only where dilution and the resulting increase in liquid mass are acceptable. It is therefore commonly used for water heating and boiler feedtank duties.

How heat is transferred

As steam bubbles condense, the process receives the latent heat of the steam and part of the sensible heat of the resulting condensate. The condensate remains in the tank.

Calculating steam consumption

ṁ_s = Q̇ / (h_g − T_f · c_p)

• ṁ_s: mean steam flowrate, kg/s
• Q̇: mean heat-transfer rate, kW
• h_g: total steam enthalpy upstream of the control valve, kJ/kg
• T_f: final water temperature
• c_p: water specific heat capacity

Increase in liquid mass and level

Final liquid mass = initial mass + injected steam mass

The resulting increase must be considered in tank freeboard and overflow design.

Factors affecting bubble condensation

Bubble size

Small bubbles provide more surface area per unit volume and condense more readily.

Differential pressure

Lower differential pressure limits bubble expansion and discharge velocity.

Liquid head

Greater liquid depth increases backpressure and residence time. A 1 m head of water represents approximately 0.1 bar.

Liquid temperature

As liquid temperature approaches steam temperature, the driving temperature difference and condensation rate decrease.

Sparge pipes

A sparge pipe is a blanked horizontal pipe with small holes along its length. It is simple and inexpensive but can become noisy at higher steam pressure.

For the simple arrangement discussed here, steam pressure is recommended to be limited to approximately 2 bar(g).

Hole orientation

Holes are practically positioned around the 4 and 8 o'clock locations. They should not discharge directly toward a nearby tank floor or wall.

Steam velocity in the sparge

• sparge shorter than 1 m: approximately 15 m/s,
• longer sparge: up to approximately 25 m/s.

Worked heating example

• water mass: 12,000 kg
• temperature: 8°C → 60°C
• heat-up time: 2 h
• steam pressure upstream of control valve: 2.6 bar(g)
• h_g = 2,733.89 kJ/kg

Water-heating duty: 363 kW

Steam for water heating: 0.146 kg/s

Steam for tank heating: 0.0052 kg/s

Steam for heat losses: 0.0071 kg/s

Total mean steam flow ≈ 570–572 kg/h

Sparge sizing

Estimated sparge pressure is approximately 2.1 bar(a), or 1.1 bar(g).

At 570 kg/h and 25 m/s, the calculated bore is 82.6 mm and DN100 is selected.

Sparge position and holes

• sparge length: approximately 2.5 m,
• height above tank floor: at least 0.33 m,
• water head above sparge: approximately 1 m,
• hole differential pressure: approximately 1 bar,
• hole diameter: 4 mm,
• flow per hole: approximately 8 kg/h,
• number of holes: approximately 72,
• hole centres: approximately 70 mm.

Final mass increase

At 572 kg/h for 2 h, 1,144 kg of water is added. The tank contents increase from 12,000 to 13,144 kg and the level rises by approximately 127 mm in a 3 × 3 m tank.

Steam injectors

A steam injector draws colder liquid into the body and mixes it intensively with steam. The resulting turbulent mixing promotes circulation and reduces temperature stratification.

Relevant current categories include steam injectors, control valves and upstream pipeline strainers.

Injector noise and vibration

Normal running

Above about 2 bar(g), a soft continuous roar may occur. Injector discharge is approximately 10°C hotter than incoming recirculated water.

Incomplete condensation

At liquid temperatures typically above about 90°C, bumping, vibration and some steam escape may occur.

Low flow

Below approximately 1.5 bar(g), characteristic crackling and cavitation-like behaviour may occur.

Injector-selection example

For approximately 572 kg/h at 1 bar(g), an injector with 400 kg/h capacity is insufficient on its own:

572 / 400 = 1.43 → 2 injectors required

Alternative heat-balance calculation

m · h₁ + m_s · h_g = (m + m_s) · h₂

For the same operating data, the heat-balance method gives the same overall mean requirement of approximately 570 kg/h.

Sparge pipe or steam injector?

Conclusion

Direct steam injection is suitable where dilution and liquid-mass increase are acceptable. Correct performance depends on bubble size, pressure differential, liquid depth, temperature, equipment position and steam-flow control.

A sparge pipe provides a simple low-pressure solution, while a steam injector provides stronger mixing, improved circulation and operation at higher steam pressures.

Frequently Asked Questions

When can direct steam injection be used?

When steam condensate can mix with the process fluid and dilution is acceptable.

Why are small bubbles preferable?

They provide more surface area per unit volume and condense more readily.

How does liquid depth help?

It increases backpressure and bubble residence time.

What pressure is recommended for a sparge pipe?

Approximately up to 2 bar(g) for the simple arrangement discussed here.

Why does tank level rise?

Because injected steam condenses and remains in the tank as water.

What advantage does a steam injector provide?

It increases mixing and circulation and reduces temperature stratification.

What can injector crackling indicate?

Very low flow or pressure, incomplete condensation and possible oversizing.