Module 09 · Heat Exchangers and Users4 min read
Jacketed Vessels, Coils and Direct Steam Injection
A batch of liquid can be heated through a wall — a jacket around the vessel or a coil inside it — or by blowing steam straight into it. Indirect heating keeps steam and product apart; direct injection uses every kilojoule of the steam but adds its water to the product.
You'll learn
- Describe jackets, coils and direct injection
- Calculate steam needed for a batch, direct and indirect
- Know when dilution rules out direct injection
- Drain and vent jackets and coils correctly
Before this: Latent heat, U-value, LMTD and fouling
BeginnerThe idea in plain words, and where you meet it. No maths.
Through a wall, or straight in
- Jacketed vessel: steam fills a jacket around the vessel wall and heats the contents through it. Common in food, pharmaceuticals and chemicals.
- Coil: a steam coil immersed in a tank heats the liquid around it. Cheaper than a jacket for large tanks.
- Direct steam injection: steam is blown straight into the liquid through a sparger or injector and condenses in it.
Jackets and coils keep the product clean and its quantity unchanged. Direct injection heats faster and wastes nothing, but the condensed steam becomes part of the product.
IntermediateThe formula, its units, and a worked example on real plant numbers.
How much steam each takes
Worked example
Heating 1,000 kg of water from 30 to 90 °C with 4 kg/cm²(g) steam
Given
- Heat needed: 251.2 MJ.
- Indirect (jacket or coil), condensate leaving at saturation: only latent heat is used, 2,109.4 kJ/kg. Steam: 119.1 kg.
- Direct injection: the steam gives up its latent heat and also cools to 90 °C in the product, 2,370.5 kJ/kg. Steam: 106.0 kg.
Answer: Direct injection uses 11% less steam, but adds 106.0 kg of water to the batch — 10.6% dilution.
Indirect heating can recover much of that difference by returning the hot condensate to the boiler, which direct injection cannot do: its condensate is in the product.
ExpertThe fine print: where the simple model breaks, and what it costs.
Doing each well
- Jackets: steam in at the top, trap at the lowest point, air vent at the point farthest from the inlet, and a vacuum breaker so the jacket drains when steam is shut off.
- Coils: slope them to drain continuously to the trap; a coil that lifts condensate is prone to waterlogging and stall on temperature control.
- Direct injection: use a purpose-made injector or sparger, not an open pipe, to condense the steam quietly and evenly; check valves stop product being drawn back into the steam line when steam is shut off.
- Steam quality for injection must suit the product: culinary or clean steam, free of boiler chemicals, for food and pharmaceuticals.
Key takeaways
- Jackets and coils heat indirectly; injection heats directly and dilutes the product.
- Heating 1,000 kg of water 30→90 °C takes 119 kg of steam indirectly, 106 kg directly.
- Indirect heating can recover its condensate; direct injection cannot.
- Drain jackets at the lowest point, vent air, fit vacuum breakers; use clean steam for injection.