Choosing a Heating Medium for a Process
Before choosing a boiler, choose how the heat should reach the product. Process temperature, how precisely it must be held, whether combustion gases may touch the product, and how many users there are narrow the choice quickly — often to one answer.
You'll learn
- Work through the questions that decide a heating medium
- Relate process temperature to steam pressure
- Know when direct and indirect heating apply
- Recognize when a plant needs more than one medium
Before this: Steam vs hot water vs thermic fluid, Conduction, convection and radiation
Five questions
- What temperature does the product need?
- How tightly must that temperature be held?
- Can combustion gases or water touch the product?
- How many users are there, and how far apart?
- Is the heat needed steadily or in batches?
The first question usually decides most of it. Low temperatures favor hot water; moderate temperatures with many users favor steam; high temperatures favor thermic fluid; drying and curing favor hot air.
Temperature and pressure
| Steam temperature, °C | Pressure, kg/cm²(g) |
|---|---|
| 120 | 1.0 |
| 150 | 3.8 |
| 180 | 9.2 |
| 200 | 14.8 |
| 230 | 27.5 |
| 250 | 39.5 |
Computed from IAPWS-IF97. Add a temperature difference over the product temperature for the heating surface.
Up to about 200 °C, steam pressure stays moderate and steam is usually the simplest medium for several users. Above that the pressure climbs steeply and thermic fluid, which reaches 250–300 °C near atmospheric pressure, usually wins.
- Indirect heating: the medium never touches the product — steam in a jacket, thermic fluid in a coil, hot air from a heat exchanger.
- Direct heating: steam injected into the product, or combustion gases blown through it, as in some dryers. Highly efficient, but only where contact is acceptable.
The trade-offs
- Steam gives the most even heating and the simplest control, but needs traps, condensate return and water treatment.
- Hot water is safe and simple below 100 °C but needs large flows and pumps.
- Thermic fluid reaches high temperatures at low pressure, with no condensate or water treatment, but the fluid degrades and heat transfer is lower.
- Hot air suits drying, curing and spray drying; indirect air heaters keep combustion products away from food or pharmaceuticals.
- Direct firing is the cheapest heat per unit where the product tolerates combustion gases, as in some agro and mineral drying.
Many plants run two media: steam for general heating and sterilizing, thermic fluid for one high-temperature machine, or hot air for a dryer. Forcing one medium onto every user usually means oversizing pressure for the many to suit the few.
Key takeaways
- Product temperature, control, contact, number of users and load pattern decide the medium.
- Steam needs about 15 kg/cm²(g) to condense at 200 °C; above that, thermic fluid usually wins.
- Indirect heating keeps the medium off the product; direct heating is cheapest where contact is allowed.
- A plant often needs more than one medium.