Steam, Hot Water or Thermic Fluid: Choosing a Heating Medium
A boiler house can deliver heat as steam, as hot water or as hot thermal oil. Each carries heat differently, reaches different temperatures at different pressures, and falls under different rules. The process temperature usually narrows the choice to one.
You'll learn
- Compare steam, hot water and thermic fluid
- Know which temperatures each reaches, and at what pressure
- Understand why steam carries so much heat per kilogram
- Pick a medium for a given process
Before this: Latent heat, Saturation pressure and temperature
Three ways to carry heat
- Steam gives up its latent heat by condensing at a fixed temperature. Small pipes carry a lot of heat, and the process temperature is set by steam pressure.
- Hot water gives up sensible heat by cooling. Simple and safe below 100 °C, but it needs pumps and larger pipes.
- Thermic fluid, a thermal oil, also gives up sensible heat, but stays liquid up to around 300 °C at near-atmospheric pressure.
As a rough rule: below 100 °C with no special need, hot water. Up to about 200 °C with several users, steam. Above about 200 °C, where steam would need very high pressure, thermic fluid.
Temperature, pressure and heat carried
| Steam | Hot water | Thermic fluid | |
|---|---|---|---|
| How it gives heat | Condensing (latent) | Cooling (sensible) | Cooling (sensible) |
| Temperature control | By pressure | By flow and supply temperature | By flow and supply temperature |
| Pressure for high temperature | High | High above 100 °C | Low, near atmospheric |
| Pumps needed | Feed pump only; flows by pressure | Circulation pump | Circulation pump |
| Under the Boilers Act | Yes, above s. 2(c) thresholds | Not below 100 °C (s. 2(c)(iii)) | No: generates no steam |
| Typical uses | Most process heating, sterilizing, cooking, turbines | Space and low-temperature process heating | Textile stenters, oil and chemical heating, presses |
Pressure is what pushes high-temperature work towards thermic fluid. For steam to condense at 250 °C it must be at about 39.5 kg/cm²(g), which means a heavier, regulated boiler and pipework. Thermic fluid reaches 250 °C with only the pressure its pump develops.
Steam’s advantage is how much heat each kilogram carries. A kilogram of steam condensing at 4 kg/cm²(g) gives 2,109 kJ; to deliver the same from hot water cooling by 20 °C takes about 25 kg of water.
The trade-offs behind the rule of thumb
Steam’s constant condensing temperature gives even heating and simple control, and condensing coefficients are high, so heat exchangers are small. Against that: traps, condensate return, flash losses, water treatment and blowdown. A steam system that is poorly maintained loses heat at every trap and leak.
Thermic fluid systems have no condensate, no traps and little water treatment, and their pipework is light. But the fluid gives up heat by cooling, so the heater must be sized for the flow and temperature drop, heat transfer coefficients are lower, and the oil itself degrades with time and overheating. Fluid life depends on keeping film temperatures in the heater coil within the oil’s limit, which is why flow must never fall while the burner fires.
Hot water is the most forgiving, and below 100 °C it is outside the Act. Above 100 °C, pressurized hot water behaves like a boiler’s water and has to be treated as one. Many plants run more than one medium: steam for general process heat, thermic fluid for one high-temperature machine.
Key takeaways
- Steam carries latent heat and holds a fixed temperature; hot water and thermic fluid carry sensible heat.
- Steam at 250 °C needs about 40 kg/cm²(g); thermic fluid reaches it near atmospheric pressure.
- Rough rule: hot water below 100 °C, steam to about 200 °C, thermic fluid above.
- Thermic fluid heaters generate no steam and sit outside IBR; steam boilers above the thresholds do not.